Effects of non-uniform interstellar magnetic field on synchrotron X-ray and inverse-Compton γ-ray morphology of supernova remnants

Abstract

Context. Observations of SuperNova Remnants (SNRs) in X-ray and γ-ray bands promise to contribute important information to our understanding of the kinematics of charged particles and magnetic fields in the vicinity of strong non-relativistic shocks and, therefore, the nature of Galactic cosmic rays. The accurate analysis of SNR images collected in different energy bands requires theoretical modeling of synchrotron and inverse Compton emission from SNRs. Aims. We develop a numerical code (remlight) to synthesize, from MHD simulations, the synchrotron radio, X-ray, and inverse Compton γ-ray emission originating in SNRs expanding in a non-uniform interstellar medium (ISM) and/or non-uniform interstellar magnetic field (ISMF). As a first application, the code is used to investigate the effects of non-uniform ISMF on the SNR morphology in the non-thermal X-ray and γ-ray bands. Methods. We perform 3D MHD simulations of a spherical SNR shock expanding through a magnetized ISM with a gradient of ambient magnetic field strength. The model includes an approximate treatment of upstream magnetic field amplification and the effect of shock modification due to the back reaction of accelerated cosmic rays, assuming both effects to be isotropic. From the simulations, we synthesize the synchrotron radio, X-ray, and inverse Compton γ-ray emission with the synthesis code remlight, making different assumptions about the details of acceleration and the injection of relativistic electrons. Results. A gradient in the ambient magnetic field strength induces asymmetric morphologies in radio, hard X-ray, and γ-ray bands independently of the model of electron injection if the gradient has a component perpendicular to the line-of-sight (LoS). The degree of asymmetry of the remnant morphology depends on the details of the electron injection and acceleration and is different in the radio, hard X-ray, and γ-ray bands. In general, the non-thermal X-ray morphology is the most sensitive to the gradient, showing the highest degree of asymmetry. The IC γ-ray emission is weakly sensitive to the non-uniform ISMF, the degree of asymmetry of the remnant morphology being the lowest in this band.