Featured image: In the left panel: a streak of debris from a disrupted star is falling toward the disk, while the hot “corona” is still emitting X-rays (the ball of white light above the black hole). In the right panel: the debris has dispersed some of the gas, causing the corona to disappear. Credit: Robert Hurt, NASA / JPL.
At the center of a far-off galaxy, a giant, or “super-massive”, black hole is slowly consuming a disk of gas that swirls around it like water circling a drain. As the gas is pulled into the black hole, it heats up and emits radiation ranging from the visible to the X-rays – radiation that is clearly seen 300 million light years away on Earth.
In most such systems, it’s not unusual to see the radiation change in luminosity, getting 10 times brighter or fainter as the rate at which the black hole accretes material fluctuates. But two years ago, a team of researchers led by Dr. Benny Trakhtenbrot and Dr. Iair Arcavi, both from the Department of Astrophysics at Tel Aviv University, identified strange variations in the behavior of a black hole known as 1ES 1927+654. The ASAS-SN sky survey measured a 50-fold increase in the visible radiation emitted around the black hole, and observations taken by the researchers using the Las Cumbres network of robotic telescopes showed rapid changes in the form and source of the radiation.
A few weeks later, the team pointed NASA’s Swift, NuSTAR and NICER space telescopes, as well as the European Space Agency’s XMM-Newton space telescope at the black hole, and noticed a 10,000-fold decrease in the X-ray radiation coming from the black hole’s vicinity. “We’ve never seen a black hole behave this way”, says Dr. Trakhtenbrot. “Usually, the amount of radiation from the vicinity of a black hole is directly linked to the rate at which it accretes material. So the sharp rise in the visible radiation was telling us that the accretion rate is increasing, while the decrease in X-ray radiation was telling us that the accretion rate is actually decreasing”.
“It was so strange that, at first, we thought maybe there was something wrong with the data”, said Claudio Ricci, an assistant professor at Diego Portales University in Santiago, Chile. Dr. Ricci is leading a new study of the black hole. In this new study, the investigators suggest that a rogue star got too close to the black hole and was torn apart by the strong gravitational forces there. In such a scenario, the remnants of the disrupted star could crash onto the disk of gas that was there earlier, heat it up (creating more visible radiation), and cause some of it to disperse (thus reducing the X-ray emission). “We’ve seen several cases of black holes tear apart stars that got too close, but until now we’ve never seen it happen around a black hole with a pre-existing disk of material, nor the collision that ensues”, says Dr. Arcavi.
Almost every galaxy contains a super-massive black hole in its center, which can have a mass of a million or even a billion times the mass of the sun, but it’s still not clear how such high masses are reached. One possibility is that black holes grow by steadily accreting gas that’s around them. Recently, the possibility that an accelerated ingestion of stars could provide enough material for the black hole is also being investigated. The recent event in 1ES 1927+654 provides a glimpse into the combination of both processes.
Although a drifting star seems the most likely culprit, the authors note that there could be other explanations for the unprecedented event. One remarkable feature of the observations is the fact that the overall drop in X-ray brightness wasn’t a smooth transition: Day to day, the NICER telescope, installed on the International Space Station, detected dramatic variation, sometimes changing in brightness by a factor of 100 in a few hours. such rapid changes occurring continuously for months, have never been seen before. “This data set has a lot of puzzles in it,” said Dr. Ricci. “But that’s exciting, because it means we’re learning something new about the universe”.