An OH(1720 MHz) Maser and a Nonthermal Radio Source in Sgr B2(M): A SNR--Molecular Cloud Interaction Site?

Abstract

Sgr B2 is a well-known star forming molecular cloud complex in the Galactic center region showing evidence of high energy activity as traced by the K$\alpha$ neutral FeI line at 6.4 keV, as well as GeV and TeV $\gamma$-ray emission. Here we present VLA and GMRT observations with respective resolutions of $\approx 3.5"\times 1.2"$ and 25$"\times 25"$ and report the detection of an OH(1720 MHz) maser, with no accompanying OH 1665, 1667 and 1612 MHz maser emission. The maser coincides with a 150 MHz nonthermal radio source in Sgr B2(M). This rare class of OH(1720 MHz) masers or the so-called supernova remnant (SNR) masers, with no main line transitions, trace shocked gas and signal the interaction of an expanding SNR with a molecular cloud. We interpret the 150 MHz radio source as either the site of a SNR -- molecular gas interaction or a wind-wind collision in a massive binary system. The interaction of the molecular cloud and the nonthermal source enhances the cosmic-ray ionization rate, allows the diffusion of cosmic rays into the cloud and produces the variable 6.4 keV line, GeV and TeV $\gamma$-ray emission from Sgr B2(M). The cosmic ray electron interaction with the gas in the Galactic center can not only explain the measured high values of cosmic ray ionization and heating rates but also contribute to nonthermal bremsstrahlung continuum emission, all of which are consistent with observations.

15 pages, 4 figures, ApJL (in press)

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• Discovery of New Interacting Supernova Remnants in the Inner Galaxy
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• Very Large Array Observations of Galactic Center OH 1720 MHz Masers in Sagittarius A East and in the Circumnuclear Disk
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Cited by in corpus (5)

• The physical and chemical structure of Sagittarius B2 -- V. Non-thermal emission in the envelope of Sgr B2
• A Large--Scale Spectroscopic Survey of Methanol and OH Line Emission from the Galactic Center: Observations and Data
• NuSTAR and XMM-Newton observations of the Arches cluster in 2015: fading hard X-ray emission from the molecular cloud
• The physical and chemical structure of Sagittarius B2, VIII. Full molecular line survey of hot cores
• Self-modulation of cosmic rays in molecular clouds: Imprints in the radio observations