EDP Sciences
Free access
Volume 492, Number 2, December III 2008
Page(s) 557 - 563
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:200809715
Published online 30 October 2008

A&A 492, 557-563 (2008)
DOI: 10.1051/0004-6361:200809715

Research Note

The INTEGRAL long monitoring of persistent ultra compact X-ray bursters

M. Fiocchi1, A. Bazzano1, P. Ubertini1, A. J. Bird2, L. Natalucci1, and V. Sguera3

1  Instituto di Astrofisica Spaziale e Fisica Cosmica di Roma (INAF), via Fosso del Cavaliere 100, Roma 00133, Italy
    e-mail: mariateresa.fiocchi@iasf-roma.inaf.it
2  School of Physics and Astronomy, University of Southampton, SO17 1BJ, UK
3  Instituto di Astrofisica Spaziale e Fisica Cosmica di Bologna (INAF), via Gobetti 101, Bologna 40129, Italy

Received 4 March 2008 / Accepted 10 October 2008

Context. The combination of compact objects, short period variability and peculiar chemical composition of the ultra compact X-ray binaries make up a very interesting laboratory to study accretion processes and thermonuclear burning on the neutron star surface. Improved large optical telescopes and more sensitive X-ray satellites have increased the number of known ultra compact X-ray binaries allowing their study with unprecedented detail.
Aims. We analyze the average properties common to all ultra compact bursters observed by INTEGRAL from ~0.2 keV to ~150 keV.
Methods. We have performed a systematic analysis of the INTEGRAL public data and Key-Program proprietary observations of a sample of the ultra compact X-ray binaries. In order to study their average properties in a very broad energy band, we combined INTEGRAL with BeppoSAX and SWIFT data whenever possible. For sources not showing any significant flux variations along the INTEGRAL monitoring, we build the average spectrum by combining all available data; in the case of variable fluxes, we use simultaneous INTEGRAL and SWIFT observations when available. Otherwise we compared IBIS and PDS data to check the variability and combine BeppoSAX with INTEGRAL /IBIS data.
Results. All spectra are well represented by a two component model consisting of a disk-blackbody and Comptonised emission. The majority of these compact sources spend most of the time in a canonical low/hard state, with a dominating Comptonised component and accretion rate $\dot {M}$ lower than ~ 10-9 ${M_{\odot}}/$yr, not depending on the model used to fit the data.

Key words: gamma rays: observations -- stars: neutron -- X-rays: binaries

© ESO 2008