Most galaxies are built around huge black holes. While many of them are relatively docile, like the one at the center of our Milky Way, some are ferocious – gobbling up surrounding material and releasing huge jets of extremely bright high-energy particles far into space.
Using data from the recently deployed Imaging X-ray Polarimetry Explorer (IXPE) orbital observatory, researchers on Wednesday offered an explanation for how these jets get so bright: subatomic particles called electrons become excited by shock waves moving at supersonic speed away from the black hole.
Researchers have studied an exotic object called a blazar at the center of a large elliptical galaxy named 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, 5.9 trillion miles (9.5 trillion km).
Blazars are a subset of objects called quasars that are powered by supermassive black holes feeding on gas and other material at the centers of galaxies and sending two jets of particles in opposite directions out into space. The Blazars are oriented so that one of their two jets from our vantage point on Earth is pointing directly at us.
“Blazars are the brightest objects in the observable universe. They’re 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 ESO Finnish Center for Astronomy, lead author of the research published in the journal Nature.
Scientists have long sought to understand how jets launched from blazars get so bright and the behavior of the particles they contain. Jets from this blazar extend over a distance of about one million light-years.
IXPE, launched last December as part of a collaboration between the US space agency NASA and the Italian space agency, measures the luminosity and polarization – a property of light involving the orientation of electromagnetic waves – of light X-rays from cosmic sources. Different phenomena, such as shock waves or turbulence, present “signatures” of polarization.
The researchers found evidence that particles in the jet become excited when hit by a shock wave propagating outwards inside the stream and emit X-rays as they accelerate. A shock wave is produced when something travels faster than the speed of sound through a medium such as air – as a supersonic jet does as it flies through the Earth’s atmosphere – or a region with particles and magnetic fields called plasma, as in this case.
“The light we see from the jets comes from the electrons,” said Alan Marscher, a Boston University astrophysicist and co-author of the study. “X-rays of the type we observe 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 strong that not even light can escape. The supermassive black hole at the center of Markarian 501 has a mass about a billion times the mass of our sun. That’s about 200 times larger than 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 cannot replicate on Earth,” Liodakis said.
“However, before we can use them as such, we need to understand all of the physical processes that take place. For many years we have observed the high energy light coming from these sources and had some theories as to how the particles that emit this light would be energized. IXPE’s X-ray polarization capabilities allowed us for the first time to directly test our theories,” Liodakis said.
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