Continuum sources from the THOR survey between 1 and 2 GHz
  We carried out a large program with the Karl G. Jansky Very Large Array
(VLA): "THOR: The HI, OH, Recombination line survey of the Milky Way". We
observed a significant portion of the Galactic plane in the first quadrant of
the Milky Way in the 21cm HI line, 4 OH transitions, 19 radio recombination
lines, and continuum from 1 to 2 GHz. In this paper we present a catalog of the
continuum sources in the first half of the survey (l=14.0-37.9deg and
l=47.1-51.2deg, |b|<1.1deg) at a spatial resolution of 10-25", with a spatially
varying noise level of ~0.3-1 mJy/beam. The catalog contains ~4400 sources.
Around 1200 of these are spatially resolved, and ~1000 are possible artifacts,
given their low signal-to-noise ratios. Since the spatial distribution of the
unresolved objects is evenly distributed and not confined to the Galactic
plane, most of them are extragalactic. Thanks to the broad bandwidth of the
observations from 1 to 2 GHz, we are able to determine a reliable spectral
index for ~1800 sources. The spectral index distribution reveals a
double-peaked profile with maxima at spectral indices of alpha = -1 and alpha =
0 , corresponding to steep declining and flat spectra, respectively. This
allows us to distinguish between thermal and non-thermal emission, which can be
used to determine the nature of each source. We examine the spectral index of
~300 known HII regions, for which we find thermal emission with spectral
indices around alpha = 0. In contrast, supernova remnants (SNR) show
non-thermal emission with alpha = -0.5 and extragalactic objects generally have
a steeper spectral index of alpha = -1. Using the spectral index information of
the THOR survey, we investigate potential SNR candidates. We classify the
radiation of four SNR candidates as non-thermal, and for the first time, we
provide strong evidence for the SNR origin of these candidates.
