XMM-Newton Observation and Optical Monitoring of the Candidate Redback Millisecond Pulsar 1FGL J0523.5$-$2529

TL;DR

This study presents simultaneous X-ray and optical observations of the candidate redback millisecond pulsar 1FGL J0523.5-2529, revealing flares, orbital modulation, and insights into the intrabinary shock and companion wind interactions.

Contribution

First simultaneous X-ray and optical monitoring of 1FGL J0523.5-2529, providing detailed analysis of its orbital modulation, flares, and shock properties, enhancing understanding of redback pulsar systems.

Findings

X-ray luminosity in an intermediate state with photon index of 1.53

Frequent flares superposed on broad orbital modulation

Optical light dominated by ellipsoidal modulation of the companion

Abstract

1FGL J0523.5$-$2529 is a Fermi selected redback millisecond pulsar candidate that exhibited luminous optical and X-ray flares in 2020-2021. We obtained a simultaneous X-ray and $U$-band observation with XMM-Newton in 2025, the first to cover the 16.5 hr orbit of 1FGL J0523.5$-$2529. The X-ray luminosity was in an intermediate state with a power-law photon spectral index of $\Gamma=1.53\pm0.02$. Frequent flares were superposed on a broad, single-peaked modulation, the latter characteristic of intrabinary shock models in which the shock front is wrapped around the pulsar. We speculate that density enhancements in the shocked companion wind cause flares, as well as variable optical recombination lines. The $U$-band light curve was dominated by ellipsoidal modulation of the nearly Roche lobe filling companion star, similar to that seen in ground-based optical photometry. We also used this effect in 10 years of ATLAS monitoring to improve the precision of the orbital period to 0.6881366(19) days. Considering that searches for radio pulsations from 1FGL J0523.5$-$2529 at all orbital phases have been unsuccessful, the shocked wind usually surrounds the pulsar.