Fermi-LAT detection of enhanced gamma-ray activity from the FSRQ PKS 1743+173

ATel #18081; P. Monti-Guarnieri (University of Trieste and INFN, Trieste), C. C. Cheung (NRL), P. Van Zyl (South African Radio Astronomy Observatory, SARAO), on behalf of the Fermi Large Area Telescope Collaboration
on 24 Sep 2026; 20:11 UT
Credential Certification: Giovanni La Mura (giovanni.lamura@inaf.it)

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The Large Area Telescope (LAT), one of the two instruments onboard the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar PKS 1743+173, also known as FL16Y J1745.7+1720 (Ballet et al. 2026, arXiv:2602.22148), with coordinates R.A. = 266.39670 deg, Dec. = +17.33373 deg (J2000; Johnston et al. 1995, AJ, 110, 880), and redshift z=1.702 (Pursimo et al 2013, ApJ, 767, 14). This source was not reported in the fourth Fermi-LAT catalog (Abdollahi et al. 2022, ApJS, 260, 53).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state from September 17-24, 2026, with a weekly averaged gamma-ray flux (E>100MeV) of (1.0 +/- 0.4) X 10^-7 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase by a factor of 20 relative to the average flux reported in the 16 years source list (FL16Y). The corresponding photon index is 2.1 +/- 0.2, indicating a harder spectrum than the FL16Y value of 2.5 +/- 0.1.

The transient was identified thanks to the method implemented within the Fermi-LAT Collaboration known as "Fermi All-sky Variability Analysis" 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). The report for this flare detection is available at: FAVA, week 946, flare 14.

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 persons are P. Monti-Guarnieri (pietro.monti-guarnieri@phd.units.it) and C. C. Cheung (chi.c.cheung2.civ@us.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.