The INTEGRAL source IGR J16328-4726: a High Mass X-ray Binary from the Beppo SAX era

Abstract: We report on temporal and spectral analysis of the INTEGRAL fast transient candidate IGR J16328-4726 observed with Beppo SAX in 1998 and more recently with INTEGRAL. The MECS X-ray data show a frequent micro activity typical of the intermediate state of Supergiant Fast X-ray Transients and a weak flare with duration of ~4.6 ks. The X-ray emission in the 1.5-10 keV energy range is well described through the different time intervals by an absorbed power law model. Comparing spectra from the lower emission level up to the peak of the flare, we note that while the power-law photon index was constant (~2), the absorption column density varied by a factor of up to ~6-7, reaching the value of ~2x10^{23}cm^{-2} at the peak of the flare. Analysis of the long-term INTEGRAL/IBIS light curve confirms and refines the proposed ~10.07 day period, and the derived ephemeris places the Beppo SAX observations away from periastron. Using the near and the mid-IR available observations, we constructed a spectral infrared distribution for the counterpart of IGR J16328-4726, allowing us to identify its counterpart as a High Mass OB type star, and to classify this source as a firm HMXB. Following the standard clumpy wind theory, we estimated the mass and the radius of the clump responsible of the flare. The obtained values of M~4x10^{22}g and R~4.4x10^6 km are in agreement with expected values from theoretical predictions.