Fermi-LAT detection of a gamma-ray flare of the blazar BL Lacertae coincident with an optical flare

ATel #13933; C. C. Cheung (NRL), on behalf of the Fermi Large Area Telescope Collaboration
on 10 Aug 2020; 18:05 UT
Credential Certification: Teddy Cheung (Teddy.Cheung@nrl.navy.mil)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed recent increasing gamma-ray activity from BL Lacertae (4FGL J2202.7+4216; The Fermi-LAT collaboration 2020, ApJS, 247, 33). Preliminary analysis indicates that BL Lacertae was detected on 2020 August 9, in a bright state with a daily averaged gamma-ray flux (E>100MeV) of (1.5+/-0.2) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only) and a corresponding photon index of 2.0 +/- 0.1, significantly smaller than the photon index reported in the 4FGL catalog of 2.23 +/- 0.01. The gamma-ray flare on August 9 is coincident with the optical maximum reported on the same day in ATel #13930.

Previous flaring activity in gamma-rays was reported in May 2019 (ATel #12718) when the source also reached a daily flux level (E > 100MeV) of (1.5 +/- 0.2) x 10^-6 photons cm^-2 s^-1.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. The source is included in the "LAT Monitored Sources" and consequently, a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT is publicly available ( https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/BL_Lac ). We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is Simone Garrappa (simone.garrappa at desy.de).

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.