Non-thermal Emission in Sagittarius B?

TL;DR

Recent studies suggested non-thermal radio components in Sagittarius B, but new high-resolution data show the radio emission is likely dominated by thermal HII regions with complex spectral behavior.

Contribution

This paper provides new VLA observations at 327 MHz and 1.4 GHz, challenging previous claims of non-thermal emission in Sagittarius B by showing thermal components dominate the radio spectrum.

Findings

No evidence for large-scale non-thermal emission at observed frequencies

Spectral behavior explained by multiple thermal HII regions with diverse properties

Apparent spectral index influenced by summation of thermal components

Abstract

We summarize three recent publications which suggest that the Galactic center region Sagittarius B (Sgr B) may contain non-thermal radio components (Crocker et al. 2007, Hollis et al. 2007 and Yusef-Zadeh et al. 2007a). Based on new VLA matched-resolution continuum data at 327 MHz and 1.4 GHz, we find no evidence for large scale non-thermal radio emission at these frequencies; the spectral behavior is likely determined by the complex summation of multiple HII region components with a wide range of emission measures and hence radio turn-over frequencies. Also, we discuss a possible additional interpretation of the radio continuum spectrum of individual component Sgr B2-F carried out by Yusef-Zadeh et al; confusion from nearby HII components with widely different turn-over frequencies may contribute to the the change in slope of the radio continuum in this direction at low frequencies. Finally, we discuss the uncertainties in the determination of the spectral index of the GBT continuum data of Sgr B carried out by Hollis et al. We find that the apparent spectral index determined by their procedure is also likely due to a summation over the many diverse thermal components in this direction.