Variable X-Ray Sources Associated with Extragalactic Supernova Remnants

We report a serendipitous and unexpected discovery from a study of long-term X-ray variability in the nearby galaxy M83: a substantial fraction of X-ray sources classified as supernova remnants (SNRs) exhibit significant variability. This behavior is surprising because soft X-ray emission from remnants older than ∼100 yr is generally expected to evolve smoothly, with little variation on decade-long timescales. At least one source in our sample is associated with the historical supernova SN 1957D, for which the X-ray emission and variability are well explained by shock interaction between the expanding ejecta and the surrounding circumstellar medium. For most sources, however, the observed variability is difficult to reconcile with standard models of SNR evolution. Instead, the light curves and spectra are more naturally explained by the presence of an accretion-powered source coincident with the remnant. Plausible explanations include fallback accretion onto the compact object formed in the SN explosion or an X-ray binary physically associated with the remnant. The variable X-ray sources are statistically correlated with high-mass stars, indicating potential donor stars and supporting a high-mass X-ray binary interpretation. Their luminosities and spectral properties are also consistent with emission from cool accretion disks around stellar-mass black holes. Although the current data do not uniquely determine the nature of these systems, the results suggest that accretion-powered sources associated with SNRs may be more common than previously recognized.