SiS in the circumstellar envelope of IRC +10126: maser and quasi-thermal
  emission
  We present new Effelsberg-100 m, ATCA, and VLA observations of rotational SiS
transitions in the circumstellar envelope (CSE) of IRC +10216. Thanks to the
high angular resolution achieved by the ATCA observations, we unambiguously
confirm that the molecule's J=1-0 transition exhibits maser action in this CSE,
as first suggested more than thirty years ago. The maser emission's radial
velocity peaking at a local standard of rest velocity of -39.862$\pm$0.065 km/s
indicates that it arises from an almost fully accelerated shell. Monitoring
observations show time variability of the SiS (1-0) maser. The two lowest-$J$
SiS quasi-thermal emission lines trace a much more extended emitting region
than previous high-J SiS observations. Their distributions show that the SiS
quasi-thermal emission consists of two components: one is very compact
(radius<1.5", corresponding to <3$\times 10^{15}$ cm), and the other extends
out to a radius >11". An incomplete shell-like structure is found in the
north-east, which is indicative of existing SiS shells. Clumpy structures are
also revealed in this CSE. The gain of the SiS (1-0) maser (optical depths of
about -5 at the blue-shifted side and, assuming inversion throughout the entire
line's velocity range, about -2 at the red-shifted side) suggests that it is
unsaturated. The SiS (1-0) maser can be explained in terms of ro-vibrational
excitation caused by infrared pumping, and we propose that infrared continuum
emission is the main pumping source.
