X-Ray Polarimetry Reveals the Magnetic-field Topology on Sub-parsec Scales in Tycho’s Supernova Remnant

Abstract

Supernova remnants are commonly considered to produce most of the Galactic cosmic rays via diffusive shock acceleration. However, many questions regarding 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 degree of polarization of the X-ray synchrotron emission is 9% ± 2% averaged over the whole remnant, and 12% ± 2% at the rim, higher than the value of polarization of 7%–8% observed in the radio band. In the west region, the degree of polarization is 23% ± 4%. The degree of X-ray polarization in Tycho is higher than for Cassiopeia A, suggesting a more ordered magnetic field or a larger maximum turbulence scale. The measured tangential direction of polarization corresponds to the 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 orientation of the magnetic field is preferentially radially oriented due to hydrodynamical instabilities.

Magnetic Structures and Turbulence in SN 1006 Revealed with Imaging X-Ray Polarimetry

“…There was an offset of the IXPE pointing in the two epochs of observations for SN 1006. A misalignment was also found in the Cas A observations with the order of ¢ 2.5 (Vink et al 2022b) and in one of the Tycho SNR observations (Ferrazzoli et al 2023). The focal pointing (x, y) has an offset due to the boom bending and the difficulty of the bending correction for extended sources (Weisskopf et al 2022).…”

“…But these three SNRs do not necessarily share the origin of their radial magnetic morphology, since one mechanism may dominate over others in specific cases. Compared to Cas A and Tycho (Vink et al 2022b;Ferrazzoli et al 2023), the radial magnetic morphology of SN 1006 northeast likely favors the efficient quasi-parallel CR acceleration in the preexisting field over the MHD instabilities from density fluctuations.…”

“…IXPE (Weisskopf et al 2022) opened the imaging X-ray polarization window to probe magnetic field distribution and turbulence in high-energy sources, including SNRs. The very recent X-ray polarization measurements of the young SNRs Cas A (Vink et al 2022b) and Tycho (Ferrazzoli et al 2023) showed radially distributed magnetic fields. Cas A has a relatively low PD of ∼5% at the outer rim, while Tycho is more polarized, with a PD of ∼12%.…”

“…The background analysis is crucial for faint or extended sources observed with IXPE (Xie et al 2021;Di Marco et al 2023;Ferrazzoli et al 2023). Among the three SNRs observed so far by IXPE, SN 1006 is the faintest; thus, the background contribution needs to be considered.…”

Rankine–Hugoniot Relations and Magnetic Field Enhancement in Turbulent Shocks

“…We have shown that the effective magnetic amplification may be much larger. The spatial scales of the magnetic fluctuations are substantially larger than the gyroradii of thermal electrons, but may be substantially smaller than the gyroradius of a high-energy accelerated electron and certainly smaller than the subparsec scales resolved by X-ray polarimetry of SNR shocks (Ferrazzoli 2023). Therefore, thermal electrons emit electromagnetic radiation in a local magnetic field that may be much stronger than the present estimates of the rms magnetic field for the subparsec-scale magnetic turbulence.…”