English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

HD49798 / RXJ0648.0-4418 is the only confirmed X-ray binary in which the massdonor is a hot subdwarf star of O spectral type and, most likely, it contains amassive white dwarf (1.28$\pm$0.05 M$_{\rm SUN}$) with a very fast spin periodof 13.2 s. Here we report the results of new XMM-Newton pointings of thispeculiar binary, carried out in 2018 and in 2020, together with a reanalysis ofall the previous observations. The new data indicate that the compact object isstill spinning-up at a steady rate of $(-2.17\pm0.01)\times10^{-15}$ ss$^{-1}$, consistent with its interpretation in terms of a young contractingwhite dwarf. Comparison of observations obtained at similar orbital phases, farfrom the ecplise, shows evidence for long term variability of the hard ($>$0.5keV) spectral component at a level of $\sim$(70$\pm$20)\%, suggesting thepresence of time-dependent inhomogeneities in the weak stellar wind of theHD49798 subdwarf. To investigate better the soft spectral component thatdominates the X-ray flux from this system, we computed a theoretical model forthe thermal emission expected from an atmosphere with element abundances andsurface gravity appropriate for this massive white dwarf. This model gives abest fit with effective temperature of T$_{\rm eff}$=2.25$\times$10$^5$ K andan emitting area with radius of $\sim$1600 km, larger than that found withblackbody fits. This model also predicts a contribution of the pulsed emissionfrom the white dwarf in the optical band significantly larger than previouslythought and possibly relevant for optical variability studies of this system.

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