Timing and spectral properties of Be/Xray pulsar 4U 1901+03 during 2019 outburst

TL;DR

This study analyzes the timing and spectral behavior of the Be/X-ray pulsar 4U 1901+03 during its 2019 outburst, revealing changes in pulse profiles, spectral features, and accretion regimes using multiple space observatories.

Contribution

It provides new insights into the spectral features, pulse profile evolution, and accretion state transitions of 4U 1901+03 during an outburst, including phase-dependent absorption features.

Findings

Pulse profiles changed significantly with time and energy.

An absorption feature at 10 keV correlated with luminosity but remains uncertain.

A 30 keV absorption feature was detected and studied across pulse phases.

Abstract

We have studied the timing and spectral properties of the BeXB 4U 1901+03 during the 2019 outburst using \textit{NuSTAR}, \textit{Swift}, and \textit{NICER} observations. Flares are in all observations and were of tens to hundreds of seconds duration. Pulse profiles were changing significantly with time and the luminosity of the source. An increase in the height of the peak of the pulse profiles was observed with energy. The pulse fraction increases with energy and at the end of the outburst. The variation of the pulse profile with time indicates the transition of the pulsar in different accretion regimes. The absorption like feature at 10 keV shows a positive correlation with the luminosity and along with other spectral parameters this feature was also pulse phase dependent. As the distance to the source is not precisely known hence we cannot confirm this feature to be CSRF and also cannot ignore other possible explanations of the feature. Another absorption like feature about 30 keV was observed in the spectra of the last two \textit{NuSTAR} observations and has line energy of about 30.37$\pm$0.55 and 30.23$\pm$0.62 keV respectively. We have also studied the variation of the line energy, width, and optical depth of this feature with pulse phase. The softening of the spectrum along with the increase in pulse fraction at the end of the outburst and absence of pulsation after 58665.09 MJD suggest that pulsar has entered propeller phase, also abrupt decrease in \textit{Swift}-XRT flux supports the fact.