Spectral evolution of X-ray pulsar 4U 1901+03 during the 2019 outburst based on Insight-HXMT and NuSTAR observations

Abstract

We report on detailed spectral analysis of emission from X-ray pulsar 4U 1901+03 using the data obtained by the Insight-HXMT and NuSTAR observatories during the 2019 outburst of the source. Thanks to the extensive coverage of the outburst by Insight-HXMT, we were able to investigate spectral evolution of the source as a function of flux, and compare these results to the previous reports with focus on the properties of a putative absorption feature at around 10 keV. In particular, we demonstrate that the broadband X-ray continuum of 4U 1901+03 can be well described with a two-component continuum model without absorption line at 10 keV, which casts doubt on its interpretation as a cyclotron line. The high quality of the data has also allowed us to perform both phase-averaged and phase-resolved spectral analysis as function of luminosity. Finally, we performed a detailed investigation of another absorption feature in the spectrum of the source around 30 keV recently reported in the NuSTAR data. In line with the previous findings, we show that this feature appears to be significantly detected, albeit only in the pulse-phase resolved spectra at relatively high luminosities. The non-detection of the line in the averaged spectrum is possibly associated with the observed strong variations of its parameters with the pulse phase.

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

“…As observed by Nabizadeh et al (2020) and Coley et al (2019) when Obs3 and Obs4+Swift spectra were fitted some negative residuals were observed near 30 keV and fitting the spectra with absorption model we estimated the line energy about 30.37 and 30.23 keV for these observations respectively and were consistent with the value estimated by the authors. However, no negative residuals near 30 keV were observed in the first two NuSTAR spectra which were having higher flux compared to the last two observations.…”

“…In NuSTAR spectra weak residuals are observed around 10 keV due to tungsten L-edge of the NuSTAR optics (Madsen et al 2015;Fürst et al 2013). The 10 keV feature in other pulsars were present in the spectra of different instruments of different satellites (Coburn et al (2002)) like in the case of 4U 1901+03 where this feature was observed by RXTE (Reig & Milonaki 2016) and Insight -HMXT (Nabizadeh et al 2020), thus clearing doubt about the instrumental origin of the feature. We have also seen that the feature is present even if we used another continuum model COMPTT instead of CUTOFFPL.…”

“…We have also seen that the feature is present even if we used another continuum model COMPTT instead of CUTOFFPL. Nabizadeh et al (2020) showed that when NuSTAR Obs3 spectra were fitted bya two components model consisting of two POWERLAW*HIGHECUT along with GAUSSIAN and PHABS models no residuals were left near 10 keV and also no additional absorption model around 10 keV was needed when this two component model was used to fit Insight -HMXT spectra. However, the authors also argued that transition from the typical cutoff power-law spectral shape to twocomponent spectral shape occurs at low luminosities about 10 34−36 erg s −1 , which indicates that the source distance must be small.…”

“…The authors also estimated the distance to the pulsar to be 12.4±0.2 kpc. Nabizadeh et al (2020) studied the spectral evolution of the source using Insight -HXMT and NuSTAR observations. They also studied the 10 keV feature of the source and also observed 30 keV feature in the NuSTAR data.…”