SWIFT OBSERVATIONS OF THE HIGH-MASS X-RAY BINARY IGR J16283-4838 UNVEIL A 288 DAY ORBITAL PERIOD

We report on the temporal and spectral properties of the high-mass X-ray binary IGR J16283-4838 in the hard X-ray band. We searched the first 88 months of Swift Burst Alert Telescope (BAT) survey data for long-term periodic modulations. We also investigated the broad band (0.2-150 keV) spectral properties of IGR J16283-4838 complementing the BAT dataset with soft X-ray data from the available Swift-XRT pointed observations. The BAT light curve of IGR J16283-4838 revealed a periodic modulation at P-o = 287.6 +/- 1.7 days (with a significance higher than 4 standard deviations). The profile of the light curve folded at P-o shows a sharp peak lasting similar to 12 days over a flat plateau. The long-term light curve also shows a similar to 300 day interval of prolonged enhanced emission. The observed phenomenology suggests that IGR J16283-4838 has a Be nature, where the narrow periodic peaks and the similar to 300 day outburst can be interpreted as Type I and Type II outbursts, respectively. The broad band 0.2-150 keV spectrum can be described with an absorbed power-law and a steepening in the BAT energy range.