Astronomers from Massachusetts Institute of Technology (MIT) have detected X-ray pulses originating from an area very close to a black hole's event horizon, and they believe the source of this pulse could be a white dwarf.
On November 2014, a rare event occurred in the universe as a supermassive black hole ripped apart a passing star. According to scientists, this black hole sits at the heart of a galaxy about 300 million light-years away from Earth.
Such an event, where a black hole tears apart a star, is known as the tidal disruption flare (TDF). The TDF that was spotted in November 2014 was called ASASSN-14li. As a result of the TDF, a burst of X-ray activity was created near the centre of the galaxy.
In the current study, researchers further investigated the ASASSN-14li using the ground-based All-Sky Automated Survey for SuperNovae (ASASSN), and also explored the archived datasets from three observatories: the European Space Agency's XMM-Newton space observatory, NASA's space-based Chandra Observatory, and NASA's Neil Gehrels Swift Observatory.
They noticed an intense, stable X-ray signal emanating from an area close to the black hole's event horizon - the point beyond which material is swallowed by the black hole. The signal was found to be periodically brightening and fading every 131 seconds and persisting over at least 450 days.
Researchers calculate the spinning speed of the black hole to be about 50 per cent of the speed of light. This is the first time that researchers have used TDF to calculate the spinning speed of a black hole.
The team suspects that the source of the periodic signal is likely a white dwarf that must be orbiting very close to the black hole. It likely sits just outside the event horizon, but near the Innermost Stable Circular Orbit (ISCO) - the smallest orbit in which a particle can safely travel around a black hole.
This white dwarf would not have been able to emit any detectable radiation on its own. But, when the black hole tore a second star passing close to it, some of the stellar debris must have remained in the innermost stable orbit.
When the white dwarf comes into contact with this stellar material, it likely illuminates the white dwarf, emitting X-rays each time it circles the black hole, every 131 seconds.
"The problem with this scenario is that, if you have a black hole with a mass that's one million times that of the Sun, and a white dwarf is circling it, then at some point over just a few hundred years, the white dwarf will plunge into the black hole," says Dheeraj Pasham, a post-doctoral scolar in MIT's Kavli Institute for Astrophysics and Space Research, and the first author of the study.
"We would've been extremely lucky to find such a system. But at least in terms of the properties of the system, this scenario seems to work."
Researchers hope to detect more of such events in near future.
The findings of the study are published in the journal Science.
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