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

arXiv:1105.6227v1 [astro-ph.HE] 31 May 2011 Draft version February 3, 2018 Preprint typeset using LATEX style emulateapj v. 11/10/09 X-RAY AND OPTICAL OBSERVATIONS OF THE UNIQUE BINARY SYSTEM HD 49798/RXJ0648.0–4418 S. Mereghetti1, N. La Palombara1, A. Tiengo1, F. Pizzolato1, P. Esposito2, P. A. Woudt3, G. L. Israel4, L. Stella4 1 INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, I-20133 Milano, Italy; sandro@iasf-milano.inaf.it 2 INAF - Osservatorio Astronomico di Cagliari, localit´a Poggio dei Pini, strada 54, I-09012 Capoterra, Italy 3 Astronomy Department, and Astrophysics, Cosmology and Gravity Centre, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa and 4 INAF - Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monteporzio Catone, Italy Draft version February 3, 2018 ABSTRACT We report the results of XMM-Newton observations of HD 49798/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⊙. Its X-ray emission consists of a strongly pulsed, soft component, well fit by a blackbody with kTBB ∼40 eV, accounting for most of the luminosity, and a fainter hard power-law component (photon index ∼1.6). A luminosity of ∼1032 erg s−1 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 2×1030 erg s−1, can be attributed to HD 49798 and represents the first detection of a hot sub-dwarf star in the X-ray band. HD 49798/RXJ0648.0–4418 is a post-common envelope binary which most likely originated from a pair of stars with masses ∼8–10 M⊙. 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. Subject headings: binaries: close – subdwarfs: individual (HD 49798) – white dwarfs – X-rays: indi- vidual (RX J0648.0–4418) 1. INTRODUCTION Interacting binaries containing accreting white dwarfs constitute a large fraction of the population of bright Galactic X-ray emitters. These systems, including both persistent and transient sources, display a rich variety of properties, determined mainly by the strength of the white dwarf’s magnetic field and the nature of the mass donor star (see, e.g., Kuulkers et al. (2006)). Most ac- creting white dwarfs are found in cataclysmic variables, where accretion proceeds through Roche-lobe overflow. The mass donor in these systems is typically a main se- quence star or another white dwarf, but also sub-giant and giant companion stars have been observed. Sym- biotic systems, where a white dwarf accretes from the stellar wind of a massive supergiant, have also been de- tected as X-ray sources. The system discussed here is the only known X-ray bi- nary composed by a white dwarf (RX J0648.0–4418) and a hot sub-dwarf star (HD 49798). Hot sub-dwarfs are evolved low mass stars that lost most of their hydrogen envelopes and are believed to be in the helium core burn- ing stage (for a recent review on hot sub-dwarfs see Heber (2009)). They are spectroscopically classified in different types: sdB (with effective temperature T∼25,000–28,000 K), sdOB (with T∼33,000–40,000 K), and sdO (with T>40,000 K). One possible mechanism responsible for the loss of their hydrogen envelopes is non-conservative mass transfer in a binary (Podsiadlowski & Han 2004). Being one of the brightest hot sub-dwarfs (apparent magnitude V=8.3), HD 49798 has been the object of several studies in the optical/UV wavebands. The first spectroscopic observations led to its classification as a sub-dwarf of O6 spectral type (Jaschek & Jaschek 1963) and showed radial velocity variations, later found to be caused by orbital motion with a period of 1.55 days (Thackeray 1970; Stickland & Lloyd 1994). No spectro- scopic evidence for a secondary could be found, leading to the early suggestion that the companion could be a white dwarf (Thackeray 1970). Soft X-ray emission from the direction of HD 49798 was first detected with the Einstein Observatory in 1979, but it could be studied in detail only with a ROSAT satellite observation per- formed in 1992. The ROSAT data showed a very soft spectrum and led to the discovery of a strong modula- tion with a period of 13.2 s (Israel et al. 1997). While the periodicity clearly indicated the presence o

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