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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Julius-Maximilians-Universität Würzburg, Fakultät für Physik und Astronomie, Institut für Theoretische Physik und Astrophysik, Lehrstuhl für Astronomie, Emil-Fischer-Str. 31, D-97074 Würzburg, Germany

^⋆ Corresponding author; dwkim@mpifr-bonn.mpg.de

Received: 2 August 2024
Accepted: 6 November 2024

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  ∝  ν^α) between the bands 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.