The powerful jets shooting out from accretion disks
surrounding black holes provide astronomers with some of their best
opportunities to study physics at the extreme. The X-Ray Polarimetry Explorer
(IXPE) has demonstrated shockwaves within these jets that help explain their
extraordinary brightness.
The polarization of light from astronomical objects
is often used to probe conditions within or around the object. Despite decades
of research on the polarization of visible light, the X-ray part of the spectrum
has been a mystery in this regard, as X-ray telescopes have been unable to
measure polarization.
That changed with the launch of IXPE last year, a
few weeks before JWST but with a fraction of the attention. The IXPE’s capacity
to measure the extent of polarization of X-Rays has been put to use on the
black hole system Markarian 501, with the results published in Nature.
“This is a 40-year-old mystery that we’ve solved,”
Dr Yannis Liodakis of the Finnish Centre for Astronomy with ESO said in a
statement. “We finally had all of the pieces of the puzzle, and the picture
they made was clear.”
Markarian 501 is a blazer – a supermassive black
hole where one of the jets happens to be pointed towards Earth – making it
exceptionally bright, considering its immense distance. Blazars are known to be
bright in the X-ray part of the spectrum as well as in ultraviolet and visible
light.
IXPE showed for the first time that not only is
Markarian 501 a powerful X-ray emitter, but its X-rays show about 10 percent
polarization, around twice that seen at optical wavelengths. Radio waves are
even less polarized, but all are in the same alignment with the direction of
the jet. Since the polarization is a product of magnetic fields, the pattern
reveals these fields are very strong when the X-rays are produced, but
subsequently weaken.
Combining the observations taken with IXPE and
telescopes in other parts of the spectrum, Liodakis and co-authors concluded a
shock wave is helping power the jet, causing magnetic fields to drive particles
with terra electronvolt energies. The cause of the shock wave remains unknown,
but like all such waves, it is produced when something moves faster than the
speed of sound in a material.
“As the shock wave crosses the region, the magnetic
field gets stronger, and energy of particles gets higher,” said co-author
Professor Alan Marscher of Boston University. “The energy comes from the motion
energy of the material making the shock wave.”
Initially, the particles emit X-rays or even gamma
rays, but gradually interactions with slower-moving material within the jets
create turbulence and shed energy. As a result, the photons emitted become
progressively lower energy, first ultraviolet, then optical, and finally radio
waves. Alternative explanations for the acceleration of the particles would
produce weak and erratic polarization, rather than the pattern seen.
IXPE will observe other blazers to replicate its
observations, as well as checking in on Markarian 501 later in its two-year
mission. Blazars undergo outbursts where X-ray emissions can jump by a factor
of 10, and the authors are keen to know if the polarization changes during
these times.
X-ray astronomy lags far behind other parts of the
spectrum because the atmosphere blocks observations, so we are dependent on
instruments in space, none of which could measure polarization before IXPE.
Reference: Nature
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