Two-week outburst event from black hole sheds light on black hole processes.
For 25 years, the black hole V404 Cygni was silent; in June of last year, it suddenly flared up. For the next two weeks, it released an “intensely, violently, variable” barrage of light, as a research team studying the event described it in their paper. Its brightness increased by a million times over a few days, making it the brightest X-ray source in the sky for a short time. And then, unceremoniously, it ended.

Luckily, NASA’s Swift spacecraft detected the strange outburst as soon as it began, and researchers quickly trained the Gran Telescopio CANARIAS (GTC) and other instruments on the event, allowing them to examine the spectrum of light emitted by V404 Cyg.

This isn’t the first time a black hole has been observed to have a period of extreme activity. V404 Cyg itself had a similar burst in 1989 before its quarter-century of quiescence. However, this one is different from the others in a few respects: it was much shorter (others have been known to last months to a year) and it stopped abruptly. The study of this event allowed researchers to gain important insights into the processes surrounding black holes.

Outburst

Black holes can emit jets of material, which are often bright in the radio wave portion of the electromagnetic spectrum. We observed that behavior at V404 Cyg, but that wasn’t the main source of the burst. The burst apparently resulted from a huge amount of matter being expelled from the black hole’s accretion disk (the disk of matter spiraling into the black hole) at about one percent of the speed of light.

The matter that was being ejected was mostly neutral hydrogen and helium. “Neutral” in this context means that it had no net electrical charge, neither positive nor negative. That’s interesting for a number of reasons. To begin with, it's much easier for the black hole's environment to accelerate charged particles, which can interact with magnetic fields. And there should be lots of charged particles around; material in churning, hot cauldrons like black hole accretion disks generally becomes ionized.

In order for neutral hydrogen and helium to exist, the temperature has to be relatively low—less than roughly ten thousand Kelvin. That’s pretty hot by earthly standards (for comparison, molten lava is about 1,200K, and even the surface of the Sun is about 5,700K). But for V404 Cyg’s accretion disk, it’s actually relatively cool. Too cool, in fact, for the material to have originated in the inner, hotter part of the accretion disk. So regardless of what was causing the matter to be ejected, it has to be coming from the outer part of the disk.

The burst will regulate how fast the hole can consume matter. That's because it limits how quickly the inner part of the accretion disk can be re-filled with matter after it has been consumed. The disk as a whole is still being fed by a nearby star orbiting the black hole, so it will likely take time for new material to make it to the inner portion and be consumed.

https://www.youtube.com/watch?v=MEJbo9LGwvw

After leaving the disk, the ejected material was expected to cool and expand, forming a nebula. And sure enough after the outburst ended, its spectrum showed signs of nebulosity. Specifically, its Balmer decrement—the ratio of intensities among hydrogen emission lines in the spectrum—increased, reaching values that are typically associated with nebulae.

It’s possible that a nebula also formed after the 1989 event, but if so, researchers at the time weren’t able to observe it. They were only able to this time around because there was an “intense observing campaign” (as the researchers put it) during the outburst, which allowed them to gather ample data and study the event in detail. In any case, the nebula phase was very short-lived.

Causes

One thing that remains unclear about the outburst is what caused it. One option is radiation pressure. Light from the infalling matter is so intense it can actually put outward pressure on material in the disk. However, the researchers ruled this one unlikely as, if the black hole was producing enough light to create the necessary pressure, it should have been brighter.

Another option is a thermal wind. In this model, the accretion disk gas just gets hot enough to escape the black hole's gravity on its own. After all, heat is really just a measure of the speed of the individual particles in the gas, so when the gas heats up, the particles become fast enough to escape. After running the numbers on this one, the researchers found that this model is the most consistent with the data.

The outburst was a huge upset for the black hole’s accretion disk, as much of the disk’s material was ejected. But there were signs that a significant amount of the disk survived. Once the black hole had returned to its quiet state, it remained brighter than other black holes in their quiescent states. That implied matter was still falling into the hole from an active disk. About six months after the outburst (bringing us to December), a second, much weaker outburst occurred. While nowhere near as prominent as the first, this outburst was another sign that the accretion disk was going strong.

That’s probably because V404 Cyg has a large accretion disk to begin with, as evidenced by the long time it takes material to orbit the black hole: about 6.5 days. Most stellar-mass black holes have orbital periods of about two days.

Given that a large disk is probably necessary for massive outflows like this, the researchers suggest that other black holes with large accretion disks might also experience outflows like this one. In fact, some of these have experienced series of brief outbursts. The accretion disk with the longest orbital period is associated with the black hole GRS 1915 + 105, and that has been permanently in outburst for the past 23 years.

Conclusions

This particular process—a massive outflow of neutral material coming from the disk’s outer portion—is unlike other outflows seen before. The researchers believe that the huge amount of mass expelled explains why the event ended so abruptly. The outer disk simply ran out of material to eject. This implies that the process seen at V404 Cyg could be a significant one that tempers the rate at which black holes consume matter.

We'll need other observations to pin down what's going on in these extreme environments.