Scientists have discovered black holes with distorted accretion disks

A black hole with a distorted disk. Image credit: Author John Paice, Five Mile River School of Astronomy, University of Southampton, UK

An international team of astronomers from South Africa, the United Kingdom, France and the United States found that there was a large variation in the brightness observed around the closest black hole to the Milky Way, which was about 9,600 light-years from Earth, which they believed was caused by a huge distortion in the accretion disk. The research was recently published in the Monthly Journal of the Royal Astronomical Society.

The object, called MAXI J1820+070, erupted in March 2018 as a new type of X-ray transient discovered by the Japanese X-ray Telescope aboard the International Space Station. The transient system, which scientists believe is a binary system, consisting of a low-mass star similar to the sun and a denser object— a white dwarf, neutron star, or black hole. In the case of a black hole, the researchers believe that MAXI J1820+070 will contain a black hole with a mass of at least 8 times that of the Sun.

The discovery was made by an extensive and detailed light curve obtained by amateurs around the world at the American Association of Variable Star Observers (AAVSO) over the course of nearly a year. MAXI J1820+070 is one of the three brightest X-ray transient phenomena observed to date due to its proximity to Earth and beyond the fuzzy plane of the Milky Way. It's bright for months, so it's possible for so many amateurs to follow it.

Phil Charles, a professor at the University of Southampton and a member of the research team, explains: "Material from normal stars is pulled into the spiral gas accretion disk around it by a dense object. When the material in the disk becomes hot and unstable, a large-scale explosion occurs and gathers on the black hole before crossing the event horizon to release a large amount of energy. The process is chaotic and varies greatly, with time scales ranging from milliseconds to months. ”

The research team made a visualization system that shows how a huge X-ray output is emitted from a place very close to the black hole, and then radiates the surrounding material, especially the accretion disk, heating it to a temperature of about 10,000K, which is the visible light emitted. So when the X-ray burst weakens, so does the visible light.

But something unexpected happened nearly 3 months after the eruption, when the light curve began to modulate dramatically — similar to an up-down dimming switch, with brightness almost doubling at peaks for about 17 hours. However, the X-ray output has not changed at all and remains stable. Although small quasi-periodic visible modulations have been observed in past X-ray transient bursts, no modulation of this magnitude has ever been seen.

What caused this unusual behavior? Charles said: "From the perspective of the system shown in the figure, we can quickly rule out the usual explanation that X-rays illuminate the inner surface of the donor star because the brightening occurs at the wrong time." Nor is it possible that as the modulation gradually moves relative to the orbit, the light emitted from where the mass transfer stream hits the disk changes. ”

What caused this unusual behavior? Charles said: "From the perspective of the system shown in the figure, we can quickly rule out the usual explanation that X-rays illuminate the inner surface of the donor star because the brightening occurs at the wrong time." Nor is it possible that as the modulation gradually moves relative to the orbit, the light emitted from the mass transfer meteoric disk hits the disk changes. ”

This leaves only one possible explanation, the huge stream of X-rays irradiating the accretion disk and causing it to twist (as shown in the figure). This distortion greatly increases the area of the disc that can be illuminated, so that when viewed over a certain period of time, the visual light energy increases dramatically. This behavior has been observed in X-ray binary stars with larger-mass donors, but has never been seen in black hole transients with low-mass donors like this. The authors say this opens up a whole new avenue for studying the structure and properties of twisted accretion disks.

"In an already interesting set of objects, this object has unusual properties that allow us to learn a lot about the endpoints of stellar evolution and the formation of dense objects." Charles said, "We already know that there are dozens of black hole binary systems in the Milky Way, all of which have masses in the range of 5-15 times the mass of the Sun. They all grow through the accumulation of matter, a phenomenon we have seen here. ”

Since about 5 years ago, major scientific research projects on transient objects at the South African Large Telescope (SALT) have made many important observations of compact binary stars, including black hole systems like MAXI J1820+070. David Buckley, principal investigator of the south African Observatory, said: "SALT is the perfect tool to study the changing behavior of these X-ray binary stars during an outbreak, it can be monitored regularly for weeks to months, and can be coordinated with observations from other telescopes, including space telescopes. (Feng Weiwei)

Related paper information:

https://doi.org/10.1093/mnras/stab3033

Source: China Science Daily