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

TL;DR

This study presents new X-ray observations of the unique hot subdwarf binary HD49798 / RXJ0648.0-4418, revealing ongoing white dwarf spin-up, spectral variability, and detailed atmospheric modeling that enhances understanding of this rare system.

Contribution

The paper provides new XMM-Newton data, confirms steady white dwarf spin-up, detects spectral variability, and introduces a detailed atmospheric model for the white dwarf in this binary.

Findings

White dwarf is still spinning up at a steady rate.

Spectral variability observed in the hard X-ray component.

Atmospheric model suggests a larger emitting area and higher temperature.

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$0.05 M$_{\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 $(-2.17\pm0.01)\times10^{-15}$ 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$20)\%, suggesting the presence of time-dependent inhomogeneities in the weak stellar wind of the HD49798 subdwarf. To investigate better the soft spectral component that dominates the X-ray flux from this system, we computed a theoretical model for the thermal emission expected from an atmosphere with element abundances and surface gravity appropriate for this massive white dwarf. This model gives a best fit with effective temperature of T$_{\rm 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.