RoboPol’s predictions on black hole jets verified by NASA’s IXPE

The universe blazes with energy, not just among stars, nebulae, and teeming galactic nurseries, but also erupting as high-speed jets of ionized matter from some of the most powerful destructive sources ever known – active galactic nuclei known as “blazars.”

Radio, infrared, optical and X-ray polarization observations of Mrk 421 for both observations in June 2022. The top panel shows the polarization degree and the bottom panel the polarization angle. The X-ray polarization degree is more than two times the lower energy observations consistent with all other IXPE observations of blazars. The polarization angle is consistent within uncertainties for all wavelengths, but the X-rays that show a roughly 400o rotation. This provides strong evidence that emission regions in astrophysical jets are not co-spatial. 

A blazar is so identified because it expels its powerful beam of ionized matter nearly directly at an observer – in this case, Earth-based ground and space telescopes and satellite imagers. That bright, highly detectable “blaze” of electromagnetic radiation results from the effects of relativistic space-time on the jet’s light as the jet hurtles away from the galaxy’s center at or near the speed of light, often creating a beam millions of light-years in length. Such jets may be expelled by a variety of powerful, heavy-mass phenomena, including pulsars, neutron stars, or the supermassive black holes to fuel blazars and other active galactic nuclei. Despite decades of study, scientists still don’t fully grasp the physical processes that shape the dynamics and emission of relativistic jets expelled by blazars.

RoboPol, a novel polarimeter mounted at the Skinakas observatory’s 1.3m telescope has been studying blazars in polarized light for a decade now. The main goal of RoboPol has been to understand a peculiar phenomenon that seems to be unique to blazars called rotations of the polarization angle. That is when the orientation of the polarized light change monotonically for often more than 180o. The origin of those rotations is still not fully understood. However, RoboPol found that not all jets show rotations, but rather a specific subclass of blazars and predicted in a paper published in the Monthly Notices of the Royal Astronomical Society (Angelakis et al., 2016) that another subclass of blazars will show rotations in X-rays.

Seven years since that prediction, an international team of astrophysicists, using data from NASA’s Imaging X-ray Polarimetry Explorer (IXPE), discovered an X-ray polarization angle rotation in the blazar dubbed Markarian 421, an active galactic nucleus and powerful gamma-ray source in the constellation Ursa Major, roughly 400 million light years from Earth. IXPE was launched Dec. 9, 2021. The new study detailing the IXPE team’s findings at Markarian 421 confirming the RoboPol prediction was published in Nature Astronomy (Di Gesu et al., 2023). IXPE’s groundbreaking X-ray polarimetry capability gives them an unprecedented view of these targets, their physical geometry, and where their emissions originate. Research models for the typical outflow of the powerful jets typically depict a spiraling helix structure, similar to that of the human DNA chain – but IXPE found unexpected variability during three prolonged observations of Markarian 421 in May and June 2022.

Remarkably, the initial analysis of the polarization data from IXPE appeared to show the polarization dropped to zero between the first and second observations. It was then found that while the polarization degree remained constant, its direction was changing fast. During the six days of the IXPE observations, the direction of the polarization made slightly over a full 360 degree turn. Sebastian Kiehlmann, a postdoctoral researcher at the Institute of Astrophysics (IA), led the analysis to test the random rotation hypothesis while several students and researchers of IA-FORTH were also involved in the interpretation of the data presented in the paper. It was found that the observed rotation is unlikely to happen by random motions of the polarization angle. This would imply that the rotation is likely caused by real changes in the magnetic field of the jet.

Stranger still was that concurrent optical measurements from RoboPol. Those showed no change in stability or structure at all – even when the polarized X-ray emissions deviated. This is strong evidence that the X-ray emission traces a different region from the optical light. The concept of a shockwave accelerating the jet’s particles is consistent with theories about the documented behavior of Markarian 501, the first blazar to be observed by IXPE that led to a published study in late 2022. The new era of multiwavelength polarization with instruments like RoboPol and IXPE make for a very exciting time for studies of astrophysical jets.

Article: Di Gesu et al. "Discovery of X-ray polarization angle rotation in the jet from blazar Mrk 421", 2023, Nature Astronomy, (DOI 10.1038/s41550-023-02032-7)