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

Abstract

Context. The HAGAR Telescope Array at Hanle, Ladakh has been regularly monitoring the nearby blazar Mkn 421 for the past seven years. 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 carried out 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 very high-energy (VHE) bands, and compute the correlations between various wavebands using the z-transformed discrete correlation function. We construct the spectral energy distribution (SED) contemporaneous with HAGAR observation seasons and fit this SED with a one-zone synchrotron self-Compton model to study the spectral variability. Results. The flux is found to be highly variable across all timescales. The variability is energy dependant and is maximum in the X-ray and VHE bands. A strong correlation is found between the Fermi-LAT (gamma) and radio bands and between Fermi-LAT and optical, but no correlation is found 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 behaviour. The SED can be well fit by a one-zone SSC model, and variations in the flux states can be attributed mainly to changes in the particle distribution. A strong correlation is seen between the break energy γ b of the particle spectrum and the total bolometric luminosity.

Broadband study of blazar 1ES 1959+650 during flaring state in 2016

“…Variability seems to increase with frequency from radio to Xrays and decrease in high energy γ−rays compared to hard Xrays. Similar trend was seen in Mkn 421 (Sinha et al 2016). …”

“…However no such correlation was found when source was observed in low state during 2007 -2011 (Aliu et al 2013). Spectral hardening with increase in flux is seen in several other blazars including Mkn 421 (Sinha et al 2016); Mkn 501 (Krawczynski et al 2000).…”

On the Origin of the Hard X-Ray Excess of High-Synchrotron-Peaked BL Lac Object Mrk 421

“…It is so bright that it can be detected by modern IACTs within several minutes (Aleksić et al 2015). Its X-ray and TeV emissions often show well-correlated variability, as do its optical and GeV emissions (e.g., Bartoli et al 2016;Kapanadze et al 2016;Ahnen et al 2016;Sinha et al 2016). Including γ-rays, its broadband SED can be well fitted by a one-zone synchrotron self-Compton (SSC) model (e.g., Cao & Wang 2013;Zheng et al 2014;Zhu et al 2016;Baloković et al 2016).…”

Investigating Multiwavelength Lognormality with Simulations: Case of Mrk 421

“…These were selected as they have long and continuous coverage with the least amount of gaps. In principle, this could be extended to include the other lightcurves in Sinha et al [5]. The radio data obtained from the OVRO are at 15 GHz from the publicly available lightcurves as a part of the Fermi blazar monitoring campaign [8].…”

“…Variability properties of Mrk 421 at different wavelengths have been derived from multiple observation campaigns listed before. Sinha et al [5] observed this source at the full range of wavelengths ranging from radio with Ovens Valley Radio Observatory (OVRO) data to The High Altitude GAmma Ray (HAGAR) telescope system at gamma-ray energies. Amongst other observables, the flux probability distribution or the flux PDF was also estimated from all these lightcurves by histogramming the observed fluxes.…”