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

¹ Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, 400 005 Mumbai, India
e-mail: atreyee@tifr.res.in
² Indian Institute of Astrophysics, II Block, Koramangala, 560 034 Bangalore, India
³ Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, 700 064 Kolkata, India
⁴ Institute for Theoretical Physics and Astrophysics, Universitat Wurzburg, 97074 Wurzburg, Germany
⁵ Nicolaus Copernicus Astronomical Center, 87-100 Torun, Poland
⁶ Dept. of Physics, University of the Free State, PO Box 339, 9300 Bloemfontein, South Africa

Received: 18 January 2016
Accepted: 11 March 2016

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.