Written by Michael Allen
An international team of astronomers
using NASA’s IXPE (Imaging X-ray Polarimetry Explorer), has challenged our understanding of what happens to matter in the
direct vicinity of a black hole.
With IXPE, astronomers can study
incoming X-rays and measure the polarization, a property of light that
describes the direction of its electric field.
The polarization degree is a measurement
of how aligned those vibrations are to each other. Scientists can use a black
hole’s polarization degree to determine the location of the corona – a region
of extremely hot, magnetized plasma that surrounds a black hole – and how it
generates X-rays.
This illustration of material swirling around a black
hole highlights a particular feature, called the “corona,” that shines brightly
in X-ray light. In this depiction, the corona can be seen as a purple haze
floating above the underlying accretion disk, and extending slightly inside of
its inner edge. The material within the inner accretion disk is incredibly hot
and would glow with a blinding blue-white light, but here has been reduced in
brightness to make the corona stand out with better contrast. Its purple color
is purely illustrative, standing in for the X-ray glow that would not be
obvious in visible light. The warp in the disk is a realistic representation of
how the black hole’s immense gravity acts like an optical lens, distorting our
view of the flat disk that encircles it.
In April, astronomers used IXPE to measure a 9.1% polarization degree for
black hole IGR J17091-3624, much higher than they expected based on theoretical
models.
“The black hole IGR J17091-3624 is
an extraordinary source which dims and brightens with the likeness of a
heartbeat, and NASA’s IXPE allowed us to measure this unique source in a
brand-new way.” said Melissa Ewing, the lead of the study based at Newcastle
University in Newcastle upon Tyne, England.
In X-ray binary systems, an
extremely dense object, like a black hole, pulls matter from a nearby source,
most often a neighboring star. This matter can begin to swirl around,
flattening into a rotating structure known as an accretion disc.
The corona, which lies in the inner
region of this accretion disc, can reach extreme temperatures up to 1.8 billion
degrees Fahrenheit and radiate very luminous X-rays. These ultra-hot coronas
are responsible for some of the brightest X-ray sources in the sky.
Despite how bright the corona is in
IGRJ17091-3624, at some 28,000 light-years from Earth, it remains far too small
and distant for astronomers to capture an image of it.
“Typically, a high polarization
degree corresponds with a very edge-on view of the corona. The corona would
have to be perfectly shaped and viewed at just the right angle to achieve such
a measurement,” said Giorgio Matt, professor at the University of Roma Tre in
Italy and a co-author on this paper. “The dimming pattern has yet to be
explained by scientists and could hold the keys to understanding this category
of black holes.”
The stellar companion of this black
hole isn’t bright enough for astronomers to directly estimate the system’s
viewing angle, but the unusual changes in brightness observed by IXPE suggest
that the edge of the accretion disk was directly facing Earth.
The researchers explored different
avenues to explain the high polarization degree.
In one model, astronomers included
a “wind” of matter lifted from the accretion disc and launched away from the
system, a rarely seen phenomenon. If X-rays from the corona were to meet this
matter on their way to IXPE, scattering would occur, leading to these
measurements.
Fast Facts
- Polarization measurements from IXPE carry information
about the orientation and alignment of emitted X-ray light waves. The high
the degree of polarization, the more the X-ray waves are traveling in
sync.
- Most polarization in the
corona comes from a process known as Compton scattering, where
light from the accretion disc bounces off the hot plasma of the corona,
gaining energy and aligning to vibrate in the same direction.
“These winds are one of the most
critical missing pieces to understand the growth of all types of black holes,”
said Maxime Parra, who led the observation and works on this topic at Ehime
University in Matsuyama, Japan. “Astronomers could expect future
observations to yield even more surprising polarization degree measurements.”
Another model assumed the plasma in
the corona could exhibit a very fast outflow. If the plasma were to be
streaming outwards at speeds as high as 20% the speed of light, or roughly 124
million miles per hour, relativistic effects could boost the observed
polarization.
In both cases, the simulations
could recreate the observed polarization without a very specific edge-on view.
Researchers will continue to model and test their predictions to better
understand the high polarization degree for future research efforts.
More about IXPE
IXPE, which continues to provide
unprecedented data enabling groundbreaking discoveries about celestial objects
across the universe, is a joint NASA and Italian Space Agency mission with
partners and science collaborators in 12 countries. IXPE is led by NASA’s
Marshall Space Flight Center in Huntsville, Alabama. BAE Systems, Inc.,
headquartered in Falls Church, Virginia, manages spacecraft operations together
with the University of Colorado’s Laboratory for Atmospheric and Space Physics
in Boulder.
Learn more about IXPE’s ongoing mission here: https://www.nasa.gov/ixpe
Source: NASA IXPE’s ‘Heartbeat Black Hole’ Measurements Challenge Current Theories - NASA
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