Galactic Electron-Positron Annihilation Emission from X-Ray Binaries with Transient and Persistent Jets

Abstract

The Galactic Center exhibits a persistent 0.511 MeV emission characteristic of electron-positron annihilation. Various astrophysical sources may be responsible for supplying the positrons, yet their contributions with respect to the total annihilation rate is uncertain. X-ray binaries have historically been a primary candidate, possibly contributing a significant fraction of the Galactic positrons. X-ray binaries can produce positrons through the Blandford-Znajek mechanism, two-photon pair production in the corona, and/or one-photon pair production for strongly-magnetized compact objects. These positrons may then be channeled into the Interstellar Medium through jets or winds. In this work, we argue that X-ray binaries cannot contribute to the electron-positron annihilation flux alone. We present 0.511 MeV line fluxes and all-sky maps produced by X-ray binaries and stellar flares from the Galactic stellar population, along with the gamma-ray continuum from positrons produced by X-ray binaries. To do so, we solve the particle transfer equation using Green's functions. The total morphology is slightly asymmetric towards negative longitudes and positive latitudes. We find five 0.511 MeV flux hotspots in the generated all sky maps and scrutinize the contribution of individual sources to these hotspots. Observations of these hotspots can reveal the contribution of X-ray binaries to the Galactic positrons. The upcoming Compton Spectrometer and Imager (COSI) mission could image a hotspot near Cygnus X-1. The detection of the other hotspots requires a telescope with improved arcminute angular resolution.

25 pages, 9 figures, submitted to The Astrophysical Journal