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

TL;DR

This study reports a significant change in the optical spectrum of the MeV Blazar PKS 0446+112, indicating high jet activity and an intermediate classification between FSRQ and BL Lac, linked to its gamma-ray activity.

Contribution

First optical spectroscopic confirmation of PKS 0446+112's redshift at z=2.153 and analysis of spectral variability related to jet activity.

Findings

Detected Lyα emission line confirming redshift

Observed a tenfold increase in continuum brightness

Noted a fivefold decrease in CIV emission line flux

Abstract

Following the high activity of the gamma-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 Ang identified as Ly$\alpha$ that confirms the redshift of source at z=2.153. Comparing with previous spectroscopy, we find an increase of the continuum by a factor $\sim$10, and a significant decrease of the CIV 1550 emission line flux by a factor $\sim$5. This produces a dramatic drop of the Equivalent Width from $\sim$20 Ang to 0.8 Ang, which is suggestive of a very high jet activity. The Full Width Half Maximum of the emission lines are midway (1000 - 2000 km/s) 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.