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

TL;DR

This study investigates accretion dynamics and spin evolution in the Be/X-ray binary RX J0520.5--6932 during its 2024 outburst using NICER and AstroSat for timing and broadband spectroscopy. Pulsations near $ u \approx 0.124$ Hz exhibit energy- and intensity-dependent profiles, with a red-noise PDS and seven short flares indicating rapid changes in accretion geometry. Broadband modelling requires a soft excess at $kT_{BB} \approx 0.09$ keV, a ~1 keV emission line, and a cyclotron line at $E_{\rm cyc} \approx 33$ keV, with $L_X \approx 2.2\times10^{38}$ erg s$^{-1}$ at 50 kpc. The spin evolution shows accretion-driven spin-up with $\dot{\nu} \approx 2.4\times10^{-11}$ Hz s$^{-1}$ and a declining torque ($\ddot{\nu} \approx -2.1\times10^{-17}$ Hz s$^{-2}$), consistent with magnetospheric accretion torque models and highlighting rapid changes in accretion flow during outburst.

Abstract

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. Our primary objective is 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. The AstroSat/LAXPC and NICER light curves reveal pronounced short-duration flaring activity lasting ~400-700 s, with enhancements by a factor of ~2. 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 reveals a soft excess and an emission feature at ~1 keV, likely arising from reprocessed emission in the accretion disc 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, a declining spin-up rate is observed during the outburst, likely due to a gradual reduction in mass accretion rate. 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 behaviour highlights rapid changes in the accretion geometry and torque as a function of accretion rate. [Abridged]

Figures (12)

• Figure 1: The 2--20 keV light curve of RX J0520.5--6932 during its 2024 outburst from MAXI-GSC binned at two days. The MAXI data points represent the source intensity, whereas the solid red vertical line marks the epoch of AstroSat observation, and the blue tick markers indicate the epoch of NICER pointings.
• Figure 2: The AstroSat/LAXPC light curve of RX J0520.5--6932 during its 2024 outburst in the 3--25 keV energy range, binned at 50 seconds. The bottom subplot highlights a zoomed-in segment of the light curve (from the second orbit) showing short-timescale flaring events, where different colors (red, cyan, and green) mark pre-flare, flare, and post-flare intervals, respectively. The bottom panel of each subplot displays the hardness ratio, defined as the ratio of count rates in the 8--25 keV and 3--8 keV energy bands. The shaded region highlights the detected flaring events.
• Figure 3: Power density spectrum of RX J0520.5--6932 in the 3--25 keV band obtained from AstroSat/LAXPC data. The continuum was modeled using a combination of three Lorentzian components representing broad noise features at different characteristic frequencies. Sharp peaks due to the neutron star's spin frequency and its harmonics were excluded from the fit.
• Figure 4: Spin frequency evolution of RX J0520.5--6932 during the 2024 outburst. Top: Barycentre-corrected spin frequencies from NICER, AstroSat, Fermi/GBM and NuSTAR, overlaid with the composite model $\nu(t) = \nu_{\rm int}(t) - \nu_{\rm orb}(t)$ (solid red line), which combines the intrinsic quadratic spin evolution with the fixed orbital parameters from Yang25 (Solution II). Middle: Spin frequencies after correcting for orbital motion. A quadratic model (solid line) fits all data points and captures the evolving spin-up trend, while a linear model (dashed line) fits only the early part of the outburst (excluding the final NICER point). Bottom: Residuals of the quadratic and linear fits. The residuals clearly favor the quadratic model, supporting the presence of a varying spin-up rate during the outburst.
• Figure 5: Energy-resolved dynamic pulse profiles of RX J0520.5--6932 obtained from AstroSat/LAXPC data during the 2024 outburst. The colormap represents the normalized pulse intensity across different energy bands. Strong energy dependence is evident, with the secondary peak most prominent in the 7--14 keV range and broadening of off-pulse appearing at higher energies. The energy-average pulse profile is overplotted as a dashed line.