Fermi LAT detection of historical maximum gamma-ray flux from 3C 138
ATel #17808; F. Casaburo (INAF - OAR, INFN - Rome Tor Vergata, Sapienza University Rome), G. La Mura (INAF-OAC), C. C. Cheung (Space Science Division, Naval Research Laboratory), and S. Ciprini (INFN - Rome Tor Vergata, ASI- SSDC), on behalf of the Fermi Large Area Telescope Collaboration
on 22 May 2026; 17:01 UT
Credential Certification: Giovanni La Mura (giovanni.lamura@inaf.it)
Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Quasar
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, 1), with coordinates R.A. = 80.29119 deg, Dec. = 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 21, 2026, with a daily averaged gamma-ray flux (E>100MeV) of (5.5 +/- 0.4) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of more than 1300 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT source catalog (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546). The corresponding photon index is 1.9 +/- 0.1, indicating a significantly harder spectrum than the 4FGL-DR4 value of 2.23 +/- 0.13. This is the highest LAT daily flux ever observed for this source. The Fermi-LAT Collaboration has previously reported flaring activity from this source in ATels #17786, #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 3C 138 can be accessed via the Fermi-LAT Light Curve Repository at 4FGL_J0521.2+1637. Since this source also belongs to the list of LAT daily monitored sources, a daily light curve can also be accessed at 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.
