NICER / ISS Science Nugget for January 10, 2019
Nature magazine highlights NICER black hole result with cover story
On March 11, 2018, the JAXA/MAXI payload on ISS detected a new transient X-ray source in the sky and designated it "MAXI J1820+070" (MAXI J1820). Shortly after the discovery, NICER began monitoring the source and determined that it is a black-hole binary system in which the black hole's mass is several times that of our Sun. Measurements by ESA's Gaia satellite determined that this system is about 10,000 light years away. Within days of the discovery, MAXI J1820 became one of the brightest X-ray sources in the sky. NICER's high throughput allowed it to track the evolution of this system with precise timing and spectroscopy.
In MAXI J1820, the black hole siphons 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 Celsius, 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 suddenly rush inward toward the black hole, like a gaseous avalanche. Above the disk is the corona, 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.
NICER measurements have helped us understand how the inner edge of a black hole's accretion disk – and the corona above it – change in size and shape as a black hole consumes material from a companion star. Through a technique called "reverberation mapping", NICER measured the delay of X-ray light echoes that originate from the corona and then reflect off the accretion disk, to measure how close the disk is to the black hole event horizon as well as how far off the disk the corona extends.
This diagram is illustrative of NICER's powerful dynamic range of roughly a million-fold, across which NICER provides high throughput X-ray variability and spectroscopy data, responding to a wealth of sources in the X-ray sky.
In NICER's first Nature paper by Erin Kara, et al. (Nature, Volume 565, pages 198-201, 2019), it is reported that the corona contracted from 100 miles to 10 miles high above the disk. The Nature paper is available online, as it University of Maryland's press release. A video describing the discovery can be seen here.