Synthetic direct impact light curves of the ultracompact AM CVn binary systems V407 Vul and HM Cnc

Abstract

The interacting binary white dwarf (AM CVn) systems HM Cnc and V407 have orbital periods of 5.4 and 9.5 min, respectively. The two systems are characterized by an ‘on/off’ behaviour in the X‐ray light curve, and optical light curves that are nearly sinusoidal and which lead the X‐ray light curves in phase by about 0.2 in both systems. Of the models that have been proposed to explain the observations, the one that seems to require the least fine‐tuning is the direct impact model of Marsh & Steeghs. In this model, the white dwarf primary is large enough relative to the semimajor axis that the accretion stream impacts the surface of the primary white dwarf directly without forming an accretion disc. Marsh & Steeghs proposed that in this situation there could be a flow setup around the equator with a decreasing surface temperature, the further one measured from the impact point. In this study, we estimate the light curves that might result from such a temperature distribution, and find them to be reasonable approximations to the observations. One unexpected result is that two distinct X‐ray spots must exist to match the shape of the X‐ray light curves.

Magnetic interaction in ultra-compact binary systems

“…Although the model is generally consistent with existing observations (see Cropper et al 2004), there are some concerns regarding whether or not it is applicable to RX J1914+24 and RX J0806+15, which are presumably compact white-dwarf pairs (e.g. Barros et al 2005Barros et al , 2007Laine, Lin & Dong 2008;Wood 2009). Some concerns, e.g.…”

“…If the binary orbit is too compact, the formation of an accretion disk might be prohibited. Mass transfers directly via a gas stream from the inner Lagrangian point of the secondary white dwarf to the surface of the primary white dwarf (Marsh & Steeghs 2002, see also Wood 2009). The mass transfer dynamics of these double white dwarfs are analogous to those of the Algol binaries.…”

Detection of a 23.6 min periodic modulation in the optical counterpart of 3XMMJ051034.6–670353

“…This could be a clue to the location and extent of the emission region. For instance, Dolence et al (2008) and Wood (2009) modelled the Xray and optical phase folded light curves to place constraints on the extent of the X-ray generating spot assuming a direct impact model.…”

AM CVn Stars: Status and Challenges

“…The ram pressure of the stream is high enough to make it penetrate to about 100 km depth, and the up-dwelling of hot (∼65 eV) material will produce the soft X-rays observed about 60-65°downstream from the impact point. Simulations show (Wood 2009) that two X-ray spots, one for the accretion point and one for the up-dwelling, are necessary to explain the sharp rise and first more gradual, then sharp, fall of the X-ray light curve (Fig. 13).…”