The environment around a stellar-mass black hole, with a mass equal to 10 times that of the Sun, has been charted by astronomers, using NASA’s Neutron star Interior Composition Explorer (NICER) payload on board the International Space Station.
X-ray light from the recently found black hole, named MAXI J1820+070 (J1820 for short), was detected by NICER, as the black hole devoured material from a companion star. X-ray waves formed “light echoes” that were reflected from the swirling gas close to the black hole and exposed the variations in the shape and size of the environment.
NICER has allowed us to measure light echoes closer to a stellar-mass black hole than ever before. Previously, these light echoes off the inner accretion disk were only seen in supermassive black holes, which are millions to billions of solar masses and undergo changes slowly. Stellar black holes like J1820 have much lower masses and evolve much faster, so we can see changes play out on human time scales.
Erin Kara, Astrophysicist, University of Maryland.
Kara, who is also an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, presented the findings at the 233rd American Astronomical Society meeting in Seattle.
A paper outlining the outcomes of the study, headed by Kara, was published in the January 10th issue of Nature and can be accessed online.
J1820 is located nearly 10,000 light-years afar toward the constellation Leo. The system’s companion star was recognized during a survey by European Space Agency’s (ESA) Gaia mission, which enabled scientists to predict its distance. Until March 11th, 2018, astronomers were unaware of the presence of the black hole, before an outburst was observed by the Monitor of All-sky X-ray Image (MAXI) Japan Aerospace Exploration Agency, also on board the space station. Over a period of just a few days, J1820 transformed from a completely obscure black hole to one of the most brilliant sources in the X-ray sky. NICER quickly moved to capture this breathtaking transformation and is still pursuing the fading tail of the eruption.
NICER was designed to be sensitive enough to study faint, incredibly dense objects called neutron stars. We’re pleased at how useful it’s also proven in studying these very X-ray-bright stellar-mass black holes.
Zaven Arzoumanian, Study Co-Author and NICER Science Lead, Goddard Space Flight Center.
A black hole has the ability to drain gas from an adjacent companion star into a ring of material known as an accretion disk. The disk is heated to a temperature of millions of degrees by magnetic and gravitational forces, rendering it sufficiently hot to generate X-rays at the inner parts of the disk, close to the black hole. Eruptions take place a flood of gas moves inward due to instability in the disk, toward the black hole, similar to an avalanche. Insights into the causes of disk instabilities are lacking.
The corona located above the disk is a region of subatomic particles with a temperature of about 1 billion degrees Celsius (1.8 billion degrees Fahrenheit) glowing in higher energy X-rays. There are still many mysteries about the origin and evolution of the corona. According to certain theories, the structure could depict an early form of the high-speed particle jets usually emitted by systems of this type.
Astrophysicists intend to gain better insights into how the shape and size of the inner edge of the accretion disk and the corona above it change when a black hole siphons material from its companion star. An understanding of how and why these variations occur in stellar-mass black holes over a period of several weeks would enable researchers to throw light on how supermassive black holes evolve over millions of years and the ways in which they have an impact on the galaxies in which they dwell.
X-ray reverberation mapping, which involves the use of X-ray reflections exactly like the way sound waves are used by sonar to map undersea terrain, is one method used to chart those variations. Although a portion of the X-rays from the corona move straight toward the Earth, others light up the disk and are reflected back at different angles and energies.
X-ray reverberation mapping of supermassive black holes has revealed that the accretion disk’s inner edge is very close to the event horizon, which is the point of no return. Moreover, the corona is compact, located very close to the black hole and not over much of the accretion disk. However, prior observations of X-ray echoes from stellar black holes indicated that the accretion disk’s inner edge could be very distant, nearly hundreds of times the size of the event horizon. However, the behavior of the stellar-mass J1820 was more analogous to its supermassive cousins.
Upon analyzing NICER’s observations of J1820, Kara’s group observed a fall in the delay, or lag time, between the initial X-ray flare emitted directly from the corona and the echo of the flare from the disk, suggesting that the X-rays moved shorter and shorter distances before being reflected. They predicted that from 10,000 light-years away, the corona underwent a vertical contraction from approximately 100 to 10 miles—similar to viewing something the size of a blueberry contract to something the size of a poppy seed at the distance of Pluto.
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. 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.
Jack Steiner, Study Co-Author and Astrophysicist, Kavli Institute for Astrophysics and Space Research, MIT.
The scientists confirmed that the decrease in lag time was caused by a variation in the corona and not the disk by using a signal known as the iron K line, which is produced when X-rays escaping the corona collide with iron atoms in the disk, making them fluoresce. According to Einstein’s theory of relativity, in stronger gravitational fields and under higher velocities, time runs slower. When the light from the corona’s core bombards the iron atoms located very close to the black hole, the X-ray wavelengths emitted by them get stretched since time moves slower for them than for the observer (which, in this case, is the NICER).
Kara’s group found that the stretched iron K line of J1820 stayed constant, suggesting that the disk’s inner edge remained close to the black hole—akin to a supermassive black hole. If the inner edge of the disk that moves even further inward causes the decrease in lag time, then the iron K line would be even more stretched.
These observations offer researchers an innovative understanding of the way material funnels in to the black hole and the way energy is liberated in this process.
NICER’s observations of J1820 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. 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.
Philip Uttley, Study Co-Author and Astrophysicist, University of Amsterdam.
Watch how X-ray echoes, mapped by NASA’s Neutron star Interior Composition Explorer (NICER), revealed changes to the corona of black hole MAXI J1820+070. (Video credit: NASA’s Goddard Space Flight Center)