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

ATel #16845; Ettore Bronzini (University of Bologna & INAF-OAS), C. C. Cheung (NRL), G. La Mura (INAF-O. A. Cagliari)
on 4 Oct 2024; 22:13 UT
Credential Certification: Ettore Bronzini (ettore.bronzini@inaf.it)

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The Large Area Telescope (LAT), one of the two instruments onboard the Fermi Gamma-ray Space Telescope, has observed enhanced 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 from September 23.0-30.0, 2024, with a weekly averaged gamma-ray flux (E>100MeV) of (2.2+/-0.6) X 10^-7 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 50 relative to the average flux reported in the fourth Fermi-LAT catalog Data-Release 4 (4FGL-DR4, Ballet et al. 2023, arXiv, arXiv:2307.12546). The corresponding photon index is 2.3+/-0.2, and is consistent with the 4FGL-DR4 value of 2.23 +/- 0.13 within the uncertainties. The transient was identified thanks to the method implemented within the Fermi-LAT Collaboration known as "Fermi All-sky Variability Analysis" that searches the sky for high-energy transients on weekly time scales (Ackermann et al. 2013, ApJ, 771, 57; Abdollahi et al. 2017, ApJ, 846, 34). 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 https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=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.