On the correlation between X-rays and TeV gamma-rays in HBL Blazars

TL;DR

This study investigates the correlation between X-ray and TeV gamma-ray fluxes in HBL blazars, confirming a general linear relationship with notable exceptions during extreme flaring events, to understand emission mechanisms.

Contribution

It extends long-term multiwavelength correlation analysis to four HBL blazars, revealing consistent flux correlations and highlighting deviations during high-flux episodes, thus providing insights into emission processes.

Findings

Confirmed linear flux correlation in three blazars.

PKS 2155-304 shows a quadratic correlation during flares.

High-flux outliers suggest additional emission mechanisms.

Abstract

The gamma-ray emission in blazars can be attributed to the leptonic Synchrotron Self-Compton (SSC) model, photo-hadronic interactions, or a combination thereof. While evidence supports both models, their specific contributions remain uncertain. One supportive piece of evidence for the SSC model is the correlation between synchrotron and SSC fluxes in some blazar's Spectral Energy Distribution (SED), indicating the relative contributions of leptonic and hadronic mechanisms. Observational studies of the HBL blazar Markarian 421 over several years, spanning TeV gamma rays and X-rays, have reported a linear correlation across various timescales, which breaks at the highest gamma-ray fluxes. Extending this analysis to four High synchrotron peaked BL Lac (HBL) blazars -- Markarian 501, 1ES 1959+650, PKS 2155-304 and 1ES 2344+514.-- we utilize multiwavelength data from ground-based Imaging Atmospheric Cherenkov Telescopes (IACTs) for gamma rays and satellite observations for X-rays. Our long-term study confirms a linear correlation between fluxes across these energy bands, except for Markarian 501, which shows a correlation index of $1.45 \pm 0.01$. Notably, the exceptional flaring episode of PKS 2155-304 exhibits a correlation index of 2 with extreme values of gamma-ray fluxes. We observe outliers with high gamma-ray fluxes, suggesting the involvement of another mechanism, either of hadronic or leptonic origin. Finally, all other correlations exhibit alignment with a general correlation, suggesting a common acceleration mechanism among them with slight variations likely due to individual magnetic field strengths.