Warped Accretion Disk Spotted around Stellar-Mass Black Hole

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Variations in the brightness of light seen from around a stellar-mass black hole in the binary system MAXI J1820+070 were caused by a huge warp in its accretion disk, according to a paper to be published in the Monthly Notices of the Royal Astronomical Society. The discovery was made from an extensive and detailed light-curve obtained over almost a year by amateur astronomers who are part of the AAVSO (American Association of Variable Star Observers).

An artist’s impression of the binary system MAXI J1820+070. Image credit: John Paice.

An artist’s impression of the binary system MAXI J1820+070. Image credit: John Paice.

MAXI J1820+070 is located approximately 9,600 light-years away in the constellation of Ophiuchus.

This binary system consists of a low-mass star similar to our Sun and an 8-solar-mass black hole.

MAXI J1820+070 was discovered as a new X-ray transient in March 2018 by a Japanese X-ray telescope onboard the International Space Station.

It was also recorded as an optical transient, named ASASSN-18ey, a few days earlier.

It became one of the brightest (in both X-rays and optical) of such objects ever seen, and remained active and bright for many months, undergoing X-ray state transitions and several subsequent outburst.

“Material from the normal star is pulled by the compact object into its surrounding accretion disk of spiraling gas,” said Professor Phil Charles, an astronomer in the Department of Physics & Astronomy at the University of Southampton.

“Massive outbursts occur when the material in the disk becomes hot and unstable, accretes onto the black hole and releases copious amounts of energy before traversing the event horizon.”

“This process is chaotic and highly variable, varying on timescales from milliseconds to months.”

Professor Charles and colleagues created a visualization of the MAXI J1820+070 system, showing how a huge X-ray output emanates from very close to the black hole, and then irradiates the surrounding matter, especially the accretion disk, heating it up to a temperature of around 10,000 K, which is seen as the visual light emitted. That is why, as the X-ray outburst declines, so does the optical light.

But something unexpected happened almost 3 months after the outburst began when the optical light curve started a huge modulation on a period of about 17 hours. Yet there was no change whatsoever in the X-ray output, which remained steady.

While small, quasi-periodic visible modulations had been seen in the past during other X-ray transient outbursts, nothing on this scale had ever been seen before.

“What was causing this extraordinary behavior? With the angle of view of the system as shown in the pictorial, we could quite quickly rule out the usual explanation that the X-rays were illuminating the inner face of the donor star because the brightening was occurring at the wrong time,” Professor Charles said.

“Nor could it be due to varying light from where the mass transfer stream hits the disk as the modulation gradually moved relative to the orbit.”

This left only one possible explanation, the huge X-ray flux was irradiating the disk and causing it to warp.

The warp provides a huge increase in the area of the disk that could be illuminated, thereby making the visual light output increase dramatically when viewed at the right time.

Such behavior had been seen in X-ray binaries with more massive donors, but never in a black-hole transient with a low mass donor like this.

It opens a completely new avenue for studying the structure and properties of warped accretion disks.

“This object has remarkable properties amongst an already interesting group of objects that have much to teach us about the end-points of stellar evolution and the formation of compact objects,” Professor Charles said.

“We already know of a couple of dozen black hole binary systems in our Galaxy, which all have masses in the 5-15 solar mass range.”

“They all grow by the accretion of matter that we have witnessed so spectacularly here.”

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Jessymol K. Thomas et al. 2021. Large optical modulations during 2018 outburst of MAXI J1820+070 reveal evolution of warped accretion disc through X-ray state change. MNRAS, in press; arXiv: 2108.05447

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