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

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 $Γ=1.53\pm0.02$. 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.

Figures (5)

• Figure 1: Panels a--g: $r$-band time-series photometry of 1FGL J0523.5$-$2529 during seven runs on the MDM 1.3 m from 2023 to 2026, folded according to the orbital ephemeris described in Section \ref{['sec:ephem']}. A log of these observations is given in Table \ref{['tab:optlog']}. The data points are color coded and labeled by the day of the month. A fixed model of ellipsoidal modulation (not a fit to the data) is superposed as an aid to visualizing deviations and variability. Panel h is the ZTF $r$-band data from an overlapping period, which is consistent with the MDM data except for an apparent $\approx 0.07$ mag offset in calibration.
• Figure 2: ATLAS forced photometry in the $o$ filter folded according to the orbital ephemeris described in Section \ref{['sec:ephem']}. Each panel contains either one or two observing seasons. A fixed model of ellipsoidal modulation (not a fit to the data) is superposed as an aid to visualizing deviations and long-term variability.
• Figure 3: Total XMM-Newton spectrum and power-law fit, with pn data in blue and MOS in black and red. The bottom panel shows the residuals from the best fit.
• Figure 4: XMM-Newton light curves in 200 s bins, with orbital phases marked. Top: The gap in the OM data is due to a telemetry drop that caused a telecommand failure of three of the 13 OM observations. Otherwise, the light curve is broadly consistent with the ellipsoidal model except perhaps for some flaring around day 732.5. Bottom: Background subtracted and summed X-ray count rates from the pn and MOS detectors showing a broad minimum around phase 0.25 and nearly continuous flaring.
• Figure 5: The X-ray light curve of Figure \ref{['fig:xmm']} in 400 s bins, with orbital phases marked.