Fermi-LAT detection of renewed gamma-ray activity from the CSS Quasar 3C 138
ATel #17107; Pietro Monti-Guarnieri (University of Trieste and INFN Trieste), Giovanni La Mura (INAF-OAC), on behalf of the Fermi Large Area Telescope Collaboration
on 24 Mar 2025; 20:18 UT
Credential Certification: Chiara Bartolini (chiara.bartolini-1@unitn.it)
Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Blazar, Quasar
Referred to by ATel #: 17142
The Large Area Telescope (LAT), one of the two instruments onboard 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 and 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 March 23, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (0.6+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 150 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 2.6+/-0.3, statistically compatible with the 4FGL-DR4 value of 2.2+/-0.1. The Fermi LAT Collaboration has previously reported gamma-ray flaring activity from 3C 138 in ATel #16845. Recent radio flaring activity has also been reported in ATels #17077 and #17104.
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.
