MEREGHETTI, SandroSandroMEREGHETTIPintore, FabioFabioPintoreRauch, T.T.RauchLA PALOMBARA, NICOLANICOLALA PALOMBARAEsposito, P.P.EspositoGeier, S.S.GeierPelisoli, I.I.PelisoliRIGOSELLI, MICHELAMICHELARIGOSELLISchaffenroth, V.V.SchaffenrothTIENGO, ANDREAANDREATIENGO2022-06-072022-06-0720210035-8711http://hdl.handle.net/20.500.12386/32217HD 49798/RX J0648.0-4418 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<SUB>⊙</SUB>) 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<SUP>-15</SUP> s s<SUP>-1</SUP>, 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) per cent, suggesting the presence of time-dependent inhomogeneities in the weak stellar wind of the HD 49798 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<SUB>eff</SUB> = 2.25 × 10<SUP>5 </SUP>K and an emitting area with a 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.STAMPAen10.1093/mnras/stab1004https://academic.oup.com/mnras/article/504/1/920/62198492021MNRAS.504..920MFIS/05 - ASTRONOMIA E ASTROFISICA