Finding a 61.0 d orbital period for the HMXB 4U 1036-56 with the Swift-BAT monitoring

Abstract

Since 2004 November, 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. Here, we study the properties of the High mass X-ray binary (HMXB) 4U 1036-56 using both the BAT survey data and those from a Swift-X-ray Telescope (XRT) observation. A folding analysis performed on the BAT light curve of the first 100 months of survey unveils a periodic modulation with a period of ̃61.0 d, tied to the presence in the BAT light curve of several intensity enhancements lasting ̃1/4 of P0. We explain this modulation as the orbital period of the binary system. The position of 4U 1036-56 on the Corbet diagram, the derived semimajor orbit axis (≃180 R⊙) and the bulk of the source emission observed in a limited portion of the orbit are consistent with a Be companion star. The broad-band 0.2- 150 keV spectrum is well modelled with a flat absorbed power law with a cutoff at ~16 keV. Finally, we explore the possible association of 4U 1036-56 with the γ -ray source AGL J1037-5808, finding that the BAT light curve does not show any correlation with the γ -ray outburst observed in 2012 November. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.