NuSTAR and NICER observations of X Persei

Abstract

We have studied the timing and spectral properties of the persistent Be/X-ray pulsar namely, X Persei using the NuSTAR and NICER observations of 2019. The pulsar is continuously in spin-up phase since 2003 with a spin-up rate of ∼(-3.18±0.04)×10⁻⁴ $y{r}^{-1}$. A broadband 0.7-79 keV energy spectral fitting is performed using NuSTAR and NICER observations. It is found that the two component model considered here fits the spectrum more satisfactorily than the single-component model. An additional gabs model was used to fit an absorption-like feature found in the residuals of the spectrum fitted by the two components model. We interpret the absorption-like feature in the spectrum with a cyclotron line as reported in the literature. The centroid energy of the absorption-like feature is found to lie in the energy range of 32.5- 36.0 keV. The absorption feature found here is independent of the models used in fitting the spectrum. The broadband spectrum is also fitted making use of the physical bulk and thermal Comptonization model suggesting that thermal Comptonization dominates over bulk Comptonization.