New X-Ray Observations of the Old Nova CP Puppis and of the More Recent Nova V351 Puppis

We present X-ray observations of the field containing Nova Puppis 1942 (CP Pup) and Nova Puppis 1991 (V351 Pup), done with ASCA in 1998, and with XMM-Newton in 2005. The X-ray and UV luminosity of CP Pup seem to have remained approximately constant since the last X-ray observations of the 1980s, while the optical luminosity has decreased. The X-ray properties of this nova are explained by a high-mass white dwarf (WD) accreting at low rate, in agreement with the nova theory given the large amplitude and other characteristics of the 1942 outburst. Assuming a distance of 1600 pc, the X-ray luminosity of CP Pup is L x = 2.2 × 1033 erg s-1 in the 0.15-10 keV range covered with EPIC, compatible with a magnetic system. The RGS grating spectrum shows a few prominent emission lines, and it is fitted with a cooling flow with mass accretion rate \dot{m}<1.6 × 10-10 M sun yr-1. We also detected the O VII complex at 21.6-21.8 Å that does not arise in the cooling flow. Most likely this feature originates in a wind or in the nova shell. The RGS and EPIC spectra are fitted only with thermal models with a very high shock temperature, T > 60 keV, indicating a WD with M > 1.1 M sun. The X-ray flux is modulated with the spectroscopic period of 1.47 hr detected in the optical. Since CP Pup is not an eclipsing system, this is better understood if magnetic accretion occurs: we discuss this possibility and its implications in detail. V351 Pup was detected with XMM-Newton, but not with ASCA. It is a faint, nonsupersoft X-ray source with luminosity L x sime 3 × 1031 erg s-1, a factor of 50 less than measured with ROSAT in 1993.