Kavli Affiliate: Grzegorz Madejski
| First 5 Authors: Laura Di Gesu, Herman L. Marshall, Steven R. Ehlert, Dawoon E. Kim, Immacolata Donnarumma
| Summary:
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.
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