- 3928 - GALAXY CENTER - what is going on there? Advancements in optics, spectrometers, and interferometry, astronomers have been able to peer into Galactic Center. The “Event Horizon Telescope” (EHT), the world got to see the first image of Sagittarius A* (Sgr A*) in May, 2022.
------------ 3928 - GALAXY CENTER - what is going on there?
- A
mysterious blob near the Milky Way’s supermassive black hole at the center of
our galaxy might finally have an explanation?
At the center of the Milky Way,
there is a massive persistent radio source known as Sagittarius A*. Since the
1970s, astronomers have known that this source is a supermassive black hole roughly 4 million times the mass of our Sun.
-
- Scientists
have been observing a mysterious elongated object around the Sgr A* (named X7)
and wondered what it was. It could be a debris cloud created by a stellar
collision.
-
- Using the
Keck Observatory’s 10-meter Telescopes on Mauna Kea, the team has been measuring
the star closest to Sgr A* (S0-2) for more than twenty years, since 1995. They
are one of only two groups in the world to have observed S0-2 make a full orbit
of Sgr A*, a process that takes 16
years, testing Einstein’s Theory of
General Relativity. The team has spent that same time monitoring the object
known as “X7”, a dust and gas cloud of about 50 Earth masses that takes 170
years to orbit the blackhole.
-
- “X7” has
become elongated and stretched by tidal forces as it has been pulled closer to
Sag A*. Within the next few decades, they anticipate that X7 will disintegrate
as the dust and gas that make it up are accreted onto the face of the
blackhole.
-
- No other
object in this region has shown such an extreme evolution. It started off
comet-shaped and people thought maybe it got that shape from stellar winds or
jets of particles from the black hole. But as they followed it for 20 years,
they saw it becoming more elongated. Something must have put this cloud on its
particular path with its particular orientation.
-
- X7 has
similar properties to other strange dusty objects orbiting Sag A*. These
objects look like dust clouds but behave like stars and were identified using
12 years of spectroscopic measurements made using Keck’s OH-Suppressing Infrared
Imaging Spectrograph.
-
- However,
X7’s shape and velocity have changed more dramatically than “Gobjects”,
reaching speeds of up to 1,126.5 km/s (700 miles per second).
-
- These results
are the most robust analysis to date of X7’s changes in appearance, shape, and
behavior and the first estimate of X7’s slightly elliptical orbit. While the
origins of X7 are still the subject of debate, the team’s finding suggests that
it resulted from a collision between two stars orbiting Sgr A*.
-
- Such mergers
are very common, especially in the vicinity of black holes. This merger is
likely to have ejected gas and dust, which could have formed a shell that is
concealing the merged star while the rest became the X7 object.
-
- The stars
circle each other, get closer, merge, and the new star is hidden within a cloud
of dust and gas. X7 could be the dust
and gas ejected from a merged star that’s still out there somewhere.
-
- Based on
its trajectory, the team estimates that X7 will make its closest approach to
Sgr A* sometime in 2036 and then spiral inward to be devoured. In the meantime,
the research team will continue to monitor X7 using the Keck Observatory and
watch as the powerful gravity of Sgr A* pulls it apart.
-
- In another
galaxy a star has survived a close encounter with a black hole, but the black
hole has been able to sneak a second bite.
A captured star has experienced multiple close encounters with a
supermassive black hole in this distant galaxy and possibly even survived
having material ripped away by immense gravitational tidal forces.
-
- 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” 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.
-
- Various X-ray telescopes, including 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. What was going on?
-
- 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 we 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. In their model,
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, so by the time
astronomers saw the system flare, the star was safe, near the most distant
point of its orbit.
-
- But as 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 we can calculate 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, 2023, when the
core of the star comes back around, and brighten again in March, 2025,
when new material begins accreting onto the black hole.
-
- 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, then we expect the star to survive for many more
encounters, whereas if it is closer to 10%, the star may have already been
destroyed.
-
- 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. 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 over cosmic history.
-
March 23, 2023 GALAXY
CENTER - what is going on there? 3928
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