The Exceptional 2017 γ -Ray Flare of the Radio Galaxy NGC 1275: VERITAS and Multiwavelength Observations

The radio galaxy NGC 1275 is the brightest cluster galaxy in the Perseus Cluster. It is well studied across all wave bands, including very high-energy (VHE; E ≥ 100 GeV) γ -rays, and with radio observations over the last 20 yr tracking an unusual radio component, “C3.” NGC 1275 was observed in an exceptional VHE-flaring state between 2016 December 31 and 2017 January 3. The flare peak reached ∼1.5 Crab units as measured by the MAGIC observatory. We report on the observations of NGC 1275 conducted by VERITAS and multiwavelength data collected during this flaring state and, for context, data taken between 2009 and 2017 inclusively. VERITAS detected the declining state of the flare on 2017 January 2 (MJD 57755) and 3 (MJD 57756) at an average flux state of 0.5 Crab units. VERITAS spectra show an overall long-term trend of harder when brighter. During the flare, the γ -ray spectrum obtained from the combined Fermi-LAT, MAGIC, and VERITAS observations changes from a power law with an exponential cutoff on January 1 to a log-parabola on January 2. To study the evolution of the flare in more detail, multiband spectral energy distributions (SEDs) were constructed for the nights of 2017 January 1 and 2 corresponding to the shift from the peak to the decline of the flare. A blob-in-jet modeling of the SEDs results in support for a two-component model with a θ = 10 ∘ jet angle to the line of sight and the γ -ray emission zone located in the vicinity of the C3 radio component.