New observations of black hole devouring a star reveal rapid disk
formation

Date:
August 26, 2020
Source:
University of California - Santa Cruz
Summary:
When a star passes too close to a supermassive 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 behavior of the supermassive black holes lurking at
the centers of galaxies, and new observations help resolve crucial
details of this process.



FULL STORY ==========================================================================
When a star passes too close to a supermassive 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 behavior of
the supermassive black holes lurking at the centers of galaxies.


==========================================================================
New TDE observations led by astronomers at UC Santa Cruz now provide
clear evidence that debris from the star forms a rotating disk, called
an accretion disk, around the black hole. Theorists have been debating
whether an accretion disk can form efficiently during a tidal disruption
event, and the new findings, accepted for publication in the Astrophysical Journaland available online, should help resolve that question, said
first author Tiara Hung, a postdoctoral researcher at UC Santa Cruz.

"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," Hung said. "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, published in 2018,
that can explain why x-rays are usually not observed in TDEs despite
the formation of an accretion disk. The new observations provide strong
support for this model.

"This is the first solid confirmation that accretion disks form in
these events, even when we don't see x-rays," Ramirez-Ruiz said. "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 telltale evidence for an accretion
disk comes from spectroscopic observations. Coauthor Ryan Foley,
assistant professor of astronomy and astrophysics at UCSC, and his
team began monitoring the TDE (named AT 2018hyz) after it was first
detected in November 2018 by the All Sky Automated Survey for SuperNovae (ASAS-SN). Foley noticed an unusual spectrum while observing the TDE
with the 3-meter Shane Telescope at UC's Lick Observatory on the night
of January 1, 2019.

"My jaw dropped, and I immediately knew this was going to be interesting,"
he said. "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." Foley explained that 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 spiraling 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.

"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," Foley said. "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, which
indicates that disk formation took place relatively quickly, in a matter
of weeks after the disruption of the star. The findings suggest that
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.

"I think we got lucky with this one," Ramirez-Ruiz said. "Our simulations
show that what we observe is very sensitive to the inclination. There is a preferred orientation to see these double-peak features, and a different orientation to see x-ray emissions." He noted that Hung's analysis
of multi-wavelength follow-up observations, including photometric and spectroscopic data, provides unprecedented insights into these unusual
events. "When we have spectra, we can learn a lot about the kinematics
of the gas and get a much clearer understanding of the accretion process
and what is powering the emissions," Ramirez-Ruiz said.


========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Cruz. Original written by Tim
Stephens. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Tiara Hung, Ryan J. Foley, Enrico Ramirez-Ruiz, Jane L. Dai, Katie
Auchettl, Charles D. Kilpatrick, Brenna Mockler, Jonathan S. Brown,
David A. Coulter, Georgios Dimitriadis, Thomas W.-S. Holoien, Jamie
A.P. Law- Smith, Anthony L. Piro, Armin Rest, Ce'sar Rojas-Bravo,
Matthew R.

Siebert. Prompt Accretion Disk Formation in an X-Ray Faint Tidal
Disruption Event. Astrophysical Journal, 2020 [abstract] ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200826152727.htm

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