Fermi-LAT detection of historical maximum gamma-ray activity from the low redshift source PKS 0521-36

ATel #18118; S. Wagner (IFAE, Barcelona), P. Monti-Guarnieri (University of Trieste and INFN Trieste), on behalf of the Fermi Large Area Telescope Collaboration
on 23 Oct 2026; 20:25 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 increasing gamma-ray activity from a source positionally consistent with the low redshift source PKS 0521-36, also known as 4FGL J0522.9-3628 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A. = 80.74160 deg, Dec. = -36.45857 deg (J2000; Charlot et al. 2020, A&A, 644, 159), and redshift z=0.05655 +/-0.00018 (Jones et al. 2009, MNRAS, 389, 683).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on October 22, 2026, with a daily averaged gamma-ray flux (E>100MeV) of (3.0 +/- 0.3) 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). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.0 +/- 0.1, indicating a significantly harder spectrum than the 4FGL-DR4 value of 2.45 +/- 0.01. Further analysis on shorter timescales reveals that the peak 3-hr flux was 60 times its average, i.e., (6.7+/-0.9) X 10^-6 photons cm^-2 s^-1, observed in the time interval from 15:00 to 18:00 UT on October 22, 2026.

The spectral hardening led to the detection of six high-energy photons associated with the source at a confidence level p > 0.99. The highest-energy photon was observed at 75 GeV on October 22 at 06:59:59 UT. Further likelihood analysis of the >10 GeV data over the period from October 12, 00:00:00 UT to October 22, 23:59:59 UT revealed a flux of (4.5 +/- 1.6) X 10^-9 photons cm^-2 s^-1, a 60-fold increase over the 4FGL-DR4 catalog value of (7.09 +/- 0.01) X 10^-11 photons cm^-2 s^-1. The Fermi LAT Collaboration has previously reported gamma-ray flaring activity from PKS 0521-36 in ATels #2683 and #18032.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. This source belongs to the "LAT Monitored Sources" and consequently, a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT is publicly available (MSL: 4FGL J0522.9-3628 ). A preliminary light curve for PKS 0521-36 can be accessed via the Fermi-LAT Light-Curve Repository (LCR: 4FGL J0522.9-3628 ). We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is F. D'Ammando (filippo[dot]dammando[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.