This artist’s rendering shows the tidal disruption event named
ASASSN-14li, where a star wandering too close to a 3-million-solar-mass
black hole was torn apart. The debris gathered into an accretion disk
around the black hole. New data from NASA's Swift satellite show that
the initial formation of the disk was shaped by interactions among
incoming and outgoing streams of tidal debris. Credit: NASA's Goddard Space Flight Center. Hi-res image
Some 290 million years ago, a star much like the sun wandered too
close to the central black hole of its galaxy. Intense tides tore the
star apart, which produced an eruption of optical, ultraviolet and X-ray
light that first reached Earth in 2014. Now, a team of scientists using
observations from NASA's Swift satellite have mapped out how and where
these different wavelengths were produced in the event, named
ASASSN-14li, as the shattered star's debris circled the black hole.
"We discovered brightness changes in X-rays that occurred about a
month after similar changes were observed in visible and UV light," said
Dheeraj Pasham, an astrophysicist at the Massachusetts Institute of
Technology (MIT) in Cambridge, Massachusetts, and the lead researcher of
the study. "We think this means the optical and UV emission arose far
from the black hole, where elliptical streams of orbiting matter crashed
into each other."
This animation illustrates how debris from a
tidally disrupted star collides with itself, creating shock waves that
emit ultraviolet and optical light far from the black hole. According to
Swift observations of ASASSN-14li, these clumps took about a month to
fall back to the black hole, where they produced changes in the X-ray
emission that correlated with the earlier UV and optical changes. Credits: NASA's Goddard Space Flight Center. This video is public domain and can be downloaded from the Scientific Visualization Studio.
Astronomers think ASASSN-14li was produced when a sun-like star wandered too close to a 3-million-solar-mass black hole similar to the one at the center of our own galaxy. For comparison, the event horizon of a black hole like this is about 13 times bigger than the sun, and the accretion disk formed by the disrupted star could extend to more than twice Earth's distance from the sun.
When a star passes too close to a black hole with 10,000 or more
times the sun's mass, tidal forces outstrip the star's own gravity,
converting the star into a stream of debris. Astronomers call this a
tidal disruption event. Matter falling toward a black hole collects into
a spinning accretion disk, where it becomes compressed and heated
before eventually spilling over the black hole's event horizon, the
point beyond which nothing can escape and astronomers cannot observe.
Tidal disruption flares carry important information about how this
debris initially settles into an accretion disk.
Astronomers know the X-ray emission in these flares arises very close
to the black hole. But the location of optical and UV light was
unclear, even puzzling. In some of the best-studied events, this
emission seems to be located much farther than where the black hole's
tides could shatter the star. Additionally, the gas emitting the light
seemed to remain at steady temperatures for much longer than expected.
ASASSN-14li was discovered Nov. 22, 2014, in images obtained by the All Sky Automated Survey for SuperNovae
(ASASSN), which includes robotic telescopes in Hawaii and Chile.
Follow-up observations with Swift's X-ray and Ultraviolet/Optical
telescopes began eight days later and continued every few days for the
next nine months. The researchers supplemented later Swift observations
with optical data from the Las Cumbres Observatory headquartered in Goleta, California.
In a paper
describing the results published March 15 in The Astrophysical Journal
Letters, Pasham, Cenko and their colleagues show how interactions among
the infalling debris could create the observed optical and UV emission.
Tidal debris initially falls toward the black hole but overshoots,
arcing back out along elliptical orbits and eventually colliding with
the incoming stream.
"Returning clumps of debris strike the incoming stream, which results
in shock waves that emit visible and ultraviolet light," said Goddard's
Bradley Cenko, the acting Swift principal investigator and a member of
the science team. "As these clumps fall down to the black hole, they
also modulate the X-ray emission there."
Future observations of other tidal disruption events will be needed
to further clarify the origin of optical and ultraviolet light.
Goddard manages the Swift mission in collaboration with Pennsylvania
State University in University Park, the Los Alamos National Laboratory
in New Mexico and Orbital Sciences Corp. in Dulles, Virginia. Other
partners include the University of Leicester and Mullard Space Science
Laboratory in the United Kingdom, Brera Observatory and the Italian
Space Agency in Italy, with additional collaborators in Germany and
Japan.
By Francis Reddy
NASA's Goddard Space Flight Center in Greenbelt, Md.
Editor: Karl Hille
Source: NASA/Solar System and Beyond
