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 Ψ. Here we report the discovery of a Ψx
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, Ψx rotated in total by ≥360∘.
Over the two respective date ranges, we find constant, within uncertainties,
rotation rates (80±9 and 91±8∘/day) and polarization
degrees (Πx=10%±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/optical emission sites, as no similar rotation of
Ψ 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