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