This diagram shows how a shifting feature, called a corona, can create a
flare of X-rays around a black hole
Image credit: NASA/JPL-Caltech. › Full image and caption
Image credit: NASA/JPL-Caltech. › Full image and caption
The baffling and strange behaviors of black holes have become
somewhat less mysterious recently, with new observations from NASA's
Explorer missions Swift and the Nuclear Spectroscopic Telescope Array,
or NuSTAR. The two space telescopes caught a supermassive black hole in
the midst of a giant eruption of X-ray light, helping astronomers
address an ongoing puzzle: How do supermassive black holes flare?
The results suggest that supermassive black holes send out beams of
X-rays when their surrounding coronas -- sources of extremely energetic
particles -- shoot, or launch, away from the black holes.
"This is the first time we have been able to link the launching of
the corona to a flare," said Dan Wilkins of Saint Mary's University in
Halifax, Canada, lead author of a new paper on the results appearing in
the Monthly Notices of the Royal Astronomical Society. "This will help
us understand how supermassive black holes power some of the brightest
objects in the universe."
Supermassive black holes don't give off any light themselves, but
they are often encircled by disks of hot, glowing material. The gravity
of a black hole pulls swirling gas into it, heating this material and
causing it to shine with different types of light. Another source of
radiation near a black hole is the corona. Coronas are made up of highly
energetic particles that generate X-ray light, but details about their
appearance, and how they form, are unclear.
Astronomers think coronas have one of two likely configurations. The
"lamppost" model says they are compact sources of light, similar to
light bulbs, that sit above and below the black hole, along its rotation
axis. The other model proposes that the coronas are spread out more
diffusely, either as a larger cloud around the black hole, or as a
"sandwich" that envelops the surrounding disk of material like slices of
bread. In fact, it's possible that coronas switch between both the
lamppost and sandwich configurations.
The new data support the "lamppost" model -- and demonstrate, in the
finest detail yet, how the light-bulb-like coronas move. The
observations began when Swift, which monitors the sky for cosmic
outbursts of X-rays and gamma rays, caught a large flare coming from the
supermassive black hole called Markarian 335, or Mrk 335, located 324
million light-years away in the direction of the constellation Pegasus.
This supermassive black hole, which sits at the center of a galaxy, was
once one of the brightest X-ray sources in the sky.
"Something very strange happened in 2007, when Mrk 335 faded by a
factor of 30. What we have found is that it continues to erupt in flares
but has not reached the brightness levels and stability seen before,"
said Luigi Gallo, the principal investigator for the project at Saint
Mary's University. Another co-author, Dirk Grupe of Morehead State
University in Kentucky, has been using Swift to regularly monitor the
black hole since 2007.
In September 2014, Swift caught Mrk 335 in a huge flare. Once Gallo
found out, he sent a request to the NuSTAR team to quickly follow up on
the object as part of a "target of opportunity" program, where the
observatory's previously planned observing schedule is interrupted for
important events. Eight days later, NuSTAR set its X-ray eyes on the
target, witnessing the final half of the flare event.
After careful scrutiny of the data, the astronomers realized they
were seeing the ejection, and eventual collapse, of the black hole's
corona.
"The corona gathered inward at first and then launched upwards like a
jet," said Wilkins. "We still don't know how jets in black holes form,
but it's an exciting possibility that this black hole's corona was
beginning to form the base of a jet before it collapsed."
How could the researchers tell the corona moved? The corona gives off
X-ray light that has a slightly different spectrum -- X-ray "colors" --
than the light coming from the disk around the black hole. By analyzing
a spectrum of X-ray light from Mrk 335 across a range of wavelengths
observed by both Swift and NuSTAR, the researchers could tell that the
corona X-ray light had brightened -- and that this brightening was due
to the motion of the corona.
Coronas can move very fast. The corona associated with Mrk 335,
according to the scientists, was traveling at about 20 percent the speed
of light. When this happens, and the corona launches in our direction,
its light is brightened in an effect called relativistic Doppler
boosting.
Putting this all together, the results show that the X-ray flare from this black hole was caused by the ejected corona.
"The nature of the energetic source of X-rays we call the corona is
mysterious, but now with the ability to see dramatic changes like this
we are getting clues about its size and structure," said Fiona Harrison,
the principal investigator of NuSTAR at the California Institute of
Technology in Pasadena, who was not affiliated with the study.
Many other black hole brainteasers remain. For example, astronomers
want to understand what causes the ejection of the corona in the first
place.
NuSTAR is a Small Explorer mission led by Caltech and managed by
NASA's Jet Propulsion Laboratory in Pasadena, California, for NASA'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.,
Dulles, Virginia. NuSTAR's mission operations center is at UC Berkeley,
and the official data archive is at NASA's High Energy Astrophysics
Science Archive Research Center. ASI provides the mission's ground
station and a mirror archive. JPL is managed by Caltech for NASA.
Media Contact
Whitney Clavin
Jet Propulsion Laboratory, Pasadena, California
818-354-4673
whitney.clavin@jpl.nasa.gov
