2021MNRAS.504..920M -
Mon. Not. R. Astron. Soc., 504, 920-925 (2021/June-2)
New X-ray observations of the hot subdwarf binary HD 49798/RX J0648.0-4418.
MEREGHETTI S., PINTORE F., RAUCH T., LA PALOMBARA N., ESPOSITO P., GEIER S., PELISOLI I., RIGOSELLI M., SCHAFFENROTH V. and TIENGO A.
Abstract (from CDS):
HD 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☉) 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) x 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) 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 Teff = 2.25 x 105 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.
Abstract Copyright:
© 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society
Journal keyword(s):
subdwarfs - white dwarfs - X-rays: binaries
Simbad objects:
1
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