X-Ray Spectra and Light Curves of Cooling Novae and a Nova-Like

We observed with XMM-Newton the recent novae V2491 Cyg (Nova Cyg 2011) and KT Eri (Nova Eri 2009) and the dwarf nova EY Cyg, which is likely to have had a nova outburst long ago (Sion et al. 2004, AJ, 128, 1795). We also obtained a Chandra high-resolution spectrum of the nova-like V794 Aql. For V2491 Cyg, we detected the ~38 minutes period in the X-ray light curve that was already measured in outburst, giving substantial support to the classification of this nova as an intermediate polar. The X-ray spectrum shows the possible presence of a non-thermal component, a plasma at ~4 keV, and a very soft component that we could fit with a blackbody at a temperature of 82 eV and luminosity above to 10³⁴ ergs/s. We speculate that the latter component may be due to a shrunk, confined nuclear burning zone. The X-ray spectrum of KT Eri also shows a luminous blackbody-like component, at about 30 eV. The EY Cyg spectrum can be fitted with a stationary cooling flow model, a moderately massive white dwarf of about 0.9 solar mass, and an accretion rate of 2x10^-11 Solar mass per year. The high-resolution X-ray spectrum of V794 Aql is "harder" than all other V Scl systems, is rich in emission lines, and indicative of a very massive white dwarf accreting at a high rate.