Swift discovery of the orbital period of the HMXB IGR J015712–7259 in the SMC.

Context. In the last years the hard X-ray astronomy has made a significant step forward, thanks to the monitoring of the IBIS/ISGRI telescope on board the INTEGRAL satellite and of the Burst Alert Telescope (BAT) on board of the Swift observatory. This has provided a huge amount of novel information on many classes of sources. Aims. We have been exploiting the BAT survey data to study the variability and the spectral properties of the new high mass X-ray binary sources detected by INTEGRAL. In this letter we investigate the properties of IGR J015712–7259. Methods. We perform timing analysis on the 88-month BAT survey data and on the XRT pointed observations of this source. We also report on the broad-band 0.2–150 keV spectral analysis. Results. We find evidence for a modulation of the hard-X-ray emission with period Po = 35.37 days. The significance of this modulation is 4.5 standard deviations. The broad band spectrum is modeled with an absorbed power law with photon index Γ ∼ 0.47 and a steepening in the BAT energy range modeled with a cutoff at an energy of ∼ 17 keV