Fermi-LAT detection of renewed gamma-ray activity from the FSRQ 3C 345
ATel #17497; M. Giroletti (INAF-IRA), G. La Mura (INAF - Astronomical Observatory of Cagliari), C. Bartolini (University of Trento & INFN Bari), on behalf of the Fermi Large Area Telescope Collaboration
on 17 Nov 2025; 07:38 UT
Credential Certification: Marcello Giroletti (giroletti@ira.inaf.it)
Subjects: Gamma Ray, AGN, Blazar, Quasar
Referred to by ATel #: 17498
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 flat-spectrum radio quasar 3C 345, also known as 4FGL J1642.9+3948 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A. = 250.74504 deg, Dec. = +39.81028 deg (J2000; Xu et al. 2019, ApJS 242, 5), and redshift z=0.5934 (Albareti et al. 2015, MNRAS 452, 4153).
Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on November 15, 2025 , with a daily averaged gamma-ray flux (E>100MeV) of (1.2 +/- 0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 20 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT catalog (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546). The corresponding photon index is 1.9+/-0.1, indicating a significantly harder spectrum than the 4FGL-DR4 value of 2.42+/-0.02. The Fermi-LAT collaboration has previously reported gamma-ray flaring activity positionally consistent with this source in ATels #2226, #10453, #15313, #15613, and #16274.
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for 3C 345 can be accessed via the Fermi-LAT Light-Curve Repository and the Monitored Source List. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is Janeth Valverde (valverde@llr.in2p3.fr).
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.
