Probing accretion dynamics and spin evolution in the X-ray pulsar RX J0520.5–6932 during its 2024 outburst

¹ Inter-University Centre for Astronomy and Astrophysics (IUCAA) Ganeshkhind Pune 411007, India
² Raman Research Institute, C V Raman Avenue Sadashivanagar Bangalore 560080, India
³ Indian Institute of Science Education and Research (IISER) Mohali Punjab 140306, India
⁴ School of Physics & Astronomy, University of Southampton Southampton Hampshire SO17 1BJ, UK
⁵ Indian Institute of Astrophysics Koramangala II Block Bangalore 560034, India
⁶ Università degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu KM 0.7 09042 Monserrato, Italy
⁷ Max-Planck-Institut für extraterrestrische Physik Gießenbachstraße 1 D-85748 Garching bei München, Germany
⁸ National Astronomical Observatories, Chinese Academy of Sciences 20A Datun Road Beijing 100101, China
⁹ School of Astronomy and Space Science, University of Chinese Academy of Sciences 19A Yuquan Road Beijing 100049, China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 October 2025
Accepted: 16 November 2025

Abstract

Context. After nearly a decade of quiescence, the transient Be/X-ray binary pulsar RX J0520.5–6932 underwent an outburst in 2024. We performed X-ray monitoring of the source with NICER and AstroSat near the peak of the event.

Aims. Our primary objective was to investigate the energy and luminosity dependence of the pulsed emission, characterize the spin evolution, and study the broadband X-ray spectral properties of RX J0520.5–6932 during the outburst.

Methods. We extracted light curves and spectra from NICER and AstroSat observations carried out during the outburst. Pulsations were detected using epoch-folding techniques, enabling a detailed study of pulse-profile evolution as a function of energy and intensity. Broadband spectral modeling was performed using simultaneous data from SXT, LAXPC, and NICER. The spectra from individual NICER observations were used to study spectral variability.

Results. The AstroSat/LAXPC and NICER light curves reveal pronounced short-duration flaring activity lasting ∼400–700 s with enhancements in intensity by about a factor of two. The pulse profile exhibits a strong dependence on both energy and intensity, evolving from a simple single-peaked structure at low energies to complex multi-peaked shapes at intermediate energies and reverting to simpler morphologies at higher energies. Pulse profiles during the flares differ significantly from those in the persistent state, indicating changes in the pulsed beam pattern with a change in the intensity on a short timescale. Broadband spectral analysis revealed a soft excess and an emission feature at ∼1 keV likely arising from reprocessed emission in the accretion disk and fluorescence from Ne K and Fe L ions. Continuous NICER monitoring over nearly one orbital cycle enabled us to track spin evolution with accretion-driven spin-up and spectral variability in the soft X-ray band. Additionally, we observed a declining spin-up rate during the outburst, likely due to a gradual reduction in mass accretion rate.

Conclusions. Our results provide a comprehensive view of the complex accretion dynamics in RX J0520.5–6932 during its 2024 outburst. The strong variability in pulse shape and spin behavior highlights rapid changes in the accretion geometry and torque as a function of accretion rate.