This illustration shows a black hole surrounded by a disk of gas. In the left panel, a streak of debris falls toward the disk. In the right panel, the debris has dispersed some of the gas, causing the corona (the ball of white light above the black hole) to disappear.Credits: NASA/JPL-Caltech
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.
Digger 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
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.
Editor: Tony Greicius
For more information on NuSTAR, visit: https://www.nasa.gov/mission_pages/nustar/main/index.html / https://www.nustar.caltech.edu/
Editor: Tony Greicius
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
Source: NASA/Nustar
