Very-High-Energy and Ultraviolet Studies of Markarian 421 during 2007-2008

Abstract

We have investigated the very-high-energy (VHE) gamma-ray emission and the ultra- violet emission of Markarian 421 recorded during 2007-2008. The VHE dataset is reasonably spread out over nine successive months of observation, with the presence of periodic time gaps due to the moon phase. During this observational season, VERI- TAS recorded the Markarian 421 averaged emission (Fmean = 2.71 × 10^10 erg/cm2/s) and its historically largest known VHE flare (Fmax = 1.18 × 10^9 erg/cm2/s). The Swift/UVOT ultraviolet dataset consists primarily of three intensive observational periods corresponding to the VHE active emission states of Markarian 421. The analysis of this dataset has returned the averaged emissions (Fmean: 11.9, 12.6, 11.6 mJy) and the maximum seasonal emissions (Fmax: 14.7, 19.4, 17.8 mJy) of Markarian 421 in the three Swift/UVOT ultraviolet bands (UVW1, UVM2, UVW2), respectively. For VHE flux variations, there is evidence of correlation between flux increases and spectral hardening, obtaining a proportionality constant of 0.86 ± 0.42 over the VHE portion of the spectrum. We find no conclusive evidence for any correlation between the VHE and UV datasets. A multiwavelength study of the typical flare profiles has also been carried out by introducing structure function analysis and obtaining a value for the ratio of the rise and fall timescales of tau_ratio,VERITAS = 2.7 ± 0.7. We measure a significantly different profile for the flares at ultraviolet bands (tau_ratio,UVOT = 0.9±0.3). At both VHE and in the ultraviolet band, the power spectral density of Markarian 421 is effectively described by P (f ) proportional to f^-alpha with alpha = 2.6 ± 0.1, thus, located between the random walk noise (alpha = 2) and the flicker random noise (alpha = 3) behaviors.