Supergiant X-ray binaries observed by Suzaku

Abstract

Suzaku observations are presented for the high-mass X-ray binaries IGR J16207−5129 and IGR J17391−3021. For IGR J16207−5129, we provide the first X-ray broadband (0.5–60 keV) spectrum from which we confirm a large intrinsic column density (NH = 1.6 ×10²³ cm⁻² ), and we constrain the cutoff energy for the first time (Ecut = 19 keV). A prolonged (> 30 ks) attenuation of the X-ray flux was observed which we tentatively attribute to an eclipse of the probable neutron star by its massive companion, in a binary system with an orbital period between 4 and 9 days, and inclination angles > 50 degrees. For IGR J17391−3021, we witnessed a transition from quiescence to a low-activity phase punctuated by weak flares whose peak luminosities in the 0.5–10 keV band are only a factor of 5 times that of the pre-flare emission. These micro flares are accompanied by an increase in NH which suggests the accretion of obscuring clumps of wind. We now recognize that these low-activity epochs constitute the most common emission phase for this system, and perhaps in other supergiant fast X-ray transients (SFXTs) as well. We close with an overview of our upcoming program in which Suzaku will provide the first ever observation of an SFXT (IGR J16479−4514) during a binary orbit enabling us to probe the accretion wind at every phase.