This image from a computer simulation shows the rapid formation of an
accretion disk during the disruption of a star by a supermassive black
hole. (Image credit: Jamie Law-Smith and Enrico Ramirez-Ruiz)
A model of ultraviolet and optical emission from the tidal disruption
event AT 2018hyz is shown in this schematic diagram. As an accretion
disk forms quickly after the TDE, it generates x-ray emission (black
arrows) at small radii, which is only visible through the vertical
funnel. In other directions, x-rays are reprocessed by the photosphere
or wind, powering the ultraviolet and optical emissions. Hydrogen
emission is produced at two distinct sites outside of the photosphere: a
large elliptical disk (color-coded by velocity to show rotation) joined
by the fallback material, and a broad emission line region (BLR) that
is likely created by a radiation-driven wind (purple shaded area). See larger image. (Image credit: Tiara Hung) First clear confirmation of accretion disk formation in a tidal
disruption event without x-ray emissions supports theoretical
predictions
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 Journal and 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.”
Telltale evidence
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.
In addition to Hung, Foley, Ramirez-Ruiz, and other members of the
UCSC team, the coauthors of the paper also include scientists at the
Niels Bohr Institute in Copenhagen (where Ramirez-Ruiz holds a Niels
Bohr Professorship); University of Hong Kong; University of Melbourne,
Australia; Carnegie Institution for Science; and Space Telescope Science
Institute.
Observations were obtained at Lick Observatory, the W. M. Keck
Observatory, the Southern Astrophysical Research (SOAR) telescope, and
the Swope Telescope at Las Campanas Observatory in Chile. This work was
supported in part by the National Science Foundation, the Gordon and
Betty Moore Foundation, the David and Lucile Packard Foundation, and the Heising-Simons Foundation.
By
Tim Stephens
Source: US Santa Cruz
