2025A&A...701A.213E

The Be/X-ray binary pulsar RX J0440.9+4431 underwent a giant outburst in late 2022 that lasted three months. Insight-HXMT observed this source at several instances during the outburst in 2022-2023. We used these bright outburst observations of the pulsar to study its X-ray spectral and timing variability. The pulse profiles obtained at similar luminosities during the progress and declining phases of the outburst show a similar shape behavior. With the increase in source luminosity, the complex pulse profile with multiple peaks at low luminosity becomes a single peaked pulse profile at the high luminosity of the outburst peak. The phase-averaged spectra of the pulsar in 1-120 keV are explained with an absorbed cutoff power-law continuum model. During the outburst phases, we found evidence of a cyclotron resonance scattering feature in the spectra that varies in energy (∼33.6 - 41.6 keV) and has a broad linewidth of > 5 keV. In declining phases of the outburst, we also found hints of first cyclotron harmonic varying in 65-75 keV. The application of a thermal and bulk Comptonization model to the phase-averaged and phase-resolved spectra reveals a high surface magnetic field (B ∼ 10¹³ G) for the pulsar.