A&A 465, 953-963 (2007)
DOI: 10.1051/0004-6361:20066678

Six new candidate ultracompact X-ray binaries

J. J. M. in 't Zand^{1, 2}, P. G. Jonker^{1, 2, 3}, and C. B. Markwardt<sup>4, 5

1</sup>  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 -ray sources. We discuss the peculiar light curves of some new UCXB candidates.

Key words: X-rays: binaries -- X-rays: bursts -- accretion, accretion disks

© ESO 2007

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

• If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
• You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
• You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.