X-ray pulse detected near event horizon as black hole devours star

An artist's impression showing hot gas orbiting in a disk around a rapidly-spinning black hole

An artist's impression showing hot gas orbiting in a disk around a rapidly-spinning black hole

This artist's illustration shows the region around a supermassive black hole after a star wandered too close and was ripped apart. Scientists discovered that the burst of X-ray emissions - which grew to be six times as bright as the Crab Nebula and was about 10,000 light-years away from Earth - came from a black hole that was in the middle of an outburst.

Astronomers have discovered a surprisingly long-lived X-ray pulse from the debris of a shredded star as it inexorably spirals closer to a black hole's mouth. Since black holes don't emit many X-rays on their own, a group of researchers made a decision to home in on the event.

Astronomers back in 2014 were searching the night sky for something cool and found what they were looking for. "A loud quasi-periodic oscillation after a star is disrupted by a massive black hole".

Chasing "echoes" within the eruption, the team found evidence that as the black hole consumes enormous amounts of stellar material, its corona significantly shrinks.

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 researchers think the bits of leftover star providing energy pulses circle around the innermost stable circular orbit (ISCO) of the black hole - the last point of safety before destruction. But researchers suspect that the black hole moved at around half the speed of light.

The spin the team estimates - at least 70% of the black hole's theoretical maximum, or at least 50% the speed of light - continues the trend followed by other supermassive black holes we have spins for, nearly all of which spin at least 60% of their max.

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Stellar black holes-like the one studied-form when the center of a huge star collapses in on itself, causing a supernova. The major takeaway for the team was the the black hole's corona was shrinking. The team's results give a way to measure the spin of supermassive black holes using tidal disruption flares.

The black hole was given the designation MAXI J1820+070, named for the instrument that first detected it, according to Phys.org. The signal lasted a surprisingly long time - at least 450 days, a duration that Pasham says is "really freaky". While fairly common in the universe (there is one dormant giant at the center of the Milky Way), black holes remain a mystery. He chose to apply his code to the three datasets for ASASSN-14li, to see if any common periodic patterns would rise to the surface. "Stellar black holes like J1820 have much lower masses and evolve much faster, so we can see changes play out on human time scales". The researchers then calculated how tight this circuit would be, based on estimates of the black hole's mass (about a million Suns). Alone, it would not have been enough to emit any sort of detectable radiation. This marks the first time that astronomers used X-rays, which orbit the black hole every 131 seconds, to calculate its incredible speed.

"We would've been extremely lucky to find such a system", says Pasham.

Sometime around November 22, 2014, a second star passed close enough to the system that the black hole tore it apart in a tidal disruption flare that emitted an enormous amount of X-ray radiation, in the form of hot, shredded stellar material. "But at least in terms of the properties of the system, this scenario seems to work".

However, "if you have a high-spin black hole, supermassive black hole, that's telling us that maybe steady accretion was dominant", Pasham said during a news conference at AAS Wednesday. "Their result suggests that tidal disruption events should be regularly followed up by x-ray telescopes to maximise our knowledge of their properties".

Pasham plans to look for, and hopefully spot, more of these dramatic events in the cosmos.

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