The redshift and the host galaxy of the neutrino candidate 4FGL J0955.1+3551 (3HSP J095507.9+355101)

The BL Lac object 4FGLJ0955.1+3551 has been suggested as a possible source of ultra energetic neutrinos detected by the Icecube observatory. The target was observed in January 2020 at the Large Binocular Telescope. Our spectroscopy (4100-8500 Ang) yields a firm redshift z = 0.557 as deduced by the absorption lines of the host galaxy. The upper limit of the minimum equivalent width on emission lines is 0.3 Ang. From the source image we are able to resolve the host galaxy for which we measure an absolute magnitude M® = -22.9 and Re = 8 kpc, that is values which are typical of the host galaxies of BL Lacs.