New X-ray observations of the hot subdwarf binary HD49798 / RXJ0648.0-4418

Abstract

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$\pm$0.05 M${}_{\mathrm{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 $(-2.17\pm 0.01)\times 10^{-15}$ 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 $\sim$(70$\pm$20)\%, suggesting the presence of time-dependent inhomogeneities in the weak stellar wind of the HD49798 subdwarf. To investigate better the soft spectral component that dominates the X-ray flux from this system, we computed a theoretical model for the thermal emission expected from an atmosphere with element abundances and surface gravity appropriate for this massive white dwarf. This model gives a best fit with effective temperature of T${}_{\mathrm{eff}}$=2.25$\times$10${}^5$ K and an emitting area with radius of $\sim$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.

arXiv commentAccepted for publication on MNRAS

References in corpus (17)

• The European Photon Imaging Camera on XMM-Newton: The MOS Cameras
• Hot subluminous stars
• An ultra-massive fast-spinning white dwarf in a peculiar binary system
• The discovery of 13s X-ray pulsations from the hydrogen depleted subdwarf O6 star binary HD49798
• X-ray and optical observations of the unique binary system HD49798/RXJ0648.0-4418
• Including All the Lines
• Formation of hot subdwarf B stars with neutron star components
• Discovery of spin-up in the X-ray pulsar companion of the hot subdwarf HD 49798
• A likely candidate of type Ia supernova progenitors: the X-ray pulsating companion of the hot subdwarf HD 49798
• X-ray emission from the luminous O-type subdwarf HD 49798 and its compact companion
• Model Photospheres with Accelerated Lambda Iteration
• A young contracting white dwarf in the peculiar binary HD 49798/RX J0648.0--4418?
• A model for the population of helium stars in the Galaxy I. Low-mass stars
• HD 49798: Its History of Binary Interaction and Future Evolution
• X-rays from Hot Subdwarfs
• Off-centre carbon burning in He-accreting carbon-oxygen white dwarfs
• Hot subdwarf wind models with accurate abundances I. Hydrogen dominated stars HD 49798 and BD+18${}^{\circ }$2647

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