An OH(1720 MHz) Maser and a Nonthermal Radio Source in Sgr B2(M): A
  SNR--Molecular Cloud Interaction Site?
  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 $\approx3.5"\times1.2"$ and 25$"\times25"$ 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.
