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

ATel #17786; F. Giacchino (USAL - Universidad de Salamanca, Spain, and INFN - Rome Tor Vergata, Sapienza University Rome) and G. La Mura (INAF - O. A. Cagliari), on behalf of the Fermi Large Area Telescope Collaboration
on 9 May 2026; 20:28 UT
Credential Certification: Federica Giacchino (federica.giacchino@roma2.infn.it)

Tweet

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 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A.=80.29119deg, Dec.=+16.63946deg (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 8, 2026, with a daily averaged gamma-ray flux (E>100MeV) of (1.9 +/- 0.4) e-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of more than 450 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT catalog (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.4 +/- 0.2, is consisted with the 4FGL-DR4 value of 2.2+/- 0.1. The Fermi-LAT Collaboration has previously reported flaring activity from this source in ATels #17180 #17461, #17180, #17107, and #16845.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for this source 3C 138 can be accessed via the Fermi-LAT Light-Curve Repository at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J0521.2+1637. This source belongs to the list of daily monitored LAT sources, therefore a daily lightcurve can also be accessed at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/3C_138. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact persons are Ettore Bronzini (ettore.bronzini@inaf.it) and Teddy Cheung (chi.c.cheung2.civ@us.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.