Fermi-LAT detection of renewed high gamma-ray activity from the BL Lac PKS 0537-441

ATel #17755; S. Rani (Michigan Technological University), E. Bronzini (INAF-OAS), P. Monti-Guarnieri (University of Trieste and INFN Trieste), T. Lewis (Michigan Technological University) on behalf of the Fermi Large Area Telescope Collaboration
on 16 Apr 2026; 19:40 UT
Credential Certification: Ettore Bronzini (ettore.bronzini@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 renewed gamma-ray activity from a source positionally consistent with the BL Lac object PKS 0537-441, also known as 4FGL J0538.8-4405 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A. = 84.70984 deg, Dec. = -44.08581 deg (J2000; Johnston et al. 1995, AJ, 110, 880), and redshift z = 0.894 (Peterson et al. 1976, ApJ, 207, L5).

Preliminary analysis indicates that this source was in an elevated gamma-ray state on April 15, 2026, with a daily averaged gamma-ray flux (E > 100 MeV) 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 5 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT catalogue (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546) and it is comparable with previously reported peak activity from this source. The corresponding photon index is 1.9 +/- 0.1, indicating a harder spectrum than the 4FGL-DR4 value of 2.14 +/- 0.01. The spectral hardening led to the detection of a high-energy photon (E = 27 GeV) associated with the source at a confidence level p > 0.999. The source has shown sustained elevated emission over multiple days, confirming that the observed activity represents a persistent high state rather than a transient fluctuation. The Fermi-LAT Collaboration has previously reported flaring activity from this source in ATels #2591, #2124 and #1759.

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for 4FGL J0538.8-4405 can be accessed via the Fermi-LAT Light-Curve Repository at 4FGL J0538.8-4405. This source belongs to the list LAT daily monitored sources and therefore a daily light-curve can also be accessed at PKS 0537-441. We encourage multifrequency observations of this source. The Fermi-LAT contact person for this source is Gino Tosti (tosti@pg.infn.it).

The Fermi LAT is a pair conversion telescope designed to cover the energy band 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.