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How does one measure vector magnetic field of astrophysical object?

How does one measure vector magnetic field of astrophysical object?


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Magnetic field strength is measured using Zeeman splitting. This is the one of the way Sun's magnetic field strength is measured. Now, how one measure vector magnetic field? Vector magnetic field = Magnetic field and it's components.


Using spectropolarimeter one can measure vector Magnetic field.

A spectropolarimeter based on a ferroelectric liquid-crystal modulator is described. An optical system with spatial modulation of the positions of the components of Zeeman splitting is a specific feature of this instrument. In comparison to the familiar instruments, the developed spectropolarimeter utilizes the light flux more efficiently and contains only one photodetector array. An operating spectropolarimeter developed at the Sayan Solar Observatory is considered as an example. Comparative estimates of noises in different operating modes are presented.

(Ref: Kolobov, D.Y., Kobanov, N.I. & Grigoryev, V.M. Instrum Exp Tech (2008) 51: 124. https://doi.org/10.1134/S0020441208010156)


Astronomers image magnetic fields at the edge of M87’s black hole

MIT Haystack Observatory is one of the 13 stakeholder institutions that constitute the Event Horizon Telescope (EHT) collaboration, which produced the first-ever image of a black hole. The EHT revealed today a new view of the massive object at the center of the M87 galaxy: how it looks in polarized light. This is the first time astronomers have been able to measure polarization, a signature of magnetic fields, this close to the edge of a black hole. The observations are key to explaining how the M87 galaxy, located 55 million light-years away, is able to launch energetic jets from its core.

Haystack Research Scientist Vincent Fish said, “Hundreds of people around the world in the EHT collaboration, including scientists and engineers at Haystack, have worked very hard to investigate the role of magnetic fields in shaping jets around black holes. Can magnetic fields build up and dominate over the intense pull of gravity? Our data provide an answer.”

On 10 April 2019, scientists released the first ever image of a black hole, revealing a bright ring-like structure with a dark central region — the black hole’s shadow. Since then, the EHT collaboration has delved deeper into the data on the supermassive object at the heart of the M87 galaxy collected in 2017. They have revealed that the famous ring of light at the edge of the M87 black hole was polarized across the ring.

“Astronomers have obtained a new tool to study the magnetism of a black hole with the direct imaging of the polarization of light,” explains Kazunori Akiyama, a Coordinator of the EHT Imaging WG and Research Scientist at Haystack Observatory. “This remarkable feat by the Event Horizon Telescope was truly achieved by years of international efforts to develop the state-of-the-art techniques in every single stage of the complex signal processing, from the telescopes to the images.”

Light becomes polarized when it goes through certain filters, like the lenses of polarized sunglasses, or when it is emitted in hot regions of space that are magnetized. In the same way polarized sunglasses only transmit a specific orientation of the electric field from the Sun’s light rays, astronomers can obtain information about the electric-field orientation of light coming from outer space, by using polarizers installed in their telescopes. Specifically, polarization allows astronomers to map the magnetic field lines present at the inner edge of the black hole.

“Polarization is a powerful tool available to astronomers to probe the physical conditions in one of the most extreme environments in the universe. It can provide clues not only to the strength and orientation of magnetic fields, but also how well ordered those fields are, and possibly even something about the otherwise invisible material that lies between us and the material that is emitting the radio waves,” said Colin Lonsdale, Director of MIT Haystack Observatory and Chair of the Event Horizon Telescope Board.

The bright jets of energy and matter that emerge from M87’s core and extend at least 5000 light-years from its center are one of the galaxy’s most mysterious and energetic features. Most matter lying close to the edge of a black hole falls in. However, some of the surrounding particles escape moments before capture and are blown far out into space in the form of jets.

Astronomers have relied on different models of how matter behaves near the black hole to better understand this process. But they still don’t know exactly how jets larger than the galaxy are launched from its central region, which is as small in size as the Solar System, nor how exactly matter falls into the black hole. With the new EHT image of the black hole and its shadow in polarized light, astronomers managed for the first time to look into the region just outside the black hole where this interplay between matter flowing in and being ejected out is happening.

The observations provide new information about the structure of the magnetic fields just outside the black hole. The team found that only theoretical models featuring strongly magnetized gas can explain what they are seeing at the event horizon.

“New polarization images suggest that the powerful jet is formed by plasma flow arrested by aligned magnetic fields in the vicinity of the black hole, resisting its strong gravitational pull,” explained Kotaro Moriyama, an Overseas Postdoctoral Fellow of the Japan Society for the Promotion of Science at Haystack Observatory.

To observe the heart of the M87 galaxy, the collaboration linked eight telescopes around the world, including ALMA (the Atacama Large Millimeter/submillimeter Array) and APEX (the Atacama Pathfinder Experiment), in northern Chile, to create a virtual Earth-sized telescope, the EHT. The impressive resolution obtained with the EHT is equivalent to that needed to measure the length of a credit card on the surface of the Moon.

“ALMA plays a central role in the entire process: it is centrally located to tie the EHT array together, and it is also the most sensitive telescope in the array, so it is crucial to making the most of the EHT data,” said Geoff Crew, Haystack Research Scientist. “In addition, the years of work on the ALMA polarimetry analysis has delivered far more than we imagined.”

This resolution allowed the team to directly observe the black hole shadow and the ring of light around it, with the new polarized-light image clearly showing that the ring is magnetized. The results are published today in two separate papers in The Astrophysical Journal Letters by the EHT collaboration. The research involved over 300 researchers from multiple organizations and universities worldwide.

A third paper, “Polarimetric properties of Event Horizon Telescope targets from ALMA,” was also published in the Astrophysical Journal Letters, led by Ciriaco Goddi, a scientist at Radboud University and Leiden Observatory, the Netherlands, and including Haystack Research Scientists Geoff Crew and Lynn Matthews and based on data from ALMA.

Composite visual of M87 and ring in polarization

Goddi said, “The ALMA data were acquired simultaneously with the VLBI observations conducted in Apr 2017 with the EHT (and the GMVA) in this sense they are a ‘byproduct’ of the VLBI operations. ALMA data were crucial to calibrate, image, and interpret the EHT polarization observations, providing tight constraints on the theoretical models that explain how matter behaves near the black hole event horizon. This data also provides a description of the magnetic field structure along the powerful relativistic jets that extend far beyond the M87 galaxy. The combined information from the EHT and ALMA allows scientists to investigate the role of magnetic fields from the vicinity of the event horizon to far beyond the M87 galaxy along its powerful relativistic jets (on scales of thousands of light-years).”

Crew added, “ALMA bridges the gap in resolution between the ultra-high resolution of the VLBI arrays and that obtained with other measurement techniques. In combination, this wealth of new polarimetry data should allow us to make progress on understanding this fascinating object.”


Harvard, Smithsonian Astronomers Help Capture First Image of Black Hole’s Magnetic Fields

This is the first time scientists have imaged magnetic fields so close to a black hole.

Cambridge, MA – Astronomers have now obtained a new view of the supermassive black hole at the center of galaxy M87. Images released today by the Event Horizon Telescope (EHT) collaboration reveal how the black hole, some 55 million light-years away, appears in polarized light.

The image marks the first time astronomers have captured and mapped polarization, a sign of magnetic fields, so close to the edge of a black hole.

Scientists still don't understand how magnetic fields — areas where magnetism affects how matter moves — influence black hole activity. Do they help direct matter into the hungry mouths of black holes? Can they explain the mysterious jets of energy that extend out of the galaxy's core?

"We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy," says Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor at Radboud University in the Netherlands

In two studies published today in The Astrophysical Journal Letters , EHT astronomers reveal their latest findings and how magnetic fields may be influencing the black hole at the center of M87.

"One of the main science drivers of the EHT is distinguishing different magnetic field configurations around the black hole," says Angelo Ricarte, a co-author and researcher at the Center for Astrophysics | Harvard & Smithsonian. “Polarization is one of the most direct probes into the magnetic field that nature provides."

The EHT collaboration has been studying the supermassive object at the heart of M87 for well over a decade. In April 2019, the team's hard work paid off when they revealed the very first image of a black hole. Since then, the scientists have delved deeper into the data, discovering that a significant fraction of the light around the M87 black hole is polarized.

Light becomes polarized when it goes through certain filters, like the lenses of polarized sunglasses, or when it is emitted in hot regions of space that are magnetized. In the same way polarized sunglasses help us see better by reducing reflections and glare from bright surfaces, astronomers can sharpen their view of the black hole by looking at how light originating from there is polarized. Specifically, polarization allows astronomers to map the magnetic field lines present around the inner edge of the black hole.

"In order to gain confidence in our analysis, we used as many as five distinct methods to calibrate the data and reconstruct polarimetric images," says Maciek Wielgus, a researcher at Harvard's Black Hole Initiative and the Center for Astrophysics (CfA) who participated in the study. "This huge team effort paid off as we found very good consistency between results obtained with all the different techniques."

"The newly published polarized images are key to understanding the powerful jets that are launched from this region," says EHT Collaboration member Andrew Chael, a Hubble Fellow at the Princeton Center for Theoretical Sciences and the Princeton Gravity Initiative in the U.S.

One of M87's most mysterious features is the bright jet of matter and energy that emerges from its core and extends at least 100,000 light years away. Most matter lying close to the edge of a black hole falls in. However, some of the surrounding particles escape moments before capture and are blown far out into space in the form of these jets.

Astronomers don't know how jets larger than the galaxy itself are launched from its core, nor how only certain matter falls into the black hole.

Now, with the new image of the black hole in polarized light, the team has looked directly into the region just outside the black hole where this interplay between inflowing and ejected matter occurs.

The observations provide new information about the structure of the magnetic fields just outside the black hole, revealing that only theoretical models featuring strongly magnetized gas can explain what astronomers are seeing at the event horizon.

"Magnetic fields are theorized to connect black holes to the hot plasma surrounding them," says Daniel Palumbo, a co-author and researcher at the Center for Astrophysics. "Understanding the structure of these fields is the first step in understanding how energy can be extracted from spinning black holes to produce powerful jets."

To observe the heart of the M87 galaxy, the EHT collaboration linked eight telescopes around the world, including the Smithsonian Astrophysical Observatory's Submillimeter Array, to create a virtual Earth-sized telescope. The impressive resolution obtained with the EHT is equivalent to that needed to image a credit card on the surface of the Moon.

This unprecedented resolving power allowed the team to directly observe the black hole with polarized light, revealing the presence of a structured magnetic field near the event horizon.

"This first polarized image of the black hole in M87 is just the beginning," says Dominic Pesce, CfA researcher and study co-author. "As the EHT continues to grow, future observations will refine the picture and allow us to study how the magnetic field structure changes with time."

Sheperd Doeleman, founding director of the EHT, added, "Even now we are designing a next-generation EHT that will allow us to make the first black hole movies. Stay tuned for true black hole cinema."

The EHT collaboration involves more than 300 researchers from across the globe and includes 30 scientists and engineers at the Center for Astrophysics | Harvard & Smithsonian.

About the Event Horizon Telescope (EHT) Collaboration

The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, North and South America. The international collaboration is working to capture the most detailed black hole images ever obtained by creating a virtual Earth-sized telescope. Supported by considerable international investment, the EHT links existing telescopes using novel systems — creating a fundamentally new instrument with the highest angular resolving power that has yet been achieved.

The individual telescopes involved are: ALMA, APEX, the IRAM 30-meter Telescope, the IRAM NOEMA Observatory, the James Clerk Maxwell Telescope, the Large Millimeter Telescope, the Submillimeter Array, the Submillimeter Telescope, the South Pole Telescope, the Kitt Peak Telescope and the Greenland Telescope.

The EHT consortium consists of 13 stakeholder institutions: the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona, the Center for Astrophysics | Harvard & Smithsonian, the University of Chicago, the East Asian Observatory, Goethe-Universitaet Frankfurt, Institut de Radioastronomie Millimétrique, Large Millimeter Telescope, Max Planck Institute for Radio Astronomy, MIT Haystack Observatory, National Astronomical Observatory of Japan, Perimeter Institute for Theoretical Physics and Radboud University.

About the Center for Astrophysics | Harvard & Smithsonian


This is the Milky Way’s Magnetic Field

The Milky Way galaxy has its own magnetic field. It’s extremely weak compared to Earth’s thousands of times weaker, in fact. But astronomers want to know more about it because of what it can tell us about star formation, cosmic rays, and a host of other astrophysical processes.

A team of astronomers from Curtin University in Australia, and CSIRO (Commonwealth Scientific and Industrial Research Organization) have been studying the Milky Way’s magnetic field, and they’ve published the most comprehensive catalogue of measurements of the Milky Way’s magnetic field in 3D.

The paper is titled “Low-frequency Faraday rotation measures towards pulsars using LOFAR: probing the 3D Galactic halo magnetic field.” It was published in Monthly Notices of the Royal Astronomical Society in April 2019. The lead author is Dr. Charlotte Sobey, a university associate at Curtin University. The team includes scientists from Canada, Europe, and South Africa.

The team worked with LOFAR, or the Low-Frequency Array, a European radio telescope. LOFAR works in radio frequencies below 250 MHz and consists of many antennae spread over a 1500 km area in Europe, with its core in the Netherlands.

<Click to Enlarge> LOFAR sites are spread around Europe, with the concentrated central core in the Netherlands. Image Credit: LOFAR

The team assembled the largest catalogue to date of magnetic field strengths and directions towards pulsars. With that data in hand, they were able to estimate the Milky Way’s decreasing field strength with distance from the plane of the galaxy, where the spiral arms are.

In a press release, lead author Sobey said “We used pulsars to efficiently probe the Galaxy’s magnetic field in 3-D. Pulsars are distributed throughout the Milky Way, and the intervening material in the Galaxy affects their radio-wave emission.”

Free electrons and the magnetic field in our Galaxy between the pulsar and us affect the radio waves emitted by the pulsars. In an email interview with Dr. Sobey she told us, “Although these effects need to be corrected in order to study the pulsars’ signals, they are really useful for providing information about our Galaxy that would not be possible to obtain otherwise.”

An illustration of a pulsar. Pulsars emit electromagnetic energy along the magnetic axis. Image Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

As the pulsar’s radio waves travel through the galaxy, they’re subject to an effect called dispersion, due to intervening free electrons. This means that higher frequency radio waves arrive sooner than lower frequency waves. Data from LOFAR allows astronomers to measure this difference, called the “dispersion measure” or DM. DM tells astronomers how many free electrons are in between us and the pulsar. If the DM is higher, that means either the pulsar is farther away, or the interstellar medium is denser.

That’s just one of the factors in the measurement of the Milky Way’s magnetic field. The other involves electron density and the magnetic field of the interstellar medium.

Pulsar emissions are often polarized, and when polarized light travels through a plasma with a magnetic field, the plane of rotation rotates. That’s called Faraday Rotation or the Faraday Effect. Radio telescopes can measure that rotation, and it’s called the Faraday Rotation Measure (RM). According to Dr. Sobey, “This tells us the number of free electrons and the strength of the magnetic field parallel to the line of sight, as well as the net direction. The larger the absolute RM means more electrons and/or greater field strengths, due to larger distances or towards the plane of the Galaxy.”

With that data in hand, the researchers then estimated the average magnetic field strength of the Milky Way towards each pulsar in the catalogue, by dividing the Rotation Measure by the Dispersion Measure. And that’s how they created the map. Each single pulsar measurement is one point on the map. As Dr. Sobey told Universe Today, “Obtaining these measurements for large numbers of pulsars (which have distance measurements or estimates) allows us to reconstruct a map of the structure of the Galactic electron density and magnetic field in 3-D.”

A representation of how our Galaxy would look in the sky if we could see magnetic fields. The plane of the Galaxy runs horizontally through the middle, and the Galactic centre direction is the middle of the map. Red–pink colours show increasing Galactic magnetic field strengths where the direction is pointing towards the Earth. Blue–purple colours show increasing Galactic magnetic field strengths where the direction is pointing away from the Earth. The background shows the signal reconstructed using sources outside our Galaxy. The points show the current measurements for pulsars. The squares show the measurements from this work using LOFAR pulsar observations. Image Credit: Sobey et al, 2019.

So what good does it do to have a map of the Milky Way’s magnetic structure in 3D?

The galaxy’s magnetic field affects all kinds of astrophysical processes across different strength and distance scales.

The magnetic field shapes the path that cosmic rays follow. So when astronomers are studying a distant source of cosmic rays, like an active galactic nucleus (AGN), knowing the strength of the magnetic field can help them understand their object of study.

The galaxy’s magnetic field also plays a role in star formation. Though the effect is not fully understood, the strength of a magnetic field may affect molecular clouds. As Dr. Sobey told UT, “At smaller scales (on the order of parsecs), magnetic fields play a role in star formation, with too weak or strong a field in a molecular cloud possibly inhibiting the collapse of a cloud into a stellar system.”

Dr. Sobey chilling in a telescope. Image Credit: CSIRO

This new catalogue is based on observations of 137 pulsars in the northern sky. The authors say that their catalogue “improves the precision of existing RM measurements on average by a factor of 20…” They also say “Overall, our initial low-frequency catalogue provides valuable information about the 3D structure of the Galactic magnetic field.”

But Dr. Sobey isn’t finished mapping the Milky Way’s magnetic field strength yet. She’s now using Australia’s Murchison Widefield Array to map the magnetic field in the southern sky. And both of these mapping endeavours are leading up to something better.

Artist’s impression of the 5km diameter central core of Square Kilometre Array (SKA) antennas. Image Credit: SPDO/TDP/DRAO/Swinburne Astronomy Productions – SKA Project Development Office and Swinburne Astronomy Productions

The world’s largest radio telescope is now in the planning phase. It’s called the Square Kilometer Array (SKA) and it will be built in both Australia and South Africa. Its receiving stations will extend out to 3,000 kilometers (1900 miles) from its central core. Its massive size and distance between receivers will give us our highest resolution images in all of astronomy.

In a CSIRO blog post, Dr. Sobey said “My work in the future will focus on building towards doing science with the SKA telescope, which is currently entering the final stages of the planning phase. One long-term goal for SKA science is to revolutionize our understanding of our galaxy, including producing a detailed map of our galaxy’s structure (which is difficult because we’re located inside it!), particularly its magnetic field.”


Helical Motion

Helical motion results when the velocity vector is not perpendicular to the magnetic field vector.

Learning Objectives

Describe conditions that lead to the helical motion of a charged particle in the magnetic field

Key Takeaways

Key Points

  • Previously, we have seen that circular motion results when the velocity of a charged particle is perpendicular to the magnetic field. The speed and kinetic energy of the particle remain constant, but the direction is altered at each instant by the perpendicular magnetic force.
  • If the velocity is not perpendicular to the magnetic field, we consider only the component of v that is perpendicular to the field when making our calculations.
  • The component of the velocity parallel to the field is unaffected, since the magnetic force is zero for motion parallel to the field. This produces helical motion.
  • Charges may spiral along field lines. If the strength of the magnetic field increases in the direction of motion, the field will exert a force to slow the charges and even reverse their direction. This is known as a magnetic mirror.

Key Terms

  • helical motion: The motion that is produced when one component of the velocity is constant in magnitude and direction (i.e., straight-line motion) while the other component is constant in speed but uniformly varies in direction (i.e., circular motion). It is the superposition of straight-line and circular motion.
  • magnetic mirror: A magnetic field configuration where the field strength changes when moving along a field line. The mirror effect results in a tendency for charged particles to bounce back from the high field region.

Helical Motion

In the section on circular motion we described the motion of a charged particle with the magnetic field vector aligned perpendicular to the velocity of the particle. In this case, the magnetic force is also perpendicular to the velocity (and the magnetic field vector, of course) at any given moment resulting in circular motion. The speed and kinetic energy of the particle remain constant, but the direction is altered at each instant by the perpendicular magnetic force. quickly reviews this situation in the case of a negatively charged particle in a magnetic field directed into the page.

Circular Motion of Charged Particle in Magnetic Field: A negatively charged particle moves in the plane of the page in a region where the magnetic field is perpendicular into the page (represented by the small circles with x’s—like the tails of arrows). The magnetic force is perpendicular to the velocity, and so velocity changes in direction but not magnitude. Uniform circular motion results.

What if the velocity is not perpendicular to the magnetic field? Then we consider only the component of v that is perpendicular to the field when making our calculations, so that the equations of motion become:

The component of the velocity parallel to the field is unaffected, since the magnetic force is zero for motion parallel to the field. This produces helical motion (i.e., spiral motion) rather than a circular motion.

shows how electrons not moving perpendicular to magnetic field lines follow the field lines. The component of velocity parallel to the lines is unaffected, and so the charges spiral along the field lines. If field strength increases in the direction of motion, the field will exert a force to slow the charges (and even reverse their direction), forming a kind of magnetic mirror.

Helical Motion and Magnetic Mirrors: When a charged particle moves along a magnetic field line into a region where the field becomes stronger, the particle experiences a force that reduces the component of velocity parallel to the field. This force slows the motion along the field line and here reverses it, forming a “magnetic mirror. “

The motion of charged particles in magnetic fields are related to such different things as the Aurora Borealis or Aurora Australis (northern and southern lights) and particle accelerators. Charged particles approaching magnetic field lines may get trapped in spiral orbits about the lines rather than crossing them, as seen above. Some cosmic rays, for example, follow the Earth’s magnetic field lines, entering the atmosphere near the magnetic poles and causing the southern or northern lights through their ionization of molecules in the atmosphere. Those particles that approach middle latitudes must cross magnetic field lines, and many are prevented from penetrating the atmosphere. Cosmic rays are a component of background radiation consequently, they give a higher radiation dose at the poles than at the equator.

Charged Particles Spiral Along Earth’s Magnetic Field Lines: Energetic electrons and protons, components of cosmic rays, from the Sun and deep outer space often follow the Earth’s magnetic field lines rather than cross them. (Recall that the Earth’s north magnetic pole is really a south pole in terms of a bar magnet. )


Extended Data Fig. 1 Summary of OTFMAP polarimetric observations.

Columns, from left to right: filter central wavelength, filter bandwidth, angular resolution of the observations, scan rate, scan phase, scan amplitude, scan duration, number of observation sets obtained, and total observation time on-source.

Extended Data Fig. 2 Polarization measurements of the several regions of the galactic disk.

Columns, from left to right: region of the galaxy, median magnetic field orientation, uncertainty of the magnetic field orientation, median polarization degree, uncertainty of the polarization degree.

Extended Data Fig. 3 Physical regions based on B-field orientation and degree of polarization.

Histograms of P (a) and PA (b) of polarization for measurements with P/σP > 3. Three distinct regions are found for the PA of polarization, which are identified with the west (orange), east (red) and low polarized (black) regions. The boundaries of each region are shown with vertical black dashed lines. (c), The spatial correspondence of the three regions identified using the PA distributions are shown with the same colors as the plots at b. The total intensity contours are shown as in Fig. 1. A legend polarization of 10% (black) and beam size of 7.8” (red circle) are shown.

Extended Data Fig. 4 Magnetic field of the central 50” x 50” (0.8 x 0.8 kpc 2 ) of Centaurus A.

a, Total flux (colorscale) with overlaid B-field orientations (white lines). b, Polarized flux (colorscale) with overlaid B-field orientation (white lines). A legend polarization of 5% (black) and beam size of 7.8” (red circle) are shown.

Extended Data Fig. 5 Parameters of the magnetic field morphological model.

Columns, from left to right: Free parameters used in the magnetic field model, symbols associated with the free parameter, boundaries of the flat pior distribution, median value of the posterior distribution with 1σ uncertainty values.

Extended Data Fig. 6 Posterior distributions of the magnetic field morphological model.

A reference of the parameter definitions, used priors, and median values is shown in Extended Data Fig. 5.

Extended Data Fig. 7 Polarization map vs physical parameters.

Temperature (a) and column density (b) maps of Centaurus A with overlaid B-field orientation (while lines) with P/σP > 2.5 and PI/σPI > 2. Temperature contours start at 20 K increasing in steps of 0.5 K, and column density density contours start at log(NH+H2 [cm −2 ]) = 20.6 increasing in steps of 0.1. 12 CO(1-0) integrated line emission (c) and velocity dispersion (d) of the warped disk of Centaurus A with overlaid B-field orientation (white lines) with P/σP > 2.5 and PI/σPI > 2.

Extended Data Fig. 8 Polarized flux vs. total intensity plots.

P-I and PI-I plots at 89 μm vs temperature (a,b) and column density (c,d). The trend of the bulk of the P-I plot, P ∝ τ −1 , is shown as a black solid line in panels (a) and (c). The uncertainties of the debiased polarized intensity in plots (b) and (d) are shown. The blue dotted vertical lines at I = 1000 and 2700 MJy sr −1 show the limits of the three physical regions found in this analysis. The black dotted lines in panels (b) and (d) show the maximum expected polarization, P ∝ I 0 = 15, 6.5, and 1.5% for each of these physical regions, respectively.

Extended Data Fig. 9 Power-law index of plots from Fig. 5.

Columns, from left to right: Parameters of the y-axis used in each fit, regions of the galaxy used for the fit, power-law indexes for the parameters used in the x-axis T, NH+H2, and σν, 12 CO(1-0).

Extended Data Fig. 10 Velocity dispersion of the outer and molecular disk.

12 CO(1-0) velocity dispersion histograms of the outer disk (red) and molecular disk (blue) as identified in Fig. 4. The median (solid line) and 1σ (dashed line) are shown for each physical structure. These values correspond to σv,12CO(1−0) = 18.4 ± 9.2 (km s −1 ), and σv,12CO(1−0) = 6.4 ± 6.0 (km s −1 ) for the molecular disk and outer disk, respectively.


The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, North and South America. The international collaboration is working to capture the most detailed black hole images ever obtained by creating a virtual Earth-sized telescope. Supported by considerable international investment, the EHT links existing telescopes using novel systems — creating a fundamentally new instrument with the highest angular resolving power that has yet been achieved.

The individual telescopes involved are: ALMA, APEX, the IRAM 30-meter Telescope, the IRAM NOEMA Observatory, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope (LMT), the Submillimeter Array (SMA), the Submillimeter Telescope (SMT), the South Pole Telescope (SPT), the Kitt Peak Telescope, and the Greenland Telescope (GLT).

The EHT consortium consists of 13 stakeholder institutes: the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona, the University of Chicago, the East Asian Observatory, Goethe-Universitaet Frankfurt, Institut de Radioastronomie Millimétrique, Large Millimeter Telescope, Max Planck Institute for Radio Astronomy, MIT Haystack Observatory, National Astronomical Observatory of Japan, Perimeter Institute for Theoretical Physics, Radboud University and the Smithsonian Astrophysical Observatory.


Magnetic Reconnection in Astrophysical and Laboratory Plasmas

Magnetic reconnection is a topological rearrangement of magnetic field that converts magnetic energy to plasma energy. Astrophysical flares, from the Earth's magnetosphere to γ-ray bursts and sawtooth crashes in laboratory plasmas, may all be powered by reconnection. Reconnection is essential for dynamos and the large-scale restructuring known as magnetic self-organization. We review reconnection theory and evidence for it. We emphasize recent developments in two-fluid physics, and the experiments, observations, and simulations that verify two-fluid effects. We discuss novel environments such as line-tied, relativistic, and partially ionized plasmas, focusing on mechanisms that make reconnection fast, as observed. Because there is evidence that fast reconnection in astrophysics requires small-scale structure, we briefly introduce how such structure might develop. Several areas merit attention for astrophysical applications: development of a kinetic model of reconnection to enable spectroscopic predictions, better understanding of the interplay between local and global scales, the role of collisionless reconnection in large systems, and the effects of flows, including turbulence.


How does one measure vector magnetic field of astrophysical object? - Astronomy

We present a vector magnetic field map obtained on 7 December 2003, below and around a filament located not so far from the active region NOAA 517, whose one spot is also found on the map of 240× 340 arcsec. This region was itself located near the disk center, so that the longitudinal (resp. transverse) field is nearly the vertical (resp. horizontal) one. The THEMIS telescope was used in its spectropolarimetric multiline mode MTR ("MulTiRaies"). The noise level is 5-10 Gauss in the longitudinal field and 50-100 Gauss in the transverse field, while the pixel size is 0.45 arcsec. Fundamental ambiguity is not solved, and the atmosphere is assumed to be homogeneous. The magnetic field derivation method described in this paper was validated on eight test points submitted to the UNNOFIT inversion code, and the results are found in agreement within 14% discrepancy. Two main results appear on the map: (i) a strong spatial correlation between the longitudinal and transverse field resulting in an inclined field vector (making a most probable angle of 60° or 120° with the line-of-sight in the filament region) and (ii) homogeneity of the field direction (inclination and azimuth) in the filament region. Parasitic polarities were also detected: first those located at the filament feet, as theoretically expected, on the one hand and then weak opposite polarity regular patterns that appear between the network field (strong field at the frontiers of supergranules), on the other. The exact superimposition of the magnetic field map derived from the Fe I 6302.5 Å line and of the Hα map, which enabled association of the parasitic polarities with the filament feet, was possible because these two maps were simultaneously obtained, thanks to a unique facility available in the multiline mode of THEMIS.


Astronomers Measure Magnetic Fields at Sagittarius A*

In this artist’s conception, the black hole at the center of our galaxy is surrounded by a hot disk of accreting material. Blue lines trace magnetic fields. It found the fields in the disk to be disorderly, with jumbled loops and whorls resembling intertwined spaghetti. In contrast, other regions showed a much more organized pattern, possibly in the region where jets (shown by the narrow yellow streamer) would be generated. Credit: M. Weiss/CfA

Using the Event Horizon Telescope, astronomers have measured the magnetic fields just outside the event horizon of Sagittarius A* for the first time.

Most people think of black holes as giant vacuum cleaners sucking in everything that gets too close. But the supermassive black holes at the centers of galaxies are more like cosmic engines, converting energy from infalling matter into intense radiation that can outshine the combined light from all surrounding stars. If the black hole is spinning, it can generate strong jets that blast across thousands of light-years and shape entire galaxies. These black hole engines are thought to be powered by magnetic fields. For the first time, astronomers have detected magnetic fields just outside the event horizon of the black hole at the center of our Milky Way galaxy.

“Understanding these magnetic fields is critical. Nobody has been able to resolve magnetic fields near the event horizon until now,” says lead author Michael Johnson of the Harvard-Smithsonian Center for Astrophysics (CfA). The results appear in the December 4th issue of the journal Science.

“These magnetic fields have been predicted to exist, but no one has seen them before. Our data puts decades of theoretical work on solid observational ground,” adds principal investigator Shep Doeleman (CfA/MIT), who is assistant director of MIT’s Haystack Observatory.

A delightful comic illustrates how the Event Horizon Telescope can measure magnetic fields at our galaxy’s core. Credit: Event Horizon Telescope

This feat was achieved using the Event Horizon Telescope (EHT) – a global network of radio telescopes that link together to function as one giant telescope the size of Earth. Since larger telescopes can provide greater detail, the EHT ultimately will resolve features as small as 15 micro-arcseconds. (An arcsecond is 1/3600 of a degree, and 15 micro-arcseconds is the angular equivalent of seeing a golf ball on the moon.)

Such resolution is needed because a black hole is the most compact object in the universe. The Milky Way’s central black hole, Sgr A* (Sagittarius A-star), weighs about 4 million times as much as our Sun, yet its event horizon spans only 8 million miles – smaller than the orbit of Mercury. And since it’s located 25,000 light-years away, this size corresponds to an incredibly small 10 micro-arcseconds across. Fortunately, the intense gravity of the black hole warps light and magnifies the event horizon so that it appears larger on the sky – about 50 micro-arcseconds, a region that the EHT can easily resolve.

The Event Horizon Telescope made observations at a wavelength of 1.3 mm. The team measured how that light is linearly polarized. On Earth, sunlight becomes linearly polarized by reflections, which is why sunglasses are polarized to block light and reduce glare. In the case of Sgr A*, polarized light is emitted by electrons spiraling around magnetic field lines. As a result, this light directly traces the structure of the magnetic field.

Sgr A* is surrounded by an accretion disk of material orbiting the black hole. The team found that magnetic fields in some regions near the black hole are disorderly, with jumbled loops and whorls resembling intertwined spaghetti. In contrast, other regions showed a much more organized pattern, possibly in the region where jets would be generated.

They also found that the magnetic fields fluctuated on short time scales of only 15 minutes or so.

“Once again, the galactic center is proving to be a more dynamic place than we might have guessed,” says Johnson. “Those magnetic fields are dancing all over the place.”

These observations used astronomical facilities in three geographic locations: the Submillimeter Array and the James Clerk Maxwell Telescope (both on Mauna Kea in Hawaii), the Submillimeter Telescope on Mt. Graham in Arizona, and the Combined Array for Research in Millimeter-wave Astronomy (CARMA) near Bishop, California. As the EHT adds more radio dishes around the world and gathers more data, it will achieve greater resolution with the goal of directly imaging a black hole’s event horizon for the first time.

“The only way to build a telescope that spans the Earth is to assemble a global team of scientists working together. With this result, the EHT team is one step closer to solving a central paradox in astronomy: why are black holes so bright?” states Doeleman.

EHT research at CfA and MIT is supported by supported by grants from the National Science Foundation and from the Gordon and Betty Moore Foundation. The Submillimeter Array (SMA) is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics. The Submillimeter Telescope is part of the Arizona Radio Observatory, which is partially supported through the NSF University Radio Observatories program. The James Clerk Maxwell Telescope was operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK, the Netherlands Organization for Scientific Research, and the National Research Council of Canada. Funding for CARMA development and operations was supported by NSF and the CARMA partner universities.


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1 Introduction (pp1-4) [ pdf (25kb) ]

2 Jet acceleration in YSOs and AGN (pp5-27) [ pdf (1.4Mb) ]

this chapter also published as a paper

  • 2.1 Introduction
  • 2.2 Non-relativistic (YSO) jets
    • 2.2.1 Fluid equations
    • 2.2.2 Numerical solution
    • 2.2.3 Initial conditions
    • 2.2.4 Results
    • 2.2.5 Steady wind solution
    • 2.2.6 Terminal wind velocities as a function of heating rate
    • 2.3.1 Fluid equations
    • 2.3.2 Scaling
    • 2.3.3 Numerical Solution
    • 2.3.4 Initial Conditions
    • 2.3.5 Results
    • 2.3.6 Steady wind solution
    • 2.3.7 Terminal wind velocities and Lorentz factors as a function of heating rate

    3 Smoothed Particle Hydrodynamics (pp29-72) [ pdf (2.4Mb) ] [ Errata ]

    • 3.1 Introduction
    • 3.2 Basic formalisms
      • 3.2.1 Interpolant
      • 3.2.2 Errors
      • 3.2.3 First derivatives
      • 3.2.4 Second derivatives
      • 3.2.5 Smoothing kernels
      • 3.2.6 A general class of kernels
      • 3.2.7 Kernel stability properties
      • 3.3.1 Continuity equation
      • 3.3.2 Equations of motion
      • 3.3.3 Energy equation
      • 3.3.4 Variable smoothing length terms
      • 3.4.1 Variational principle
      • 3.4.2 General alternative formulation
      • 3.4.3 Ott & Schnetter formulation
      • 3.5.1 Artificial viscosity and thermal conductivity
      • 3.5.2 Artificial dissipation switches
      • 3.6.1 Predictor-corrector scheme
      • 3.6.2 Reversible integrators
      • 3.6.3 Courant condition
      • 3.7.1 Implementation
      • 3.7.2 Propagation and steepening of sound waves
      • 3.7.3 Sod shock tube
      • 3.7.4 Blast wave
      • 3.7.5 Cartesian shear flows
      • 3.7.6 Toy stars

      4 Smoothed Particle Magnetohydrodynamics (pp73-122) [ pdf (5.8Mb) ]

      NB: parts of this chapter were published as two papers (paper I and paper II), although the thesis chapter presents a more up-to-date version of this work.


      Watch the video: 2012: Αναστροφή του γήινου μαγνητικού πεδίου 13 (May 2022).