PSR J1311-3430 is one of the most extreme examples of the ‘black widow’ pulsar class. Here a sub-stellar companion, tidally locked to the millisecond pulsar, orbits in 1.5hours. The pulsar heats the facing side of the companion to ~10,000 K. Since its discovery, J1311 has been observed to exhibit flares at X-ray and optical wavelengths. The peak energy flux during these flares is higher than the intercepted pulsar spindown power, implying an energy storage mechanism. We report on a set of simultaneous optical and X-ray observations with LCO (g’), XMM Newton (EPIC and OM u’), and NICER, designed to probe the flare SED and its emission mechanism.
We find that the X-ray flare spectra are well-fit by a power law model, but this power law can not be connected to the optical u’ and g’ band fluxes. This suggests that the flares have two separate emission components. The spectral index between the u’ and g’ band is close to zero, albeit with large uncertainties. Based on the limited information from these two optical bands, we suggest that the flares consist of a blackbody component with temperatures of ~13,000-25,000K, and a power law component in the X-ray. The power law is estimated to be the more energetic of the two, implying that it traces the accelerated particles, which heat a dense medium to produce a secondary thermal component. In this way, they may be analogous to solar flares, where reconnection in the companion magnetosphere generates non-thermal elections, which then heat the companion photosphere, creating thermal blackbody emission. Our planned observations had more broad-band optical coverage; with only g and u’ band fluxes, we are unable to fully constrain this component. Simultaneous multi-band optical observations (eg. ug’riz) will be needed to check our interpretation of the soft component, while hard X-ray observations above 10 keV could constrain the full extent of the power law emission.