Fermi-LAT detection of enhanced gamma-ray activity from the FSRQ 4C +51.37

ATel #17696; S. Rani (Michigan Technological University), T. Lewis (Michigan Technological University), P. Monti-Guarnieri (University of Trieste and INFN Trieste), on behalf of the Fermi Large Area Telescope Collaboration
on 24 Feb 2026; 01:25 UT
Credential Certification: Israel Martinez (imc@umd.edu)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed an enhanced gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar 4C +51.37, associated with 4FGL J1740.5+5211 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A. = 265.15407 deg, Dec. = 52.19539 deg (J2000), and a redshift of z = 1.381 (Xiao et al. 2022, ApJ, 936, 146).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on February 22, 2026, with a daily averaged gamma-ray flux (E > 100 MeV) of  (0.6 +/- 0.1) e-06 ph cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase by a factor of more than 10 relative to the average flux reported in the fourth Fermi-LAT catalog, Data Release 4 (4FGL-DR4; Ballet et al. 2023, arXiv:2307.12546). The corresponding photon index is 2.41 +/- 0.18, consistent within uncertainties with the 4FGL-DR4 value of 2.47 +/- 0.02. This is the highest daily flux observed by LAT from this source so far.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. We encourage multifrequency follow-up observations to investigate the nature and evolution of this flare. A preliminary light curve for 4FGL J1740.5+5211 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_J1740.5+5211. For this source, the LAT contact persons are S. Rani (rani@mtu.edu) and P. Monti-Guarnieri (pietro.monti-guarnieri@phd.units.it).

The Fermi-LAT is a pair-conversion telescope designed to cover the energy range from 20 MeV to beyond 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.