Near-infrared observations of the 1E 1740.7-2942 field at the galactic center

1E 1740.7-2942 is thought to be a Cygnus X-1-like black hole near the Galactic center, although several important questions regarding its nature and possible connection to the narrow e⁺-e^- annihilation line seen from this region remain unanswered. Here we report on HK photometry of this field which shows that the distribution of near-infrared stars is consistent with a population lying near the Galactic center similar to that of the late-type Baade's window population with an average foreground extinction A_V = 30 ± 5 mag and a substantially greater value of A_V = 55 ± 5 mag associated with stars centered at the position of the molecular cloud enshrouding 1E 1740.7-294.2. We argue that the known distribution of bulge stars and the observed "shadowing" by the cloud constrain its position to lie within ∼300 pc of the Galactic center. We show that the implied total hydrogen column density in this direction is (1.05 ± 0.53) × 10²³ atoms cm^-2, of which (0.48 ± 0.24) × 10²³ cm^-2 is attributable to the cloud itself. This is consistent with the value required to account for the attenuation in the 4-300 keV spectrum of 1E 1740.7-2942 in its standard state, and we therefore conclude that 1E 1740.7-2942 is at least as distant as the molecular cloud. The total mass of the observed obscuring material associated with the cloud is (4.6 ± 2.3) × 10³ M_⊙, an order of magnitude below that derived under the assumption that the molecular cloud is predominantly composed of gas traced by HCO⁺ emission with an average number density greater than 5 × 10⁴ atoms cm^-3. The corresponding average cloud density of ∼5 × 10³ cm^-3 appears to be inconsistent with the value required to account for the narrow electron-positron line flux variations if the leptons trace out straight-line trajectories.