Title:	Probing the energy and luminosity-dependent spectro-timing properties of RX J0440.9+4431 with AstroSat
Authors:	Sharma, Rahul Mandal, Manoj Pal, Sabyasachi Paul, Biswajit Jaisawal, G.K. Ratheesh, Ajay
Issue Date:	1-Oct-2024
Publisher:	Oxford University Press for Royal Astronomical Society
Abstract:	The Be/X-ray binary pulsar RX J0440.9+4431 went through a giant outburst in December 2022 with a peak flux of ∼∼2.3 Crab in 15--50 keV. We studied the broad-band timing and spectral properties of RX J0440.9+4431 using four AstroSatAstroSat observations, where the source transited between subcritical and supercritical accretion regimes. Pulsations were detected significantly above 100 keV. The pulse profiles were found to be highly luminosity- and energy-dependent. A significant evolution in the pulse profile shape near the peak of the outburst indicates a possible change in the accretion mode and beaming patterns of RX J0440.9+4431. The rms pulsed fraction was luminosity- and energy-dependent, with a concave-like feature around 20--30 keV. The depth of this feature varied with luminosity, indicating changes in the accretion column height and proportion of reflected photons. The broad-band continuum spectra were best fitted with a two-component Comptonization model with a blackbody component or a two-blackbody component model with a thermal Comptonization component. A quasi-periodic oscillation at 60 mHz was detected at a luminosity of 2.6×10372.6×1037 erg s−1−1, which evolved into 42 mHz at 1.5×10371.5×1037 erg s−1−1. The QPO rms were found to be energy dependent with an overall increasing trend with energy. For the first time, we found the QPO frequency varying with photon energy in an X-ray pulsar, which poses a challenge in explaining the QPO with current models such as the Keplarian and beat frequency model. Hence, more physically motivated models are required to understand the physical mechanism behind the mHz QPOs.
Description:	Open Access
URI:	http://hdl.handle.net/2289/8338
Alternative Location:	https://ui.adsabs.harvard.edu/abs/2024MNRAS.534.1028S/abstract https://inspirehep.net/literature/2829374 http://arxiv.org/abs/2409.11121 https://doi.org/10.1093/mnras/stae2175
Copyright:	2024 The Author(s)
Appears in Collections:	Research Papers (A&A)

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