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| A disk of hot gas swirls around a black hole in this illustration. The stream of gas stretching to the right is what remains of a star that was pulled apart by the black hole. A cloud of hot plasma (gas atoms with their electrons stripped away) above the black hole is known as a corona. Credit: NASA/JPL-Caltech |
Recent observations of a black hole devouring a wandering star may help
scientists understand more complex black hole feeding behaviors.
Multiple NASA telescopes recently observed a massive black hole tearing
apart an unlucky star that wandered too close. Located about 250 million
light-years from Earth in the center of another galaxy, it was the
fifth-closest example of a black hole destroying a star ever observed.
Once the star had been thoroughly ruptured by the black hole’s gravity,
astronomers saw a dramatic rise in high-energy X-ray light around the black
hole. This indicated that as the stellar material was pulled toward its
doom, it formed an extremely hot structure above the black hole called a
corona. NASA’s NuSTAR (Nuclear Spectroscopic Telescopic Array) satellite is
the most sensitive space telescope capable of observing these wavelengths of
light, and the event’s proximity provided an unprecedented view of the
corona’s formation and evolution, according to a new study published in the
Astrophysical Journal.
The work demonstrates how the destruction of a star by a black hole – a
process formally known as a tidal disruption event – could be used to better
understand what happens to material that’s captured by one of these
behemoths before it’s fully devoured.
Most black holes that scientists can study are surrounded by hot gas that
has accumulated over many years, sometimes millennia, and formed disks
billions of miles wide. In some cases, these disks shine brighter than
entire galaxies. Even around these bright sources, but especially around
much less active black holes, a single star being torn apart and consumed
stands out. And from start to finish, the process often takes only a matter
of weeks or months. The observability and short duration of tidal disruption
events make them especially attractive to astronomers, who can tease apart
how the black hole’s gravity manipulates the material around it, creating
incredible light shows and new physical features.
“Tidal disruption events are a sort of cosmic laboratory,” said study
co-author Suvi Gezari, an astronomer at the Space Telescope Science
Institute in Baltimore. “They’re our window into the real-time feeding of a
massive black hole lurking in the center of a galaxy.”
A Surprising Signal
The focus of the new study is an event called AT2021ehb, which took place in
a galaxy with a central black hole about 10 million times the mass of our
Sun (about the difference between a bowling ball and the Titanic). During
this tidal disruption event, the side of the star nearest the black hole was
pulled harder than the far side of the star, stretching the entire thing
apart and leaving nothing but a long noodle of hot gas.
Scientists think that the stream of gas gets whipped around a black hole
during such events, colliding with itself. This is thought to create shock
waves and outward flows of gas that generate visible light, as well as
wavelengths not visible to the human eye, such as ultraviolet light and
X-rays. The material then starts to settle into a disk rotating around the
black hole like water circling a drain, with friction generating low-energy
X-rays. In the case of AT2021ehb, this series of events took place over just
100 days.
The event was first spotted on March 1, 2021, by the Zwicky Transient
Facility (ZTF), located at the Palomar Observatory in Southern California.
It was subsequently studied by NASA’s Neil Gehrels Swift Observatory and
Neutron star Interior Composition Explorer (NICER) telescope (which observes
longer X-ray wavelengths than Swift).
Then, around 300 days after the event was first spotted, NASA’s NuSTAR began
observing the system. Scientists were surprised when NuSTAR detected a
corona – a cloud of hot plasma, or gas atoms with their electrons stripped
away – since coronae usually appear with jets of gas that flow in opposite
directions from a black hole. However, with the AT2021ehb tidal event, there
were no jets, which made the corona observation unexpected. Coronae emit
higher-energy X-rays than any other part of a black hole, but scientists
don’t know where the plasma comes from or exactly how it gets so hot.
“We’ve never seen a tidal disruption event with X-ray emission like this
without a jet present, and that’s really spectacular because it means we can
potentially disentangle what causes jets and what causes coronae,” said
Yuhan Yao, a graduate student at Caltech in Pasadena, California, and lead
author of the new study. “Our observations of AT2021ehb are in agreement
with the idea that magnetic fields have something to do with how the corona
forms, and we want to know what’s causing that magnetic field to get so
strong.”
Yao is also leading an effort to look for more tidal disruption events
identified by ZTF and to then observe them with telescopes like Swift,
NICER, and NuSTAR. Each new observation offers the potential for new
insights or opportunities to confirm what has been observed in AT2021ehb and
other tidal disruption events. “We want to find as many as we can,” Yao
said.
Reference:
Yuhan Yao et al. The Tidal Disruption Event AT2021ehb: Evidence of
Relativistic Disk Reflection, and Rapid Evolution of the Disk–Corona
System. The Astrophysical Journal, Volume 937, Number 1. DOI: 10.3847/1538-4357/ac898a
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Space & Astrophysics