Discovery Of X-ray Polarization Angle Rotation In The Jet From Blazar Mrk 421

Monday, July 17, 2023

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.”

This week, an international team of astrophysicists, using data from NASA’s Imaging X-ray Polarimetry Explorer (IXPE), published new findings about 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.

Markarian 421 (Mrk421) is a nearby (redshift z= 0.0308) HBL that has been intensively studied at many wavelengths. It is among the first blazers detected at both gigaelectronvolt and teraelectronvolt energies. It is bright and well-monitored in the X-ray band, where the synchrotron SED peaks at a high flux level, making a it a prime target for linear polarization observations by the IXPE.

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

Marshall continued, saying, “Remarkably, the polarization appeared to drop to zero between the first and second observations, until we recognized that the polarization was actually about the same but its direction literally pulled a U turn, rotating nearly 180 degrees in two days!  Then, surprising us again, the direction continued to rotate at the same rate for the third observation that started a day later.”

Based on the IXPE data, the polarization rotation lends weight to a model where a shock propagates along spiraling magnetic fields inside the expelled jet.  “When the optical polarization didn’t vary during the IXPE observations, we had evidence that the X-ray emission maps the spiral while the electrons producing the optical light are much further down the jet in a more uniform, straighter region,” Marshall said.

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