Nonthermal radio emission from the galactic center arc

Abstract

Observations at three frequencies - 160 MHz, 327 MHz, and 4.8 GHz - of the continuum arc near the galactic center are presented and discussed. A map of the 160 MHz radio continuum and another of the polarized 4.8 GHz emission indicate clearly that nonthermal processes are responsible for the emission arising from one limited segment of the arc: that arising from G0.16-0.15, a broad intensity maximum situated on the southern half of the system of parallel filaments oriented perpendicular to the galactic plane. Other portions of this vertical system of filaments have radio characteristics consistent with thermal emission. A model is developed in which the filaments forming the core of the arc are uniformly nonthermal emitters, but are surrounded by a nonuniform distribution of thermal plasma which preferentially absorbs low-frequency radiation, thus flattening the nonthermal spectrum, and depolarizes the nonthermal emission from the filaments everywhere except in the vicinity of G0.16-0.15. 23 references.