Fermi-LAT gamma-ray flare in BL Lacertae contemporaneous with optical flaring activity
ATel #14330; Sara Cutini (INFN Perugia) on behalf of the Fermi-LAT collaboration
on 18 Jan 2021; 19:35 UT
Credential Certification: Dario Gasparrini (dario.gasparrini@asdc.asi.it)
Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Blazar
The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed high gamma-ray activity from BL Lacertae (4FGL J2202.7+4216; The Fermi-LAT Collaboration 2020, ApJS, 247, 33).
This activity coincides with optical brightening reported recently in ATel #14318.
On 2021 January 17 Fermi-LAT detected strong enhancement, with a flux = (4.2 +/-0.3) x 10^-6 photons cm^-2 s^-1 (E > 100 MeV) on a daily time scale, and (5.1 +/- 1.0) x 10^-6 photons cm^-2 s^-1 on the six-hour time scale (statistical uncertainties only). This flux is comparable to the 2020 October 6 (ATel #14072) maximum observed flux of BL Lacertae since the beginning of Fermi-LAT operations in Aug 2008 with a flux of (5+/-1) x 10^-6 photons cm^-2 s^-1 in the daily time scale.
The corresponding photon index was 1.8 +/- 0.1 which is smaller than the photon index of 2.23 +/- 0.01 reported in the 4FGL catalog, indicating a spectral hardening accompanying the flux increase.
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.
