Broadband study of the Be X-ray binary RX J0520.5-6932 during its outburst in 2024

TL;DR

This study presents a comprehensive broadband analysis of the 2024 outburst of the Be X-ray binary RX J0520.5-6932, revealing spectral features, luminosity variations, and complex pulse profiles with phase-dependent properties.

Contribution

It provides the first detailed broadband spectral and timing analysis of this source during its outburst, including constraints on the cyclotron line and orbital parameters, and reports novel energy-dependent pulse profile features.

Findings

Detection of a cyclotron line at 32.2 keV with no energy change since 2014

Observation of energy-dependent pulse profile shapes and pulsed fraction variations

Refined orbital period estimate of 24.39 days and evidence of long-term spin-down

Abstract

A new giant outburst of the Be X-ray binary RX J0520.5-6932 was detected and subsequently observed with several space-borne and ground-based instruments. This study presents a comprehensive analysis of the optical and X-ray data, focusing on the spectral and timing characteristics of selected X-ray observations. A joint fit of spectra from simultaneous observations performed by the X-ray telescope (XRT) on the Neil Gehrels Swift Observatory (Swift) and Nuclear Spectroscopic Telescope ARray (NuSTAR) provides broadband parameter constraints, including a cyclotron resonant scattering feature (CRSF) at 32.2(+0.8/-0.7) keV with no significant energy change since 2014, and a weaker Fe line. Independent spectral analyses of observations by the Lobster Eye Imager for Astronomy (LEIA), Einstein Probe (EP), Swift-XRT, and NuSTAR demonstrate the consistency of parameters across different bands. Luminosity variations during the current outburst were tracked. The light curve of the Optical Gravitational Lensing Experiment (OGLE) aligns with the X-ray data in both 2014 and 2024. Spin evolution over 10 years is studied after adding Fermi Gamma-ray Burst Monitor (GBM) data, improving the orbital parameters, with an estimated orbital period of 24.39 days, slightly differing from OGLE data. Despite intrinsic spin-up during outbursts, a spin-down of ~0.04s over 10.3 years is suggested. For the new outburst, the pulse profiles indicate a complicated energy-dependent shape, with decreases around 15 keV and 25 keV in the pulsed fraction, a first for an extragalactic source. Phase-resolved NuSTAR data indicate variations in parameters such as flux, photon index, and CRSF energy with rotation phase.