Kavli Affiliate: Deepto Chakrabarty
| First 5 Authors: Pragati Pradhan, Carlo Ferrigno, Biswajit Paul, Enrico Bozzo, Ileyk El Mellah
| Summary:
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.
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