Discovery of a new blazar surprised astronomers

NASA’s IXPE fires up astronomers with new blazar findings.


The magnetic-field conditions in astrophysical relativistic jets can be probed by multiwavelength polarimetry, which has been recently extended to X-rays. A new study reported the discovery of a blazar Markarian 421 at an average flux state.

The discovery is made by an international team of astrophysicists using data from IXPE. This blazar, around 400 million light-years away from Earth in the constellation Ursa Major, stunned scientists by proving that the magnetic field has a spiral shape in the region of the jet where particles are being accelerated.

Italian Space Agency astrophysicist Laura Di Gesu, lead author of the new paper, said, “Markarian 421 is an old friend for high-energy astronomers. We were sure the blazar would be a worthwhile target for IXPE. Still, its discoveries were beyond our best expectations, successfully demonstrating how X-ray polarimetry enriches our ability to probe the complex magnetic field geometry and particle acceleration in different regions of relativistic jets.”

The Markarian 421 jet, for example, can reach millions of light-years in length. As particles approach the speed of light, they emit enormous amounts of energy and act in peculiar ways that Einstein anticipated, which is why they are particularly luminous. Blazar jets are exceptionally bright because, like an ambulance siren, the light pointed in our direction seems brighter as it gets closer. Because of this, blazars can glow brighter than every star in the galaxies they inhabit.

Despite decades of research, scientists still don’t fully understand the physical mechanisms that influence the behavior and emission of blazar jets. However, they now have a previously unprecedented understanding of these targets, their actual shape, and the source of their emissions thanks to IXPE’s ground-breaking X-ray polarimetry, which quantifies the typical direction of the electric field of light waves.

Similar to how human DNA is arranged, research models for the usual outflow of the strong jets typically show a spiraling helix pattern. But the presence of particle acceleration areas within the helical structure was not anticipated by physicists.

During three lengthy studies of Markarian 421 in May and June 2022, IXPE discovered unexpected polarization angle variations.

Herman Marshall, a research physicist at the Massachusetts Institute of Technology in Cambridge and a co-author of the paper, said, “We had anticipated that the polarization direction might change, but we thought large rotations would be rare, based on previous optical observations of many blazars. So, we planned several observations of the blazar, with the first showing a constant polarization of 15%.”

“Remarkably, initial analysis of the polarization data from IXPE appeared to show it dropped to zero between the first and second observations.”

“Then we recognized that the polarization was about the same, but its direction pulled a U-turn, rotating nearly 180 degrees in two days. It then surprised us again during the third observation, which started a day later, to observe the direction of polarization continuing to rotate at the same rate.”

Di Gesu, Marshall, and their colleagues are eager to conduct further observations of Markarian 421 and other blazars to learn more about these jet fluctuations and how frequently they occur.

Journal Reference:

  1. Di Gesu, L., Marshall, H.L., Ehlert, S.R. et al. Discovery of X-ray polarization angle rotation in the jet from blazar Mrk 421. Nat Astron (2023). DOI: 10.1038/s41550-023-02032-7
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