A team of astronomers led by Erin Kara, the Neil Gehrels Prize Postdoctoral Fellow in the University of Maryland’s Department of Astronomy, has provided the clearest picture to date of exactly how black holes generate massive X-ray outbursts. And their findings may help settle a long-standing debate about where around a black hole these outbursts originate.
By measuring the differences, or lag times, between these X-rays and the “echoes” of these X-rays reflected off swirling gas near the black hole, the researchers revealed information on how the black hole changed during the outburst. In a study published January 10th in the journal Nature, the team reports evidence that as the black hole consumed material from a nearby star, its corona—the halo of highly-energized particles that surrounds a black hole—shrank significantly.
“We don’t really understand the source of these relativistic jets [X-ray burst] that are basically common in many accreting systems. However, these results indicate [the process] really is driven by the change of corona,” said Kara, the lead author of the paper, who is also a Hubble Fellow with a co-appointment at NASA’s Goddard Space Flight Center and the Joint Space-Science Institute, a UMD and NASA Goddard collaboration.
Black hole J1820 was studied by the team and is located about 10,000 light-years from Earth. Its existence was unknown until March 11, 2018, when the outburst was spotted by the Japanese Aerospace and Exploration Agency’s Monitor of All-sky X-ray Image (MAXI), also aboard the space station. In the space of a few days, it went from a totally unknown black hole to one of the brightest sources in the X-ray sky. NICER was used to quickly capture this dramatic transition and continues to follow the fading tail of the eruption.
“NICER is the only instrument out there capable of making these measurements,” said Kara. “So we were lucky in that we saw this incredibly bright object, but we also were prepared to study it with this new instrument [NICER] on the international Space Station.”
X-ray Insight into Black Hole Evolution
A black hole can siphon gas from a nearby companion star and into a ring of material called an accretion disk. Gravitational and magnetic forces heat the disk to millions of degrees Celsius, making it hot enough to produce X-rays at the inner parts of the disk, near the black hole.
Above the disk is the corona of a black hole, a region of subatomic particles heated to 1 billion degrees Celsius that glows in higher-energy X-rays. Many mysteries remain about the origin and evolution of a black hole’s corona. Some theories suggest the structure could represent an early form of the high-speed particle jets these types of systems often emit.
Astrophysicists want to better understand how the inner edge of the accretion disc (the spiraling ring of material being pulled in by a black hole)—and the corona above it—change in size and shape as a black hole consumes material from a companion star. If scientists can understand these changes in stellar-mass black holes over a period of weeks, they could gain new insights into how supermassive black holes evolve over millions of years and how they affect the galaxies where they reside.
In this case the research team used a method called X-ray reverberation mapping to study changes in this black hole during its X-ray outburst. This technique uses X-ray reflections in the environment of a black hole in much the same way radar is uses sound wave reflections to map undersea terrain. Some X-rays from the black holes corona travel straight toward us, while others light up the disk and reflect back at different energies and angles, plotting these reflections over time allows changes in the black hole to be mapped.
X-ray reverberation mapping of supermassive black holes has shown that the inner edge of the accretion disk is very close to a black hole’s event horizon— the point beyond which no matter or energy can escape. The corona is also compact, lying closer to the black hole rather than over much of the accretion disk.
Previous observations of X-ray echoes from stellar mass black holes suggested the inner edge of the accretion disk could be quite distant—up to hundreds of times the size of the event horizon. However, J1820 behaved more like its supermassive cousins.
As they examined NICER’s observations of J1820, Kara’s team saw a decrease in the delay, or lag time, between the initial flare of X-rays coming directly from the corona and the flare’s echo off of the disk. This indicated that the X-rays traveled over shorter and shorter distances before they were reflected.
To confirm that the decrease in lag time was due to a change in the corona and not the accretion disk, the researchers used a signal called the iron K line, which is created when X-rays from the corona collide with iron atoms in the disk, causing them to fluoresce.
According to Einstein’s theory of relativity, time runs slower in strong gravitational fields and at high velocities. When the iron atoms closest to the black hole are bombarded by light from the core of the corona, the wavelengths of the X-rays they emit get stretched because time is moving slower for them than for the observer.
Kara’s team discovered that J1820’s stretched iron K line remained constant, which means the inner edge of the disk remained close to the black hole. This indicated that the disk was not the source of the X-rays. These observations give scientists new insights into how material funnels into a black hole and how energy is released in this process.
The research team and other scientists say that these new findings point to the corona and not the disk as the driver of the evolution of X-ray outbursts in stellar-size black holes, but other studies in similarly sized black holes are needed.
“These observations also offer a new framework through which to study the evolution of accretion in supermassive black holes,” Kara said.
This work was supported by NASA (Award Nos. HST-HF2-51360.001-A, NAS5-26555, and PF5-160144), the National Science Foundation (Award No. AST-1351222), and the Royal Society. The content of this article does not necessarily reflect the views of these organizations.