The physical and chemical structure of Sagittarius B2 -- V. Non-thermal emission in the envelope of Sgr B2

TL;DR

This study investigates the non-thermal radio emission in the Sgr B2 molecular cloud's envelope, revealing extended non-thermal components likely caused by relativistic electrons accelerated via Fermi processes, distinct from thermal emission.

Contribution

First detailed analysis of non-thermal emission in Sgr B2 envelope, demonstrating Fermi acceleration as a plausible mechanism for relativistic electron production.

Findings

Detection of non-thermal radio emission with spectral index -1.2 to -0.4

Non-thermal emission is more extended and diffuse than thermal emission

Fermi acceleration model reproduces observed flux and spectral index

Abstract

The giant molecular cloud Sagittarius B2 (hereafter SgrB2) is the most massive region with ongoing high-mass star formation in the Galaxy. In the southern region of the 40-pc large envelope of SgrB2, we encounter the SgrB2(DS) region which hosts more than 60 high-mass protostellar cores distributed in an arc shape around an extended HII region. We use the Very Large Array in its CnB and D configurations, and in the frequency bands C (4--8 GHz) and X (8--12 GHz) to observe the whole SgrB2 complex. Continuum and radio recombination line maps are obtained. We detect radio continuum emission in SgrB2(DS) in a bubble-shaped structure. From 4 to 12 GHz, we derive a spectral index between -1.2 and -0.4, indicating the presence of non-thermal emission. We decompose the contribution from thermal and non-thermal emission, and find that the thermal component is clumpy and more concentrated, while the non-thermal component is more extended and diffuse. The radio recombination lines in the region are found to be not in local thermodynamic equilibrium (LTE) but stimulated by the non-thermal emission. The thermal free-free emission is likely tracing an HII region ionized by an O7 star, while the non-thermal emission can be generated by relativistic electrons created through first-order Fermi acceleration. We have developed a simple model of the SgrB2(DS) region and found that first-order Fermi acceleration can reproduce the observed flux density and spectral index.