Fermi-LAT detection of enhanced gamma-ray activity from the FSRQ PKS 2155-83

ATel #17229; Adithiya Dinesh (Universidad Complutense de Madrid), Giovanni La Mura (INAF-O. A. Cagliari), on behalf of the Fermi Large Area Telescope Collaboration
on 15 Jun 2025; 17:45 UT
Credential Certification: Adithiya Dinesh (adinesh@ucm.es)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar PKS 2155-83, also known as 4FGL J2201.5-8339 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 330.58014 deg, Dec. = -83.63662 deg (J2000; Petrov et al., 2011, MNRAS, 414, 2528), and redshift z=1.867 (Chen et al., 2024, ApJS, 271, 20).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on June 13, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (1.1+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 30 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 2.5+/-0.2, and is consistent with the 4FGL-DR4 value of 2.50+/-0.02 within the uncertainties. The Fermi-LAT Collaboration has previously reported flaring activity from this source in ATel #2373.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for PKS 2155-83 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 Eric Wallace (wallacee@uw.washington.edu).

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.