A Multiwavelength Study of a Long-duration Very-high-energy Flare from BL Lacertae with VERITAS

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Abstract We report the first observations of a long-duration very-high-energy (VHE; E>100 GeV) flare from BL Lacertae (VER J2202+422), taken with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). On 2022 October 15, the Fermi-Large Area Telescope (LAT) detected elevated GeV activity originating from this blazar. This triggered a multiwavelength campaign, which includes observations from VERITAS, Swift, NuSTAR, and select optical and radio observatories. VERITAS observed the source for a total of ∼9.8 hr between 2022 September 1 and 2022 December 1. An analysis of these data yields a ∼28 σ detection of the source. While previously observed VHE flares from BL Lacertae have lasted on timescales of minutes to days, VERITAS continued to detect flaring activity from the source for over a month (∼40 days) after the original flaring activity was detected with Fermi-LAT. Broadband spectral modeling shows that a synchrotron self-Compton (SSC) model with an external inverse-Compton component is preferred over a one-zone SSC model.

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