Maser Activity of Organic Molecules toward Sgr B2(N)

Abstract

At centimeter wavelengths, single-dish observations have suggested that the Sagittarius (Sgr) B2 molecular cloud at the Galactic Center hosts weak maser emission from several organic molecules, including CH _2 NH, HNCNH, and HCOOCH _3 . However, the lack of spatial distribution information on these new maser species has prevented us from assessing the excitation conditions of the maser emission as well as their pumping mechanisms. Here, we present a mapping study toward Sgr B2 north (N) to locate the region where the complex maser emission originates. We report the first detection of the Class I methanol (CH _3 OH) maser at 84 GHz and the first interferometric map of the methanimine (CH _2 NH) maser at 5.29 GHz toward this region. In addition, we present a tool for modeling and fitting the unsaturated molecular maser signals with non-LTE radiative transfer models and Bayesian analysis using the Markov Chain Monte Carlo approach. These enable us to quantitatively assess the observed spectral profiles. The results suggest a two-chain-clump model for explaining the intense CH _3 OH Class I maser emission toward a region with low continuum background radiation. By comparing the spatial origin and extent of maser emission from several molecular species, we find that the 5.29 GHz CH _2 NH maser has a close spatial relationship with the 84 GHz CH _3 OH Class I masers. This relationship serves as observational evidence to suggest a similar collisional pumping mechanism for these maser transitions.

Keywords

• Interstellar masers
• Radiative transfer
• Galactic center
• Spectral line identification