Investigating the physical origin of X-ray variability in HMXBS with NuSTAR

We investigate the X-ray variability in two HMXBs OAO 1657-415 and 4U 0114+65 with NuSTAR and NICER, exploiting a joint spectral and timing analysis for both. We made use of an adaptive rebinning technique for the hardness ratio computed from the source's energy-resolved lightcurves to appropriately select time intervals for the spectral extraction and therefore search for rapid spectral variations. We show that the spectral variations are all consistent with being caused by the accretion of the neutron star from the clumpy wind of its massive companion. We do not find any evidence for the debated cyclotron line in the X-ray spectra with NuSTAR for both the sources. We will compare our findings with those reported previously in the literature and discuss the accretion physics in the wider context of other similar supergiant HMXBs.