Scientists have reported a “major leap forward” in understanding light and other electromagnetic radiation emitted by black holes with the help of NASA’s newly installed $188 million IXPE Space Telescope.
According to research paper published this week in Nature, beams of electrons slam into slower-moving particles, causing a shock wave that results in electromagnetic radiation across frequency bands ranging from X-rays to visible light.
In the early 1960s, astronomers observed quasi-stellar radio sources, or quasars, for the first time. This new class of astronomical objects was a mystery. They looked like stars, but they were also very bright at radio frequencies, and their optical spectra contained strange emission lines not associated with “normal” stars. In fact, these strange objects are giant black holes at the centers of distant galaxies.
Particle acceleration in the jet of a supermassive black hole. Photo credit: Liodakis et al/Nature
Advances in radio astronomy and X-ray observation satellites have helped scientists understand that the anomalous radiation is caused by a stream of charged particles being accelerated to nearly the speed of light. If it points to 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.
However, it remained a mystery how the very fast particles emit the radiation.
To shed light on the phenomenon, Ioannis Liodakis, a postdoctoral researcher 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 new kit’s ability to measure the polarization of X-rays (X-ray polarimetry) to try to uncover the crucial finding.
By comparing polarized X-ray data with optically polarized visible light data, scientists concluded that the electromagnetic radiation originated from a shock wave in the stream of charged particles emitted from the black hole (see figure).
In an accompanying article, Lea Marcotulli, NASA Einstein Postdoctoral Fellow at Yale University, said: “Such shock waves occur naturally when particles traveling at near the speed of light encounter slower material in their path. Particles traveling through this shock wave lose radiation quickly and efficiently — producing polarized X-rays in the process. As the particles move away from the collision, the light they emit radiates at progressively lower frequencies and becomes less polarized.”
Marcotulli said Liodakis’ work was the first blazar ever observed through the lens of an X-ray polarimeter and the results were “stunning”.
“Blazar jets are among 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 discovered and are accessible at any wavelength, but the mechanisms by which the particles are emitted and accelerated remain elusive.
“This major leap forward brings us 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 Kennedy Space Center in Florida. It was designed to observe the remnants of supernovae, supermassive black holes and other high-energy objects.
The project first got the green light in 2017 and was expected to cost $188 million – a modest price compared to NASA’s largest missions in the flagship program, which are often estimated at over $1 billion. ®
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