Timing and Spectral Analysis of HMXB OAO 1657-415 with NuSTAR

TL;DR

This study provides a detailed timing and spectral analysis of the HMXB pulsar OAO 1657-415 using NuSTAR data, revealing a cyclotron absorption feature and insights into its magnetic field and spin behavior over 43 years.

Contribution

First robust detection of a cyclotron absorption feature in OAO 1657-415, enhancing understanding of its magnetic field and spectral properties.

Findings

Detected X-ray pulsations up to 70 keV

Identified a cyclotron absorption feature around 40 keV

Observed a long-term spin-down phase with increased spin period

Abstract

This work presents a comprehensive timing and spectral analysis of high-mass X-ray binary pulsar, OAO 1657-415 by using the observation made with Nuclear Spectroscopic Telescope Array (NuSTAR) on June 2019. During this observation, OAO 1657-415 exhibited X-ray variability by a factor of about 3. X-ray pulsations at 37.03322(14) s were observed up to 70 keV. OAO 1657-415 was undergoing a spin-down phase with $\dot{P} = 9(1) \times 10^{-8}$ s s$^{-1}$. This is an order of about 100 larger than the long-term spin-up rate. The pulse profile evolved marginally during the observation. We have discussed the long-term pulse period history of the source spanning a time-base of 43 years, including the latest Fermi/GBM data. The 3-70 keV source spectrum is described by a partially covered cutoff power-law, an Fe K$_{\alpha}$ line at 6.4 keV and a Compton shoulder at 6.3 keV. We report the presence of a cyclotron absorption feature around 40 keV, which is indicative of a surface magnetic field strength of $3.59 \pm 0.06 \ (1 + z)^{-1} \times 10^{12}$ and $3.29_{-0.22}^{+0.23} \ (1 + z)^{-1} \times 10^{12}$ G. This work shows the first robust presence of cyclotron absorption feature in the source.