Pulsar Reveals Clues to Black Hole's Light Appetite

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Not often do I get to talk about two of my favorite, most extreme objects in the Universe in the same story: pulsars and black holes. The recent discovery of a very special pulsar very close to our Galaxy’s supermassive black hole has astronomers, and this blogger, pretty darn excited.

First, let’s set the scene. In the center of our Milky Way Galaxy, as in most galaxies, there lies a supermassive black hole. Ours is millions of times the mass of the sun. Others are larger and much more active, but ours is the closest, and thus easier to study in many ways. This black hole, dubbed Sagittarius A* (pronounced “A-star”), or Sgr A* for short, is pulling in, or “eating” some of the gas around it, but very inefficiently. It is a typical quiescent, or quiet black hole.

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The feeding process gives off radio and X-ray emission that has been studied at great length, but the magnetic field strength around the black hole must also be known to complete the physical picture.

One way to determine this is to look at the Faraday rotation of the emission. This process just means that the polarization of the light it rotated, or turned, as it goes through a region with a magnetic field. This was actually discovered in the lab by Michael Faraday in 1845, showing that light and electromagnetism were indeed interlinked.

The Faraday rotation effect depends on the strength of the magnetic field and the wavelength of the light in question. Since an object like Sgr A* gives off a range of wavelengths, telescopes measure the polarization of light at different wavelengths and can “back out” how much it has been rotated, thus measuring the magnetic field strength. It’s actually a pretty clever technique.

And though the Faraday rotation has been measured for the immediate area around the black hole itself, we had no idea what the magnetic fields were like just outside of it, where much of the gas it is eating resides.

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This is where the new pulsar comes in. PSR J1745-2900 is no ordinary pulsar. In fact, it was first discovered as a bright X-ray flare by the Swift satellite. Further observations with the X-ray telescope NuSTAR showed that the emission was pulsating with a period of almost 4 seconds.

Finally, it was detected in the radio with the 100-meter radio telescope in Effelsberg, a dish that has been used to search for pulsars near the Galactic Center for 40 years. Several other radio telescopes joined in to measure the long-awaited pulsar, determining that it was indeed very close, less than a light-year away, from the supermassive black hole at the Galactic Center.

This pulsar is one with an extreme magnetic field of its own, a magnetar. This is an incredibly rare type of pulsar, indicating that there should be many more “normal” pulsars close to the black hole as well.

Faraday rotation measurements were made of the region around this magnetar, making a measurement of the magnetic field in the hot gas falling into the supermassive black hole. This measurement is the final piece of the puzzle to explain the radio and x-ray emission seen from Sgr A*.

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How strong is the magnetic field after all? The Faraday rotation measurements indicate that it is a few millionths of a gauss, where the unit of measure is named after the 19th century physicist and mathematician, Johann Carl Friedrich Gauss. For comparison, the magnetic field on the surface of the Earth is about half of one gauss, and your refrigerator magnet is close to 100 gauss.

These tiny magnetic fields make a big difference when calculating the physics of such a dynamic region, and the synthesis of all these different telescopes certainly makes this discovery an exciting one.

Image: Artist’s impression of PSR J1745-2900, a pulsar with a very high magnetic field (“magnetar”) in direct vicinity of Sagittarius A*. Credit: MPIfR/Ralph Eatough

This research has been published in Nature, and a press release with more links is available.

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