When a star weighs a little too close to a black hole, we know – broadly – what is happening. The intense tidal forces tore the star apart in what is called a tidal disruption event, releasing one last burst of light before the star’s rumble passes the event horizon.
The exact details of this swallow have been a little harder to pin down. Theoretically, the clutter should accumulate in a disk as it circles and falls on the black hole – but most of the tidal disturbance events (TDEs) we have been able to observe show no evidence of the X-ray emission present. can fix this accretion disk.
“In classical theory, the TDE torch is driven by an accretion disk, which produces X-rays from the inner region where hot gas spirals into the black hole,” said astronomer Tiara Hung of the University of California Santa Cruz.
“But for most TDEs, we do not see X-rays – they shine mostly in the ultraviolet and optical wavelengths – so it was suggested that instead of a disk, we see emissions from the collision of stellar currents.”
This has led some astronomers to speculate that a stellar tidal event is too short to create an action disk. But new research has shown otherwise. Using optical and ultraviolet observations of a tidal disruption event, astronomers have found clear evidence of the shifting light expected for a rotating action disk.
“This is the first solid confirmation that recovery disks form in these events, even if we do not see X-rays,” said astrophysicist Enrico Ramirez-Ruiz of UCSC.
The disturbing event took place in the center of a galaxy called 2MASS J10065085 + 0141342, 624 million light-years away.
Towards the end of 2018, astronomers saw the telltale flare indicating that the supermassive black hole inside it disturbed a star, and the researchers set out to see in multiple wavelengths as light evolved. They named the event for tide disturbance AT 2018hyz.
They could later calculate that a supermassive black hole had disturbed the star about several million times the mass of the sun. But there was also something else in the spectroscopic observations – a double peak in what is known as Balmer emission, formed when electrons in hydrogen atoms transition to a lower energy level.
“My jaw dropped, and I knew right away that this was going to be interesting,” said UCSC astrophysicist Ryan Foley, who saw the early signature. “What stood out was the hydrogen line – the emission of hydrogen gas – which had a colon profile that was different from any other TDE we saw.”
This double peak of broad Balmer emission in an active galactic nucleus is interpreted as evidence of an action disk. Where these lines fall into the spectrum may show evidence of motion in their Doppler shift.
Waves of light emitted from something moving towards us are shortened to the blue side of the spectrum, or blueshifke. But light waves of a returning object are extended, or shifted red. You can see examples of both in the diagram below:
(ATNF / CSIRO)
If you look at something like a disc from just the right angle, you will see evidence of both shifts – blueshift from the side facing you, and red from the side facing away. It can not only be used to determine rotation, but the speed of that rotation.
“I think we’m lucky with this,” said Ramirez-Ruiz, who co-authored a paper in 2018 that presented a uniform model for events of tide disorder.
“Our simulations show that what we observe is very sensitive to inclination. There is a preferred orientation to see these features with double peak, and a different orientation to see X-ray emission.”
As AT 2018hyz evolved over the course of several months, the team continued to take observations of multiple wavelengths, compared to other TDEs, such as simulations and models.
They decided that the accreditation disk consisted of about 5 percent of the star’s first mass – and that it was unimaginably rapidly formed within a month.
The team’s interpretation of a disc descent for the double-peak Balmer release stands alone. Independently, an international team of researchers led by astronomer Phil Short of the University of Edinburgh in Scotland reached the same conclusion.
In a pre-print paper submitted to the Monthly announcements from the Royal Astronomical Society, Koarte and his team note that “AT 2018hyz is the first TDE in which clear, double-peak emission lines have been observed and provide strong observational evidence that acquisition disks form at least some TDEs and are an important source of perceived luminosity”,
However, there are still mysteries to be solved. Short and his team note that the double spikes only appeared briefly before disappearing, and they do not know why. In addition, Hung and her team call attention to the lack of double-heads in other TDE observations.
Both groups suggest that we may do well to pay closer attention to TDEs in the future.
The research of Hung’s team has been accepted in The Astrophysical Journal, and is available on arXiv.
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