A strong magnetic field around the Milky Way's black hole

Aug 14, 2013
Pulsar and Milky Way's Central Supermassive Black Hole: In this artist's conception, the magnetic field from a rotating disk surrounding the supermassive black hole at the center of the Milky Way (left) extends outward to encompass the closest pulsar yet found to the Galaxy's core. The pulsar (right) has a strong magnetic field, and emits lighthouse-like beams of radio waves outward from the poles of its own magnetic field. Those beams are detected and analyzed by radio telescopes on Earth. Credit: Bill Saxton, NRAO/AUI/NSF.

(Phys.org) —Astronomers have made an important measurement of the magnetic field emanating from a swirling disk of material surrounding the black hole at the center of our Milky Way Galaxy. The measurement, made by observing a recently-discovered pulsar, is providing them with a powerful new tool for studying the mysterious region at the core of our home galaxy.

Like most galaxies, the Milky Way harbors a at its center, some 26,000 light-years from Earth. The Milky Way's central black hole is some four million times more massive than the Sun. Black holes, concentrations of mass so dense that not even light can escape them, can pull in material from their surroundings. That material usually forms a swirling disk around the black hole, with material falling from the outer portion of the disk inward until it is sucked into the black hole itself.

Such disks concentrate not only the matter pulled into them but also the magnetic fields associated with that matter, forming a giant, twisting magnetic field that is thought to propel some of the matter back outward along its poles in superfast "jets."

The region near the black hole is obscured from visible-light observations by gas and dust, and is an exotic, still little-understood by astronomers. The magnetic field in the central portion of the region is an important component that affects other phenomena.

Artist’s impression of PSR J1745-2900, a pulsar with a very high magnetic field ("magnetar") potentially just less than half a light year from the supermassive black hole at the centre of our Galaxy. Measurements of the Faraday effect, imprinted on the pulsar emission, imply that a strong magnetic field also exists in the environment around the black hole. Credit: Ralph Eatough/MPIfR

The first link to measuring the magnetic field near the black hole came last April when NASA's Swift satellite detected a flare of X-rays from near the Milky Way's center. Observers soon determined that the X-rays were coming in regular pulses. Follow-on observations with , including ones in Germany, France, and the National Science Foundation's Karl G. Jansky Very Large Array (VLA), showed radio pulses identically spaced. The astronomers concluded the object, called PSR J1745-2900, is a , a highly-magnetized pulsar, or spinning neutron star.

The pulsar is the closest yet found to the black hole, possibly within less than half a light-year. Analysis of the radio waves coming from the pulsar showed that they are undergoing a dramatic twist as they travel from the pulsar to Earth. Such a twist, called Faraday rotation, comes when the waves travel through charged gas that is within a magnetic field.

This video is not supported by your browser at this time.
Credit: MPIfR/David Champion & Ralph Eatough.

The charged gas, the astronomers said, is somewhere roughly 150 light-years from the black hole, directly between the pulsar and Earth. Measuring the twist in the waves caused by their passage through this gas allowed the scientists to calculate the strength of the magnetic field. The magnetic field is a crucial part of the black hole's environment, affecting the structure of the flow of material into the black hole, and even regulating that flow.

"The lucky alignment of this gas with a pulsar so close to the black hole has given us a valuable tool for understanding this difficult-to-observe environment," said Paul Demorest, of the National Radio Astronomy Observatory.

The measured strength of the magnetic field at the presumed distance of the gas cloud from the black hole is about what astronomers expected, based on the intensity of X-rays and radio waves coming from the area closest to the black hole. The measurements also indicate that the field is relatively well-ordered, instead of turbulent, the scientists said.

"The closer you get to the black hole and the disk surrounding it, the stronger the magnetic field should become," Demorest said. "Our measurement shows the field strength we would expect at the distance we believe that gas cloud is from the black hole," he added.

The scientists plan to continue watching PSR J1745-2900, because they expect to detect changes as it moves in its orbital motion around the black hole. This will provide additional measurements of the magnetic-field strength in different gas clouds. Also, they expect—and hope—to find more pulsars that will allow them to use the same technique to make a detailed map of the near the black hole.

Explore further: Millisecond pulsars clearly demonstrate that pulsars are neutron stars

More information: Paper: dx.doi.org/10.1038/nature12499

Related:

Swift Discovery of a new soft gamma repeater, SGR J1745-29, near Sagittarius A* Astrophysical Journal Letters 770, L24, 2013 (arxiv.org/abs/1305.2128)
K. Mori et al.

NuSTAR discovery of a 3.76-second transient magnetar near Sagittarius A* Astrophysical Journal Letters 770, L23, 2013 (arxiv.org/abs/1305.1945)
R. M. Shannon, S. Johnston

Radio properties of the magnetar near Sagittarius A* from observations with the Australia Telescope Compact Array Monthly Not. Roy. Astron. Soc. (MNRAS) Letters, August 14, 2013 (de.arxiv.org/abs/1305.3036)

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User comments : 11

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barakn
3.7 / 5 (6) Aug 14, 2013
Queue the EU police to complain about the phrase "charged gas"...
cantdrive85
1 / 5 (11) Aug 14, 2013
Queue the EU police to complain about the phrase "charged gas"...


There are much larger issues that pertain to the nonsensical pseudo-science that's being blathered. Magnetars, black holes, and frozen-in fields oh my!
savas_ozyurt_1
not rated yet Aug 15, 2013
I would buy that idea of a rotating disk in the pict. And that magnetic wave causing the earth to topple and waggle. But if the disk is correct, than the black hole isn't a hole but a collapsed system into a disk(?) shape.
GSwift7
5 / 5 (5) Aug 15, 2013
and frozen-in fields oh my!


I'm not sure why you find that term so offensive. Can you please explain what you think the term "frozen field" means?

I strongly suspect that you do not understand what the term means, and that your argument against the concept is based only on an obscure quote from Alfven.

Here's a recent paper from Cornell University that goes into detail about when the concept does and does not apply.

http://arxiv.org/abs/1208.2961

Here's Alfven's outdated 1976 paper on the subject:

http://plasma.col...1976.pdf

Take note of the Cornell paper's introduction, where they talk about Alfven's objections to certain uses of the term, though it was his idea to start with. (click on the PDF link and you can see the full paper)

full paper here:

http://arxiv.org/...61v1.pdf
Infinum
1 / 5 (2) Aug 15, 2013
black hole mass to galaxy mass ratio of (roughly) 1:1000 seems to be connected to the strength of the magnetic field of the black hole which is directly proportional to the mass of the black hole. Mystery solved? ;)
cantdrive85
1 / 5 (5) Aug 16, 2013
Alfven tried to enlighten them 40+ years ago, yet here we are going round and round.

the frozen-in field concept is the "bed-rock"concept underlying ideal MHD

There's that "ideal MHD", I think Alfven had something to say about that.
"The cosmical plasma physics of today is far less advanced than the thermonuclear research physics. It is to some extent the playground of theoreticians who have never seen a plasma in a laboratory. Many of them still believe in formulae which we know from laboratory experiments to be wrong. The astrophysical correspondence to the thermonuclear crisis has not yet come." Alfven, Nobel Lecture Speech

And it still has yet to come, the continued used of these fundamentally incorrect models is the problem. Proving this and that based upon a flawed model is of little use.

"The velocity of the spread of relevant knowledge to astrophysicists seems to be much below the velocity of light.
http://kth.diva-p...XT01.pdf
cantdrive85
1 / 5 (4) Aug 16, 2013
Sorry, I'm not trying to take credit.

"The velocity of the spread of relevant knowledge to astrophysicists seems to be much below the velocity of light." Alfven

GSwift7
5 / 5 (1) Aug 16, 2013
Once again, outdated quotes. That has ZERO relevance today. Alfven's concernes are addressed regularly in modern cosmology, as you can see from the Cornell paper. We have gone way past Alfven's work in recent decades. You are bailing water from a boat that is sitting on the beach. Stop bailing and just step out and walk.

In other words, stop arguing a dead question and just go read some of the many papers with the answers you're looking for.

So I ask again. What do you think a frozen in field is?
cantdrive85
1.7 / 5 (6) Aug 16, 2013
It's a moot point, you're ask me to define something based on "models we know are incorrect". If you insist on an answer I put it the same way as always. These pseudoscientists are "creating" pseudoscience claptrap based upon a hypothetical construct, ideal MHD. It's not about quotes, it's not about when, it's about astrophysicists ignoring 100 years of plasma experiment in favor of a theoretical framework insisting their plasma is different. The difference between you and I, you insist theoretical models are correct, despite experiment, I insist experiment and theory must agree. And before you claim I am changing the subject, an analog of what you are asking I do is plot the path of comet Ison using epicycles. I ask, what would I gain from answering such a question? Little more than a false answer.
GSwift7
5 / 5 (2) Aug 16, 2013
based on "models we know are incorrect".


No, frozen fields were Alfven's idea, so I'm sure he wasn't suggesting an idea that he thought was wrong.

Can you explain to me what Alfven meant when he proposed the idea of frozen fields?

BTW, we have been able to make observations in the past couple decased which prove Alfven was correct on this, and we've taken his work even farther, observing how Alfven waves actually move in the photosphere. He wasn't right about everything, but some of his work is the basis for some of what we do today.
cantdrive85
1 / 5 (5) Aug 17, 2013
Alfven was very clear that he saw FF as probably the biggest mistake of his career, something he tried to highlight in numerous papers since he introduced it in 1950. He explained it as a useful pedagogical tool, but warned that it didn't describe birkeland currents, instabilities, and a number of other plasma phenomena that are ubiquitous to astrophysical plasmas.

Alfven waves exist, no argument there. What it all comes down to is treating plasma as an ideal gas with MHD considerations. His objection was, before the "thermonuclear crisis" ideal MHD was the basis for nuclear research, it failed miserably. What nuclear physicists learned was that the particle and circuit approach to explaining the plasma was required. That crisis has yet to reach the astrophysicists, in spite of the constant surprises and need to revise or adjust hypotheses.

Yep, "some" of his work is being used.
http://www.plasma...asma.pdf