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 (similar to 100 deg(2)) of the Galactic plane in the first quadrant of the Milky Way in the 21 cm 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 = 14.0-37.9 degrees and l = 47.1-51.2 degrees, vertical bar b vertical bar <= 1.1 degrees) at a spatial resolution of 10-25 '', depending on the frequency and sky position with a spatially varying noise level of similar to 0.3-1 mJy beam(-1). The catalog contains similar to 4400 sources. Around 1200 of these are spatially resolved, and similar to 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 similar to 1800 sources. The spectral index distribution reveals a double-peaked profile with maxima at spectral indices of alpha approximate to -1 and alpha approximate to 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 similar to 300 known HII regions, for which we find thermal emission with spectral indices around alpha approximate to 0. In contrast, supernova remnants (SNR) show non-thermal emission with alpha approximate to -0.5 and extragalactic objects generally have a steeper spectral index of alpha approximate to -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.