X-ray polarimetry reveals the magnetic field topology on sub-parsec scales in Tycho's supernova remnant

TL;DR

This study uses X-ray polarimetry to map the magnetic field structure in Tycho's supernova remnant, revealing a predominantly radial magnetic field and providing insights into turbulence and particle acceleration near shock fronts.

Contribution

First detection of localized X-ray polarization in Tycho's supernova remnant, revealing magnetic field topology and turbulence characteristics close to shock regions.

Findings

X-ray polarization degree is 9% overall, 12% at the rim, and 23% in the west region.

Magnetic field is predominantly radial, consistent with radio observations.

Magnetic-field amplification factor is approximately 3.4, indicating anisotropic turbulence.

Abstract

Supernova remnants are commonly considered to produce most of the Galactic cosmic rays via diffusive shock acceleration. However, many questions about the physical conditions at shock fronts, such as the magnetic-field morphology close to the particle acceleration sites, remain open. Here we report the detection of a localized polarization signal from some synchrotron X-ray emitting regions of Tycho's supernova remnant made by the Imaging X-ray Polarimetry Explorer. The derived polarization degree of the X-ray synchrotron emission is 9+/-2% averaged over the whole remnant, and 12+/-2% at the rim, higher than the 7-8% polarization value observed in the radio band. In the west region the polarization degree is 23+/-4%. The X-ray polarization degree in Tycho is higher than for Cassiopeia A, suggesting a more ordered magnetic-field or a larger maximum turbulence scale. The measured tangential polarization direction corresponds to a radial magnetic field, and is consistent with that observed in the radio band. These results are compatible with the expectation of turbulence produced by an anisotropic cascade of a radial magnetic-field near the shock, where we derive a magnetic-field amplification factor of 3.4+/-0.3. The fact that this value is significantly smaller than those expected from acceleration models is indicative of highly anisotropic magnetic-field turbulence, or that the emitting electrons either favor regions of lower turbulence, or accumulate close to where the magnetic-field orientation is preferentially radially oriented due to hydrodynamical instabilities.