Runaway Star Might Explain Black Hole's Disappearing Act
The telltale sign that the black hole was feeding vanished, perhaps when a star interrupted the feast. The event could lend new insight into these mysterious objects.
At the center of a far-off galaxy, a black hole
is slowly consuming a disk of gas that swirls around it like water circling a
drain. As a steady trickle of gas is pulled into the gaping maw, ultrahot
particles gather close to the black hole, above and below the disk, generating a
brilliant X-ray glow that can be seen 300 million light-years away on Earth. These
collections of ultrahot gas, called black
hole coronas, have been known to exhibit noticeable changes
in their luminosity, brightening or dimming by up to 100 times as a black hole
feeds.
But two years ago, astronomers watched in awe
as X-rays from the black hole corona in a galaxy known as 1ES 1927+654 disappeared
completely, fading by a factor of 10,000 in about 40 days. Almost immediately
it began to rebound, and about 100 days later had become almost 20 times
brighter than before the event.
The X-ray light from a black hole corona is a
direct byproduct of the black hole's feeding, so the disappearance of that
light from 1ES 1927+654 likely means that its food supply had been cut off. In
a new study in the Astrophysical
Journal Letters, scientists hypothesize that a runaway star
might have come too close to the black hole and been torn apart. If this was
the case, fast-moving debris from the star could have crashed through part of the
disk, briefly dispersing the gas.
"We just don't normally see variations
like this in accreting black holes," said Claudio Ricci, an assistant
professor at Diego Portales University in Santiago, Chile, and lead author of
the study. "It was so strange that at first we thought maybe there was
something wrong with the data. When we saw it was real, it was very exciting.
But we also had no idea what we were dealing with; no one we talked to had seen
anything like this."
Nearly every galaxy in the universe may host a
supermassive black hole at its center, like the one in 1ES 1927+654, with
masses millions or billions of times greater than our Sun. They grow by
consuming the gas encircling them, otherwise known as an accretion disk. Because
black holes don't emit or reflect light, they can't be seen directly, but the
light from their coronas and accretion disks offers a way to learn about these
dark objects.
The authors' star hypothesis is also supported
by the fact that a few months before the X-ray signal disappeared, observatories
on Earth saw the disk brighten considerably in visible-light wavelengths (those
that can be seen by the human eye). This might have resulted from the initial
collision of the stellar debris with the disk.
Digging Deeper
The disappearing event in 1ES
1927+654 is unique not only because of the dramatic
change in brightness, but also because of how thoroughly astronomers were able
to study it. The visible-light flare prompted Ricci and his colleagues
to request follow-up monitoring of the black hole using NASA's Neutron star Interior Composition
Explorer (NICER), an X-ray telescope aboard the International Space
Station. In total, NICER observed the system 265 times over 15 months. Additional
X-ray monitoring was obtained with NASA's Neil Gehrels Swift Observatory - which also observed the system in ultraviolet light - as well
as NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the ESA (the European Space Agency) XMM-Newton observatory (which has NASA involvement).
When the X-ray light from the corona
disappeared, NICER and Swift observed lower-energy X-rays from the system so
that, collectively, these observatories provided a
continuous stream of information throughout the event.
Although a wayward star seems the
most likely culprit, the authors note that there could be other explanations
for the unprecedented event. One remarkable feature of the observations is that the overall drop in brightness wasn't a smooth transition:
Day to day, the low-energy X-rays NICER detected showed dramatic variation,
sometimes changing in brightness by a factor of 100 in as little as eight hours.
In extreme cases, black hole coronas have been known to become 100 times
brighter or dimmer, but on much longer timescales. Such rapid changes occurring
continuously for months was extraordinary.
"This dataset has a lot of
puzzles in it," said Erin Kara, an assistant professor of physics at the
Massachusetts Institute of Technology and a coauthor of the new study. "But
that's exciting, because it means we're learning something new about the
universe. We think the star hypothesis is a good one, but I also think we're
going to be analyzing this event for a long time."
It's possible that this kind of extreme
variability is more common in black hole accretion disks than astronomers
realize. Many operating and upcoming observatories are designed to search for short-term
changes in cosmic phenomena, a practice known as "time domain astronomy,"
which could reveal more events like this one.
"This new study is a great example of how
flexibility in observation scheduling allows NASA and ESA missions to study
objects that evolve relatively quickly and look for longer-term changes in
their average behavior," said Michael Loewenstein, a coauthor of the study
and an astrophysicist for the NICER mission at the University of Maryland
College Park and NASA's Goddard Space Flight Center (GSFC) in Greenbelt,
Maryland. "Will this feeding black hole return to the state it was in
before the disruption event? Or has the system been fundamentally changed?
We're continuing our observations to find out."
More About
the Missions
NICER is an Astrophysics Mission of Opportunity within NASA's
Explorer program, which provides frequent flight opportunities for world-class
scientific investigations from space utilizing innovative, streamlined and
efficient management approaches within the heliophysics and astrophysics
science areas.
NuSTAR recently celebrated eight years in space, having launched
on June 13, 2012. A Small Explorer mission led by Caltech and managed by NASA's
Jet Propulsion Laboratory in Southern California for the agency's Science
Mission Directorate in Washington, NuSTAR was developed in partnership with the
Danish Technical University and the Italian Space Agency (ASI). The spacecraft
was built by Orbital Sciences Corp. in Dulles, Virginia. NuSTAR's mission
operations center is at the University of California, Berkeley, and the
official data archive is at NASA's High Energy Astrophysics Science Archive
Research Center at GSFC. ASI provides the mission's ground station and a mirror
data archive. Caltech manages JPL for NASA.
ESA's XMM-Newton observatory was launched in December 1999 from
Kourou, French Guiana. NASA funded elements of the XMM-Newton instrument
package and provides the NASA Guest Observer Facility at GSFC, which supports
use of the observatory by U.S. astronomers.
GSFC
manages the Swift mission in collaboration with Penn State in University Park,
Pennsylvania, the Los Alamos National Laboratory in New Mexico and Northrop
Grumman Innovation Systems in Dulles, Virginia. Other partners include the
University of Leicester and Mullard Space Science Laboratory of the University
College London in the United Kingdom, Brera Observatory in Italy, and the
Italian Space Agency.
For more information on
NuSTAR, visit:
https://www.nasa.gov/mission_pages/nustar/main/index.html
https://www.nustar.caltech.edu/
For more information on
NICER, visit:
https://www.nasa.gov/nicer
https://nicer.gsfc.nasa.gov
For more information on
Swift, visit:
https://www.nasa.gov/mission_pages/swift/main
https://swift.gsfc.nasa.gov/
For more information on XMM-Newton,
visit:
https://www.nasa.gov/xmm-newton
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