An updated gamma-ray analysis of the Be-BH binary HD~215227

Abstract

We report an updated analysis of the gamma-ray source AGL J2241+4454 that was detected as a brief two-day flare in 2010 by the AGILE satellite. The high-energy emission of AGL J2241+4454 has been attributed to the binary system HD 215227, which consists of a Be star being orbited by a black hole making it the first known Be-black hole binary system. We have analyzed the AGILE data and find a gamma-ray flux of $(1.8\pm0.7)\times10^{-6}$ ph cm$^{-2}$ s$^{-1}$, in agreement with the initial report. Additionally, we examined data from the Fermi LAT over several time intervals including the two day flare, the folded orbital phase, and the entire mission ($\sim$6-years). We do not detect AGL J2241+4454 over any of these time periods with Fermi and find upper limits of $1.1\times10^{-7}$ ph cm$^{-2}$ s$^{-1}$ and $5.2\times10^{-10}$ ph cm$^{-2}$ s$^{-1}$ for the flare and the full mission, respectively. We conclude that the HD 215227 Be-black hole binary is not a true gamma-ray binary as previous speculated. While analyzing the Fermi data of the AGL J2241+4454 region, we discovered a previously unknown gamma-ray source with average flux of $(13.56\pm0.02)\times10^{-8}$ ph cm$^{-2}$ s$^{-1}$ that is highly variable on monthly timescales. We associate this emission with the known quasar 87GB 215950.2+503417.