Fermi-LAT detection of enhanced gamma-ray activity from the blazar BL Lacertae

ATel #16849; P. V. van Zyl (SARAO), G. La Mura (INAF-O. A. Cagliari), Denis Bernard (LLR, Ecole Polytechnique & CNRS / IN2P3), on behalf of the Fermi Large Area Telescope Collaboration
on 6 Oct 2024; 23:27 UT
Credential Certification: Janeth Valverde (valverde@llr.in2p3.fr)

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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), with coordinates R.A. = 330.68038 deg, Decl. = +42.27778 deg (J2000; Johnston et al. 1995, AJ, 110, 880), and redshift z = 0.0686 (Vermeulen et al. 1995, ApJ, 452, L5).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on 2024 October 5, with a daily averaged gamma-ray flux (E>100MeV) of (10.4+/-0.5) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), corresponding to a flux increase of a factor of about 20 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL-DR4). The photon index is 1.9 +/- 0.1, corresponding to a significantly harder spectrum than the 4FGL catalog value of 2.13 +/- 0.01. The spectral hardening resulted in the detection of nine high-energy (E > 10 GeV) photons, having probability to be associated with the source larger than p = 0.999, the highest energy of which was close to 180 GeV. We have previously reported enhanced activity from this source in ATels #15688, #14330 and #14072.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. Preliminary light curves for BL Lacertae can be accessed via the Fermi-LAT Monitored Source List at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/BL_Lac; and via the Fermi-LAT Light-Curve Repository at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/lcr/source.html?source_name=4FGL_J2202.7+4216. 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.