The Unique WD-Sbd Binary System RX J0648/HD 49798

The cataclysmic variable interacting binary class of accreting white dwarfs play an important role in the production of Galactic X-rays, constituting a large fraction of the total stellar population of bright X-ray emitters in the Milky Way. Variable binary systems contaning white dwarfs (WDs) display an immense variety of observational properties; due to both the mass of its companion star, the binary separation (and subsequently the systems Roche-lobe geometry) and also due to the strength of the WDs dipole magnetic field (Hellier 2001; Kuulkers et al. 2006).

Most, if not all, WDs will undergo accretion via Roche-lobe overflow of the donor star. The donor star is often found to be on the main sequence (Patterson 1984), with both sub-giant (e.g. Knigge et al. 2003) and giant (Hachisu & Kato, 2001) companion stars also being observed in a few limited systems. Symbiotic systems, where a white dwarf accretes from the stellar wind of a massive supergiant, have also been detected as X-ray sources (e.g. Masetti et al. 2007).

However, something rarer than all these systems would be a WD with a hot sub-dwarf (Sbd) companion. Hot sub-dwarfs (a.k.a “extreme horizontal branch stars”), of spectral types O or B, represent stars in the late stage of their evolution: when a red giant star loses its outer H envelope at which point the core contracts and begins to fuse H into He (Hellier 2001; Heber 2009).

Such a system represents an event in stellar evolution which occurs for a very short time, and thus, is harder to observe within our current limited time frames of a few decades since the invention of high-resolution telescopes. In principle, other systems of this kind, with a luminosity of the order of 10³² erg s⁻¹, could be detected even at distances of several kpc by the current X-ray satellites. However, due to the very soft spectrum, the observed flux is very sensitive on the amount of interstellar absorption, and hence they would be hard to find.

The only WD-Sbd system discovered thus far is the X-ray binary known as RXJ0648.0–4418/HD49798. It is believed to be the outcome of a common envelope evolution, most likely of an original pair of stars with mass of 8–10(Mereghetti et al. 2011). Adding to its uniqueness as the only WD-Sbd observed in the Milky Way, the compact counterpart of the system (RX J0648.0–4418) is one of the most massive white dwarfs currently known with a dynamically measured mass of 1.28±0.05 (Mereghetti et al. 2011) and the one with the shortest spin period of 13.2s (Israel et al. 1997). In a similar vein, the optical counterpart (HD49798) is one of the brightest Sbds known with an apparent magnitude of V = 8.3, leading to its spectral classification as an O6 type (Jaschek & Jaschek 1963).

Coupled to these amazingly unique properties, RX J0648.0–4418 is believed to be an ONe white dwarf. Therefore, the most likely outcome is an accretion induced collapse (AIC) to a neutron star, as opposed to a supernovae Type Ia explosion, leading to the formation of a millisecond pulsar (MSP) (Bailyn & Grindlay 1990). This has interesting consequences for the zoology of MSPs, as the current theory to their nature is due to the accretion of H/He onto an older neutron star, thus “spinning” it up. Such spun-up MSPs are known as recycled millisecond pulsars. This evolutionary scenario is supported by the observation of short period X–ray pulsations in the persistent and/or burst emission of several LMXRB, by the small eccentricity of MSPs with white dwarf companions in the Galactic disk and by the recent discovery of PSR J1023+0038 (Archibald et al. 2009), the long sought missing-link between the LMXRB and MSP phase.

Hence, it may be possible for future generations to observe  RX J0648/HD 49798 as the first accretion induced collapse of a fast-spinning and low magnetic field white dwarf, and witness a promising scenario for the direct formation of a non-recycled millisecond pulsar.

Journal References:

• Bisscheroux, B. C. et al. (1997) The Nature Of The Bright Sub-Dwarf HD 49798 & Its X-Ray Pulsating Companion. Astronomy & Astrophysics, 317 pp.815-822.

• Israel, G.L. (1997) The Discovery Of 13 Second X-Ray Pulsations From The Hydrogen-Depleted Sub-Dwarf O6 Star Binary HD 49798. The Astrophysical Journal: Letters, 474 (1) pp.L53.

• Archibald, A.M. et al. (2009) A Radio Pulsar/X-ray Binary Link. Science, 324 (5933) pp. 1411-1429.

• Wang, B.; Han, Z.-W. (2010) A Likely Candidate Of Type Ia Supernova Progenitors: The X-Ray Pulsating Companion Of The Hot Sub-Dwarf HD 49798. Astronomy & Astrophysics Research, 10 (7) pp. 681-688.

• Mereghetti, S. et al. (2011) X-Ray & Optical Observations Of The Binary System HD49798/RXJ0648. The Astrophysical Journal, 737 (2) Article I.D.: 51.