- 3945 - STAR
SPAGEHTTIFICATION? - A captured star has experienced multiple
close encounters with a supermassive black hole in a distant galaxy, and
possibly even survived having material ripped away by immense gravitational
tidal forces.
------------ 3945
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- Star
survives spaghettification by black hole.
A star has survived a close encounter with a black hole, but the black
hole has been able to sneak a second bite.
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- The
destruction of a star by the gravitational forces of a supermassive black hole
is a violent affair known as a “tidal disruption event” (TDE). Gas is torn from
the star and undergoes "spaghettification," in which it is shredded
and stretched into streams of hot material that flow around the black hole,
forming a temporary and very bright accretion disk. From our point of view, the
center of the galaxy housing the supermassive black hole seems to flare.
-
- On
September 8, 2018, the “All-Sky Automated Survey for Supernovae” (ASASSN)
spotted a flare in the nucleus of a distant galaxy 893 million light-years
away. Cataloged as AT2018fyk, the flare had all the hallmarks of a TDE.
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- Various
X-ray telescopes, NASA's Swift, Europe's XMM-Newton, the NICER instrument
mounted to the International Space Station, and Germany's eROSITA, observed the
black hole brightening dramatically. Ordinarily,
-
- TDEs
exhibit a smooth decline in brightness over several years, but when astronomers
looked again at AT2018fyk about 600 days after it had first been noticed, the
X-rays had quickly vanished. Even more puzzling, about 600 days after that, the
black hole suddenly flared up again.
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- Until now,
the assumption has been that when we see the aftermath of a close encounter
between a star and a supermassive black hole, the outcome will be fatal for the
star; that is, the star is completely destroyed. But contrary to all other TDEs we know of,
when they pointed our telescopes to the same location again several years
later, we found that it had re-brightened.
-
- The
repeated flares were the signature of a star that had survived a TDE and
completed another orbit to experience a second TDE. The star was once a member of a binary system
that passed too close to the black hole at the center of its galaxy. The black
hole's gravity flung one of the stars away, which transformed into a runaway
hypervelocity star racing at 600 miles per second out of the galaxy.
-
- The other
star became tightly harnessed to the black hole, on a 1,200-day elliptical
orbit that took it toward what scientists call the tidal radius, the distance
from the black hole at which a star starts to be ripped apart by the
gravitational tides emanating from the black hole.
-
- Because the
star was not fully within the tidal radius, only some of its material was
stripped away, leaving a dense stellar core that continued on its orbit around
the black hole. It takes approximately 600 days for the material pulled from
the star by the black hole to form the accretion disk.
-
- The star's core began to approach the black
hole again, about 1,200 days after its first encounter, the star began to steal
back some of its material back from the accretion disk, causing the X-ray
emission to suddenly fade. When the core
returns to the black hole it essentially steals all the gas away from the black
hole via gravity, and as a result there is no matter to accrete and hence the
system goes dark.
-
- The black
hole's gravity soon returns the favor, stealing more material at the star's close
approach. As during the initial encounter, there's a 600-day lag from the black
hole snacking on the star to the formation of the accretion disk, explaining
why the X-ray flare switched back on when it did.
-
- From the
star's orbit they calculated that the black hole has a mass nearly 80 million
times that of our sun, or about 20 times more massive than the black hole at
the center of our Milky Way galaxy, Sagittarius A*.
-
- The
scientists predict that AT2018fyk should go dark again in August, when the core
of the star comes back around, and brighten again in March 2025 when new
material begins accreting onto the black hole.
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- However,
there's one potential complication in the amount of mass the star has lost to
the black hole. The amount of lost mass depends partly on how fast the star is
spinning, which the black hole might be affecting. If the star is spinning
nearly fast enough to break apart, then the black hole will more easily steal
material, increasing the mass loss.
-
- If the mass
loss is only at the 1% level, they expect the star to survive for many more
encounters, whereas if it is closer to 10%, the star may have already been
destroyed.
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- Regardless,
TDEs and repeating partial TDEs provide a rare window into the lives of supermassive
black holes that we cannot normally detect because they are dormant.
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- This is
important for measuring their mass and determining something about how the
black holes have evolved, and hence how the galaxy around the black hole has
also evolved.
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April 3, 2023 STAR SPAGEHTTIFICATION? 3945
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------ “Jim Detrick” -----------
---------------------
--- Tuesday, April 4, 2023 ---------------------------
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