Black hole spotted devouring a star – ‘First solid confirmation’

When stars pass close to a black hole, tidal forces tear it apart, producing a bright flare of radiation as material from the star falls into the black hole. Astronomers study the light from these Tidal Disruption Events (TDEs) for clues to the feeding behaviour of the supermassive black holes sitting at the centres of galaxies.

New TDE observations led by astronomers at University of California have now provided proof debris from the star forms a rotating disk, known an accretion disk, around the black hole.

Theorists have been debating whether an accretion disk can form efficiently during a tidal disruption event.

First author Tiara Hung, a postdoctoral researcher at University of California thicks the research can help resolve that question.

She said: “In classical theory, the TDE flare is powered by an accretion disk, producing x-rays from the inner region where hot gas spirals into the black hole.

“But for most TDEs, we don’t see x-rays—they mostly shine in the ultraviolet and optical wavelengths—so it was suggested that, instead of a disk, we’re seeing emissions from the collision of stellar debris streams.”

Coauthors Enrico Ramirez-Ruiz, professor of astronomy and astrophysics at UCSC, and Jane Dai at the University of Hong Kong developed a theoretical model that can explain why x-rays are usually not observed in TDEs despite the formation of an accretion disk.

The latest observations provide strong support for this model.

Dr Ramirez-Ruiz said: “This is the first solid confirmation that accretion disks form in these events, even when we don’t see x-rays.

“The region close to the black hole is obscured by an optically thick wind, so we don’t see the x-ray emissions, but we do see optical light from an extended elliptical disk.”

The evidence for an accretion disk arrives from spectroscopic observations.

Co-author Ryan Foley, assistant professor of astronomy and astrophysics at UCSC, and his team began monitoring the TDE ‘AT 2018hyz’ after it was first detected by the All Sky Automated Survey for SuperNovae (ASAS-SN).

Professor Foley noticed an unusual spectrum while observing the TDE with the 3-metre Shane Telescope at UC’s Lick Observatory early last year.

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He said: “My jaw dropped, and I immediately knew this was going to be interesting.

“What stood out was the hydrogen line—the emission from hydrogen gas—which had a double-peaked profile that was unlike any other TDE we’d seen.”

He added the double peak in the spectrum results from the Doppler effect, which shifts the frequency of light emitted by a moving object.

In an accretion disk spiralling around a black hole and viewed at an angle, some of the material will be moving toward the observer, so the light it emits will be shifted to a higher frequency, and some of the material will be moving away from the observer, its light shifted to a lower frequency.

Professor Foley added: “It’s the same effect that causes the sound of a car on a race track to shift from a high pitch as the car comes toward you to a lower pitch when it passes and starts moving away from you.

“If you’re sitting in the bleachers, the cars on one turn are all moving toward you and the cars on the other turn are moving away from you.

“In an accretion disk, the gas is moving around the black hole in a similar way, and that’s what gives the two peaks in the spectrum.”

The team continued to gather data over the next few months, observing the TDE with several telescopes as it evolved over time.

Hung led a detailed analysis of the data, indicative the disk formation occurred quickly, in a matter of weeks after the disruption of the star.

The findings suggest disk formation may be common among optically detected TDEs despite the rarity of double-peaked emission, which depends on factors such as the inclination of the disk relative to observers.

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