X-Ray Polarization Reveals Why Black Holes Create Such Bright Jets


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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