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

Abstract

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