Title:	Probing spectral and timing properties of the X-ray pulsar RX J0440.9 + 4431 in the giant outburst of 2022–2023
Authors:	Mandal, Manoj Sharma, Rahul Pal, Sabyasachi Jaisawal, G. K. Gendreau, Keith C. Ng, Mason Sanna, Andrea Malacaria, Christian Tombesi, Francesco Ferrara, E. C. Markwardt, Craig B. Wolff, Michael T. Coley, Joel B.
Keywords:	accretion,accretion discs –stars magnetic field –stars neutron –pulsars individual RX J0440.9 + 4431
Issue Date:	12-Sep-2023
Publisher:	Oxford University Press on behalf of Royal Astronomical Society
Citation:	Monthly Notices of the Royal Astronomical Society, 2023, Vol.526, p771-781
Abstract:	The X-ray pulsar RX J0440.9 + 4431 went through a giant outburst in 2022 and reached a record-high flux of 2.3 Crab, as observed by Swift /BAT. We study the evolution of different spectral and timing properties of the source using NICER observations. The pulse period is found to decrease from 208 s to 205 s, and the pulse profile evolves significantly with energy and luminosity. The hardness ratio and hardness intensity diagram (HID) show remarkable evolution during the outburst. The HID turns towards the diagonal branch from the horizontal branch abo v e a transition (critical) luminosity, suggesting the presence of two accretion modes. Each NICER spectrum can be described using a cutoff power law with a blackbody component and a Gaussian at 6.4 keV. At higher luminosities, an additional Gaussian at 6.67 keV is used. The observed photon index shows negative and positive correlations with X-ray flux below and abo v e the critical luminosity , respectively . The evolution of spectral and timing parameters suggests a possible change in the emission mechanism and beaming pattern of the pulsar depending on the spectral transition to sub- and supercritical accretion regimes. Based on the critical luminosity, the magnetic field of the neutron star can be estimated in the order of 10 12 or 10 13 G, assuming different theoretical models. Moreo v er, the observ ed iron emission line evolves from a narrow to a broad feature with luminosity. Two emission lines originating from neutral and highly ionized Fe atoms are evident in the spectra around 6.4 and 6.67 keV (later is seen only in higher luminosities).
Description:	Open Access
URI:	http://hdl.handle.net/2289/8267
Alternative Location:	https://ui.adsabs.harvard.edu/abs/2023MNRAS.526..771M/abstract https://arxiv.org/abs/2306.08083 https://doi.org/10.1093/mnras/stad2767
Copyright:	2023, The Author(s)
Appears in Collections:	Research Papers (A&A)

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