RX J1914.4+2456: the first double-degenerate polar?

Abstract

RX J1914.4+2456 exhibits a light curve with a strong modulation at 569 s: this period is characteristic of the white dwarf spin period in intermediate polar (IP) systems. However, the X‐ray light curve is difficult to reconcile with current models for IP emission. We argue that a simpler explanation is that the system is a polar with a degenerate secondary star, which would make it the first known system of its kind. As such it should contain the first example of an He‐dominated radial accretion flow on to the white dwarf surface, and RX J1914.4+2456 would have the shortest known orbital period of any binary system.

Ultra-Short Period Double-Degenerate Binaries

“…When Cropper et al (1998) performed a Fourier analysis of their ROSAT HRI data of RX J1914.4+2456, only one period (with its harmonics) was evident in the data, with the value of 569.38s reported by Motch k Haberl (1995). The same was found in the optical by Ramsay et al (2000).…”

“…The shape of the X-ray modulation profile is unlike that seen in any other IP, 'soft' or normal. Cropper et al (1998) argued that the shape of the X-ray emission, and its absence over half of the period, required a small emission region moving into and out of view. The accretion had to be far from symmetric about the rotation axis in order to prevent a lower region rotating into view as the upper rotated over the limb of the star; they considered absorption could not be the cause, and that pole-switching would not be possible on the observed short timescales in the system.…”

“…Cropper et al (1998) noted that for such a compact system, the magnetic field of the primary did not need to be as strong as that of normal Polars in order to achieve synchronism. The predictions from the model were that there may be detectable levels of optical circular polarisation and that the accretion flow should be mostly He.…”

“…In followup observations of the 'soft' IPs, Cropper et al (1998) were exercised by the shape of the X-ray modulation in this particular object, and by the fact that for half of the modulation phase there was no detectable X-ray emission. They noted that this was unlike the behaviour of all other IPs and proposed instead that the period was the orbital period of a rotationally sychronised white dwarf pair with a magnetic primary -the double degenerate analog of Polars.…”

The Turn‐On of Mass Transfer in AM CVn Binaries: Implications for RX J0806+1527 and RX J1914+2456

“…Both systems are soft X-ray sources whose light curves are modulated on periods of 321 s (RX J0806, hereafter J0806) and 569 s (RX J1914, hereafter J1914) (Motch et al 1996;Israel et al 1999;Burwitz & Reinsch 2001). In both cases, the X-ray light is 100% modulated for roughly half the period (Cropper et al 1998;Israel et al 1999) and the optical light is modulated on the same period as the X-ray light (Ramsay et al 2000;Israel et al 2002;Ramsay et al 2002a,b;Israel et al 2003) with little evidence for other periodicities (but see Ramsay et al 2006). It is widely believed, based partially on the stability and singularity of these periods, that the modulations are on the systems' orbital periods, P orb (although see Norton et al 2004, for an alternate interpretation).…”