The INTEGRAL Source IGR J16328-4726: A High-mass X-Ray Binary from the BeppoSAX Era

Abstract

We report on temporal and spectral analysis of the INTEGRAL fast transient candidate IGR J16328-4726 observed with BeppoSAX in 1998 and more recently with INTEGRAL. The MECS X-ray data show a frequent microactivity typical of the intermediate state of supergiant fast X-ray transients and a weak flare with a 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 a value of ~2 × 10²³ 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 BeppoSAX 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 for the flare. The obtained values of M _cl ~= 4 × 10²²g and R_cl ~= 4.4 × 10⁶ km are in agreement with expected values from theoretical predictions.