SDSS J0926+3624: the shortest period eclipsing binary star

Creators: Copperwheat, C. M.; Marsh, T. R.; Littlefair, S. P.; Dhillon, V. S.; Ramsay, G.; Drake, A. J.; Gänsicke, B. T.; Groot, P. J.; Hakala, P.; Koester, D.; Nelemans, G.; Roelofs, G.; Southworth, J.; Steeghs, D.; Tulloch, S.

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.

Additional Information

© 2010 The Authors. Journal compilation © 2010 RAS. Accepted 2010 August 11. Received 2010 August 11; in original form 2010 July 8. Article first published online: 11 Oct. 2010. CMC and TRM are supported under grant ST/F002599/1 from the Science and Technology Facilities Council (STFC). ULTRACAM, VSD and SPL are supported by STFC grants PP/D002370/1 and PP/E001777/1. DS acknowledges the support of an STFC Advanced Fellowship. The results presented in this paper are based on observations made with the WHT operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Institutio de Astrofisica de Canarias; observations made with the Liverpool Telescope operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council; and on observations obtained with XMM–Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. The CSS survey is funded by NASA under grant NNG05GF22G issued through the Science Mission Directorate Near-Earth Objects Observations Program. This research has made use of NASA's Astrophysics Data System Bibliographic Services and the SIMBAD data base, operated at CDS, Strasbourg, France.

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