First detection of the HSO radical in space?

We report the discovery of HSO towards several cold dark clouds. The detection is confirmed by the observation of the fine and hyperfine components of two rotational transitions in the protostellar core B1-b, using the Yebes 40 m and IRAM 30 m telescopes. Furthermore, all the fine and hyperfine components of its fundamental transition 1(0,?1) - 0(0,?0) at 39 GHz were also detected toward the cyanopolyyne peak of TMC-1. The measured frequencies were used to improve the molecular constants and predict more accurate line frequencies. We also detected the strongest hyperfine component of the 1(0,?1) - 0(0,?0) transition of HSO toward the cold dark clouds L183, L483, L1495B, L1527, and Lupus-1A. The HSO column densities were obtained using LTE models that reproduce the observed spectra. The rotational temperature was constrained to 4.5 K in B1-b and TMC-1 using the available Yebes 40 m and IRAM 30 m data. The obtained column densities range between 7.0x10(10) cm(-2) and 2.9x10(11) cm(-2), resulting in abundances in the range of (1.4-7.0) x 10(-12) relative to H-2. Our observations show that HSO is widespread in cold dense cores. However, more observations, together with a detailed comparison with other S-bearing species, are needed to constrain the chemical production mechanisms of HSO, which are not considered in current models.