Changing Look of the Optical Spectrum of the MeV Blazar PKS 0446+112 (4FGL J0449.1+1121)

Following the high activity of the γ-ray Fermi source 4FGL J0449.1+1121 (PKS 0446+112), possibly associated with a IceCube neutrino event IC-240105A, we obtained optical spectroscopy with the Gran Telescopio Canarias of the counterpart. We detect a clear emission line at 3830 Å identified as Lyα that confirms the redshift of source at z = 2.153. Comparing with previous spectroscopy, we find an increase of the continuum by a factor ∼10, and a significant decrease of the C IV 1550 emission line flux by a factor ∼5. This produces a dramatic drop of the equivalent width from ∼20 to 0.8 Å, which is suggestive of a very high jet activity. The full width half maximum of the emission lines are midway (1000–2000 km s ^‑1) between those typical of the broad and narrow regions of quasars. Based on this, the source classification is intermediate between flat spectrum radio quasar and BL Lac object.