PKS 1424-418: A persistent candidate source of the mm$-γ$-ray connection?

Abstract

We present a long-term strong correlation between millimeter (mm) radio and $γ$-ray emission in the flat-spectrum radio quasar (FSRQ) PKS 1424-418. The mm$-γ$-ray connection in blazars is generally thought to originate from the relativistic jet close to the central engine. We confirm a unique long-lasting mm$-γ$-ray correlation of PKS 1424-418 by using detailed correlation analyses and statistical tests, and we find its physical meaning in the source. We employed ~8.5 yr of (sub)mm and $γ$-ray light curves observed by ALMA and Fermi-LAT, respectively. From linear and cross-correlation analyses between the light curves, we found a significant, strong mm$-γ$-ray correlation over the whole period. We did not find any notable time delay within the uncertainties for the mm$-γ$-ray correlation, which means zero lag. The mm wave spectral index values (S$_ν$ $\propto$ $ν_α$) between the band 3 and 7 flux densities indicate a time-variable opacity of the source at (sub)mm wavelengths. Interestingly, the mm wave spectral index becomes temporarily flatter (i.e., $α$ > $-$0.5) when the source flares in the $γ$-rays. We relate our results with the jet of PKS 1424-418, and we discuss the origin of the $γ$-rays and opacity of the inner (sub)parsec-scale jet regions.