X-ray and Gamma-Ray Shock Emission from Spider Binaries

Radio, optical and X-ray follow-up of unidentified Fermi sources has expanded the number of known galactic-field "black widow" and "redback" millisecond pulsar (MSP) binaries from four to nearly 30. Several systems observed by Chandra, XMM, Suzaku, and NuSTAR exhibit double-peaked X-ray orbital modulation. Orbital modulation has also been detected recently from two MSP binaries by Fermi. Such emission is attributed to electrons accelerated in an intrabinary shock, which cool via synchrotron and inverse Compton channels, and whose emission is Doppler-boosted along the shock in a mildly relativistic bulk flow. We present new transport and radiation calculations that may simultaneously model orbital-phase-resolved light curves and spectra from UV to TeV energies. The structure of the orbital X-ray light curves depends upon the binary inclination, shock geometry, and accelerated particle spectrum, energy losses and transport in the shock environment. In particular, the spatial variation along the shock of the underlying electron power-law index yields energy-dependent light curves, motivating future high-energy phase-resolved spectroscopic studies to probe the unknown physics of pulsar winds and relativistic shock acceleration therein. By fitting the observed X-ray and gamma-ray spectra and light curves, we are able to constrain the particle spectrum and shock synchrotron emission spectrum downstream of the shock as well as observer geometry. Inverse-Compton emission from nearby pulsars with hot or flaring companions may be promising targets for the future Cherenkov Telescope Array (CTA) and may also be detectable by Fermi LAT for optimistic parameter choices.