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

Abstract

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 10$^{36}$~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 ($\phi=0.35$) in this source detected by other telescopes. We conclude that this flare occurs as a result of the periastron passage of the neutron star, rather than due to the presence of a corotating interaction region (CIR). 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 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 of 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.