On the origin of asymmetries in bilateral supernova remnants

¹ INAF – Osservatorio Astronomico di Palermo “G.S. Vaiana”, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: orlando@astropa.inaf.it
² Consorzio COMETA, via Santa Sofia 64, 95123 Catania, Italy
³ Dip. di Scienze Fisiche & Astronomiche, Univ. di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁴ Institute for Applied Problems in Mechanics and Mathematics, Naukova St. 3-b Lviv 79060, Ukraine
⁵ Astronomical Observatory, National University, Kyryla and Methodia St. 8 Lviv 79008, Ukraine

Received: 14 July 2006
Accepted: 30 March 2007

Abstract

Aims.We investigate whether the morphology of bilateral supernova remnants (BSNRs) observed in the radio band is determined mainly either by a non-uniform interstellar medium (ISM) or by a non-uniform ambient magnetic field.

Methods.We perform 3D MHD simulations of a spherical SNR shock propagating through a magnetized ISM. Two cases of shock propagation are considered: 1) through a gradient of ambient density with a uniform ambient magnetic field; 2) through a homogeneous medium with a gradient of ambient magnetic field strength. From the simulations, we synthesize the synchrotron radio emission, making different assumptions about the details of acceleration and injection of relativistic electrons.

Results.We find that asymmetric BSNRs are produced if the line-of-sight is not aligned with the gradient of ambient plasma density or with the gradient of ambient magnetic field strength. We derive useful parameters to quantify the degree of asymmetry of the remnants that may provide a powerful diagnostic of the microphysics of strong shock waves through the comparison between models and observations.

Conclusions.BSNRs with two radio limbs of different brightness can be explained if a gradient of ambient density or, most likely, of ambient magnetic field strength is perpendicular to the radio limbs. BSNRs with converging similar radio arcs can be explained if the gradient runs between the two arcs.