Post-adiabatic supernova remnants in an interstellar magnetic field: oblique shocks and non-uniform environment

Abstract

We present very-high-resolution 1D MHD simulations of the late-stage supernova remnants (SNR). In the post-adiabatic stage the magnetic field has an important and significant dynamical effect on the shock dynamics, the flow structure, and hence the acceleration and emission of cosmic rays. We find that the tangential component of the magnetic field provides pressure support that to a fair degree prevents the collapse of the radiative shell and thus limits the total compression ratio of the partially or fully radiative forward shock. A consequence is that the spectra of cosmic rays would not be as hard as in hydrodynamic simulations. We also investigated the effect on the flow profiles of the magnetic-field inclination and a large-scale gradient in the gas density and/or the magnetic field. A positive density gradient shortens the evolutionary stages whereas a shock obliquity lowers the shock compression. The compression of the tangential component of the magnetic field leads to its dominance in the downstream region of post-adiabatic shocks for a wide range of orientation of the upstream field, which may explain why one preferentially observes tangential radio polarization in old SNRs. As most cosmic rays are produced at late stages of SNR evolution, the postadiabatic phase and the influence of the magnetic field during it are most important for modeling the cosmic-ray acceleration at old SNRs and the gamma-ray emission from late-stage SNRs interacting with clouds.

Magneto-hydrodynamic simulations of young supernova remnants and their energy-conversion phase

“…In particular, Blondin et al (1998) analysed different setups and demonstrated that this is a physical effect and not the artefact of simulation features. Our 1D MHD simulations of post-adiabatic shocks (Petruk et al 2016(Petruk et al , 2018 confirm this behaviour in magnetized systems, although the oscillations may be suppressed by a sufficiently strong MF (see fig. 4 in Petruk et al 2016).…”

“…In other words, if magnetic energy in the supernova progenitor was low comparing to the thermal one then the dynamical role of magnetic fields in SNR is small enough and its influence on the HD structures may be neglected as long as SNR evolves adiabatically. Instead, the tangential component of MF is a critical component determining the flow evolution in the post-adiabatic era (Petruk et al 2016(Petruk et al , 2018.…”

“…Hydrodynamical properties of the post-adiabatic SNRs were studied already before by Cioffi et al (1988) and Blondin et al (1998). Magnetohydrodynamics of SNR during this transition was considered by Petruk et al 2016Petruk et al , 2018 It is worth noting that SNR evolve all the time till the end of Sedov stage in the adiabatic regime. Therefore, the term 'adiabatic' is appropriate to refer all the time from the supernova explosion to the prominent radiative losses, not only to the Sedov phase.…”

“…The Sedov description is valid for the adiabatic shock. Radiative cooling of the plasma is represented by the term L = −n e n H (T) in the energy conservation equation, where is the cooling coefficient (see equation 1 in Petruk et al 2018). When it becomes relevant, the post-adiabatic stage begins, and a smooth decrease of the parameter m commences (Fig.…”

“…Woltjer 1972). Instead, MHD simulations have shown that magnetic field and its orientation is crucial in evolution of the post-adiabatic shocks (Petruk et al 2016(Petruk et al , 2018; in particular, by limiting the compression of cooling plasma, as predicted by Woltjer (1972).…”