After decades of effort, scientists have finally discovered the secret mechanism that powers the brightest light shows in the universe, which are emitted by absurdly energetic beams that shoot out of explosive galaxies known as blazars, reports a new study.
The breakthrough was made possible by a new space mission that can see, for the first time, the mind-boggling physics that fuels these astrophysical jets, which are made of ultrafast particles and can shine with the brightness of 100 billion Suns.
Though our own galaxy, the Milky Way, is in a sleepy phase at the moment, many other “active” galaxies are bursting at the seams with energetic matter that is juiced up by the supermassive black holes that lurk at their centers. Intense interactions between the huge black holes and their gassy surroundings can cause radiant jets to erupt from these galaxies at close to the speed of light; some jets extend for more than a million light years into deep space.
Blazars are active galaxies with jets that point directly at Earth. These objects are located many millions or billions of light years away, so they don’t pose any risk to our planet, though their jets are so bright that they can be spotted even across those vast distances. Thousands of blazars have been observed by astronomers, but nobody has ever been able to explain the precise mechanisms that made them so overwhelmingly luminous—until now.
Scientists led by Ioannis Liodakis, a Gruber Fellow at the Finnish Centre for Astronomy with the European Southern Observatory at the University of Turku, were able to solve this mystery at last, thanks to the Imaging X-ray Polarimetry Explorer (IXPE), a joint mission between NASA and the Italian Space Agency that launched into orbit in December 2021.
Liodakis and his colleagues used IXPE to examine an extremely bright blazar called Markarian 501, which is located more than 300 million light years from Earth. Because IXPE is the first mission that can capture a pattern called polarization in X-ray light, the researchers were able to show that the particles in these jets are supercharged by shock fronts, resolving a longstanding “unanswered question” about the dynamics of these brilliant objects, according to a study published on Wednesday in Nature.
“We’ve known about these sources from the 60s,” Liodakis said in an email to Motherboard, referring to blazar jets. “They are among the brightest objects in X-rays and for years we did not know how the X-rays are made. We had a few theories, but the radio and optical data we could get are not able to tell us much.”
“That is because those come far from the acceleration site, whereas X-rays come right from the heart of the accelerator,” he continued. “They really let us look at the acceleration region and physical conditions there, making them the ideal tool to address our questions.”
Put another way, each band of the light spectrum tells a different story about the nature of these jets, and scientists have been missing the key X-ray chapter. In particular, researchers have sought to capture the polarization of X-rays in the jets, which is essentially a pattern embedded in the configuration of light waves that contains information about how and where the light was produced.
While scientists have long been able to study the polarization of blazar jets in many different bands of the light spectrum, only IXPE can resolve these patterns in the kind of high-energy X-ray light that illuminates the initial process that sends the jet particles careening into deep space at unthinkable energies. Liodakis said that the mission has been on the wishlist of astronomers for decades, and that its observations have helped to open “a new window to the Universe” that has enabled scientists to “be able to do the observations and after all those years to directly test our models.”
Indeed, IXPE’s view of Markarian 501, which was captured in March 2022, suggest that the particles in a jet are accelerated when they slam into slower-moving material in the galaxy, which produces a shock wave that spreads through the jet and boosts the particles to incredibly high energy levels. Particles that travel in this wave produce highly polarized X-ray light; as they move beyond it, their emission becomes less polarized.
These results confirm models that predicted the central role of shock waves in powering these cosmic particle accelerators, which are natural laboratories for studying the behavior of light and matter at extremely high energies. To that end, Liodakis and his colleagues hope that IXPE, and similar instruments, will continue to expose the secrets of blazars and their pyrotechnic jets, including Markarian 501.
“Our observations were done when Markarian 501 was in sort of an average activity state,” Liodakis said. “Those sources are always active, but there are periods of time that they go into these outbursts that can make them more than 100 times brighter. We are not sure our findings apply in those states.”
“We have planned more observations that will hopefully take place soon, and we will be able to figure out what is happening in the jets during these outbursts,” he concluded.