World of Physics: The Glow of Blazers

Some black holes at the centers of distant galaxies shine unusually brightly. A research team has now discovered the cause of one of these celestial bodies: a strong pressure wave around a black hole accelerates electrons to extremely high energies, which then emit these particles again. According to the scientists in the journal “Nature”, it is now necessary to investigate whether this mechanism is the cause in all such objects.

Almost all galaxies have a black hole at their center that is millions or even billions of times the mass of the Sun. Most of these massive objects do not emit radiation. But some are active: gas from the environment falls into the black hole and lights up in the process. Part of the incident matter is also deflected by magnetic fields and hurled far into space in the form of bundled beams of matter at the black hole’s poles – known as jets.

If such a jet is aimed at Earth, the active black hole lights up especially brightly as seen from Earth. Talk about a blazer. Astronomers have long known that the radiation from blazars comes from high-energy, electrically charged particles such as electrons. However, until now it was not clear how these particles obtain their high energies. Yannis Liodakis from the University of Turku in Finland and his colleagues have now investigated this question by studying radiation from a blazar called Markarian 501, located 500 million light-years away.

Using the Italian space agency ASI and NASA’s IXPE space telescope – Imaging X-ray Polarimetry Explorer, the researchers analyzed radiation from Markarian 501 in the X-ray range. It turned out that electromagnetic waves are polarized, that is, they oscillate in a preferred direction. “Until now, such measurements were only possible in the radio range and the optical range,” explained Liodakis and his colleagues. But this radiation comes from particles that were released into space from several days to several years ago. “The origin of the acceleration can be seen only with the help of X-ray measurements.”

The measured polarization “points to a shock wave as the cause of the acceleration,” according to the scientists. Researchers call a shock wave a particularly strong pressure wave that propagates faster than sound. This shock wave is created when matter bound in the jet accelerates to nearly the speed of light and then encounters gas in the surrounding region that moves much more slowly.

It is clear, at least for the blazar Markarian 501, that the shock wave accelerates particles around the black hole. The question remains whether this is the case for all blazars – or whether there are different blazars with different mechanisms for accelerating the particles. With the IXPE space telescope, astronomers now want to investigate the X-rays of many other blazars and find the answer to this question.