Fermi Gamma-ray Space Telescope

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

Christo Venter
(Zorawar Wadiasingh, Alice K Harding, Andreas Kopp)


Black widow and redback systems are compact binaries in which the pulsar heats and may even ablate its low-mass companion by its intense wind of relativistic particles and emission. This wind drives mass loss from the companion, and an intra-binary shock forms as a site of particle acceleration. The Fermi Large Area Telescope (LAT) has detected about two dozen of these systems. Double-peaked X-ray light curves, interpreted as Doppler-beamed synchrotron shock emission, have also been observed from a number of binaries. The implied presence of relativistic leptons and a hot companion presents an ideal environment for the creation of inverse Compton emission that will be modulated at the binary period. We model the X-ray and gamma-ray spectral components from nearby spider binaries including diffusion, convection and radiative energy losses in an axially-symmetric, steady-state approach. Our new code simultaneously yields energy-dependent light curves and orbital phase-resolved spectra. By fitting the observed X-ray spectra and light curves, we can constrain certain model parameters in order to give a more robust prediction of the expected gamma-ray flux. Nearby pulsars with hot or flaring companions may be promising targets for the future Cherenkov Telescope Array (CTA) and GeV photons (in the off-peak phases of the pulsar light curve) may be detectable by Fermi LAT for optimistic parameter choices. Constraints on the inverse Compton emission will probe the particle acceleration at the shock as well as the pulsar wind content.