IXPE Observations of the Blazar Mrk 501 in 2022: A Multiwavelength View

TL;DR

This study reports IXPE X-ray polarization measurements of blazar Mrk 501 in 2022, showing consistent polarization across observations and supporting models involving shock acceleration and magnetic field anisotropy.

Contribution

First-time X-ray polarization measurement of Mrk 501 with IXPE, combined with multiwavelength data, and modeling of polarization with shock and magnetic field anisotropy.

Findings

X-ray polarization degree of 6±2% detected in 2022 July observation.

Polarization angle and degree are consistent across multiple observations.

Models suggest shock acceleration and magnetic field anisotropy explain the polarization.

Abstract

The blazar Markarian 501 (Mrk 501) was observed on three occasions over a 4-month period between 2022 March and 2022 July with the Imaging X-ray Polarimetry Explorer (IXPE). In this paper, we report for the first time on the third IXPE observation, performed between 2022 July 9 and 12, during which IXPE detected a linear polarization degree of $\Pi_X=6\pm2$ per cent at a polarization angle, measured east of north, of $\Psi_X=143^\circ\pm11^\circ$ within the 2-8 keV X-ray band. The X-ray polarization angle and degree during this observation are consistent with those obtained during the first two observations. The chromaticity of the polarization across radio, optical, and X-ray bands is likewise consistent with the result from the simultaneous campaigns during the first two observations. Furthermore, we present two types of models to explain the observed spectral energy distributions (SEDs) and energy-resolved polarization: a synchrotron self-Compton model with an anisotropic magnetic field probability distribution in the emitting volume, as well as an energy-stratified shock model. Our results support both the shock scenario as well as support that small levels of magnetic field anisotropy can explain the observed polarization.