Probing spectral and timing properties of the X-ray pulsar RX J0440.9+4431 in the giant outburst of 2022-2023

Abstract

The X-ray pulsar RX J0440.9+4431 went through a giant outburst in 2022 and reached a record-high flux of nearly 2.3 Crab, as observed by Swift/BAT. We study the evolution of different spectral and timing properties of the source using NICER observations during the outburst. The pulse period is found to decrease from 208 s to 205 s and the pulse profile evolved significantly during the outburst with energy and luminosity. The emission mechanism and beaming patterns may change significantly, which is related to the state transition of the source. The hardness ratio shows significant evolution during the outburst, and the hardness intensity diagram also shows two different branches. The HID turns towards the diagonal branch from the horizontal branch above the critical luminosity. The observed photon index shows a negative correlation with X-ray flux below the critical luminosity, which turns into a positive correlation above the critical luminosity. This indicates a spectral transition from the sub-critical to the super-critical regime. The magnetic field is estimated to be nearly 1.8 $\times$ 10$^{12}$ G using critical luminosity of $\sim$2.7 $\times$ 10$^{37}$ erg s$^{-1}$. NICER spectra can be described using a cutoff power-law model with a blackbody component and an additional 6.4 keV iron fluorescence line. The iron emission line evolves from a narrow to a broad feature with the increase in luminosity. The iron line flux is strongly correlated with the X-ray flux. Based on luminosity, the Fe band featured two emission lines at 6.4 and 6.67 keV originating from neutral and highly ionized Fe-atoms.