In a first, astronomers watch a black hole's corona disappear, then
reappear
A colliding star may have triggered the drastic transformation

Date:
July 16, 2020
Source:
Massachusetts Institute of Technology
Summary:
It seems the universe has an odd sense of humor. While a
crown-encrusted virus has run roughshod over the world, another
entirely different corona about 100 million light years from Earth
has mysteriously disappeared.

For the first time, astronomers have watched as a supermassive
black hole's own corona, the ultrabright, billion-degree ring of
high-energy particles that encircles a black hole's event horizon,
was abruptly destroyed.



FULL STORY ==========================================================================
It seems the universe has an odd sense of humor. While a crown-encrusted
virus has run roughshod over the world, another entirely different corona
about 100 million light years from Earth has mysteriously disappeared.


==========================================================================
For the first time, astronomers at MIT and elsewhere have watched as
a supermassive black hole's own corona, the ultrabright, billion-degree
ring of high-energy particles that encircles a black hole's event horizon,
was abruptly destroyed.

The cause of this dramatic transformation is unclear, though the
researchers guess that the source of the calamity may have been a star
caught in the black hole's gravitational pull. Like a pebble tossed into
a gearbox, the star may have ricocheted through the black hole's disk
of swirling material, causing everything in the vicinity, including the corona's high-energy particles, to suddenly plummet into the black hole.

The result, as the astronomers observed, was a precipitous and surprising
drop in the black hole's brightness, by a factor of 10,000, in under
just one year.

"We expect that luminosity changes this big should vary on timescales of
many thousands to millions of years," says Erin Kara, assistant professor
of physics at MIT. "But in this object, we saw it change by 10,000 over
a year, and it even changed by a factor of 100 in eight hours, which is
just totally unheard of and really mind-boggling." Following the corona's disappearance, astronomers continued to watch as the black hole began to
slowly pull together material from its outer edges to reform its swirling accretion disk, which in turn began to spin up high-energy X-rays close
to the black hole's event horizon. In this way, in just a few months,
the black hole was able to generate a new corona, almost back to its
original luminosity.



========================================================================== "This seems to be the first time we've ever seen a corona first of all disappear, but then also rebuild itself, and we're watching this in
real-time," Kara says. "This will be really important to understanding
how a black hole's corona is heated and powered in the first place."
Kara and her co-authors, including lead author Claudio Ricci of
Universidad Diego Portales in Santiago, Chile, have published their
findings today in Astrophysical Journal Letters. Co-authors from MIT
include Ron Remillard, and Dheeraj Pasham.

A nimble washing machine In March 2018, an unexpected burst lit up
the view of ASSASN, the All-Sky Automated Survey for Super-Novae, that
surveys the entire night sky for supernova activity. The survey recorded
a flash from 1ES 1927+654, an active galactic nucleus, or AGN, that is
a type of supermassive black hole with higher-than-normal brightness
at the center of a galaxy. ASSASN observed that the object's brightness
jumped to about 40 times its normal luminosity.

"This was an AGN that we sort of knew about, but it wasn't very special,"
Kara says. "Then they noticed that this run-of-the-mill AGN became
suddenly bright, which got our attention, and we started pointing
lots of other telescopes in lots of other wavelengths to look at it."
The team used multiple telescopes to observe the black hole in the
X-ray, optical, and ultraviolet wave bands. Most of these telescopes
were pointed at the the black hole periodically, for example recording observations for an entire day, every six months. The team also watched
the black hole daily with NASA's NICER, a much smaller X-ray telescope,
that is installed aboard the International Space Station, with detectors developed and built by researchers at MIT.



========================================================================== "NICER is great because it's so nimble," Kara says. "It's this little
washing machine bouncing around the ISS, and it can collect a ton of
X-ray photons.

Every day, NICER could take a quick little look at this AGN, then go off
and do something else." With frequent observations, the researchers were
able to catch the black hole as it precipitously dropped in brightness,
in virtually all the wave bands they measured, and especially in the high-energy X-ray band -- an observation that signaled that the black
hole's corona had completely and suddenly vaporized.

"After ASSASN saw it go through this huge crazy outburst, we watched
as the corona disappeared," Kara recalls. "It became undetectable,
which we have never seen before." A jolting flash Physicists are unsure exactly what causes a corona to form, but they believe it has something
to do with the configuration of magnetic field lines that run through
a black hole's accretion disk. At the outer regions of a black hole's
swirling disk of material, magnetic field lines are more or less in a straightforward configuration. Closer in, and especially near the event horizon, material circles with more energy, in a way that may cause
magnetic field lines to twist and break, then reconnect. This tangle of magnetic energy could spin up particles swirling close to the black hole,
to the level of high- energy X-rays, forming the crown-like corona that encircles the black hole.

Kara and her colleagues believe that if a wayward star was indeed the
culprit in the corona's disappearance, it would have first been shredded
apart by the black hole's gravitational pull, scattering stellar debris
across the accretion disk. This may have caused the temporary flash in brightness that ASSASN captured. This "tidal disruption," as astronomers
call such a jolting event, would have triggered much of the material in
the disk to suddenly fall into the black hole. It also might have thrown
the disk's magnetic field lines out of whack in a way that it could no
longer generate and support a high-energy corona.

This last point is a potentially important one for understanding how
coronas first form. Depending on the mass of a black hole, there is a
certain radius within which a star will most certainly be pulled in by
a black hole's gravity.

"What that tells us is that, if all the action is happening within that
tidal disruption radius, that means the magnetic field configuration
that's supporting the corona must be within that radius," Kara
says. "Which means that, for any normal corona, the magnetic fields
within that radius are what's responsible for creating a corona."
The researchers calculated that if a star indeed was the cause of
the black hole's missing corona, and if a corona were to form in a
supermassive black hole of similar size, it would do so within a radius
of about 4 light minutes - - a distance that roughly translates to about
75 million kilometers from the black hole's center.

"With the caveat that this event happened from a stellar tidal disruption,
this would be some of the strictest constraints we have on where the
corona must exist," Kara says.

The corona has since reformed, lighting up in high-energy X-rays which
the team was also able to observe. It's not as bright as it once was,
but the researchers are continuing to monitor it, though less frequently,
to see what more this system has in store.

"We want to keep an eye on it," Kara says. "It's still in this unusual
high- flux state, and maybe it'll do something crazy again, so we don't
want to miss that." This research was funded, in part, by NASA.


========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


==========================================================================


Link to news story: https://www.sciencedaily.com/releases/2020/07/200716101557.htm

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