2021/04/08 by S. Mereghetti, F. Pintore, T. Rauch +7 · 1 citation
Physics and Astronomy · #astro-ph.SR #astro-ph.HE
paper · pdf · doi:10.1093/mnras/stab1004
Accepted for publication on MNRAS
arxiv created 2021/04/08 · arxiv updated 2021/04/21
HD49798 / RXJ0648.0-4418 is the only confirmed X-ray binary in which the mass donor is a hot subdwarf star of O spectral type and, most likely, it contains a massive white dwarf (1.28±0.05 M\rm SUN) with a very fast spin period of 13.2 s. Here we report the results of new XMM-Newton pointings of this peculiar binary, carried out in 2018 and in 2020, together with a reanalysis of all the previous observations. The new data indicate that the compact object is still spinning-up at a steady rate of (-2.17±0.01)×10-15 s s-1, consistent with its interpretation in terms of a young contracting white dwarf. Comparison of observations obtained at similar orbital phases, far from the ecplise, shows evidence for long term variability of the hard (>0.5 keV) spectral component at a level of ∼(70±20)%, suggesting the presence of time-dependent inhomogeneities in the weak stellar wind of the HD49798 subdwarf. To investigate better the soft spectral component that dominates the X-ray flux from this system, we computed a theoretical model for the thermal emission expected from an atmosphere with element abundances and surface gravity appropriate for this massive white dwarf. This model gives a best fit with effective temperature of T\rm eff=2.25×105 K and an emitting area with radius of ∼1600 km, larger than that found with blackbody fits. This model also predicts a contribution of the pulsed emission from the white dwarf in the optical band significantly larger than previously thought and possibly relevant for optical variability studies of this system.