Long-term study of Mkn 421 with the HAGAR Array of Telescopes

TL;DR

This study presents a comprehensive 7-year long-term analysis of the blazar Mkn 421 using the HAGAR array, revealing energy-dependent variability, flux correlations across bands, and spectral modeling insights.

Contribution

It provides the first detailed long-term variability and spectral analysis of Mkn 421 across multiple wavelengths, including the detection of lognormal flux distribution and correlation patterns.

Findings

Flux variability is energy-dependent and maximal in X-ray and VHE bands.

Strong correlations exist between gamma-ray and radio/optical bands.

Flux distribution exhibits lognormal behavior, indicating multiplicative variability processes.

Abstract

Context:The HAGAR Telescope Array at Hanle, Ladakh has been regularly monitoring the nearby blazar Mkn 421 for the past 7yrs. Aims: Blazars show flux variability in all timescales across the electromagnetic spectrum. While there is abundant literature characterizing the short term flares from different blazars, comparatively little work has been done to study the long term variability. We aim to study the long term temporal and spectral variability in the radiation from Mkn 421 during 2009-2015. Methods: We quantify the variability and lognormality from the radio to the VHE bands, and compute the correlations between the various wavebands using the z-transformed discrete correlation function. We construct the Spectral Energy Distribution (SED) contemporaneous with HAGAR observation seasons and fit it with a one zone synchrotron self Compton model to study the spectral variability. Results: The flux is found to be highly variable across all time scales. The variability is energy dependant, and is maximum in the X-ray and Very High Energy (VHE) bands. A strong correlation is found between the Fermi-LAT (gamma) and radio bands, and between Fermi-LAT and optical, but none between Fermi-LAT and X-ray. Lognormality in the flux distribution is clearly detected. This is the third blazar, following BL~Lac and PKS~2155$+$304 to show this behavior. The SED can be well fit by a one zone SSC model, and variations in the flux states can be attributed mainly due to changes in the particle distribution. A strong correlation is seen between the break energy $\gamma_b$ of the particle spectrum and the total bolometric luminosity.