X-ray emission from the luminous O-type subdwarf HD 49798 and its compact companion

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy
e-mail: sandro@iasf-milano.inaf.it
² IUSS, Istituto Universitario di Studi Superiori, piazza della Vittoria 15, 27100 Pavia, Italy
³ INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
⁴ INFN, Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, via Bassi 6, 27100 Pavia, Italy

Received: 9 February 2013
Accepted: 4 April 2013

Abstract

The X-ray source RX J0648.0−4418 is the only confirmed binary system in which a compact object, most likely a massive white dwarf, accretes from a hot subdwarf companion, the bright sdO star HD 49798. The X-ray emission from this system is characterized by two periodic modulations caused by an eclipse, at the orbital period of 1.55 d, and by the rotation of the compact object with a spin period of 13.2 s. In 2011 we obtained six short XMM-Newton observations centered at orbital phase 0.75, in order to study the system during the eclipse, and spaced at increasingly long time intervals in order to obtain an accurate measure of the spin-period evolution through phase-connected timing. The duration of the eclipse ingress and egress,  ~500 s, indicates the presence of an X-ray emitting region with dimensions of the order of a few 10⁴ km, surrounding the pulsar and probably due to scattering in the companion’s wind. We derived an upper limit on the spin-period derivative  |Ṗ| < 6 × 10^-15 s s^-1, more than two orders of magnitude smaller than the previously available value. Significant X-ray emission is detected also during the 1.2 hours-long eclipse, with a luminosity of  ~3 × 10³⁰ erg s^-1. The eclipse spectrum shows prominent emission lines of H- and He-like nitrogen, an overabundant element in HD 49798. These findings support the suggestion that the X-ray emission observed during the eclipse originates in HD 49798 and that the processes responsible for X-ray emission in the stellar winds of massive O stars are also at work in the much weaker winds of hot subdwarfs.