Timing and spectral properties of the Be/X-ray pulsar 4U 1901+03 during its 2019 outburst

Abstract

We have studied the timing and spectral properties of the BeXB 4U 1901+03 during the 2019 outburst using NuSTAR, Swift, and 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 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 NuSTAR observations and has line energy of about 30.37 ± 0.55 and 30.23 ± 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 the pulsar has entered a propeller phase, also an abrupt decrease in Swift-XRT flux supports the fact.