Multiwavelength Observations of the Blazar VER J0521+211 During an Elevated TeV Gamma-Ray State

TL;DR

This paper presents multiwavelength observations of the blazar VER J0521+211 during a high TeV gamma-ray flux state, revealing spectral variability, flux correlations, and constraining its redshift and emission models.

Contribution

It provides the first detailed multiwavelength analysis of VER J0521+211 during an elevated gamma-ray state, including redshift constraints and spectral modeling.

Findings

Peak gamma-ray flux reached ~37% of Crab Nebula flux.

Moderate correlation observed between X-ray and TeV gamma-ray fluxes.

Optical flux remained relatively unchanged during the flare.

Abstract

We report on a long-lasting, elevated gamma-ray flux state from VER J0521+211 observed by VERITAS, MAGIC, and Fermi-LAT in 2013 and 2014. The peak integral flux above 200 GeV measured with the nightly-binned light curve is $(8.8 \pm 0.4) \times 10^{-7} \;\text{ph}\;\text{m}^{-2}\; \text{s}^{-1}$, or ~37% of the Crab Nebula flux. Multiwavelength observations from X-ray, UV, and optical instruments are also presented. A moderate correlation between the X-ray and TeV gamma-ray fluxes was observed, and the X-ray spectrum appeared harder when the flux was higher. Using the gamma-ray spectrum and four models of the extragalactic background light (EBL), a conservative 95% confidence upper limit on the redshift of the source was found to be z<=0.31. Unlike the gamma-ray and X-ray bands, the optical flux did not increase significantly during the studied period compared to the archival low-state flux. The spectral variability from optical to X-ray bands suggests that the synchrotron peak of the spectral energy distribution (SED) may become broader during flaring states, which can be adequately described with a one-zone synchrotron self-Compton model varying the high-energy end of the underlying particle spectrum. The synchrotron peak frequency of the SED, as well as the radio morphology of the jet from the MOJAVE program, are consistent with the source being an intermediate-frequency-peaked BL Lac object.