A second glance at SN 2002ap and the M 74 field with XMM-Newton

Abstract

We have re-observed the field of M 74 in January 2003 with XMM-Newton, 11 months after the X-ray detection of SN 2002ap. From a comparison of the two XMM-Newton observations we obtain more accurate values for the X-ray luminosity and colours of the source five days after the event, and a limit on its decline rate. We compare its X-ray behaviour (prompt soft X-ray emission, relatively low luminosity) with that of other type Ic SNe, and speculate that SN 2002ap may share some physical properties (low mass-loss rate and high-velocity stellar wind from the progenitor star) with the candidate hypernova/gamma-ray-burst progenitor SN 1998bw, but with a lower (non-relativistic) speed of the ejecta. We suggest that the X-ray emission observed in 2002 is likely to come from the radiatively-cooling reverse shock, at a temperature kT ≈ 0.8 keV, and that this soft component was already detected 5 d after the event because the absorbing column density of the cool shell between the forward and reverse shocks was only ∼ 10²⁰ cm^-2, i.e., the shell was optically thin in the soft X-ray band. The new XMM-Newton data also allowed us to continue monitoring two bright variable sources in M 74 that had reached peak luminosities > 10³⁹ erg s^-1 in previous XMM-Newton and Chandra observations. Finally, we used two Chandra observations from 2001 to investigate the luminosity and colour distribution of the X-ray source population of M 74, typical of moderately-active late-type spirals.