# What is the difference between $E_gamma$ and $E_{gamma,iso}$ in gamma ray bursts?

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In the literature on gamma ray bursts I see several references to $$E_gamma$$ and $$E_{gamma,iso}$$ for example as in Piran (2005)

$$E_{k,iso,52}$$ is “isotropic equivalent” kinetic energy, discussed below, in units of 1052 ergs, while $$E_{k,θ,52}$$ is the real kinetic energy in the jet i.e: $$E_{k,θ,52} = (θ^2/2)E_{k,iso,52}$$. One has to be careful which of the two energies one discusses. In the following I will usually consider, unless specifically mentioned differently, $$E_{k,iso,52}$$, which is also related to the energy per unit solid angle as: $$E_{k,iso,52}/4π$$

My interpretation is that when an observation of a GRB is made, an energy $$E$$ is recorded. $$E_{gamma,iso}$$ is the total energy that would have been emitted by the explosion if the energy was released equally in all directions and by considering our distance away from it. However, because we know that GRBs are tightly beamed, $$E_gamma$$ is the corrected energy by taking $$E_{gamma,iso}$$ and considering only a small fraction of it in a tight beam. However, this all seems a tad confusing and I can't see the point in talking about $$E_{gamma,iso}$$ when we think they are very tightly beamed.

I would appreciate some clarification on what exactly $$E_gamma$$ and $$E_{gamma, iso}$$ are and where they come from.

Thanks

I will answer with what I now believe to be the correct answer. We see some gamma-ray burst emission hit our satellite which carries an amount of energy $$E_{obs}$$. If we say the burst was emitted equally in all directions (isotropic) then we derive a quantity $$E_{iso}$$. If we say that the burst was actually emitted in a tight beam then we derive $$E_ heta$$ where $$heta$$ is the opening angle of the burst and we will find $$E_{iso} > E_{ heta}$$. $$E_{iso}$$ is used to compare burst energies in the literature without needing to find the opening angle.

## Observing the highest-energy processes in the Universe

Light is made up of waves of alternating electric and magnetic fields that can be measured by frequency (number of waves that pass by a point in one second) or wavelength (the distance from the peak of one wave to the next). The larger the frequency, the smaller the wavelength. The spectrum ranges from the very lowest frequencies of radio waves and microwaves to the mid-range frequencies found in infrared, optical (visible) and ultraviolet light to the very highest frequencies of X-rays and gamma rays. The frequency range of gamma rays is so vast that it doesn’t even have a well-defined upper limit. The gamma rays CTA will detect are about 10 trillion times more energetic than visible light.

The electromagnetic spectrum provides scientists with a variety of ways to view the Universe. As seen in the figures below, telescopes detecting different frequencies of light provide different perspectives of the Milky Way and the Crab Nebula, providing a more complete picture of the phenomena they are studying. With its ability to view the highest-energy processes in the Universe, CTA will be a vital asset in improving our understanding of these phenomena.

For example, supernova remnants – the giant explosion shells generated by dying stars – are suspected as accelerators of cosmic rays. CTA will have the resolution to identify specific regions of supernova remnants and probe the presence of high-energy cosmic rays, that serve as sources of gamma rays.

Top Background Image Credit: ESA/NASA/Felix Mirabel

### Non-Thermal Emissions

Most of the light we are used to seeing is emitted by hot objects and is known as thermal radiation. The hotter the source of this radiation, the higher the frequency of the light produced. However, it is not possible for objects to get hot enough to produce gamma rays these must be produced by a non-thermal mechanism. The mechanisms often rely on the presence of high-energy sub-atomic particles that are produced by some kind of cosmic particle accelerator.

Accelerated particles develop in special environments where a small fraction of the particles can take on an “un-fair” share (or fraction) of the energy available. In such a system, a small number of particles can be accelerated to very close to the speed of light and carry a significant fraction of the energy available. Since energy is no longer shared roughly equally among particles – as is the case in a “normal” hot environment – these processes are referred to as non-thermal processes.

These special environments are usually associated with violent events such as explosions, outbursts or powerful jets of material produced close to the giant black holes at the centre of galaxies. For this reason, gamma rays can be used to trace violent events in the universe.

### Cosmic Sources

CTA will be sensitive to the highest-energy gamma rays, making it possible to probe the physical processes at work in some of the most violent environments in the Universe. Although cosmic gamma rays cannot reach the earth’s surface, CTA can detect them from the ground using the subatomic particle cascades that they produce in the atmosphere. Charged particles in these cascades travel at very close to the speed of light and emit visible (mostly blue) light known as Cherenkov light. CTA’s large telescope mirrors and ultra-high-speed cameras can then collect and record the nanosecond flash of light so that the incoming gamma ray can be tracked back to its cosmic source. (See How CTA Works)

CTA will be able to detect hundreds of objects in our galaxy, the Milky Way. These galactic sources will include the remnants of supernova explosions, the rapidly spinning ultra-dense stars known as pulsars and more normal stars in binary systems and large clusters. Beyond the Milky Way, CTA will detect star-forming galaxies and galaxies with supermassive black holes at their centres (active galactic nuclei or AGN) and, possibly, whole clusters of galaxies. The gamma rays detected with CTA may also provide a direct signature of dark matter, evidence for deviations from Einstein’s theory of relativity and definitive answers to the contents of cosmic voids, the empty space that exists between galaxy filaments in the Universe.

#### Cosmic Rays

What makes a ray a cosmic ray? Even though they are called “rays,” cosmic rays are really just normal atomic particles. Despite being “normal” matter, cosmic rays are special because they are accelerated to extraordinarily high energies, traveling very close to the speed of light. Primarily in the form of high-energy protons and atomic nuclei, cosmic rays constantly bombard the earth, but despite a century-long search, we know very little about their sources and the role they play in our own galaxy and beyond. Gamma rays are produced in the interactions of cosmic rays and provide the most sensitive means to study cosmic rays in and around their sources.

#### Black Holes

Black holes are among the most mysterious objects in astronomy. They are thought to be very small regions in space-time with a gravitational pull so strong that nothing, not even light, can escape. And they are by no means “black” – they are some of the brightest sources of very-high energy (VHE) gamma rays.

It is believed that most black holes are the relics of massive stars following a supernova explosion. The core of the star collapses under its own gravity to form a black hole, which are typically only a few kilometers in radius but with a mass several times greater than the Sun. When black holes accrete (grow by gravitationally attracting more matter) material from their surroundings, it is a violent, highly energetic process. Much of the material is devoured by the black hole and it grows in size, and the frictional forces within the material spiraling into the black hole make the object immensely luminous.

On a very different size-scale, supermassive black holes are a million to a billion times more massive than the Sun and are assumed to exist in the centre of most galaxies, including our own. While the central black hole in the Milky Way is only detectable through the orbits of stars moving around it, about 10 percent of known galaxies, so-called “active galaxies,” harbour a supermassive black hole that is fueled by a huge accretion disk (a rotating disk of material or gas formed by the black hole’s accretion). The very hot disk can outshine all the stars in the galaxy itself and can produce powerful outflows called “jets,” in some cases longer than the diameter of the Milky Way.

The jets emitted from the centres of these active galaxies, called active galactic nuclei (AGN), offer excellent conditions for particle acceleration to the highest energies and for the emission of gamma rays. AGN account for one-third of all known very-high-energy (VHE) gamma-ray sources and are nearly the only objects we can detect at these energies that are not located in our own galaxy.

CTA aims to measure large samples of such active galaxies, and galactic black holes, to study particle acceleration and gamma-ray emission processes. These observations will give us a picture of the conditions and physical processes occurring in and around some of nature’s most mysterious objects.

#### Supernova Remnants

When certain stars reach the end of their natural lifetime they die in a gigantic explosion called a supernova. The explosion causes a large part of the stellar material to be expelled at thousands of kilometres per second into the surrounding interstellar environment. The resulting shock fronts are called supernova remnants (SNRs), which emit radiation across the whole electromagnetic spectrum and play an important role in the evolution of galaxies.

It is now known that charged particles can be accelerated by SNRs to reach energies beyond those achievable with the most powerful man-made particle accelerator, the Large Hadron Collider at CERN. SNRs may be the dominant source of the cosmic rays that bombard the Earth. Particles accelerated in SNRs are implicated in the growth of magnetic fields in the Universe and can influence star-formation in galaxies.

CTA will be able to detect a much larger number of SNRs in gamma rays than is currently possible and measure the properties of these objects in much greater detail, helping us to understand the process of particle acceleration in SNRs and the propagation of these particles away from SNRs and their subsequent impact on the interstellar medium. Crucially, for the first time, CTA will be able to probe particle acceleration up to PeV (10^15 eV) energies in these objects and for any class of objects within our galaxy. We know from the locally measured cosmic rays that something in our galaxy accelerates particles to those energies, but the sources remain unknown. There is very recent evidence of particle acceleration in the Galactic Centre, but it is not clear if it can provide the local cosmic rays.

#### Pulsars

When a massive star reaches the end of its life, it undergoes a supernova explosion, ejecting most of its outer layers. The remaining core of the star collapses and, depending on its mass, becomes a white dwarf, a neutron star or a black hole. Neutron stars are formed from the collapse of ordinary stars roughly 8-20 times the size of our sun and are incredibly dense – the equivalent of the Earth’s mass condensed into a space the size of 1-2 football stadiums.

In the collapse process, as the radius of the star decreases, the magnetic field becomes stronger and the rate of rotation increases (often rotating many times per second). As it rotates, so does its magnetic field, creating an electric field on the surface that accelerates charged particles. The radiation produced by these particles during their acceleration leads to a beam of electromagnetic emission along the axis of the magnetic field. As the neutron star rotates, the jets may swing past the Earth’s direction, much as the light from a lighthouse passes over the sea, leading to the observation of pulsed objects or “pulsars.”

The pulsar’s rotation rate slows down over time, as it uses its rotational energy to accelerate particles to high energies. These particles, trapped by the magnetic field, rotate in sync with the pulsar out to large distances. At a distance called the light cylinder, the particles would have to travel at the speed of light to continue keeping up with the pulsar. Rather than break the laws of physics, the particles are able to escape from the immediate region around the pulsar at this location, streaming away and creating what is called a pulsar wind. When this ultra-fast wind plows into the surrounding material, it creates a shock wave where particles are accelerated, spreading out into a cloud called a pulsar wind nebula (PWN).

Emissions from both pulsars and their wind nebulae have been detected at TeV energies. Pulsar wind nebulae (PWNe) are the most populous class of galactic objects in this energy range. The most famous PWN is the Crab Nebula, which formed from the Crab supernova explosion in 1054 AD, as recorded by Chinese astronomers. The Crab is one of the brightest TeV sources and was the first TeV gamma-ray source to be detected (in 1989). Pulsation from the Crab pulsar has been detected across the electromagnetic spectrum, from radio up to approximately TeV.

Observations with CTA will provide the first truly detailed gamma-ray images of PWNe and make it possible to probe the motion and cooling of high-energy particles in PWNe. CTA also will greatly increase the number of known PWNe, helping to further the understanding of their evolution and the impact of their environment. Additionally, CTA will provide insights in the central engine of PWNe, the pulsar itself, greatly expanding the number of known VHE pulsars and the precision with which they can be measured.

#### Binary Systems

Binary systems are composed of two stars that closely orbit one other, exchanging matter and energy through accretion processes or via the periodic interaction of their respective winds. Stars much more massive than the sun can have very powerful winds, which in binary systems, collide and can accelerate particles, producing gamma-ray emission. In binaries where one of the stars is a “compact object” like a black hole or a neutron star, the winds (or jets) can be travelling at close to the speed of light and particles can be accelerated to very high energies. As a binary system moves through its orbit, the physical conditions in the collision region change. For this reason, gamma-ray binaries can be thought of as a laboratory for high energy astrophysics, allowing scientists to adjust the parameters of the system and see what happens.

A few hundred of these systems have been discovered in our galaxy thanks to the advent of X-ray astronomy in the 1960’s, but only a handful of binary systems emitting VHE gamma rays have been detected in our galaxy in recent years. The improved capabilities of Cherenkov telescopes (MAGIC, VERITAS and H.E.S.S.) have made these more recent discoveries possible. Their discovery has proven to be extremely useful to study high-energy processes, in particular particle acceleration, emission and radiation reprocessing, and the dynamics of the underlying magnetized flows. CTA will greatly expand the population of gamma-ray binaries and allow us to precisely measure the behaviour of many systems as a function of orbital phase and photon energy. These measurements are expected to cast light on the physics of particle acceleration, as well as the winds of pulsars and massive stars and the way they interact.

#### Dark Matter

The nature of dark matter is one of the biggest outstanding questions in science. The material is known to exist due to its gravitational effects and in far larger quantities than normal matter, but close to nothing is known about what it is. Many hypotheses exist for dark matter, mostly postulating a new very weakly interacting particle (a weakly interacting massive particle, or WIMP). Some of the most promising theories predict WIMPs that can annihilate when they interact to produce more familiar particles. Such annihilations inevitably produce gamma rays.

There is a strong idea as to how often these annihilations should happen in order to give the WIMPs the right density in the Universe today, as well as where to look for this signal – in places where the density of dark matter is very high (the centre of our own galaxy). Up until now, there have not been instruments sensitive enough to see the predicted signal. CTA will reach this critical sensitivity and complement other searches using the Fermi satellite, the large hadron collider and deep underground direct searches for WIMPs. Together these instruments have a very good chance to solve the mystery of dark matter within this decade. More about how CTA will study dark matter.

#### Cosmic Voids

Most of the Universe is very close to empty, with matter clustered into galaxy clusters, super-clusters and filaments, separated by huge voids. How empty these voids are is a matter of great debate. In particular, are there any tiny magnetic fields in these regions that are a relic of the earliest moments of the Universe? CTA will be able to probe magnetic fields in the voids via observations of halos around active galaxies and also help to probe the proposed heating of low-density regions in the Universe by TeV photon interactions. A known ingredient of the space between galaxy clusters is the extragalactic background light, the integrated light of all processes over the history of the Universe in the infrared to ultraviolet range. CTA will be able to characterize these radiation fields via the absorption features that they leave in the spectra of the population of active galaxies seen by CTA.

## What is the difference between $E_gamma$ and $E_{gamma,iso}$ in gamma ray bursts? - Astronomy

Context: .Recently, Liang & Zhang found a tight correlation involving only observable quantities, namely the isotropic emitted energy E γ, iso , the energy of the peak of the prompt spectrum E'_p, and the jet break time t' j of Gamma Ray Bursts. This phenomenological correlation can have a first explanation in the framework of jetted fireballs, whose semiaperture angle θ j is measured by the jet break time t'_j. By correcting E γ, iso for the angle θ j one obtains the so-called Ghirlanda correlation, linking the collimation-corrected energy E_γ and E'_p.
Aims: .There are two ways to derive θ j from t' j in the "standard" scenario, corresponding to a homogeneous or to a wind-like circumburst medium. We compute and compare the E'_p-E_γ correlations derived in these two conditions and study the consistency of these model-dependent correlations with the empirical Liang & Zhang correlation. We consider the difference between the observed correlations and the ones in the comoving frame.
Methods: .We study 18 GRBs with firmly measured z, E_peak and t break and discuss the differences with previously published samples. We compute the correlations accounting for the errors on all the relevant quantities.
Results: .We show that the Ghirlanda correlation with a wind-like circumburst medium is as tight as (if not tighter) than the Ghirlanda correlation for a homogeneous medium. These two Ghirlanda correlations are both consistent with the phenomenological Liang & Zhang relation. The wind-like Ghirlanda relation, which is linear, remains linear also in the comoving frame, independently of the distribution of bulk Lorentz factors. Instead, in the homogeneous density case, one is forced to assume the existence of a strict relation between the bulk Lorentz factor and the total energy, which in turn places constraints on the radiation mechanisms of the prompt emission. The wind-like Ghirlanda correlation, being linear, corresponds to different bursts having the same number of photons.

## Gamma Sterilization Validations VDmax 25 and Method 1

The Association for the Advancement of Medical Instrumentation (AAMI) generates numerous standards used by professionals in the medical device industry. Occasionally, the AAMI Standards board provides additional guidance to specific standards in the form of a Technical Information Report (TIR). These TIRs reflect common industry practices that evolve from an accumulated process knowledge base.

When gamma irradiation is selected for product sterilization, the dose at which the product is irradiated must be established and validated in accordance with appropriate AAMI standards. ANSI/AAMI/ISO 11137: 2006 Sterilization of health care products-Radiation and TIR33: 2005 Sterilization of health care products-Radiation-Substantiation of a selected sterilization dose-Method VDmax25 kGy as a sterilization dose-Method VDmax, provide are established methods for completing a validation process.

#### Which validation is right for me?

The VDmax25 option (formerly TIR27, now in 11137: 2006) is convenient when a company wants several product lines sterilized at the same minimum dose, when product is expensive to make, or for companies with markets where a 25 kGy dose is the accepted standard. Furthermore, the validation is less expensive because fewer tests are necessary. Bioburden counts must be 1000 CFU or less.

Method 1 (from 11137-2) determines the lowest sterilization dose necessary for the determined bioburden population. This Method should be used when the lowest possible sterilization dose is desired due to cost considerations, use of gamma sensitive materials, or when the bioburden count is above 1000 CFU.

#### If one of these validations establishes my minimum dose, how do I establish a maximum dose?

Performed early in product qualification, materials can be screened for compatibility with irradiation. Pre- and postirradiation properties related to functionality and appearance must be evaluated to determine maximum dose. Irradiating your product at a dose approximately 2.0 times that of the minimum (or greater), then testing the product’s form, fit, and function, is an excellent way to establish maximum dose. Setting the maximum dose as high as possible allows the greatest flexibility in processing schedules when product is ready for routine sterilization. Accelerated aging and package testing are additional tests to be considered for product irradiated at the maximum dose.

#### Why is the verification dose experiment performed at a lower SAL than the sterilization dose?

In order to test a dose for SAL 10-6, one million products would need to be irradiated and sterility tested. SAL 10-2 (Method 1) or SAL 10-1 (VDmax) is used because only 100 or 10 products, respectively, must be used for the experiment.

#### What if my product sample is dosed at less than 90% of the target verification (sub-lethal) dose?

If the sterility test exhibits a failing number of positive tests, the verification dose experiment can be performed again and samples re-tested.

#### What if my product sample is dosed greater than the target verification (sub-lethal) dose plus 10%?

This is considered an overdose. During a verification (sublethal) dose experiment, it is not permissible to irradiate over 10% above the target. Do not sterility test the samples. Send new samples for irradiation prior to sterility testing.

#### Do I need a Biological Indicator?

Bacillus pumilus, a spore-forming microorganism, served for many years as a biological indicator to test for sterility. Its use today has been discontinued. The radiation resistance of B. pumilus is generally lower than the dose required to achieve a 10-6 SAL based on the bioburden typically seen on healthcare products.BIs also do not accurately represent natural form of bioburden on a product (spore strip vs. actually in or on product as a result of manufacture).

#### What are the basic steps?

For both types of validations, the first step is identical: Have bioburden testing performed on 10 products from three different batches, for a total of 30 products.

#### Are there other options for dose setting besides VDmax25 and Method 1?

Yes. Contained in 11137-2 and TIR 33 are additional methodsincluding Method 2 (incremental dosing) and VD max for selected doses of 15-35 kGy (in 2.5 kGy increments). Each method has specific limitations and requirements that must be fully investigated before selection.

### Definitions:

#### Bioburden:

Population of viable microorganisms on a product. In the context of irradiation sterilization, bioburden is determined immediately prior to sterilization. The unit of measurement is CFU: Colony Forming Unit.

#### Sterility Test:

Test to determine if viable microorganisms are present on a product and/or package.

#### Sterility Assurance Level (SAL):

Probability of a viable microorganism being present on a product unit after sterilization. Normally expressed as 10-n, the SAL at a particular sterilization dose estimates the likelihood of one positive sterility test out of a total of 10n sterility tests.

#### Recovery Efficiency:

Measure of the ability of a specified technique to remove microorganisms from product.

#### Bacteriostasis/Fungistasis Testing (B&F):

Test performed with selected microorganisms to demonstrate the presence of substances that inhibit the multiplication of these microorganisms. This must be retested if any changes are made to the product. It is recommended that even without changes that the test be repeated every 1-3 years to account for any changes in raw materials or suppliers. The number of samples required for this testing should be confirmed with the laboratory performing the testing (usually 3-6).

## History

E-102 Gamma was a robotic life form created by Dr. Eggman. As such, its name could refer to many characters in the Sonic universe. Nearly all E-100 models were identical in design, with slight modifications such as expanded weaponry. While they varied in rank, E-100's were considered a profound model by Eggman, and thus usually had a close role with their master, (somewhat replacing the traitorous Metal Sonic as a right-hand minion or personal muscle). Because of their form and origin, however, nearly all E-100s played a tragic role in their starring game.

E-102 Gamma was created by Dr. Eggman as the second unit in the E-100 Series. E-102 Gamma was powered by a captured pink bird held inside the robot. Although E-102 Gamma was created by and made to work under the orders of Dr. Eggman without question, which involved assisting in empowering the mystical water elemental Chaos, he eventually turned against his maker's will and devoted his short existence to freeing the animals contained in the other E-100 units.

Immediately upon his 'birth', he was ordered by Dr. Eggman to pass a unique training course and to fight his older 'brother', E-101 Beta, to become a crew member on the Egg Carrier and prove the efficiency of his design. After defeating Beta in combat, his first task on board involved capturing a frog who had absorbed Chaos' tail (namely Big's pet "Froggy", who also held a Chaos Emerald that Eggman desired). The mission also served as a contest against his younger brothers, E-103 Delta, E-104 Epsilon and E-105 Zeta, who were all modeled upon Gamma. Gamma passed the mission but as a result of his success, he was forced to watch his less-successful brothers be exiled for their failure and remodeled for generic 'Badnik' duty in undisclosed locations. Gamma was distinctly haunted by the memory of Delta turning to look at him as he was warped away this memory came up several times later in the game.

Eggman then told Gamma, now the sole elite robot on board, to acquire a bird from a girl (Amy Rose) held captive in a prison cell on the Egg Carrier. Upon searching for the cell, Gamma accidentally discovered a disabled E-101 Beta in a lab, undergoing a torturous reconstruction process through machines. Though somewhat disturbed by this sight, he quickly disregarded this and returned to his mission. After finding Amy, E-102 strictly demanded, "Give me the bird.", which then caused her to become very defensive. After explaining her and the bird's situation she proceeded by questioning him, and from this Gamma experienced illogicality in his programming due to a conflict of interests. He then inquired, "Why save that which is useless to you? Does not compute." Amy replied that he didn't need to obey Eggman, that he was "too good" to do that. After this, Birdie flew up to Gamma knowing that his pink relative was inside. After a long stare into Birdie's eyes, perhaps triggering memories from his organic battery, Gamma realized that he needed to release them he then proceeded to do this. Surprised and touched by this, Amy exclaimed that Gamma must be different from the other robots and tells him she'd be his friend.

Gamma had no time to think about what he'd just done before he was called up to the deck to dispose of a pair of trespassers: Sonic the Hedgehog and Miles "Tails" Prower. Gamma was brought up to the deck and was commanded to kill Sonic by Eggman. A grueling battle ensued between Sonic and Gamma which ended when Amy intervened, begging them to stop and explaining to Sonic and Tails that Gamma was a friend. They reluctantly submitted to Amy's pleads Gamma backed down. As the Egg Carrier began to lose altitude and slowly fall to the earth below, Amy encouraged Gamma to free himself, explaining the virtues of friendship before they evacuated. While hovering to the Mystic Ruins, Gamma's life literally flashed before his eyes with pictures of Robotnik, the other E-Series robots, Amy and the Egg Carrier, flashing faster and faster until he saw Amy one last time and realized his purpose. Gamma determines that Dr. Eggman was the enemy, and deleted his master registration program. He then decided to begin a 'rescue' mission, locating his newly registered "friends" (the rest of the E-100 series) and 'freeing' the birds or other animals trapped inside them.

Gamma first found and destroyed Delta in Windy Valley, then he located Epsilon in Red Mountain on Angel Island, and eradicated him as well, freeing the animals. Afterward, he ran a scan to locate Zeta his results showed that Zeta was not in any range of the Mystic Ruins or Station Square. Gamma realized that he must be aboard the Egg Carrier. So he returned to the Egg Carrier to find Zeta. He was horrified when he saw that Zeta had been built into a large defense system for the Egg Carrier. After destroying the upgraded and much more heavily fortified version of Zeta, he overlooked his progress, trying to locate the final E-series model, E-101 Beta, knowing that he must be on or around the Egg Carrier. He overlooked himself as the other E-Series robot that must also be "saved". On the deck of the Egg Carrier, while thinking, Beta flew by the east side of the Egg Carrier as the newly rebuilt E-101 Beta MKII. Gamma quickly moved to the center of the ship where Beta was waiting for him for the final confrontation. Despite Beta's numerous enhancements, Gamma eventually succeeded in destroying Beta.

The bird (pink) inside Gamma flying away with its family member at the end of Gamma's story.

Before exploding, however, Beta gathered up one last ounce of strength and suddenly blasted Gamma at point-blank range, severely damaging him. As Gamma limped away from the battlefield, the bird that was inside Beta flew up close and stirred one final memory in Gamma, of a family of birds together. Gamma, realizing he needed to free his bird, didn't activate his auto repair system. He then collapsed and self-destructed, freeing the pink bird inside, who united with the other bird, a family member whom it had been separated from. After that, the birds reunited with Amy's blue bird companion to become a family. The pink bird later followed Amy to Station Square after it was near destroyed by Perfect Chaos. While Gamma himself could not be there, the bird represented him in spirit.

Despite Gamma's betrayal of Eggman, the doctor continued to build robots based on the E-102 model after Gamma's demise, most likely because Eggman never found out about Gamma's betrayal.

## Title: The faster the narrower: characteristic bulk velocities and jet opening angles of Gamma Ray Bursts

30 GRBs with measured theta_jet or Gamma_0 it is known that: (i) the real energetic E_gamma, obtained by correcting the isotropic equivalent energy E_iso for the collimation factor

theta_jet^2, is clustered around 10^50-10^51 erg and it is correlated with the peak energy E_p of the prompt emission and (ii) the comoving frame E'_p and E'_gamma are clustered around typical values. Current estimates of Gamma_0 and theta_jet are based on incomplete data samples and their observed distributions could be subject to biases. Through a population synthesis code we investigate whether different assumed intrinsic distributions of Gamma_0 and theta_jet can reproduce a set of observational constraints. Assuming that all bursts have the same E'_p and E'_gamma in the comoving frame, we find that Gamma_0 and theta_jet cannot be distributed as single power-laws. The best agreement between our simulation and the available data is obtained assuming (a) log-normal distributions for theta_jet and Gamma_0 and (b) an intrinsic relation between the peak values of their distributions, i.e theta_jet^2.5*Gamma_0=const. On average, larger values of Gamma_0 (i.e. the "faster" bursts) correspond to smaller values of theta_jet (i.e. the "narrower"). We predict that

6% of the bursts that point to us should not show any jet break in their afterglow light curve since they have sin(theta_jet)<1/Gamma_0. Finally, we estimate that the local rate of GRBs is

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### Library of Past Questions and Answers

#### Electromagnetic Radiation towards the Higher Energies

Where can I get a poster of the electromagnetic spectrum? I have seen them at some places. If you know it would be greatly appreciated.

Unfortunately I do not know where to get an electromagnetic spectrum poster.

Two nice catalogs for stuff like this are:

However, neither one has an electromagnetic spectrum poster in its current catalog.

Jonathan Keohane
(for Imagine the Universe!)

PS. A reader recommended the on-line catalogue from Exploratorium in San Francisco as another possible resource. See http://www.exploratoriumstore.com/.

Where could I find some pictures of UV radiation (if it's possible). Please send me the URL address.

High-energy at the heasarc covers the energy range of 100 eV on up, X-rays and gamma-rays, which is beyond what most astronomers consider to be the UV. However, if you are open minded about it, we have some great images from the soft X-ray (100 - 2000 eV) band. Many of these can be found in the rosat (Roentgen Satellite, a German-UK-US collaboration) Guest Observer Facility pages:

in the Public Gallery area:

HEASARC has images from a vast array of experiments (66), although not all have images of astrophysical objects. These can be found at:

The Hubble Space Telescope ventures a bit into the near UV. Their pages can be found at:

In addition, the Resources button on the Imagine the Universe! pages can lead you to pages of other groups with outreach activities.

Steve Snowden for Imagine the Universe!.

What are the respective wavelength and frequency ranges for the main six subdivisions of the electromagnetic spectrum (i.e gamma, x-rays, ultraviolet, visible, infrared, radio) and what is the name of the quanta of a gamma-ray?

There are no "hard" numbers for the wavelengths/frequencies of the various parts of the electromagnetic spectrum. For example, what is considered a high-energy X-ray and what is considered a low-energy gamma-ray is very blurry. But here is a "ball park" guide:

All quanta in the electromagnetic spectrum, regardless of its wavelength, is called a photon.

Regards,
Laura Whitlock
for the Ask an Astrophysicist Team

What is a gamma-ray? And how does an X-ray in space work?

X-rays and gamma-rays are like the light we can see with our eyes and the radio waves we can detect with radio and TV sets. The only difference is how fast they vibrate. Radio waves vibrate the most slowly, then microwaves and infra-red (heat) waves, then the colors red to violet, then ultra-violet radiation (which causes sun-burn among other things) then X-rays, and finally (vibrating the most quickly) gamma-rays.

Imagine the Universe! provides a great deal of information about X-rays and gamma-rays at the high school level. If you are younger than that you might also want to take a look at the starchild site.

At the upper end of the electromagnetic spectrum are Gamma-rays. These "Gamma-rays" have the highest energy content in the electromagnetic spectrum. What is never discussed by is the following: Is there a an upper limit (frequency) to the electromagnetic spectrum? To wit: What is the "highest frequency" Gamma-ray ever detected and is there reason to believe that there are Gamma-rays with even higher levels of energy and if so. does the electromagnetic frequency spectrum have an upper limit. or does it go out to infinity?

Thank you for your very good question about the highest energy gamma-rays. Historically, all particles with frequencies greater than about 10 19 Hertz (or about 50,000 electron Volts (5x10 4 eV) where a typical optical photon carries 2-3 eV) are called gamma-rays. Theoretically, there is no hard limit to the energy that a gamma-ray can have. However, there are a number of practical considerations that one needs to take into account involving both astrophysical sources and basic physics.

Before we address this, however, let's tackle the question about the highest energy gamma-rays yet detected. The highest energy measurements of gamma-rays are accomplished using ground-based instrumentation which also measure cosmic rays. Reliable detections of very high energy gamma-ray radiation from individual astrophysical sources, specifically from a couple of active galaxies and from the Crab Nebula, have extended up to about 10 27 Hz (5 x 10 12 eV). Aside from these individual sources, there is also expected to be a diffuse emission of gamma-rays which accompany the isotropic flux of cosmic rays. This diffuse gamma-ray emission is well measured below around 10 24 Hz (10 9 eV) or so, and is expected to extend up to at least 10 30 Hz (10 15 eV). There have been reports of measurements of diffuse gamma-ray emission above 10 29 Hz, but many other groups have only reported upper limits to emission at these energies. The measurement is exceedingly difficult since cosmic rays can outnumber gamma-rays at these energies by a factor of 10,000 to 1 or more! So you have to sift through a lot of cosmic rays to try to find the gamma-ray signal - a very difficult task.

The truth is we may never actually know to how high an energy nature sees fit to produce gamma-rays. As the gamma-ray is making its way to our telescopes, it has to traverse through space, where there are photons and particles all around us, for example the microwave background. At the highest energies, these photons will scatter down to lower energies before they arrive at Earth. In addition, many sources could produce very high energy gamma-rays which are absorbed and re-processed within the source. As a result, at the most extreme energies, we should see only those gamma-rays produced by relatively nearby sources. In addition, while we expect diffuse gamma-rays up to 10 30 Hz, at energies beyond this the basic physics of particle interactions and gamma-ray production is less clear. There could be many surprises.

Nevertheless, from the distribution of gamma-ray energies observed we know we should be able to detect gamma-rays with energies higher than those stated above. There are currently a number of projects being developed that will collect ultra-high energy gamma-rays from cosmic sources, such as OWL and MILAGRO. See

for details on these exciting gamma-ray astronomy projects.

for the Ask an Astrophysicist Team

#### Space Astronomy

Hi, I was wondering what space astronomy was?

Space astronomy refers to the study of astronomical objects using instruments flown on satellites. These instruments may detect gamma-rays, x-rays, ultraviolet light, optical light or infrared. The Hubble Space Telescope, the Rossi X-ray Timing explorer, and the Extreme ultraviolet Explorer are examples of satellites used in the study of space astronomy. Space Astronomers study a wide range of objects, from normal stars to black holes to active galaxies.

As I'm sure you've noticed, our web site concentrates on x-rays and gamma-rays. It has more information about the objects we study and the satellites we use.

Jim Lochner
for Imagine the Universe!

I'm in 12th grade and plan to major in astrophysics in college. But I was wondering, what are problems with ground-based telescopes? Why is the hst more effective than telescopes on the ground?

The short answer is that ground-based telescopes have to look through the Earth's atmosphere. The stars appear to twinkle when we look up at the night sky because their light has to pass through air which is moving and is at different temperatures, and thus has different refractive indices. To get a clearer view, the Hubble Space Telescope was placed in orbit, above the Earth's atmosphere.

You can find out more about the Hubble Space Telescope at: http://www.stsci.edu/.

Damian Audley and Sean Scully

I was just wondering exactly what astrophysics is? Also what is the difference between astrophysics and high-energy astrophysics?

Astrophysics is the part of astronomy that deals with the physics of stars, stellar systems, and interstellar material. It applies the laws of physics to astronomical bodies in order to help us understand how these bodies formed, how they interact with other bodies, and how they cease to be.

High-energy astrophysics is a sub-branch of astrophysics which utilizes information obtained from astronomical objects in X-ray and gamma-ray wavelengths of the electromagnetic spectrum. X-rays and gamma-rays have higher energies than visible and ultraviolet light.

Jim Lochner

#### X-rays

I am 5 years old. How do X-rays work?

Here at nasa we catch X-rays from far away things. From these X-rays we can learn about those far away things -- like stars and galaxies.

When you go to the doctor an get an X-ray, it is similar. The doctor shoots X-rays in one side of you, and then film catches the X-rays on the other. This makes a picture of your inside, showing your bones really well.

Jonathan Keohane
for Imagine the Universe!

Can you tell me who some of the pioneers of X-ray astronomy were? I've looked at the resources that give a bit of history of rocket-born and satellite work, but I was wondering about the astronomers/astrophysicists who started the field of study.

X-ray astronomy began after WWII when a large number of captured V2 rockets were made available to scientists for small experiments in sub-orbital flight. The led to detection of X-rays from the Sun by Herbert Friedman (Naval Research Laboratory) and collaborators in the 1950s. The bigger event was the detection of X-rays from Sco X-1 by a rocket flight (not a V2) in 1962 by Bruno Rossi, Riccardo Giacconi, and Frank Paolini (MIT). They were supposedly looking for X-ray fluorescence off of the moon, an effect which wasn't actually observed until 30 years later with the rosat satellite. Giacconi went on to promote X-ray astronomy at American Science and Engineering and the Harvard Smithsonian Astrophysical Observatory, which led to the uhuru and HEAO 1 and 2 satellites.

I assume you have looked at our lab history page:

Tim Kallman

I'm a student from Belgium. I'm writing a paper on applications of foil. Can you tell me why precisely you use foil for the making of the X-ray telescope. Thank you.

Thank you for your question. The basic reason why we use foil for X-ray telescope mirrors is because X-rays only bounce at shallow angles. So the mirrors must deflect the X-rays just only a little from their path.

As you can imagine, this means that the mirrors must be shaped like a cylindrical tube.

The problem, however, with this comes in collecting area. If you have a tube shaped mirror, it will not collect very many X-rays.

The solution is to make many thin tube shaped mirrors and nest them. Put one inside the other. If they were not thin, it would be hard to put them inside each other --- hence the need for foil.

Pictures and other descriptions are at:

For more elementary information, try reading this:

Jonathan Keohane

Please send me some information about the mechanism behind X-ray radiation in the interstellar medium. For example, the plasma mechanism.

X-rays in space come from a variety of sources. These include objects, such as supernova remnant, active galactic nuclei (including quasars), stars, and compact objects (black holes or neutron stars) in binary orbits with more normal stars. In addition, X-rays are likely to be emitted by diffuse gas in the interstellar medium. The relative contributions or these various sources to the total X-ray flux received at earth is a subject of some debate, and it varies with the X-ray energy.

It is customary to divide the emission mechanisms for X-rays into "thermal" and "non-thermal", according to whether the velocity distribution of the emitting electrons is Maxwellian. Among thermal mechanisms, the most common is almost certainly bremsstrahlung, in which radiation occurs as the result of coulomb collisions between electrons and nuclei in an ionized gas. This mechanism is likely to be operating in virtually all X-ray sources, and dominates the emission from many of them. One of the most common non-thermal mechanisms is synchrotron emission, in which electrons radiate as the result of their gyroscopic motion in a magnetic field. This mechanism, and variation, called synchrotron-self Compton, is likely to dominate in some supernova remnants and in some quasars. Both of these mechanisms are described in electricity and magnetism texts, such as Jackson's "Classical Electrodynamics". More details can be found in a "Radiative Processes in Astrophysics" by Rybicki and Lightman.

Tim Kallman
for the Ask an Astrophysicist team

I have read the article on the X-ray emissions from comet Hyakutake. Your hypothesis on the water cloud around the nucleus is interesting but did you analyze the same activities on comet Hale-Bopp? If so what are other hypothesis or conclusions?

Last May in Baltimore, Maryland, USA we had a meeting of all the people interested in the cometary X-ray emission problem. the first time all of us had gotten together since the discovery in 1996. We now have detections of X-rays from some 8 comets, and all bright, nearby comets seem to emit X-rays! (Including Hale-Bopp, although it was much fainter than we expected in the X-ray for such an optically bright and productive comet. It has been proposed that the extremely large amount of dust emitted by the comet, as compared to other comets, may be somehow damping the X-ray emission.)

At the meeting, it became apparent that 3 mechanisms are possible causes of the emission, in oder of likeliness: charge exchange between solar wind heavy ions and cometary neutrals, bremsstrahlung emission, and magnetic field recombination. All of these mechanisms involve interactions between the solar wind and the comet's extended atmosphere and ionosphere.

It is clear that we need more observations to figure out exactly what is going on, though! But it does seem that we will be able to use the X-rays to probe the nature of the solar wind and magnetic field throughout the solar system.

Casey Lisse

As a Radiographer for 14 years, I am familiar with how diagnostic X-rays are produced by man. Other, than the obvious difference of mechanical means of producing the X-rays that Dr. Roentgen discovered or even the radiation such as that noted at Chernobyl (please bear my ignorance) are we talking a natural phenomena when you say High-Energy Astrophysics--X-rays & gamma-rays or something other than the mechanized means of production? By now, you see how little knowledge I have of what is titled "High- Energy Astrophysics".

Yes, the X-rays we observe are natural -- not man made. They are produced very far away in the myriad of phenomena described in Imagine the Universe! (http://imagine.gsfc.nasa.gov/). The X-rays then travel for hundreds to billions of years before they happen to hit the detectors on our X-ray telescopes.

In fact there are two basic natural methods of producing extraterrestrial X-ray -- thermal and non-thermal. Two members of our ask_astro team replied to your message -- one describing the thermal processes and the other the non-thermal processes. I have attached a brief description of each with this E-mail (see below), and as I mentioned above there is much information on this at our Web site.

Jonathan Keohane and much of the Ask an Astrophysicist Team

Appendix A: Thermal X-rays (by Mike Arida)

At 37 C the human body emits infrared radiation. This is called blackbody radiation.

The hotter the object, the higher the energy of the photon's emitted.

When you heat a piece of metal in a fire till it glows read you have energized some of those photons to the red part of the visible spectrum.

The Sun, at 5,000 C emits most of its energy in the yellow/green part of the visible spectrum.

A tungsten light bulb get to be about 10,000 C and emits in the bluish/white end of the spectrum.

X-rays, being much more energetic than visible light, require a hotter source, in the 1 - 10's of millions of degree range. So one method of X-ray production is in the very hot gas expanding outward after a supernova explosion, or the gas heated as it spirals (and accelerates) into a black hole.

Appendix B: Non-Thermal X-ray (by David Palmer)

Many X-rays studied by high-energy astronomers are produced by high-energy electrons being accelerated or decelerated, either by being deflected by a magnetic field, or by hitting other particles. X-ray tubes used in radiography work the same way: a beam of electrons is fired into a metal target, and as they stop the electrons produce X-rays.

There are also gamma-rays produced by the decay of radioactive isotopes. These are produced on Earth by reactors such as Chernobyl, and in the sky by reactors such as novae and supernovae.

I am an Italian girl and I'm eleven years old. I should like to know the principle sources known of the X-rays and Gamma-rays in the universe. Thank you very much .

Thank you for you interest. It is impressive that someone so young is interested in these topics. There are many types of sources that produce X-rays, gamma-rays or both. Two types of objects, "active galactic nuclei" (which include quasars), and "X-ray binaries" produce X-rays and to a lesser extent, gamma-rays. Active galactic nuclei are most likely powered by supermassive black holes (as massive as millions to billions of suns). Some X-ray binaries may also contain black holes. supernova explosions also produce a lot of X- and gamma radiation. There is hot gas in some galaxies, including our Milky Way, that produces X-rays. stars, including our Sun, produce X-rays, particularly in their coronae. There is also a mysterious phenomenon called "gamma-ray bursts" which are now being detected daily by the Compton Gamma-Ray Observatory. Scientists are not sure yet what these are, but they are very energetic. For more information on these topics, try looking at the Learning Center Web pages.

Have fun,
Andy Ptak and the Ask an Astrophysicist team

#### Gamma-rays

I know that gamma rays were discovered shortly after alpha and beta particles, but who is given credit for the discovery?

Paul Villard, a French physicist, is credited with discovering gamma rays. Most sources put this in 1900, although I've seen a few sources use 1898. Villard recognized them as different from X-rays (discovered in 1896 by roentgen) because the gamma rays had a much greater penetrating depth. It wasn't until 1914 that Rutherford showed that they were a form of light with a much shorter wavelength than X-rays.

A good web site on the history of the discovery of radiation, written by Michael Fowler at University of Virginia, is:

Jim Lochner

What does a Gamma-ray Astronomer do and what deals with Gamma-rays today?

Gamma-ray astronomers study the universe as revealed by the most energetic portion of the electromagnetic spectrum. They study a variety of objects, including solar flares, neutron stars, black holes, active galaxies and gamma-ray bursts. You can learn lots about these objects via our Basic and Advanced areas on this web site.

Jim Lochner

I want to know more about the gamma-rays that do make it through the atmosphere. Are these the same as the 'cosmic rays' that are constantly bombarding earth and have a bearing on mutation? Do any gamma-rays get through, what happens to them, are they measured down here? Would the quality of our atmosphere have any bearing on how many rays get through? I know that rays do not cause the ozone hole, but does the hole in the ozone let more rays in? Do our increased levels of CO2 in the atmosphere cause changes in the 'cosmic ray' barrier?

Very few gamma-rays make it through the atmosphere. The atmosphere is as thick to gamma-rays as a twelve-foot thick plate of aluminum. Gamma-rays are very very unlikely to go through that much material. However, they can strike the material and produce 'secondary' particles which are more penetrating, and can go through the material.

Most of the cosmic rays which reach the Earth's surface are 'secondary cosmic rays', produced by gamma-rays or (much more commonly) 'primary cosmic rays' hitting the top of Earth's atmosphere. These primary cosmic rays are high energy particles (such are protons and the nuclei from iron atoms) moving at very close to the speed of light. These primary cosmic rays have a hard time even getting to the top of our atmosphere--the Earth's magnetic field deflects most of them away. If Earth didn't have a magnetic field, there would be many more primary cosmic rays hitting the atmosphere, and many more secondary cosmic rays hitting us.

There is a page in Imagine the Universe! about observations of the light produced when cosmic rays and gamma-rays hit the top of the atmosphere. It is at:

The cosmic rays are not very sensitive to the quality of the air (the chemical composition--how the nitrogen, oxygen, carbon and other elements in the air are joined together to make ozone, smog and other chemicals). They are more affected by the quantity of the air, because most interactions depend only on the nuclei of the atoms, and not on entire molecules. Three O2 molecules and two O3 (ozone) molecules look exactly the same to a cosmic ray. A carbon atom looks only slightly different from an oxygen or nitrogen atom, so the increased CO2 level has almost no effect. Nothing we do is likely to significantly change the number of cosmic rays hitting Earth.

David Palmer

1. Why is the shape of shower different for a gamma-ray compared to a cosmic ray? Is it to do with the initial interaction of the gamma-ray produces a positron/electron pair that go off at some angle.
2. At the cgro learning center I was expecting to see plots of the pool of Cerenkov light to be elliptical for a gamma-ray and roughly circular for a cosmic ray. I don't see this in the plots shown, especially the cosmic ray plot, which shows a scattered distribution that I don't understand.
3. On the actual imaging telescope why are there multiple mirrors instead of just one dish with the PMTs behind focal plane. Also why are the individual mirrors hexagonal and not say squares or pentagons?
4. Since the pool of light is much bigger than the area of telescope how can the shape of the pool be determined? Does the intensity of light fall off or increase in concentric contours from the edge of the pool to the center?

> 1. why is the shape of shower different for a gamma-ray
> compared to a cosmic ray? Is it to do with the initial
> interaction of the gamma-ray produces a positron/electron
> pair that go off at some angle.

Yes, that is the key. The initial interaction of the gamma-ray is a pair production interaction which has a relatively small opening angle for the created pair. On the other hand, the first cosmic ray interaction is some exotic nuclear interaction where many different particles can result, each splintering off in different directions, some at large angles. Further interactions tend to accentuate this. Even though the gamma-ray showers broaden lower down in the atmosphere as the particles lose energy, the cosmic-ray showers are still much more extended.

> 2. At the CGRO learning center I was expecting to see
> plots of the pool of Cerenkov light to be elliptical
> for a gamma-ray and roughly circular for a cosmic ray.
> I don't see this in the plots shown, especially the
> cosmic ray plot, which shows a scattered distribution
> that I don't understand.

The plots at the learning center are actually hit positions of photons from simulated air showers. If many more events were plotted, and some smoothing were done, the shapes you describe would start to be apparent.

> 3. On the actual imaging telescope why are there multiple
> mirrors instead of just one dish with the PMTs behind
> focal plane. Also why are the individual
> mirrors hexagonal and not say squares or pentagons?

Some observatories use mirrors originally intended for other work - such as solar power studies. For this type of work, and air Cerenkov work, it is the size of the mirrored surface, not the quality of the mirror which is important. Using the Whipple Observatory example, it would be very expensive to manufacture a smooth mirror with a 10 meter diameter. On the other hand, a 10 meter diameter mirror which is a mosaic of smaller mirrors is cheap, has a large collecting area, and provides adequate optics for the relatively broad structures being measured (the size of the image is typically a few tenths of a degree). The shape of the individual facets is not really important.

> 4. Since the pool of light is much bigger than the area of
> telescope how can the shape of the pool be determined?
> Does the intensity of light fall off or increase in concentric
> contours from the edge of the pool to the center?

Detectors are not actually measuring the shape of the entire pool (which is roughly a large pancake), but the "shape" arising from the angular distribution of light in the local part of the pool that the mirror reflects. This shape changes depending upon where you are in the "pool". Even gamma-ray showers look circular at the center of the pool. The light intensity in the pool does have an interesting radial dependence. For gamma-ray initiated showers, if you measured the intensity from the center of the shower, you would find that it was approximately constant out to a distance of 100 meters or so, falling off rapidly after that.

Daryl Macomb
Compton Gamma Ray Observatory team

Can gamma-rays react with CO2 in our atmosphere? If so how?

Gamma-rays are usually absorbed in the upper atmosphere (most would not even reach the top of the Everest). Higher in the atmosphere, it can interact with CO2 molecules, but they do not react with CO2 more than with other kinds of molecules, if that was your question.

Koji Mukai for Imagine the Universe!

What happens to people who have been exposed to a lot of gamma-rays?

There is some general information concerning radiation exposure, including gamma-rays, at

Andy Ptak

Hi, I'm in 8th grade and my science class is learning about the the electromagnetic spectrum. My question is why are gamma-rays so much more harmful than radio waves? I already understand that gamma-rays have a higher energy level in them than radio waves, but what makes this energy so harmful?

The reason gamma-rays are more harmful then radio waves is because light can be thought of as particles (photons) as well as electromagnetic waves. A radio photon doesn't have much energy and doesn't travel through matter well (that's why you don't pick up radio well in a tunnel). A gamma-ray photon has enough energy to damage atoms in your body and make them radioactive, and gamma-rays can easily penetrate into your body. It's like the difference between getting hit by sand or a bullet. It takes a lot of sand to do any damage, but only one bullet.

Eric Christian

I am a PhD student at the Observatory in Torino (Italy).

I would like to ask you if is it possible to have high-energy gamma emissions (that's to say up to 30 MeV) from some kind of dissections of nuclei? I know about Carbon and Oxygen at about 5-6 MeV and I know about the decay energy emission (e.g. aluminum and iron and others) not up to 10 MeV. Can one use EGRET or other similar high-energy satellites (e.g. in the future AMS) to study stellar production of some kind of nuclei (excited for example by the passage of a shock wave)?

Does electron-positron annihilation always yields energy in the gamma-ray band?

I've been reading up on the recent positron findings at our galaxy center and wondered if we know that electron-positron annihilation always yields energy in the gamma-ray band?

My background in this area is limited to college physics, and book (net) research in plasma and astrophysics.

Thank you for your question. The answer is yes, we know the physics of electron-positron interactions quite well, because it has been measured in particle physics labs. As it turns out the mass of an electron (9.1E-28 grams) times the speed of light squared (E = m c 2 ) is 8.12E-07 ergs of energy. In more common units this is 511 keV (kilo electron volts).

When an electron and positron annihilate they produce 2 photons, each with 511 keV of energy (so no net energy is gained or lost). When we observe a spectral emission line at 511 keV, we can be pretty sure it is caused by this positron/electron interactions.

Jonathan Keohane

Recently I've heard about antimatter. Could you describe to me what it is? Could you tell me why is it so important for NASA to join matter and antimatter?

The existence of antimatter was predicted by the theory of quantum mechanics of the electron by Dirac in the 1920's. The first experimental verification came with the discovery of positrons in cosmic rays by Anderson in 1932 or thereabouts. However, positrons (anti-electrons) are found in some kinds of naturally radioactive substances also. Later anti-protons and other anti particles were produced in accelerators in the laboratory.

An anti-particle is a particle whose properties are exactly opposite to its corresponding particle. Thus a positron (the particle thought to be responsible for the gamma-rays which were in the news last week) has a charge opposite (positive) to that of the electron, and 'lepton number' -1. When positrons and electrons collide they annihilate each other, and their energies are converted into gamma-rays. If the positron and electron are at rest (which is unlikely) and their spins are oriented opposite to each other they produce 2 gamma-rays, each with energy 511,002.7 electron volts. This is the radiation which was observed by the Compton Gamma Ray Observatory, and is considered to be a unique signature of electron-positron annihilation. This signature is expected to arise from annihilation even if the positrons and electrons are not at rest, but have moderate kinetic energies.

Antimatter is interesting partly because of the spectacular and violent way in which it interacts with normal matter. It is also an open question why the universe appears to be relatively empty of antimatter theories for the big bang predict approximately equal amounts of matter and antimatter should have been produced. The positrons which produced the gamma-rays seen by the Compton Observatory were probably produced by collisions of high energy particles (predominantly ordinary protons and electrons) accelerated near a black hole.

## Fermi mission reveals its highest-energy gamma-ray bursts

Green dots show the locations of 186 gamma-ray bursts observed by the Large Area Telescope (LAT) on NASA's Fermi satellite during its first decade. Some noteworthy bursts are highlighted and labeled. Background: Constructed from nine years of LAT data, this map shows how the gamma-ray sky appears at energies above 10 billion electron volts. The plane of our Milky Way galaxy runs along the middle of the plot. Brighter colors indicate brighter gamma-ray sources. Credit: NASA/DOE/Fermi LAT Collaboration

For 10 years, NASA's Fermi Gamma-ray Space Telescope has scanned the sky for gamma-ray bursts (GRBs), the universe's most luminous explosions. A new catalog of the highest-energy blasts provides scientists with fresh insights into how they work.

"Each burst is in some way unique," said Magnus Axelsson, an astrophysicist at Stockholm University in Sweden. "It's only when we can study large samples, as in this catalog, that we begin to understand the common features of GRBs. These in turn give us clues to the physical mechanisms at work."

The catalog was published in the June 13 edition of The Astrophysical Journal and is now available online. More than 120 authors contributed to the paper, led by Axelsson, Elisabetta Bissaldi at the National Institute of Nuclear Physics and Polytechnic University in Bari, Italy, and Nicola Omodei and Giacomo Vianello at Stanford University in California.

GRBs emit gamma rays, the highest-energy form of light. Most GRBs occurs when some types of massive stars run out of fuel and collapse to create new black holes. Others happen when two neutron stars, superdense remnants of stellar explosions, merge. Both kinds of cataclysmic events create jets of particles that move near the speed of light. The gamma rays are produced in collisions of fast-moving material inside the jets and when the jets interact with the environment around the star.

Astronomers can distinguish the two GRB classes by the duration of their lower-energy gamma rays. Short bursts from neutron star mergers last less than 2 seconds, while long bursts typically continue for a minute or more. The new catalog, which includes 17 short and 169 long bursts, describes 186 events seen by Fermi's Large Area Telescope (LAT) over the last 10 years.

Fermi observes these powerful bursts using two instruments. The LAT sees about one-fifth of the sky at any time and records gamma rays with energies above 30 million electron volts (MeV)—millions of times the energy of visible light. The Gamma-ray Burst Monitor (GBM) sees the entire sky that isn't blocked by Earth and detects lower-energy emission. All told, the GBM has detected more than 2,300 GRBs so far.

Below is a sample of five record-setting and intriguing events from the LAT catalog that have helped scientists learn more about GRBs.

The short burst 081102B, which occurred in the constellation Boötes on Nov. 2, 2008, is the briefest LAT-detected GRB, lasting just one-tenth of a second. Although this burst appeared in Fermi's first year of observations, it wasn't included in an earlier version of the collection published in 2013.

"The first LAT catalog only identified 35 GRBs," Bissaldi said. "Thanks to improved data analysis techniques, we were able to confirm some of the marginal observations in that sample, as well as identify five times as many bursts for the new catalog."

Long-lived burst 160623A, spotted on June 23, 2016, in the constellation Cygnus, kept shining for almost 10 hours at LAT energies—the longest burst in the catalog. But at the lower energies recorded by Fermi's GBM instrument, it was detected for only 107 seconds. This stark difference between the instruments confirms a trend hinted at in the first LAT catalog. For both long and short bursts, the high-energy gamma-ray emission lasts longer than the low-energy emission and happens later.

The highest-energy individual gamma ray detected by Fermi's LAT reached 94 billion electron volts (GeV) and traveled 3.8 billion light-years from the constellation Leo. It was emitted by 130427A, which also holds the record for the most gamma rays—17—with energies above 10 GeV.

A popular model proposed that charged particles in the jet, moving at nearly the speed of light, encounter a shock wave and suddenly change direction, emitting gamma rays as a result. But this model can't account for the record-setting light from this burst, forcing scientists to rethink their theories.

The original findings on 130427A show that the LAT instrument tracked its emission for twice as long as indicated in the catalog. Due to the large sample size, the team adopted the same standardized analysis for all GRBs, resulting in slightly different numbers than reported in the earlier study.

The farthest known GRB occurred 12.2 billion light-years away in the constellation Carina. Called 080916C, researchers calculate the explosion contained the power of 9,000 supernovae.

Telescopes can observe GRBs out to these great distances because they are so bright, but pinpointing their exact distance is difficult. Distances are only known for 34 of the 186 events in the new catalog.

The known distance to 090510 helped test Einstein's theory that the fabric of space-time is smooth and continuous. Fermi detected both a high-energy and a low-energy gamma ray at nearly the same instant. Having traveled the same distance in the same amount of time, they showed that all light, no matter its energy, moves at the same speed through the vacuum of space.

"The total gamma-ray emission from 090510 lasted less than 3 minutes, yet it allowed us to probe this very fundamental question about the physics of our cosmos," Omodei said. "GRBs are really one of the most spectacular astronomical events that we witness."

GRB 170817A marked the first time light and ripples in space-time, called gravitational waves, were detected from the merger of two neutron stars. The event was captured by the Laser Interferometer Gravitational Wave Observatory (LIGO), the Virgo interferometer and Fermi's GBM instrument, but it wasn't observed by the LAT because the instrument was switched off as the spacecraft passed through a region of Fermi's orbit where particle activity is high.

"Now that LIGO and Virgo have begun another observation period, the astrophysics community will be on the lookout for more joint GRB and gravitational wave events" said Judy Racusin, a co-author and Fermi deputy project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "This catalog was a monumental team effort, and the result helps us learn about the population of these events and prepares us for delving into future groundbreaking finds."

## Compton Cameras for Nuclear Medical Imaging

W.L. ROGERS , . A. BOLOZDYNYA , in Emission Tomography , 2004

### 3 Polarization

Single gamma rays emitted from a nucleus are unpolar-ized. That is, all orientations of the electric vector are equally likely. When an unpolarized beam undergoes Compton scattering it becomes partially linearly polarized orthogonal to the scattering plane because the scattering cross section for the perpendicular and parallel components is not the same ( Klein and Nishina, 1929 ). The degree of polarization is a function of scattering angle and initial gamma-ray energy as illustrated by Eq. (9) ( McMaster, 1961 ) and Figure 6 :

FIGURE 6 . Gamma-ray polarization as a function of scattering angle at three primary gamma-ray energies.

Moreover, a subsequent Compton scattering of this partially polarized beam will produce an azimuthally asymmetric intensity in the scattered gamma rays that depends on the angle between the incident gamma-ray polarization, e1, and the scattered gamma-ray polarization, e2. Kamae et al., (1987) have described an application of this method to measuring the energy, direction, and polarization of incident gamma rays.

Polarization influences two aspects of Compton camera performance. First, gamma rays that scatter in the patient will be partially polarized. Those scattered gamma rays that cannot be rejected by energy windowing will have an asymmetric azimuthal scattering distribution in the first detector and should therefore be back-projected using a weighted azimuthal distribution. Because the initial scattering plane is unknown, there is no frame of reference for the asymmetry, so these events must be modeled using a uniform azimuthal distribution. The effects of this mismodeling have not been evaluated. Second, polarization effects can be used to reduce the conical ambiguity for gamma rays that have not been scattered in the patient. For gamma rays that undergo two or more scatters in a Compton camera, Dogan (1993 Dogan et al., 1992 ) has shown that it is possible to determine an azimuthal weighting function for the conical back-projection that reduces the ambiguity. One must first determine the sequence of interactions. Methods for sequencing have been described by Kamae and Hanada (1988) , Dogan (1993 Dogan et al., 1990 , 1992), and Durkee (Durkee, Antich, Tsyganov, Constantinescu, Fernando, et al., 1998 Durkee, Antich, Tsyganov, Constantinescu, Kulkarni, et al., 1998 ) and essentially consist of determining which set of energies and scattering angles calculated for each of the postulated sequences best fits the data. For n interactions, there are n! sequences to test, so untangling more than three interactions could be very time consuming.

Figure 7 illustrates the probability of Compton double scattering as a function of azimuthal angle. Results are shown for 150-keV gamma rays and two different initial scattering angles ( Dogan et al., 1992 ). Polarization effects have been included in a system design study by Chelikani et al (2004) , but the effect of this added information on improving image quality for Compton imaging has not been completely investigated to our knowledge. However, it appears from Figure 7 that one can substantially reduce the ambiguity in azimuth for low-energy gamma rays for the larger scattering angles.

FIGURE 7 . Azimuthal probability of conical distribution derived for double Compton scattering for 150-keV incident gamma rays. θ1 is the first scattering angle, and Φ is the azimuthal angle around the cone measured with respect to the second scattering plane.

The gamma irradiation process uses Cobalt 60 radiation for a variety of applications, including sterilization, decontamination and materials modification. Gamma irradiation offers good penetration of dense products and is ideal for many types of materials and their packaging.

The gamma irradiation process uses Cobalt 60 radiation to kill microorganisms on a variety of different products in a specially designed cell. Gamma radiation is generated by the decay of the radioisotope Cobalt 60, with the resultant high energy photons being an effective sterilant. A key characteristic of gamma irradiation is the high penetration capability, which allows for delivery of target radiation dose to areas of products that may be higher in density.

The unit of absorbed dose is kiloGray, expressed as kGy. Delivery and absorption of dose by product is determined by product density, packaging size, dose rate, exposure time and facility design.

### What is Gamma Irradiation Used For?

The gamma irradiation process can effectively treat a wide variety of products composed of different materials, with varying densities, configurations and orientations. Some examples of products processed include:

• Medical devices
• Pharmaceuticals
• Combination drug/device products
• Tissue-based and biological products
• Animal retail products
• Archives
• Cosmetics and toiletries
• Horticultural supplies
• Packaging materials

For a comprehensive list of products commonly treated by gamma irradiation, please see Technical Tip # 4

### What are the Benefits of Gamma Irradiation?

Gamma irradiation is safe, reliable and highly effective at treating a wide variety of products with varying densities. With the ability to penetrate products while sealed in their final packaging, gamma irradiation supports the manufacturing and distribution process by facilitating final packaged products as well as raw material needs, ensuring full sterility of the product.

#### Standards:

Gamma sterilization is supported by the internationally recognized consensus standard, ISO 11137, which describes the approach to validating a dose to achieve a defined sterility assurance level (SAL).