Probing Galactic Center Black Hole Candidates with Far-Infrared Spectroscopy

Abstract

The narrow positron annihilation line source in the Galactic center has been identified with the X-ray source 1E 1740.7-2942, which is coincident in projection with a Galactic center molecular cloud and exhibits synchrotron jets in the radio spectrum. It has been suggested that the source is powered by Bondi-Hoyle accretion from the cloud onto a compact object. Here we show that, regardless of the source of the X-ray luminosity, the irradiation of the interstellar medium (ISM) in its vicinity by ~1 keV X-rays can generate substantial luminosities in a number of far-infrared atomic fine-structure lines, such as [O I] 63 μm, [C II] 158 μm, and [Si II] 35 μm. Similar predictions hold for the nearly identical (in luminosity, spectrum, and presence of radio jets) Galactic center source GRS 1758-258. The corresponding line fluxes will be easily detectable using the Infrared Space Observatory; if the sources are embedded in dense ISM, the observations will allow estimates of the gas density, column density, and volume filling factor in the vicinity of the X-ray sources, the X-ray luminosity of the source averaged over the last t ~ 100 yr, and will constrain models for accretion from the ISM.