Six new candidate ultracompact X-ray binaries

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: jeanz@sron.nl
² Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁵ Astroparticle Physics Laboratory, Mail Code 661, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 1 November 2006
Accepted: 8 January 2007

Abstract

Ultracompact X-ray binaries (UCXBs) appear able to sustain accretion onto the compact accretor at rates lower than in wider X-ray binaries. This may be understood by the smaller accretion disks in UCXBs: a lower X-ray luminosity suffices to keep a disk completely ionized through irradiation and, thus, keep the viscosity at a sufficiently high level to allow effective transport of matter to the compact object. We employ this distinguishing factor on data from RXTE and BeppoSAX to identify six new candidate UCXBs, thus increasing the population by one quarter. The candidates are drawn from the population of persistently accreting and type-I X-ray bursting low-mass X-ray binaries. The X-ray bursts establish the low-mass X-ray binary nature and provide a handle on the accretion rate. We find that the low accretion rates are supported by the long burst recurrence times and the hard X-ray spectra of the persistent emission as derived from the 2nd INTEGRAL catalog of soft γ-ray sources. We discuss the peculiar light curves of some new UCXB candidates.