Locating the gamma-ray emission site in Fermi/LAT blazars from correlation analysis between 37 GHz radio and gamma-ray light curves

TL;DR

This study uses cross-correlation analysis of radio and gamma-ray light curves in blazars to locate the gamma-ray emission site, revealing time lags, co-spatial activity, and differences between correlated and uncorrelated sources.

Contribution

It introduces a method to constrain gamma-ray emission regions in blazars through correlation analysis and stacking of light curves, providing new insights into emission site locations.

Findings

Significant correlations found in 26 blazars.

Gamma-ray peaks generally lead radio emission by 20-690 days.

Stacked analysis suggests emission region is about 7 parsecs away.

Abstract

We address the highly debated issue of constraining the gamma-ray emission region in blazars from cross-correlation analysis using discrete correlation function between radio and gamma-ray light curves. The significance of the correlations is evaluated using two different approaches: simulating light curves and mixed source correlations. The cross-correlation analysis yielded 26 sources with significant correlations. In most of the sources, the gamma-ray peaks lead the radio with time lags in the range +20 and +690 days, whereas in sources 1633+382 and 3C 345 we find the radio emission to lead the gamma rays by -15 and -40 days, respectively. Apart from the individual source study, we stacked the correlations of all sources and also those based on sub-samples. The time lag from the stacked correlation is +80 days for the whole sample and the distance travelled by the emission region corresponds to 7 pc. We also compared the start times of activity in radio and gamma rays of the correlated flares using Bayesian block representation. This shows that most of the flares at both wavebands start at almost the same time, implying a co-spatial origin of the activity. The correlated sources show more flares and are brighter in both bands than the uncorrelated ones.