For 25 years, the black hole V404 Cygni remained silent; in June last year it suddenly flared up. Over the next two weeks, it released an “intense, violent, variable” barrage of light, as a research team studying the event described it in their paper. Its brightness increased by a million times in a few days, making it the brightest X-ray source in the sky for a short time. And then, unceremoniously, it ended.
Fortunately, NASA’s Swift spacecraft detected the strange outburst as soon as it started, 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 experienced a period of extreme activity. V404 Cyg itself had a similar outburst in 1989, before its quarter-century of dormancy. However, this one differs from the others in a few ways: it was much shorter (others have been known to last for months to a year), and it stopped abruptly. The study of this event enabled researchers to gain important insights into the processes surrounding black holes.
Black holes can emit jets of material, which are often bright in the radio wave portion of the electromagnetic spectrum. We observed that behavior in V404 Cyg, but that was not the main source of the outburst. The outburst apparently resulted from a huge amount of matter being expelled from the black hole’s accretion disk (the disk of matter spiraling towards the black hole) at about one percent of the speed of light.
The matter 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. For starters, it is much easier for the black hole environment to accelerate charged particles that can interact with magnetic fields. And there should be a lot of charged particles around; material in swirling hot cauldrons such as black hole accretion disks is generally ionized.
For neutral hydrogen and helium to exist, the temperature must be relatively low — less than about ten thousand Kelvin. That’s pretty hot by Earth 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. Actually too cool for the material to form in the inner, hotter part of the accretion disk. So regardless of what caused the matter to eject, it must be coming from the outer part of the disc.
The burst controls how fast the hole can consume matter. That’s because it limits how quickly the inner part of the accretion disk can refill with matter after it’s consumed. The disk as a whole is still fueled by a nearby star orbiting the black hole, so it will likely take some time for new material to reach the inner portion and be consumed.
After it left the disk, the ejected material was expected to cool and expand, creating a nebula. Sure enough, after the eruption ended, the spectrum showed signs of fuzziness. In particular, the Balmer decay – the ratio of intensities between hydrogen emission lines in the spectrum – increased, reaching values typically associated with nebulae.
It’s possible that a nebula also formed after the 1989 event, but if so, researchers couldn’t see it at the time. They could only this time because there was an “intense observation campaign” (as the researchers put it) during the eruption, which allowed them to collect enough data and study the event in detail. In any case, the mist phase was very short-lived.
One thing that remains unclear about the eruption is what caused it. One option is radiation pressure. Light from the incident matter is so intense that it can actually exert an outward pressure on material in the disc. However, the researchers judged this to be unlikely because if the black hole produced 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 gets just hot enough to escape the black hole’s gravity on its own. After all, heat is 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 shock to the black hole’s accretion disk, as much of the disk’s material was ejected. But there were signs that a significant portion of the disc survived. Once the black hole returned to its quiescent state, it remained brighter than other black holes in their quiescent state. That implicit matter still fell into the hole of an active disk. About six months after the eruption (which brings us to December), a second, much weaker eruption occurred. While not nearly as prominent as the first, this eruption was another sign that the accretion disk was getting strong.
That’s probably because V404 Cyg has a large accretion disk to begin with, as evidenced by the long time it takes for the material to orbit the black hole: about 6.5 days. Most stellar-mass black holes have an orbital period of about two days.
Since a large disk is likely required for this type of massive outflow, the researchers suggest that other black holes with large accretion disks may also experience such outflows. Some of them have even experienced a series of short bursts. The accretion disk with the longest orbital period is associated with the black hole GRS 1915 + 105, which has been in permanent eruption for the past 23 years.
This particular process — a massive outflow of neutral material emanating from the outer portion of the disk — is unlike any other outflow we’ve seen before. The researchers believe the sheer amount of ejected mass explains why the event ended so abruptly. The outer disc simply ran out of material to eject. This implies that the process seen at V404 Cyg may be an important one that tempers the rate at which black holes consume matter.
We need other observations to determine what is happening in these extreme environments.
Nature2015. DOI: doi:10.1038/nature17446 (About DOIs)