In this illustration of a newly
discovered black hole named MAXI J1820+070, a black hole pulls material
off a neighboring star and into an accretion disk. Above the disk is a
region of subatomic particles called the corona. Credit: Aurore Simonnet and NASA’s Goddard Space Flight Center. Hi-res image
Scientists have charted the environment surrounding a stellar-mass black hole that is 10 times the mass of the Sun using NASA’s Neutron star Interior Composition Explorer (NICER) payload aboard the International Space Station.
NICER detected X-ray light from the recently discovered black hole,
called MAXI J1820+070 (J1820 for short), as it consumed material from a
companion star. Waves of X-rays formed “light echoes” that reflected off
the swirling gas near the black hole and revealed changes in the
environment’s size and shape.
“NICER has allowed us to measure light echoes closer to a
stellar-mass black hole than ever before,” said Erin Kara, an
astrophysicist at the University of Maryland, College Park and NASA’s Goddard Space Flight Center
in Greenbelt, Maryland, who presented the findings at the 233rd
American Astronomical Society meeting in Seattle. “Previously, these
light echoes off the inner accretion disk were only seen in supermassive
black holes, which are millions to billions of solar masses and undergo
changes slowly. Stellar black holes like J1820 have much lower masses
and evolve much faster, so we can see changes play out on human time
scales.”
A paper describing the findings, led by Kara, appeared in the Jan. 10 issue of Nature and is available online.
J1820 is located about 10,000 light-years away toward the
constellation Leo. The companion star in the system was identified in a
survey by ESA’s (European Space Agency)
Gaia mission, which allowed researchers to estimate its distance.
Astronomers were unaware of the black hole’s presence
until March 11,
2018, when an outburst was spotted by the Japan Aerospace Exploration Agency’s Monitor of All-sky X-ray Image (MAXI),
also aboard the space station. J1820 went from a totally unknown black
hole to one of the brightest sources in the X-ray sky over a few days.
NICER moved quickly to capture this dramatic transition and continues to
follow the fading tail of the eruption.
“NICER was designed to be sensitive enough to study faint, incredibly
dense objects called neutron stars,” said Zaven Arzoumanian, the NICER
science lead at Goddard and a co-author of the paper. “We’re pleased at
how useful it’s also proven in studying these very X-ray-bright
stellar-mass black holes.”
A black hole can siphon gas from a nearby companion star into a ring
of material called an accretion disk. Gravitational and magnetic forces
heat the disk to millions of degrees, making it hot enough to produce
X-rays at the inner parts of the disk, near the black hole. Outbursts
occur when an instability in the disk causes a flood of gas to move
inward, toward the black hole, like an avalanche. The causes of disk
instabilities are poorly understood.
Above the disk is the corona, a region of subatomic particles around 1
billion degrees Celsius (1.8 billion degrees Fahrenheit) that glows in
higher-energy X-rays. Many mysteries remain about the origin and
evolution of the corona. Some theories suggest the structure could
represent an early form of the high-speed particle jets these types of
systems often emit.
Astrophysicists want to better understand how the inner edge of the
accretion disk and the corona above it change in size and shape as a
black hole accretes material from its companion star. If they can
understand how and why these changes occur in stellar-mass black holes
over a period of weeks, scientists could shed light on how supermassive
black holes evolve over millions of years and how they affect the
galaxies in which they reside.
One method used to chart those changes is called X-ray reverberation
mapping, which uses X-ray reflections in much the same way sonar uses
sound waves to map undersea terrain. Some X-rays from the corona travel
straight toward us, while others light up the disk and reflect back at
different energies and angles.
X-ray reverberation mapping of supermassive black holes has shown
that the inner edge of the accretion disk is very close to the event
horizon, the point of no return. The corona is also compact, lying
closer to the black hole rather than over much of the accretion disk.
Previous observations of X-ray echoes from stellar black holes, however,
suggested the inner edge of the accretion disk could be quite distant,
up to hundreds of times the size of the event horizon. The stellar-mass
J1820, however, behaved more like its supermassive cousins.
As they examined NICER’s observations of J1820, Kara’s team saw a
decrease in the delay, or lag time, between the initial flare of X-rays
coming directly from the corona and the flare’s echo off the disk,
indicating that the X-rays traveled shorter and shorter distances before
they were reflected. From 10,000 light-years away, they estimated that
the corona contracted vertically from roughly 100 to 10 miles — that’s
like seeing something the size of a blueberry shrink to something the
size of a poppy seed at the distance of Pluto.
“NICER’s observations of J1820 have taught us something new about
stellar-mass black holes and about how we might use them as analogs for
studying supermassive black holes and their effects on galaxy
formation,” said co-author Philip Uttley, an astrophysicist at the University of Amsterdam.
“We’ve seen four similar events in NICER’s first year, and it’s
remarkable. It feels like we’re on the edge of a huge breakthrough in
X-ray astronomy.”
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. NASA's Space Technology Mission
Directorate supports the SEXTANT component of the mission, demonstrating
pulsar-based spacecraft navigation.
“This is the first time that we’ve seen this kind of evidence that
it’s the corona shrinking during this particular phase of outburst
evolution,” said co-author Jack Steiner, an astrophysicist at the
Massachusetts Institute of Technology’s Kavli Institute for Astrophysics and Space Research
in Cambridge. “The corona is still pretty mysterious, and we still have
a loose understanding of what it is. But we now have evidence that the
thing that’s evolving in the system is the structure of the corona
itself.”
To confirm the decreased lag time was due to a change in the corona
and not the disk, the researchers used a signal called the iron K line
created when X-rays from the corona collide with iron atoms in the disk,
causing them to fluoresce. Time runs slower in stronger gravitational
fields and at higher velocities, as stated in Einstein’s theory of
relativity. When the iron atoms closest to the black hole are bombarded
by light from the core of the corona, the X-ray wavelengths they emit
get stretched because time is moving slower for them than for the
observer (in this case, NICER).
Kara’s team discovered that J1820’s stretched iron K line remained
constant, which means the inner edge of the disk remained close to the
black hole — similar to a supermassive black hole. If the decreased lag
time was caused by the inner edge of the disk moving even further
inward, then the iron K line would have stretched even more.
These observations give scientists new insights into how material
funnels in to the black hole and how energy is released in this process.
NASA’s Goddard Space Flight Center, Greenbelt, Md.
Editor: Rob Garner
Source: NASA/NICER
