TeV and Multi-wavelength Observations of Mrk 421 in 2006-2008

TL;DR

This paper presents detailed TeV gamma-ray observations of Mrk 421 from 2006 to 2008, revealing flux variability, spectral changes, and multi-wavelength correlations, with implications for understanding blazar emission mechanisms.

Contribution

It provides the first extensive, high-accuracy TeV gamma-ray spectral and flux variability study of Mrk 421 over multiple years, combined with multi-wavelength data and SSC modeling.

Findings

Detected bright TeV flares reaching 10 Crab flux

Observed spectral hardening with increased flux

Achieved good spectral fits with one-zone SSC models

Abstract

We report on TeV gamma-ray observations of the blazar Mrk 421 (redshift of 0.031) with the VERITAS observatory and the Whipple 10m Cherenkov telescope. The excellent sensitivity of VERITAS allowed us to sample the TeV gamma-ray fluxes and energy spectra with unprecedented accuracy where Mrk 421 was detected in each of the pointings. A total of 47.3 hrs of VERITAS and 96 hrs of Whipple 10m data were acquired between January 2006 and June 2008. We present the results of a study of the TeV gamma-ray energy spectra as a function of time, and for different flux levels. On May 2nd and 3rd, 2008, bright TeV gamma-ray flares were detected with fluxes reaching the level of 10 Crab. The TeV gamma-ray data were complemented with radio, optical, and X-ray observations, with flux variability found in all bands except for the radio waveband. The combination of the RXTE and Swift X-ray data reveal spectral hardening with increasing flux levels, often correlated with an increase of the source activity in TeV gamma-rays. Contemporaneous spectral energy distributions were generated for 18 nights, each of which are reasonably described by a one-zone SSC model.