An article published in the journal “Monthly Notices of the Royal Astronomical Society” reports a study of the black hole MAXI J1820+070, part of an X-ray binary system that includes a normal star to which the black hole steals gas. A team of astronomers coordinated by the British University of Southampton used the HiPERCAM and NICER instruments to detect visible light and X-rays respectively emitted during an outburst activity in 2018. The data collected were used to create a video slowed down ten times to allow astronomers to see the fastest flares and examine the details of that activity.
In X-ray binary systems the gas stripped from the normal star falls on its companion, a compact object with a strong gravitational field that can be a white dwarf, a neutron star or a black hole. In its fall towards that object, the gas emits X-rays which can be detected from the Earth together with the visible light emitted by the star.
In the case of the system that contains MAXI J1820+070, a low-mass X-ray binary, X-rays were detected using the NICER (Neutron star Interior Composition Explorer) instrument, mounted on the International Space Station especially to study neutron stars, while visible light was detected using the HiPERCAM instrument at the Gran Telescopio Canarias (La Palma, Canary Islands).
The MAXI J1820+070 black hole’s system is about 10,000 light-years away from the Earth, a distance at which it’s difficult to see the details of objects so small and dim. The combination of the NICER and HiPERCAM instruments allowed to record continuously the emissions coming from that system with the changes of its brightness at over 300 frames per second. The result is a movie that shows violent flares in visible light and X-rays.
John Paice, lead author of this study, presented the movie explaining that it was created using real data but slowed down to a tenth of its real speed to allow the human eye to catch the fastest flares. The materials around the black hole are so bright that they outshine the star it’s consuming. The fastest glows last only a few milliseconds with an energy equal to that emitted by a hundred suns.
The astronomers discovered that dips in X-ray levels are accompanied by a rise in visible light and vice versa. The fastest flashes in visible light were detected a fraction of a second after X-ray bursts. This is a pattern that indirectly reveals the presence of plasma, very hot ionized gas in structures deep in the black hole’s gravitational field.
The difference of a fraction of a second between X-ray and visible light emissions was also observed in two other systems hosting black holes, GX 339-4 and V404 Cyg, but never at these levels of detail. Dr. Poshak Gandhi, another of the authors of this study, stated that seeing this feature in three systems reinforces the idea that it’s unifying of black holes in their growth phase. If that’s true, it must tell us something fundamental about how plasma flows around black holes. Our best ideas consider a deep connection between the plasma bits that spiral towards the black hole and those that flow from it. These are extreme physical conditions that we can’t replicate in labs on Earth and we don’t understand how nature manages them. Those data will be crucial to guide us towards the correct theory.
This type of research can offer new information about the processes taking place in the area surrounding a black hole. The emissions of materials stripped from the companion star in an X-ray binary system offers details about an extreme environment and makes it possible to test scientific hypotheses.