Fermi-LAT detection of a flaring gamma-ray source associated with the FSRQ B3 0151+474

ATel #17920; C. C. Cheung (NRL), P. Monti-Guarnieri (University of Trieste and INFN Trieste), G. La Mura (INAF - Astronomical Observatory of Cagliari), on behalf of the Fermi Large Area Telescope Collaboration
on 22 Jul 2026; 20:04 UT
Credential Certification: Teddy Cheung (Teddy.Cheung@nrl.navy.mil)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed gamma-ray emission from a source positionally consistent with the flat-spectrum radio quasar B3 0151+474, also known as FL16Y J0155.0+4742 (Ballet et al. 2026, arXiv:2602.22148), with radio coordinates R.A. = 28.73454 deg, Dec. = 47.72404 deg (J2000; Fey et al. 2004, AJ, 127, 3587), and redshift z=1.026 (Vermeulen & Taylor 1995, AJ, 109, 1983).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state from 2026 Jul 13-20, with a weekly averaged gamma-ray flux (E>100MeV) of (1.0 +/- 0.3) X 10^-7 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 40 relative to the average flux reported in the LAT 16-year Source List (FL16Y; Ballet et al. 2026, arXiv:2602.22148). The corresponding photon index is 1.9 +/- 0.2, indicating a significantly harder spectrum than the FL16Y value of 2.30 +/- 0.17. During this time interval, the highest-energy event associated with the source was a 14 GeV photon (with association probability p > 0.99) detected at 05:55:37.621 UT on Jul 20.

The transient was identified thanks to the method implemented within the Fermi-LAT Collaboration known as "Fermi All-sky Variability Analysis" (report link is: https://fermi.gsfc.nasa.gov/ssc/data/access/lat/FAVA/SourceReport.php?week=937&flare=21) that searches the sky for high-energy transients on weekly time scales (Ackermann et al. 2013, ApJ, 771, 57; Abdollahi et al. 2017, ApJ, 846, 34). Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is C.C. Cheung (Teddy.Cheung at nrl.navy.mil).

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.