The Swift-BAT survey reveals the orbital period of three high-mass X-ray binaries

¹ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123, Palermo, Italy
e-mail: dai@fisica.unipa.it
² INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, via U. La Malfa 153, 90146 Palermo, Italy

Received: 23 December 2010
Accepted: 22 February 2011

Abstract

Aims. A growing number of previously hidden Galactic X-ray sources are now detected with recent surveys performed by the INTEGRAL and Swift satellites. Most of these new sources eluded past surveys due to their large local X-ray extinction and consequent low soft X-ray flux. The Swift-BAT performs daily monitoring of the sky in an energy band (15–150 keV) which is only marginally affected by X-ray extinction, thus allowing for the search of long periodicities in the light curve and identification of the nature of the X-ray sources.

Methods. We performed a period search using the folding technique in the Swift-BAT light curves of three INTEGRAL sources: IGR J05007−7047, IGR J13186−6257 and IGR J17354−3255. Their periodograms show significant peaks at 30.77 ± 0.01 d, 19.99 ± 0.01 d and 8.448 ± 0.002 d, respectively. We estimate the significance of these features from the χ² distribution of all the trials, finding a probability  ≤ 1.5 × 10^-4 that the detections occurred due to chance. We complement our analysis with the study of their broadband X-ray emission.

Results. We identify the periodicities with the orbital periods of the sources. The periods are typical for the wind accretors X-ray binaries and we support this identification showing that also their energy spectra are compatible with an X-ray spectral emission characteristic of high-mass X-ray binaries. The spectrum of IGR J05007−704 that resides in the Large Magellanic Cloud, does not show any intrinsic local absorption, whereas the spectra of the Galactic sources IGR J17354−3255 and IGR J13186−6257 may be affected by a local absorber. The folded light curve for IGR J13186−6257 suggests a possible Be companion star.