Fermi-LAT detection of renewed gamma-ray activity from the CSS 3C 138

ATel #17180; S. Wagner (University of Wuerzburg), on behalf of the Fermi Large Area Telescope Collaboration
on 10 May 2025; 20:15 UT
Credential Certification: Sarah Wagner (sarah.wagner@physik.uni-wuerzburg.de)

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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 compact steep spectrum quasar 3C 138, also known as 4FGL J0521.2+1637 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with radio coordinates R.A. = 80.29119 deg, Decl. = +16.63946 deg (J2000; Truebenbach & Darling, 2017, ApJS, 233, 3), and redshift z= 0.759 (Lynds et al. 1966, ApJ, 144, 1244). Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on May 9, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (1.3 +/- 0.4) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 300 relative to the average flux reported in the fourth data release of 4FGL (4FGL-DR4; Ballet et al. 2024, arXiv:2307.12546). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.3 +/- 0.2, and is consistent with the 4FGL-DR4 value of 2.2 +/- 0.1 within the uncertainties. The Fermi LAT Collaboration has previously reported gamma-ray flaring activity from 3C 138 in ATel #16845 and ATel #17107. Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for 3C 138 can be accessed via the Fermi-LAT Light-Curve Repository at 4FGL_J0521.2+1637. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact persons are Ettore Bronzini (ettore.bronzini@inaf.it) and 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.