Fermi-LAT detection of renewed high-energy gamma-ray emission from the AGN PMN J1603-4904
ATel #17439; P. Monti-Guarnieri (University of Trieste and INFN Trieste), F. Casaburo (INAF-OAR & INFN Roma Tor Vergata & Sapienza University of Roma), P. Van Zyl (South African Radio Astronomy Observatory), on behalf of the Fermi Large Area Telescope Collaboration
on 9 Oct 2025; 20:04 UT
Credential Certification: Giovanni La Mura (giovanni.lamura@inaf.it)
Subjects: Gamma Ray, >GeV, Request for Observations, AGN
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 AGN PMN J1603-4904, also known as 4FGL J1603.8-4903 (Abdollahi et al. 2022, ApJS, 260, 53) and 3FHL J1603.8-4903 (Ajello et al. 2017, ApJS, 232, 18), with coordinates R.A. = 240.96119 deg, Dec. = - 49.06819 deg (J2000; Xu et al. 2020, ApJS, 242, 5), and redshift z=0.232 (Goldoni et al. 2016, A&A 586, L2).
Preliminary analysis indicates that this source was in an elevated gamma-ray emission state between October 05, 2025 and October 08, 2025, as confirmed by the detection of seven high-energy photons associated with the source at a confidence level p > 0.9. The highest energy event was a 370 GeV photon observed at 15:23:04 on October 05. A likelihood analysis of the high-energy data over the period from October 05, 12:00:00 UTC to October 08, 23:59:59 UTC revealed a flux above 10 GeV of (1.08 +/- 0.45) X 10^-8 photons cm^-2 s^-1, a 20-fold increase over the 3FHL catalog value of (5.32 +/- 0.44) X 10^-10 photons cm^-2 s^-1. The flux above 100 MeV in the same period is (3.6 +/- 2.1) X 10^-8 photons cm^-2 s^-1, statistically compatible with the 4FGL-DR4 catalog value of (3.44 +/- 0.11) X 10^-8 photons cm^-2 s^-1 (Ballet et al. 2023, arXiv:2307.12546). The corresponding photon index is 1.2 +/- 0.2, indicating a significantly harder spectrum than the 4FGL-DR4 value of 1.99 +/- 0.02. Previous VHE flaring activity from this source was recently reported by the H.E.S.S. Collaboration (ATels #17205, #17307 and #17410).
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for PMN J1603-4904 can be accessed via the Fermi-LAT Light-Curve Repository at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/lcr/source.html?source_name=4FGL_J1603.8-4903. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is Pietro Monti-Guarnieri (pietro.monti-guarnieri@phd.units.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.
