How huge galactic black holes become the brightest objects in the universe

Some black holes release huge, incredibly bright jets of high-energy particles into space — yes, black holes, the epitome of darkness, cause some of the brightest lights in space. In particular, some huge black holes around which most galaxies are built. While many of these galactic black holes are relatively tame, like the one at the center of our Milky Way, some are ferocious: They gobble up surrounding material and release huge, incredibly bright jets.

Using data from the recently deployed Imaging X-ray Polarimetry Explorer (IXPE) orbiting observatory, the researchers have offered an explanation for how these jets get so bright: Subatomic particles called electrons are energized by shock waves as they recede at speeds supersonic from the black hole.

The researchers studied an exotic object called a blazar at the center of a large elliptical galaxy called Markarian 501 located about 460 million light-years from Earth in the direction of the constellation Hercules. A light year is the distance light travels in one year, 9.5 trillion kilometers (5.9 trillion miles).

Blazars are a subset of objects called quasars that are powered by supermassive black holes that feed on gas and other material at the centers of galaxies and send two jets of particles into space in opposite directions. The blazars are oriented such that one of their two jets from our vantage point on Earth is heading directly towards us.

‘Blazars are the most persistent bright objects in the observable universe. They are the most energetic. They have the biggest and scariest black holes. Everything that happens around them is so fascinating,’ said astronomer Yannis Liodakis of the Finnish Center in astronomy with ESO , lead author of the research published in the journal Nature.

Scientists have long sought to understand how jets fired by blazars get so bright and how the particles in them behave. The jets from this blazar extend to a distance of about a million light-years.

IXPE, launched last December in collaboration between the US space agency NASA and the Italian Space Agency, measures the brightness and polarization – a property of light involving the orientation of electromagnetic waves – of X-ray light from sources cosmic. Several phenomena, such as shock waves or turbulence, exhibit polarization “signatures”.

The researchers found evidence that particles in the jet become excited when struck by an outward-propagating shock wave within the stream, and emit X-rays as they accelerate. A shock wave is produced when something moves faster than the speed of sound through a medium such as air – such as a supersonic jet does as it passes through the earth’s atmosphere – or a region with particles and magnetic fields called plasma, such as in this case.

“The light we see from the jets comes from electrons,” said Boston University astrophysicist and study co-author Alan Marscher. “X-rays of the kind we see in Markarian 501 can only come from very high-energy electrons.”

The driving force behind this drama is a black hole, an extraordinarily dense object with gravity so powerful that not even light can escape. The supermassive black hole at Markarian 501’s center has a mass somewhere around a billion times the mass of our sun. It is about 200 times the mass of Sagittarius A*, the supermassive black hole in the Milky Way.

“Black holes are unique laboratories for studying fundamental physics under extreme conditions that we can’t replicate on Earth,” Liodakis said.

“However, before we can use them as such, we need to understand all the physical processes that take place. For many years we have observed high-energy light from those sources and had some theories about how the particles that emit that light are energized. The capabilities of X-ray polarization from IXPE allowed us for the first time to directly test our theories,” said Liodakis.

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