Hi, my name is Erin Kara, and I’m an
astrophysicist at NASA’s Goddard Space Flight Center and the University of Maryland.
I was part of a team who used X-rays to map the environment around a
recently discovered black hole, learning new details about how those
surroundings evolve as material swirls closer to the black hole.
We made the breakthrough using observations from NASA’s
Neutron star Interior Composition Explorer, or NICER, on the
International Space Station. NICER let us watch a flare
of light from the area around a black hole called
MAXI J1820+070, or J1820 for short.
This stellar-mass black hole is around ten times
the Sun’s mass, and funnels gas away from a neighboring star
and into a dense ring of material called an accretion disk.
Magnetic and gravitational forces compress and heat the gas
to millions of degrees, hot enough to glow in X-rays.
We think the flare of X-rays NICER spotted was due to an
instability in the disk, which caused a flood of material to move toward
the central black hole. Above the black hole is a region of
subatomic particles called the corona. The corona is extremely
hot — 1 billion degrees — and shines in even higher-energy
X-rays. Not a lot is known about why the corona is so
hot. This outburst provides an opportunity for us to
study how both the disk and the corona change as the black hole
consumes this material. Waves of X-rays from the corona
echo off the accretion disk like the sonar we use explore the ocean floor.
These echoes tell us about the size and shape of the
disk and corona. Iron atoms in the disk absorb
X-rays from the corona and then re-emit them. Gravitational distortion
of space-time stretches the wavelengths of the X-rays,
reducing their energy. The farther from the black hole they are, the
less the light is affected. As we watched the system over weeks,
the light echoes got closer together. This suggested that
something in the system was becoming smaller. The low-energy emission
coming from iron atoms close to the black hole, didn’t change at all,
suggesting that it was not the disk moving in, but rather the corona
shrinking. The team and I estimated that the corona
contracted from roughly 100 miles to only 10.
The discovery reveals that stellar-mass black holes
behave similarly to their supermassive cousins, which are
millions to billions of times the Sun’s mass. Those
monster objects are found in the hearts of most galaxies, like our Milky Way,
but their slower evolution over millions or billions of
years is impossible to detect on human time scales.
Stellar-mass black holes, on the other hand, evolve much more quickly.
Thanks to NICER, scientists like me
are observing the evolution of black hole systems and
learning more about how our universe works.
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