Luminous Optical and X-ray Flaring of the Putative Redback Millisecond Pulsar 1FGL J0523.5$-$2529

Abstract: Several redback and black widow millisecond pulsar binaries have episodes of flaring in X-rays and optical. We initially detected such behavior from the Fermi selected redback candidate 1FGL J0523.5$-$2529 during optical time-series monitoring. Triggered observations with the Neil Gehrels Swift Observatory over the next $\approx100$ days showed episodic flaring in X-rays with luminosity up to $8\times10^{33}$ erg s$^{-1}$ ($\sim100$ times the minimum), and a comparable luminosity in the optical/UV, with similar power-law spectra of $f_{\nu}\propto\nu^{-0.7}$. These are the most luminous flares seen in any non-accreting "spider" pulsar system, which may be related to the large size of the companion through the fraction of the pulsar wind that it or its ablated wind intercepts. Simultaneously with an optical flare, we see Balmer-line and He I emission, not previously known in this object, which is evidence of a stellar wind that may also inhibit detection of radio pulsations. The quiescent optical light curves, while dominated by ellipsoidal modulation, show evidence of variable non-uniform temperature that could be due either to large starspots or asymmetric heating of the companion by the pulsar. This may explain a previous measurement of unusual non-zero orbital eccentricity as, alternatively, distortion of the radial-velocity curve by the surface temperature distribution of the large companion.