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

The BL Lac object 4FGL J0955.1+3551 has been suggested as a possible source of ultra-energetic neutrinos detected by the IceCube observatory. The target was observed in 2020 January at the Large Binocular Telescope. Our spectroscopy (4100-8500 Å) 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 Å. From the source image, we are able to resolve the host galaxy for which we measure an absolute magnitude M(R) = -22.9 and R_e = 8 kpc, which are values which are typical of the host galaxies of BL Lacs.