Fermi-LAT detection of record gamma-ray flare in BL Lacertae contemporaneous with record X-ray flare
I. Mereu (INFN Perugia) on behalf of the Fermi Large Area Telescope Collaboration
The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed record high gamma-ray activity from BL Lacertae (4FGL J2202.7+4216; The Fermi-LAT Collaboration 2020, ApJS, 247, 33). Preliminary analysis indicates that, since the flare we reported on 2020 August 9 (ATel #13933) and 2020 August 19 (ATel #13964), BL Lacertae has remained in a bright state. On 2020 October 6, it reached its highest daily averaged gamma-ray flux (E>100MeV) of (5+/-1) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only) since the beginning of Fermi-LAT operations in Aug 2008. The corresponding photon index was 2.1 +/- 0.1, which is consistent with the photon index of 2.23 +/- 0.01 reported in the 4FGL catalog. On 2020 October 5, BL Lacertae reached a daily averaged gamma-ray flux (E>100MeV) of (4.0+/-0.6) x 10^-6 photons cm^-2 s^-1. The corresponding photon index was 1.76+/-0.09, smaller than the 4FGL value, indicating a spectral hardening accompanying the flux increase. Swift observations performed on 2020 October 5 and 6 (ATel #14065, ATel #14069) found the source at the historical maximum level in X-rays and in one of the brightest observed optical and UV states.

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.
optical,ultra-violet,x-ray,gamma ray,agn,blazar,quasar