Fermi-LAT detection of enhanced gamma-ray activity from the flat-spectrum radio source PMN J1524-5903

ATel #17564; C. C. Cheung (NRL), on behalf of the Fermi Large Area Telescope Collaboration
on 26 Dec 2025; 22:06 UT
Credential Certification: Teddy Cheung (Teddy.Cheung@nrl.navy.mil)

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The Large Area Telescope (LAT), one of the two instruments onboard the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray activity from a source positionally consistent with the flat spectrum radio source, PMN J1524-5903, also known as 4FGL J1524.8-5904 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with radio coordinates R.A. = 231.21301 deg, Dec. = -59.06103 deg (J2000; Petrov et al., 2019, MNRAS, 485, 88). There is no redshift measured for this object (see Rajagopal et al. 2020, ApJ, 898, 18).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state from December 15.0-22.0, 2025, with a weekly averaged gamma-ray flux (E>100MeV) of (5.1 +/- 1.2) X 10^-7 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase by a factor of more than 40 relative to the average flux reported in the fourth Fermi-LAT catalog Data-Release 4 (4FGL-DR4, Ballet et al. 2023, arXiv, arXiv:2307.12546). The corresponding photon index is 2.3 +/- 0.2, consistent with the 4FGL-DR4 value of 2.17 +/- 0.09. During this time interval, the highest-energy event associated with the source is a E = 24.5 GeV photon (with association probability P > 0.98) detected at 09:18:09.874 UT on December 20, 2025.

The transient was identified thanks to the method implemented within the Fermi-LAT Collaboration known as "Fermi All-sky Variability Analysis" that searches the sky for high-energy transients on weekly time scales (Ackermann et al. 2013, ApJ, 771, 57; Abdollahi et al. 2017, ApJ, 846, 34). The report for this flare detection is: https://fermi.gsfc.nasa.gov/ssc/data/access/lat/FAVA/SourceReport.php?week=907&flare=2

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is C.C. Cheung (Teddy.Cheung@nrl.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.