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

TL;DR

This study demonstrates a persistent, strong correlation between millimeter radio and gamma-ray emissions in the blazar PKS 1424-418 over 8.5 years, suggesting linked jet processes near the central engine.

Contribution

It provides the first detailed long-term analysis confirming a zero-lag, millimeter-gamma-ray correlation in PKS 1424-418, revealing insights into jet opacity and emission mechanisms.

Findings

Strong, persistent mm-gamma-ray correlation with zero time delay.

Spectral index variations indicate changing opacity during flares.

Flatter spectral index correlates with gamma-ray activity.

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 ~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$_{\nu}$ $\propto$ $\nu_{\alpha}$) 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.