Discovery of X-ray polarization angle rotation in the jet from blazar Mrk 421

In June 2022, the IXPE satellite observed a shock passing through the jet of active galaxy Markarian 421. The rotation of the X-ray-polarized radiation over a 5-day period revealed that the jet contains a helical magnetic field.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;(X) rotation in the X-ray band in the blazar Markarian 421 at an average flux state. Across the 5 days of Imaging X-ray Polarimetry Explorer observations on 4-6 and 7-9 June 2022, & psi;(X) rotated in total by & GE;360 & DEG;. Over the two respective date ranges, we find constant, within uncertainties, rotation rates (80 & PLUSMN; 9 & DEG; per day and 91 & PLUSMN; 8 & DEG; per day) and polarization degrees (& pi;(X) = 10% & PLUSMN; 1%). 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 and 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 probably lies in a sheath surrounding an inner spine where the X-ray radiation is released.