* Astronomy

Title: Finding a 60.9-day orbital period for the HMXB 4U 1036-56 with the Swift-BAT monitoring
Authors: G. Cusumano, A. Segreto, V. La Parola, N. Masetti, A. D'Aì, G. Tagliaferri

Since November 2004, the Burst Alert Telescope on board Swift is producing a monitoring of the entire sky in the 15-150 keV band, recording the timing and spectral behaviour of the detected sources. In this letter we study the properties of the HMXB 4U 1036-56 using both the BAT survey data and those from a Swift-XRT pointed observation. The timing analysis of the BAT light curve unveils a periodic modulation with a period of ~60.9 days, that we explain as the orbital period of the binary system. The position of 4U 1036-56 on the Corbet diagram and the derived semi-major orbit axis (~180 solar radii) are consistent with the Be nature of its companion star. The intensity orbital profile averaged over 88 months of observations shows a large asymmetric shape with a minimum consistent with zero intensity, that could be related to the occultation of the neutron star by the supergiant companion. The source shows also a strong long term variability, going from high intensity states to quiescent states over a time scale of ~2 years. The broad band 0.2-150 keV spectrum is well modelled with a flat absorbed power law with a cutoff at ~14 keV. Finally, we explore the possibility that 4U~1036--56 is associated to the unidentified source AGL J1037-5708, finding that the BAT light curve does not show any intensity enhancement correlated to the outburst of the Gamma-ray transient.

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