A model of the pre-Sedov expansion phase of supernova remnant-ambient plasma coupling and X-ray emission from SN 1987A

Abstract

We investigate the mechanisms that couple supernova remnants to the ambient medium during the pre-Sedov or the so-called free expansion phase, immediately following the progenitor explosion, in order to provide a physical basis for the widely assumed snowplow process in supernova remnants at early times. Applicable hydromagnetic and collisionless plasma coupling mechanisms are identified and calculated with a multifluid code. Since these coupling mechanisms are the cause of momentum and energy being transferred from the remnant to the ambient medium, they must precede the formation of shocks, since otherwise shocks cannot form. Further, because they produce intense electron and ion heating and acceleration, they will cause X-ray emission that precedes similar emissions formed by shock or reverse shock models from young remnants such as that formed by SN 1987A. The X-ray emission produced in this manner will persist much longer than the component due to Comptonized nuclear γ-rays, and it may arise via collisionless coupling mechanisms in two ways. First, X-rays are generated directly by the interaction of the high-speed portion of the expanding piston with the interstellar medium before the formation of a reverse shock, a mode which may ultimately apply to any supernova. Second, as seems appropriate for the present state of SN 1987A, X-rays are similarly generated by interaction of the high-speed portion of the remnant with material within the circumstellar envelope, produced by mass loss during earlier stages of the progenitor's evolution. An important distinction exists between previous models of X-ray production by simple shocks or reverse shocks and the X-ray production mechanism developed in this paper: the directly heated high-density remnant, that is, the piston, is the principal X-ray luminosity source in our model, not a downstream, shock- heated ambient plasma into which the remnant moves or reverse shock-heated snowplowed material. By showing that the remnant can be heated to soft X- ray temperatures, we avoid the major difficulty that some other models have in accounting for the high soft X-ray luminosity of SN 1987A. The present model predicts X-ray luminosity and brightness temperature that are consistent with spacecraft observations of SN 1987A. The possibility that hard X-rays, in addition to those from Comptonized γ-rays, are produced by the present mechanism is also examined. The X-ray emission of a young supernova remnant is expected to arise outside the periphery as defined in visible light, in agreement with observation (the visible-light remnant is identified with less rapidly expanding ejecta). Three predictions are made that can be used to test our hypothesis concerning the X-ray emission: the soft X-ray flux should remain constant at about the present level for many years; the rate of decrease of the hard X-ray flux should show substantially after the Comptonized γ-ray flux has decayed sufficiently and the present emission mechanism begins to dominate; and the hard X-ray spectrum should have roughly the same spectral slope as the soft X-ray component after the Comptonized γ-ray flux becomes negligible. Numerical results obtained with a multifluid code demonstrate these arguments.