SDSS J0926+3624: the shortest period eclipsing binary star

Abstract

With orbital periods of the order of tens of minutes or less, the AM Canum Venaticorum stars are ultracompact, hydrogen-deficient binaries with the shortest periods of any binary subclass, and are expected to be among the strongest gravitational wave sources in the sky. To date, the only known eclipsing source of this type is the P = 28 min binary SDSS J0926+3624. We present multiband, high time resolution light curves of this system, collected with William Herschel Telescope (WHT)/ULTRACAM in 2006 and 2009. We supplement these data with additional observations made with Liverpool Telescope/Rapid Imager to Search for Exoplanets (LT/RISE), XMM-Newton and the Catalina Real-Time Transient Survey. From light curve models we determine the mass ratio to be q = M 2 /M 1 = 0.041 ± 0.002 and the inclination to be 82. • 6 ± 0. • 3. We calculate the mass of the primary white dwarf to be 0.85 ± 0.04 M and the donor to be 0.035 ± 0.003 M , implying a partially degenerate state for this component. We observe superhump variations that are characteristic of an elliptical, precessing accretion disc. Our determination of the superhump period excess is in agreement with the established relationship between this parameter and the mass ratio, and is the most precise calibration of this relationship at low q. We also observe a quasi-periodic oscillation in the 2006 data, and we examine the outbursting behaviour of the system over a 4.5 year period.

Hunting for eclipses: high-speed observations of cataclysmic variables

“…The eclipses are shallow, and the bright-spot eclipse appears to arrive very late -almost after the white dwarf has come out of eclipse. This behaviour is similar to that seen in the AM CVn system SDSS J092638.71+362402.4 Copperwheat et al 2011), where the secondary star is so bright and small in size (e.g. a white dwarf), that the eclipses are shallow and the bright-spot ingress occurs after the primary white dwarf egress.…”

Double white dwarfs and LISA

“…Attention has focussed on three types of progenitor: (i) detached DWDs, (ii) evolved cataclysmic variable stars, and (iii) white dwarf / helium star accreting binaries. At the long periods of most known systems, these three models lead to rather subtle differences in mass transfer rate and other parameters (Deloye et al 2007) which even the best constrained systems are not yet capable of distinguishing (Copperwheat et al 2010). We know DWDs of short enough period to merge within a Hubble time, while there are no clear progenitors of the other two routes.…”

PTF1 J071912.13+485834.0: AN OUTBURSTING AM CVn SYSTEM DISCOVERED BY A SYNOPTIC SURVEY

“…The second group is more difficult to understand, primarily due to a lack of known recurrence times. The one published recurrence time, for SDSSJ0926, is very poorly determined (Copperwheat et al 2011). However, there does appear to be a clear gap between the determined recurrence times of PTF1J0719+4858 and the much more poorly determined recurrence time of SDSSJ0926+3624, and we question whether this difference in recurrence time is purely a result of the increased orbital period (and thus decreased mass-transfer rates) or if different parameters also place a role (such as the mass of the primary and/or the entropy of the secondary).…”

“…In outburst they are typically 3-5 mag brighter than in quiescence and feature superhumps. These outbursts tend to last for a few weeks, and recur on a timescale (where known) between 46 days (e.g., Kato et al 2000) and over a year (e.g., Copperwheat et al 2011). Between the two states, some of these systems have been observed to have a "cycling" state wherein some experience magnitude changes of ∼1 mag with a period of about a day (Patterson et al 2000).…”