The changing look of the neutrino-emitter blazar candidate 5BZQ J1243+4043

Abstract: Context. In recent years, changing-look blazars have put into question the traditional BL Lacs-flat spectrum radio quasars empirical classification of blazars based on the intensity of optical spectral lines, as they appear to transition between the two classes over time. Aims. We focus on the blazar 5BZQ J1243+4043, recently proposed as a promising candidate for the emission of high-energy neutrinos observed by the IceCube Neutrino Observatory and reported as a changing look in the literature. We study the spectral properties of this blazar, inferring its radiation field and accretion regime properties among different epochs. Methods. This study presents new optical spectroscopy observations of 5BZQ J1243+4043 taken with the Gran Telescopio Canarias. We use this new dataset and two optical spectra available from the literature to investigate the continuum and line emissions and pinpoint the physical properties of the source. Results. The newly collected data for 5BZQ J1243+4043 shows broad emission lines, consistent with the spectrum of the first epoch and the redshift $z = 1.5181\pm0.0002$ known from the literature. For the second epoch, the spectrum appears featureless, and we place limits on the emission lines and related physical properties. We observe spectral variability for both the continuum and line emissions among the three spectra. Despite this, the accretion properties of the blazar remain, overall, unvaried, indicating that the intrinsic physics remains the same across the three epochs. In the broader multi-messenger context, this suggests that, despite the changing look in the optical band, the candidate neutrino-emitter blazar 5BZQ J1243+4043 remains characterized by the presence of intense external radiation fields and radiatively efficient accretion, typical of high-excitation radio galaxies, that may foster neutrino production.