Study of long-term spectral evolution and X-ray and Gamma-ray correlation of blazars seen by HAWC

TL;DR

This study analyzes 6 years of HAWC data to investigate long-term spectral evolution and correlations between X-ray and gamma-ray emissions in blazars Mrk 421 and Mrk 501, revealing significant correlations and spectral behaviors.

Contribution

It provides the longest continuous VHE monitoring of blazars to date and demonstrates a linear correlation between X-ray and gamma-ray emissions, supporting multi-zone SSC models.

Findings

Long-term correlation observed in Mrk 501.

Strong linear correlation in Mrk 421's X-ray and gamma-ray fluxes.

Confirmation of harder-when-brighter spectral behavior in Mrk 421.

Abstract

The HAWC Observatory collected 6 years of extensive data, providing an ideal platform for long-term monitoring of blazars in the Very High Energy (VHE) band, without bias towards specific flux states. HAWC continuously monitors blazar activity at TeV energies, focusing on sources with a redshift of {z \lt 0.3}, based on the Third Fermi-LAT Catalog of High-Energy sources. We specifically focused our analysis on Mrk 421 and Mrk 501, as they are the brightest blazars observed by the HAWC Observatory. With a dataset of 2143 days, this work significantly extends the monitoring previously published, which was based on 511 days of observation. By utilizing HAWC data for the VHE {\gamma}-ray emission in the 300 GeV to 100 TeV energy range, in conjunction with Swift-XRT data for the 0.3 to 10 keV X-ray emission, we aim to explore potential correlations between these two bands. For Mrk 501, we found evidence of a long-term correlation. Additionally, we identified a period in the light curve where the flux was very low for more than two years. On the other hand, our analysis of Mrk 421 measured a strong linear correlation for quasi-simultaneous observations collected by HAWC and Swift-XRT. This result is consistent with a linear dependence and a multiple-zone synchrotron self-Compton model to explain the X-ray and the {\gamma}-ray emission. Finally, as suggested by previous findings, we confirm a harder-when-brighter behavior in the spectral evolution of the flux properties for Mrk 421. These findings contribute to the understanding of blazar emissions and their underlying mechanisms.