Bright light from black holes found to be caused by particle shock waves

Scientists have reported a "huge leap forward" in the understanding of light and other electromagnetic radiation emitted by black holes using NASA's newly deployed $188m space telescope IXPE.

Beams of electrons smash into slower-moving particles causing a shock wave which results in electromagnetic radiation across frequency bands from X-rays to visible light, according to a research paper published in Nature this week.

Astronomers first observed quasi-stellar radio sources or quasars in the early 1960s. This new class of astronomical objects was a puzzle. They looked like stars, but they also radiated very brightly at radio frequencies, and their optical spectra contained strange emission lines not associated with "normal" stars. In fact, these strange objects are gigantic black holes at the center of distant galaxies.

Advances in radio-astronomy and X-ray-observing satellites have helped scientists understand that the anomalous radiation is caused by a stream of charged particles accelerated close to the speed of light. If it points at Earth, the generating quasar can be called a blazar. Electromagnetic radiation from them can be observed from radio waves through the visible spectrum to very high-frequency gamma rays.

But the mystery has remained as to how the very fast particles end up emitting the radiation.

To shed light on the phenomenon, Ioannis Liodakis, postdoctoral research fellow at the University of Turku, Finland, used data from NASA's Imaging X-ray Polarimetry Explorer (IXPE) space telescope, designed to observe and measure X-rays.

Liodakis and his colleagues used the ability of the new kit to measure the polarization of X-rays (X-ray polarimetry) to try to gain the vital insight.

By comparing polarized X-rays data with data about optical polarized visible light, the scientists reached the conclusion that the electromagnetic radiation resulted from a shock wave in the stream of charged particles emitting from the blackhole (see figure).

In an accompanying article, Lea Marcotulli, NASA Einstein Postdoctoral Fellow at Yale University, said: "Such shock waves occur naturally when particles travelling close to the speed of light encounter slower-moving material along their path. Particles traveling through this shock wave lose radiation rapidly and efficiently – and, in doing so, they produce polarized X-rays. As the particles move away from the shock, the light they emit radiates with progressively lower frequencies, and becomes less polarized."

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Marcotulli said Liodakis's work was the first blazar ever observed through the lens of an X-ray polarimeter, and the results were "dazzling."

"Blazar jets are some of the most powerful particle accelerators in the Universe. Their conditions could never be reproduced on Earth, so they provide excellent 'laboratories' in which to study particle physics. Thousands of blazars have now been detected, and at every accessible wavelength, but the mechanisms by which the particles are emitted and accelerated remain elusive. Liodakis and colleagues' multi-wavelength polarimetric data provide clear evidence of the particle-acceleration mechanism… making the authors' results a turning point in our understanding of blazars.

"This huge leap forward brings us yet another step closer to understanding these extreme particle accelerators, the nature of which has been the focus of much research since their discovery."

In December last year, a SpaceX Falcon 9 rocket launched NASA's IXPE mission into orbit from Florida's Kennedy Space Center. It is designed to observe the remnants of supernovae, supermassive black holes, and other high-energy objects.

The project first got the go-ahead in 2017 and was expected to cost $188m – a modest price tag compared to NASA's largest missions on the flagship program often valued at over $1 billion. ®