X-ray and optical observations of the unique binary system HD49798/RXJ0648.0-4418

We report the results of XMM-Newton observations of HD49798/RXJ0648.0-4418, the only known X-ray binary consisting of a hot sub-dwarf and a white dwarf. The white dwarf rotates very rapidly (P=13.2 s) and has a dynamically measured mass of 1.28+/-0.05 M_sun. Its X-ray emission consists of a strongly pulsed, soft component, well fit by a blackbody with kT~40 eV, accounting for most of the luminosity, and a fainter hard power-law component (photon index ~1.6). A luminosity of ~10^{32} erg/s is produced by accretion onto the white dwarf of the helium-rich matter from the wind of the companion, which is one of the few hot sub-dwarfs showing evidence of mass-loss. A search for optical pulsations at the South African Astronomical Observatory 1.9-m telescope gave negative results. X-rays were detected also during the white dwarf eclipse. This emission, with luminosity 2x10^{30} erg/s, can be attributed to HD 49798 and represents the first detection of a hot sub-dwarf star in the X-ray band. HD49798/RXJ0648.0-4418 is a post-common envelope binary which most likely originated from a pair of stars with masses ~8-10 M_sun. After the current He-burning phase, HD 49798 will expand and reach the Roche-lobe, causing a higher accretion rate onto the white dwarf which can reach the Chandrasekhar limit. Considering the fast spin of the white dwarf, this could lead to the formation of a millisecond pulsar. Alternatively, this system could be a Type Ia supernova progenitor with the appealing characteristic of a short time delay, being the descendent of relatively massive stars.Comment: Accepted for publication on The Astrophysical Journa

UvA-DARE (Digital Academic Repository) The nature of the bright subdwarf HD 49798 and its X-ray pulsating companion

The nature of the bright subdwarf HD49798 and its X-ray pulsating companion Bisscheroux, B.C.; Pols, O.R.; Kahabka, P.; Belloni, T.; van den Heuvel, E.P.J. Published in: Astronomy & Astrophysics Link to publication Citation for published version (APA): Bisscheroux, B. C., Pols, O. R., Kahabka, P., Belloni, T., & van den Heuvel, E. P. J. (1997). The nature of the bright subdwarf HD49798 and its X-ray pulsating companion. Astronomy & Astrophysics, 317, 815-822. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Download date: 30 Jun 2019 Astron. Astrophys. 317, 815-822 (1997) Abstract. An analysis of the observed properties of the subdwarf O6 close binary system HD 49798 and its 13.18 s ultrasoft X-ray pulsating companion (WGA J0648.0-4418) is presented. On evolutionary grounds we show that the subdwarf must have a degenerate CO core and is in the phase of shell helium burning, which explains its high luminosity. The subdwarf, which probably has a mass between 0.7 − 1.3M , is the descendant of a massive asymptotic giant branch star that lost its hydrogen-rich outer layers in a common-envelope event. We show that all observations are consistent with the X-ray source being a weakly magnetized massive white dwarf which is accreting matter from the wind of its subdwarf companion. We exclude a neutron star companion on the ground of (1) the ultrasoft spectrum of the X-ray source; (2) the very close resemblance of the X-ray spectrum and luminosity with that of the soft intermediate polars; (3) the relative closeness of HD 49798, which implies a Galactic birthrate of such systems that is much larger than that of binary pulsars with a massive white dwarf companion, the natural descendants of systems like HD 49798 if the pulsar in this system were a neutron star