NuSTAR investigation of X-ray variability and hard X-ray spectral properties in IGR J16320-4751 and IGR J16479-4514

We present the results obtained from a comprehensive timing and spectral study of two high mass X-ray binary sources using NuSTAR observations. These two sources, IGR J16320-4751 and IGR J16479-4514 were discovered by INTEGRAL and have been characterized for the first time in the hard X-ray band (beyond 10 keV) in this work. In these sources, we observe the occurrence of intense X-ray flares, with average luminosities exceeding 10³⁶ erg s^-1. Our analysis reveals that these flares can be described consistently in the quasi-spherical accretion regime. The orbital phase of the first flare in NuSTAR observation of IGR J16479-4514 matches with the orbital phases of previous flares (ϕ = 0.35) in this source detected by other telescopes. We conclude that, this flare occurs as a result of the periastron passage of the neutron star (NS), rather than due to the presence of a corotating interaction region. Furthermore, from the energy-resolved pulse profile analysis of IGR J16320-4751, we find that the pulse fraction is lower in hard X-rays compared to the soft X-rays. We present the hard X-ray spectral parameters of these two sources using several standard spectral model components. We do not detect a cyclotron absorption feature in either target. We provide estimates to the surface magnetic field strength of NS in IGR J16320-4751, using two indirect methods. Lastly, we observe spectral hardening during flaring segments compared to the off-flaring segments which indicates that Comptonization is more effective during the flaring segments.