Detection Forecasting for Blazars at MeV Energies with Fermi-LAT Swift-BAT Cross Match

Abstract: Over the past two decades, the lack of an MeV $γ$-ray all-sky instrument has left an observational gap in blazar broadband emission. Recently, a new suite of MeV $γ$-ray instruments with sensitivity in the $10<sup>{-11}$ to $10<sup>{-12}$ $\mathrm{erg/cm<sup>{2}/s}$ range has been proposed, with the potential to far surpass the capabilities of previous all-sky MeV instruments. In this work, we forecast the capabilities of future MeV $γ$-ray instruments including COSI, GRAMS, newASTROGAM, and AMEGO-X to observe blazars. We pair total source count estimates of blazar luminosity functions with SED-modeled flux estimates of 76 blazars selected from a sample of 115 blazars cross-matched between the latest Fermi-LAT and Swift-BAT catalogs. We forecast that an instrument with a sensitivity limit of $10<sup>{-11}$ $\mathrm{erg/cm<sup>{2}/s}$ could detect 61 of the 76 SED-modeled blazars, and expect a total population of a few tens to a few hundreds of Flat-Spectrum Radio Quasars (FSRQs), as well as a few hundred BL Lacs (BLLs). An instrument with a sensitivity limit of $10<sup>{-12}$ $\mathrm{erg/cm<sup>{2}/s}$ could detect all 76 of the SED-modeled blazars, and we expect a total population of a few hundred to one thousand FSRQs and roughly one thousand BLLs. As MeV blazars have been observed to be extremely luminous and MeV $γ$-rays are minimally attenuated by the extragalactic background light, we use luminosity functions to estimate that blazars could be detected out to $z\approx6$. Lastly, we demonstrate detectability for blazars that have been proposed as neutrino-emitting candidates, as the MeV regime could provide a window into resolving lepto-hadronic degeneracies.