Fermi LAT detection of renewed gamma-ray activity from the BL Lac S5 1803+784

ATel #17776; F. Giacchino (USAL - Universidad de Salamanca, Spain, and INFN - Rome Tor Vergata, Sapienza University Rome) and P. Monti-Guarnieri (University of Trieste and INFN Trieste), on behalf of the Fermi Large Area Telescope Collaboration
on 6 May 2026; 19:24 UT
Credential Certification: Federica Giacchino (federica.giacchino@roma2.infn.it)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed renewed gamma-ray activity from a source positionally consistent with the BL Lac S5 1803+78, also known as 4FGL J1800.6+7828 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 270.19035 deg, Decl. = +78.46778 deg (J2000; Johnston et al. 1995, AJ, 110, 880), and redshift z=0.684 (Stickel et al. 1993, A&AS, 98, 393).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on May 05, 2026 with a daily averaged gamma-ray flux (E>100MeV) of (1.3+/-0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of more than 10 relative to the average flux reported in the fourth dara release of the fourth Fermi-LAT catalog (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546). This is the highest daily flux ever observed for this source by the LAT. The corresponding photon index is 2.27+/-0.09, and is consistent with the 4FGL value of 2.24+/-0.01. The Fermi-LAT Collaboration has previously reported flaring activity from this source in ATels #15292, #13633, #3322 and #2386.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. This source is one of the "LAT Monitored Sources" and consequently a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT is publicly available at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/S5_1803p78. A preliminary light curve can be accessed via the Fermi-LAT Light-Curve Repository at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J1800.6+7828. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is Tonia Venters (tonia.m.venters@nasa.gov).

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.