Scientists using NASA’s newly installed $188m space telescope IXPE have reported a “huge leap” in understanding the light and other electromagnetic radiation emitted by black holes.
According to a research paper published this week in Nature, beams of electrons break up into slow-moving particles that create a shock wave that causes electromagnetic radiation across the frequency band from X-rays to visible light.
Astronomers first observed quasi-stellar radio sources, or quasars, in the early 1960s. This new class of astronomical objects was a puzzle. They look like stars, but they radiate very brightly at radio frequencies and have strange emission lines in their optical spectra that are not associated with “normal” stars. In fact, these strange objects are supermassive black holes at the center of distant galaxies.
Particle acceleration in a jet emitted by a supermassive black hole. Illustration credit: Leodakis et al/Nature
Advances in radio-astronomy and X-ray-observing satellites have helped scientists understand that anomalous radiation is caused by streams of charged particles near the speed of light. If it points toward Earth, the resulting quasar can be called a blazar. Electromagnetic radiation from them ranges from radio waves in the visible spectrum to very high-frequency gamma rays.
But how the ultrafast particles emit radiation remains a mystery.
To shed light on the phenomenon, Ioannis Leodakis, a postdoctoral research fellow at the University of Turku in Finland, used data from NASA’s Imaging X-ray Polarimetry Explorer (IXPE) space telescope, designed to observe and measure X-rays.
Leodakis and his colleagues used the new kit’s ability to measure the polarization of X-rays (X-ray polarimetry) to try to gain important insights.
By comparing polarized X-ray data with data about optically polarized visible light, scientists concluded that a shock wave in the stream of charged particles emitted from the black hole caused electromagnetic radiation (see figure).
In an accompanying article, Leah Marcotuli, a NASA Einstein postdoctoral fellow at Yale University, said: “Such shock waves occur naturally when particles traveling near the speed of light encounter slower-moving material along their path. Particles traveling through these shock waves lose radiation. Quickly 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.”
Leodakis’ work was the first blazar seen through the lens of an X-ray polarimeter, and the results were “stunning,” Marcotuli said.
“Blazar jets are the most powerful particle accelerators in the universe. Their conditions can 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 process by which The particles are ejected and accelerated remain elusive. The multi-wavelength polarimetric data of Leodakis and colleagues provide clear evidence of a particle-acceleration process…making the authors’ results a turning point in our understanding of blazars.
“This huge advance brings us one step closer to understanding these extreme particle accelerators, whose nature 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 the Kennedy Space Center in Florida. It is designed to observe the remnants of supernovae, supermassive black holes and other high-energy objects.
The project first progressed in 2017 and was expected to cost $188m – a modest price tag compared to NASA’s largest missions in the flagship program, often costing $1 billion. ®