Clumpy wind studies and the non-detection of cyclotron line in OAO 1657-415

TL;DR

This study analyzes the wind clumpiness in the high-mass X-ray binary OAO 1657-415 using spectro-timing methods, revealing spectral variations consistent with a clumpy wind, and discusses future prospects for such measurements.

Contribution

It introduces a novel framework for measuring clump sizes and masses in HMXBs based on absorption and orbital data, and assesses current and future X-ray observational capabilities.

Findings

Spectral variations correlated with iron line and column density, indicating clumpy wind accretion.

Detection of a new dip in pulse profile near spin phase 0.15, suggesting changes in accretion geometry.

No evidence found for the previously debated cyclotron line at 36 keV.

Abstract

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 spectro-timing analysis of the HMXB OAO 1657-415 with non-simultaneous 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 hundreds to thousands of seconds. Column density and intensity of Iron K$\alpha$ 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, masses in HMXBs more accurately based on absorption measurements and orbital parameters of the source. We then discuss the limitations posed by current X-ray spacecrafts 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 $\sim$ 36\,keV in the time-averaged or the phase-resolved spectra with NuSTAR.