Astrophysicists using NASA’s Neutron Star Interior Composition Explorer (NICER) have detected X-rays from a stellar-mass black hole called MAXI J1820+070 as it consumed material from a companion star; waves of X-rays formed ‘light echoes’ that reflected off the swirling gas near the black hole and revealed changes in size and shape of the black hole’s environment.
In this illustration, MAXI J1820+070 pulls material off a neighboring star and into an accretion disk; above the disk is a region of subatomic particles called the corona. Image credit: Aurore Simonnet / NASA’s Goddard Space Flight Center.
A black hole can siphon gas from a nearby companion star into a ring of material called an accretion disk.
Gravitational and magnetic forces heat the disk to millions of degrees, making it hot enough to produce X-rays at the inner parts of the disk, near the black hole.
Outbursts occur when an instability in the disk causes a flood of gas to move inward, toward the black hole, like an avalanche. The causes of disk instabilities are poorly understood.
Above the disk is the corona, a region of subatomic particles around 1 billion degrees Celsius (1.8 billion degrees Fahrenheit) that glows in higher-energy X-rays.
Many mysteries remain about the origin and evolution of the 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 disk and the corona above it change in size and shape as a black hole accretes material from its companion star. If they can understand how and why these changes occur in stellar-mass black holes over a period of weeks, scientists could shed light on how supermassive black holes evolve over millions of years and how they affect the galaxies in which they reside.
One method used to chart those changes is called X-ray reverberation mapping, which uses X-ray reflections in much the same way sonar uses sound waves to map undersea terrain.
Some X-rays from the corona travel straight toward us, while others light up the disk and reflect back at different energies and angles.
X-ray reverberation mapping of supermassive black holes has shown that the inner edge of the accretion disk is very close to the event horizon, the point of no return.
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 black holes, however, suggested the inner edge of the accretion disk could be quite distant, up to hundreds of times the size of the event horizon.
But the stellar-mass black hole MAXI J1820+070, which is located about 10,000 light-years away toward the constellation Leo, behaved more like its supermassive cousins.
“NICER was designed to be sensitive enough to study faint, incredibly dense objects called neutron stars,” said Dr. Zaven Arzoumanian, NICER science lead at NASA’s Goddard Space Flight Center.
“We’re pleased at how useful it’s also proven in studying these very X-ray-bright stellar-mass black holes.”
As they examined NICER’s observations of this black hole, Dr. Arzoumanian and his colleagues 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 the disk, indicating that the X-rays traveled shorter and shorter distances before they were reflected.
From 10,000 light-years away, they estimated that the corona contracted vertically from roughly 100 (161 km) to 10 miles (16 km).
“This is the first time that we’ve seen this kind of evidence that it’s the corona shrinking during this particular phase of outburst evolution,” said Dr. Jack Steiner, an astrophysicist at MIT’s Kavli Institute for Astrophysics and Space Research.
“The corona is still pretty mysterious, and we still have a loose understanding of what it is. But we now have evidence that the thing that’s evolving in the system is the structure of the corona itself.”
“NICER’s observations of MAXI J1820+070 have taught us something new about stellar-mass black holes and about how we might use them as analogs for studying supermassive black holes and their effects on galaxy formation,” said Dr. Philip Uttley, an astrophysicist at the University of Amsterdam.
“We’ve seen four similar events in NICER’s first year, and it’s remarkable. It feels like we’re on the edge of a huge breakthrough in X-ray astronomy.”
The study is published in the journal Nature.
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E. Kara et al. 2019. The corona contracts in a black-hole transient. Nature 565: 198-201; doi: 10.1038/s41586-018-0803-x
