Clumpy Wind Studies and the Nondetection of a Cyclotron Line in OAO 1657-415

Winds of massive stars are suspected to be inhomogeneous (or clumpy), which biases the measures of their mass-loss rates. In high-mass X-ray binaries (HMXBs), the compact object can be used as an orbiting X-ray point source to probe the wind and constrain its clumpiness. We perform a spectrotiming analysis of the HMXB OAO 1657-415 with nonsimultaneous NuSTAR and NICER observations. We compute the hardness ratio from the energy-resolved light curves, and, using an adaptive rebinning technique, we thus select appropriate time segments to search for rapid spectral variations on timescales of a few hundred to thousands of seconds. The column density and intensity of the iron Kα line were strongly correlated, and the recorded spectral variations were consistent with accretion from a clumpy wind. We also illustrate a novel framework to measure clump sizes and masses in HMXBs more accurately based on the absorption measurements and orbital parameters of the source. We then discuss the limitations posed by current X-ray spacecraft in such measurements and present prospects with future X-ray missions. We find that the source pulse profiles show a moderate dependence on energy. We identify a previously undetected dip in the pulse profile visible throughout the NuSTAR observation near spin phase 0.15 possibly caused by intrinsic changes in accretion geometry close to the neutron star. We do not find any evidence for the debated cyclotron line at ~36 keV in the time-averaged or phase-resolved spectra with NuSTAR.