Fermi-LAT detection of renewed gamma-ray activity from the blazar BL Lacertae
ATel #17039; Chiara Bartolini (University of Trento & INFN Bari), Ettore Bronzini (University of Bologna & INAF-AOS), Pietro Monti-Guarnieri (University of Trieste & INFN Trieste) on behalf of the Fermi Large Area Telescope Collaboration
on 14 Feb 2025; 09:17 UT
Credential Certification: Giovanni La Mura (giovanni.lamura@inaf.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 recent increasing gamma-ray activity from BL Lacertae, also known as 4FGL J2202.7+4216 (4FGL-DR4; Ballet et al. 2024, arXiv:2307.12546), with coordinates R.A. = 330.68038 deg, Dec. = +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 2025 February 12, with a daily averaged gamma-ray flux (E>100MeV) of (3.9+/-0.3) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), corresponding to a flux increase of a factor of about 10 relative to the average flux reported in 4FGL-DR4. The photon index is 1.9 +/- 0.1, corresponding to a significantly harder spectrum than the 4FGL-DR4 catalog value of 2.13 +/- 0.01. The spectral hardening resulted in the detection of five high-energy (E > 10 GeV) photons, having a probability of being associated with the source equal to p >= 0.999, the highest energy of which was 56 GeV. We have previously reported enhanced activity from this source in ATels #16849, #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.
