Evolution of timing and spectral characteristics of 4U 1901+03 during its 2019 outburst using the Swift and NuSTAR observatories

TL;DR

This study analyzes the timing and spectral properties of the X-ray pulsar 4U 1901+03 during its 2019 outburst, revealing a luminosity-dependent cyclotron resonance feature at 30 keV and phase-dependent spectral characteristics.

Contribution

First detailed broadband timing and spectral analysis of 4U 1901+03 during its 2019 outburst, identifying a luminosity and phase-dependent cyclotron line at 30 keV.

Findings

Detection of a cyclotron resonance scattering feature at ~30 keV.

Pulse profiles vary with luminosity and energy.

Strong pulse phase dependence of spectral parameters.

Abstract

We report the results from a detailed timing and spectral study of transient X-ray pulsars, 4U 1901+03 during its 2019 outburst. We performed broadband spectroscopy in the 1-70 keV energy band using four observations made with Swift and NuSTAR at different intensity levels. Our timing results reveal the presence of highly variable pulse profiles dependent on both luminosity and energy. Our spectroscopy results showed the presence of a cyclotron resonance scattering feature (CRSF) at ~30 keV. This feature at 30 keV is highly luminosity and pulse-phase dependent. Phase-averaged spectra during the last two observations, made close to the declining phase of the outburst showed the presence of this feature at around 30 keV. The existence of CRSF at 30 keV during these observations is well supported by an abrupt change in the shape of pulse profiles found close to this energy. We also found that 30 keV feature was significantly detected in the pulse-phase resolved spectra of observations made at relatively high luminosities. Moreover, all spectral fit parameters showed a strong pulse phase dependence. In line with the previous findings, an absorption feature at around 10 keV is significantly observed in the phase-averaged X-ray spectra of all observations and also showed a strong pulse phase dependence.