Fermi-LAT detection of further gamma-ray brightening in the radio galaxy NGC 1275

ATel #17943; P. Monti-Guarnieri (University of Trieste and INFN Trieste), A. Zaccaro (University of Bari and INFN Bari), on behalf of the Fermi Large Area Telescope Collaboration
on 5 Aug 2026; 20:12 UT
Credential Certification: Giovanni La Mura (giovanni.lamura@inaf.it)

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In ATel #17937, we reported the Fermi-LAT detection of renewed activity from the radio galaxy NGC 1275, with a daily averaged gamma-ray flux (E>100MeV) of (1.8+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), observed on August 01, 2026.

Preliminary analysis indicates that this source further brightened on August 04, 2026, reaching a daily averaged gamma-ray flux (E>100MeV) of (4.5 +/- 0.3) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of more than 10 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). This is a factor of 2.5 brighter than observed on August 01. The corresponding photon index is 1.98 +/- 0.05, indicating a harder spectrum than the 4FGL-DR4 value of 2.12 +/ 0.01. This is the highest LAT daily flux ever observed for this source. Further analysis on shorter timescales reveals that the peak 6-hr flux was (6.0+/-0.7) X 10^-6 photons cm^-2 s^-1, observed in the time interval from 00:00:00 UT to 06:00:00 UT on August 04.

The spectral hardening led to the detection of nine high-energy photons associated with the source at a confidence level p > 0.999. The highest energy event was a 100 GeV photon observed on August 04 at 04:21:08 UT. Further likelihood analysis of the >10 GeV data over the period from August 03, 23:00:00 UT to August 04, 05:00:00 UT revealed a flux of (1.5 +/- 0.5) X 10^-7 photons cm^-2 s^-1, a 100-fold increase over the 4FGL-DR4 catalog value of (1.47 +/- 0.01) X 10^-9 photons cm^-2 s^-1. The Fermi-LAT Collaboration has previously reported flaring activity from this source in ATels #17937, #16988, #8219, #8219, #4753, and #2737.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT is publicly available via the LAT Daily Monitored Source List ( NGC_1275 ). A preliminary light curve for NGC 1275 can be accessed via the Fermi-LAT Light-Curve Repository at this link. We encourage multifrequency observations of this source. The Fermi-LAT contact persons for this source are J. Kataoka (kataoka.jun at waseda.jp) and C.C. Cheung (chi.c.cheung2.civ at 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.