Now that gravitational waves have been directly observed, the next big goal in gravity physics is to better observe black holes. While there is lots of evidence showing black holes exist, we’d really like to observe them more directly.

The Event Horizon Telescope hopes to see the region around a black hole directly. Since the black hole itself doesn’t emit any light, what we hope to observe is the hot material close to the black hole. Because the closest material is relatively near the event horizon of a black hole, the light we see would be greatly distorted by gravity, causing its appearance to warp in shape and color. The science fiction movie Interstellar, for example, used computer simulations to create a simulated black hole showing some of these effects. But the black hole in Interstellar is an enhanced Hollywood version, and not quite what we’d actually observe.

A real black hole would look somewhat less dramatic, as seen in this video. Produced in part by some of my colleagues at RIT, the video shows the hard and soft x-rays emitted by the corona and accretion disk of a black hole, including the gravitational warping and Doppler shift caused by relativity. It gives us a good idea of what we expect to see near a black hole.

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A millisecond pulsar is a neutron star that is rotating about 600 to 700 times a second. Because of their strong magnetic fields, they produce strong beams of radio energy from the regions of their magnetic poles, and as they rotate these beams can point in our direction. As a result, we observe these neutron stars as radio bursts that pulse every 1 – 10 milliseconds. Hence their name.

Millisecond pulsars are rotating about as fast as neutron star can rotate, which makes them a bit of a mystery. Left by themselves, a pulsar gradually slows down over time. That means millisecond pulsars are either very young neutron stars that formed at near maximal rotation, or there must be some mechanism that causes them to spin more rapidly.

It’s generally thought that the latter process is the more common. A neutron star that is part of a binary system with a red giant companion can capture material from the companion star. As the material is captured, the angular momentum (rotation) of the material is transferred to the neutron star, thus increasing its rotation. This would explain why millisecond pulsars are often old pulsars with a companion. While this mechanism was initially proposed decades ago, over the years we’ve gathered a lot of evidence to support it.

When neutron stars are actively capturing material from their companion, the energy released as it falls to the neutron star produces intense x-rays. Such x-ray producing systems are known as x-ray binaries. These x-ray binaries can be quite active, but the radiation emitted by them tends to push the accreting material away. Thus over time an active x-ray binary will become less active, eventually entering a quiet period after which it may become active again. In the late 1980s it was observed that some x-ray binaries in the late stage of their active period contain radio millisecond pulsars. In 1998 a millisecond pulsar was observed within an active x-ray binary. Then in 2009 an accretion disk was discovered around a millisecond pulsar, indicating that the pulsar had been accreting material in the past.

Then last year in Nature new evidence was presented that further verifies the mechanism. The paper presents observations of an x-ray transient known as IGR J18245–2452. An x-ray transient is an object that emits x-rays for a time, then goes quiet for a time. There are several types of x-ray transients, but this particular one is a neutron star with a companion. In the past it had been observed as a radio pulsar. It then entered an active period and begin emitting x-rays with millisecond pulsations. After an active period of about a month, the x-rays went quiet, and the neutron star began to emit radio pulses again.

This not only demonstrates a clear connection between x-ray binaries and millisecond pulsars, but that these objects can shift between the two states on a fairly rapid pace. It seems that some neutron stars really do eat and run.

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One of the challenges to understanding black holes is that when things get close to a black hole, things get complicated.  We actually have a good description of black holes by themselves, but the description of the heated material near a black hole is complex.  To understand the behavior of this material you need to account for not only the gravitational attraction of the black hole, but also things such as magnetic fields.  To model active black holes, you need sophisticated computer simulations, and those simulations rely on certain assumptions about how black holes interact.The assumptions we make about black holes is based upon observations we have of black holes.  Some properties, such as rotation, we’ve been able to get good measures of, but other properties such as the strength of magnetic fields near a black hole have been more challenging. Now a new paper in Nature has presented a good measure of magnetic field strength near supermassive black holes, and it is a bit surprising.

Comparison of magnetic flux vs accretion disk brightness. Credit: M. Zamaninasab, et al.

In the paper the authors looked at 76 active (radio loud) supermassive black holes. First they measured the brightness of the accretion disk of each black hole, then they measured the jets emitted from the black holes, from which they could determine the strength of their magnetic fields.   They then compared the brightness of the accretion disks with the strength of the magnetic fields.  They found the two were strongly correlated across seven orders of magnitude.

What this means is that the magnetic field plays a crucial role in the production of black hole jets across a wide range of black hole masses.  From this correlation they could also determine the strength of the magnetic field near the black hole itself. It turned out to be much stronger than expected.  So strong that it can seriously effect the behavior of the black hole accretion disk, such as compressing it magnetically.  It can even act to inhibit the infall of material into the black hole.

Basically, the magnetic fields near a black hole can be as strong as those in an MRI, and they can affect the surrounding material as strongly as the gravity of the black hole itself.  While we’ve known that magnetic fields have a significant effect on black hole dynamics, we hadn’t thought they were strong enough to seriously affect accretion rates.

So now we have a better understanding of black hole magnetic fields, and that means modeling these beasts will require a bit more animal magnetism.

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