Fermi-LAT detection of enhanced gamma-ray activity from the FSRQ PKS 1725+123 and the unassociated gamma-ray source 4FGL J0406.2+0639

ATel #17094; P. Monti-Guarnieri (University of Trieste and INFN Trieste), on behalf of the Fermi Large Area Telescope Collaboration
on 20 Mar 2025; 19:08 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 quasar PKS 1725+123, also known as 4FGL J1728.0+1216 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 262.02938 deg, Dec. = +12.26097 deg (J2000; Xu et al. 2019, ApJS, 242, 5), and redshift z=0.568 (Nanci et al. 2022, A&A, 663, A129).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on March 19, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (0.5+/-0.2) 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 release of the Fourth Fermi-LAT catalog (4FGL-DR4, Ballet et al. 2024, arXiv:2307.12546). The corresponding photon index is 1.9+/-0.2, indicating a significantly harder spectrum than the 4FGL-DR4 value of 2.47+/-0.05.

The Fermi LAT has also observed enhanced gamma-ray activity from a source positionally consistent with the unassociated gamma-ray source 4FGL J0406.2+0639, from the first data release of the Fourth Fermi-LAT source catalog (4FGL, The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 61.551 deg, Dec. = +6.652 deg and a 95% uncertainty radius of 0.007 deg.

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on March 19, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (0.7+/-0.3) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 130 relative to the average flux reported in 4FGL-DR4. The corresponding photon index is 2.4+/-0.3, consistent with the 4FGL-DR4 value of 2.4 +/- 0.1.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of these sources will continue. A preliminary light curve for PKS 1725+123 can be accessed via the Fermi-LAT Light-Curve Repository at 4FGL_J1728.0+1216. We encourage multifrequency observations of these sources. For both of these sources, 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.