Fermi-LAT detection of enhanced gamma-ray activity from the flat-spectrum radio galaxy PKS 2331-240 and the BL Lac PMN J1326-5256

ATel #18000; G. La Mura (INAF - Astronomical Observatory of Cagliari), on behalf of the Fermi Large Area Telescope Collaboration
on 23 Aug 2026; 17:13 UT
Credential Certification: Giovanni La Mura (giovanni.lamura@inaf.it)

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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 galaxy PKS 2331-240, also known as 4FGL J2333.9-2343 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A. = 353.48016 deg, Dec. = -23.72796 deg (J2000; Johnston et al., 1995, AJ, 110, 880), and redshift z=0.048 (Wills & Wills, 1976, ApJS, 31, 143).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on August 22, 2026, with a daily averaged gamma-ray flux (E>100MeV) of (0.8+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of approximately 70 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 the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.1+/-0.2, indicating a significantly harder spectrum than the 4FGL-DR4 value of 2.61 +/- 0.08.

On the same day, the LAT has also observed enhanced gamma-ray activity from a source positionally consistent with the BL Lac type source PMN J1326-5256, also known as 4FGL J1326.8-5256 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A. = 201.70512 deg, Dec. = -52.93990 deg (J2000; Fey et al. AJ, 150(2), 58), and unknown redshift.

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on August 22, 2026, with a daily averaged gamma-ray flux (E>100MeV) of (0.5+/-0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of approximately 10 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT catalog (Ballet et al. 2023, arXiv:2307.12546). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.0+/-0.2, indicating a significantly harder spectrum than the 4FGL-DR4 value of 2.32 +/- 0.03.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of these sources will continue. We encourage multifrequency observations of these sources. For these sources, the Fermi-LAT contact person is Giovanni La Mura (giovanni DOT lamura AT inaf DOT it).

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.