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Discovery of X-ray polarization angle rotation in active galaxy Mrk 421
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abstract
The magnetic field conditions in astrophysical relativistic jets can be probed by multiwavelength polarimetry, which has been recently extended to X-rays. For example, one can track how the magnetic field changes in the flow of the radiating particles by observing rotations of the electric vector position angle $\Psi$. Here we report the discovery of a $\Psi_{\mathrm x}$ rotation in the X-ray band in the blazar Mrk 421 at an average flux state. Across the 5 days of Imaging X-ray Polarimetry Explorer (IXPE) observations of 4-6 and 7-9 June 2022, $\Psi_{\mathrm x}$ rotated in total by $\geq360^\circ$. Over the two respective date ranges, we find constant, within uncertainties, rotation rates ($80 \pm 9$ and $91 \pm 8 ^\circ/\rm day$) and polarization degrees ($\Pi_{\mathrm x}=10\%\pm1\%$). Simulations of a random walk of the polarization vector indicate that it is unlikely that such rotation(s) are produced by a stochastic process. The X-ray emitting site does not completely overlap the radio/infrared/optical emission sites, as no similar rotation of $\Psi$ was observed in quasi-simultaneous data at longer wavelengths. We propose that the observed rotation was caused by a helical magnetic structure in the jet, illuminated in the X-rays by a localized shock propagating along this helix. The optically emitting region likely lies in a sheath surrounding an inner spine where the X-ray radiation is released.
Forward citations
Cited by 2 Pith papers
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DHARA: Data Handling and Automated Reduction pipeline for AIMPOL
An automated Python pipeline for AIMPOL dual-beam polarimetry recovers literature polarization values within 2σ for standards and the Alessi 1 cluster and is adaptable to similar instruments.
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IXPE Observations of the Blazar Mrk 501 in 2022: A Multiwavelength View
IXPE's third 2022 observation of Mrk 501 yields X-ray polarization degree 6±2% at angle 143±11°, consistent with earlier epochs and with shock-based jet models.
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