The origin of X-ray emission from a Galactic center molecular cloud: low-energy cosmic-ray electrons

The Galactic center region near l ≈ 0°.2 hosts a mixture of nonthermal linear filaments and thermal radio continuum features associated with the radio arc. Chandra observations of this region reveal an X-ray filament and diffuse emission with an extent of roughly 60″ × 2″ and 5′ × 3′, respectively. The X-ray filament lies at the edge of the nonthermal radio filaments and the dense molecular shell G0.13 - 0.13 that has an unusually high kinetic temperature ≥70 K. These observations demonstrate that the G0.13-0.13 molecular cloud and the non-thermal radio filaments of the arc are interacting. The diffuse X-ray emission is correlated with the molecular shell and is fitted either by two-temperature (1 and 10 keV) thermal emission or by power-law and 1 keV thermal gas. Fluorescent 6.4 keV line emission is also detected throughout the molecular shell. This cloud coincides within the error circle of a steady unidentified EGRET source, 3EG J1746-2851. We argue that low-energy cosmic-ray electrons produce the power-law continuum by bremsstrahlung and 6.4 keV line emission from the filament and the diffuse cloud with the implication on the origin of the Galactic ridge X-ray emission. The strong 6.4 keV Fe line emission seen from other Galactic center clouds could be produced in a similar fashion rather than via fluorescent emission induced by a transient hard X-ray source in the Galactic center. In addition, heating by ionization induced by low-energy cosmic-ray electrons are also responsible for the high temperature of G0.13-0.13. The gamma-ray source is a result of bremsstrahlung by the high-energy tail of the electron energy distribution.