Investigating a unique partial eclipse in the high-mass X-ray binary IGR J16393−4643 with Swift-XRT

Abstract

The orbital profile of the high-mass X-ray binary IGR J16393-4643 shows a dip in its X-ray intensity, which was previously interpreted as an eclipse. Unlike most eclipsing HMXBs, where the X-ray eclipses are about two orders of magnitude fainter compared to the out-of-eclipse emission, this particular eclipse-like feature is narrow and partial, casting doubt if it is indeed an eclipse. To further investigate the nature of this low-intensity orbital phase, we use a large number of observations with Swift-XRT, covering the entire orbital phase. The soft X-ray observations also show this low-intensity phase, which is about 30 % of the intensity during rest of the orbit. We also carried out orbital-phase-resolved spectroscopy to compare the change in the spectral parameters inside and outside of this low-intensity state. The results indicate that this low-intensity state might not be an eclipse, as previously thought but absorption in the stellar corona. We have also provided the inclination angle of the binary for grazing eclipse caused by the stellar corona.