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From: atel@astronomerstelegram.org
Subject: ATel  17180
Message-Id: <20250511070619.B0F6828807C1@server.astronomerstelegram.org>
Date: Sun, 11 May 2025 03:06:19 -0400 (EDT)

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ATEL #17180							     ATEL #17180

Title:	Fermi-LAT detection of renewed gamma-ray activity from the
		CSS 3C 138
Author:	S. Wagner (University of Wuerzburg), on behalf of the Fermi
		Large Area Telescope Collaboration
Queries:	sarah.wagner@physik.uni-wuerzburg.de
Posted:	10 May 2025; 20:15 UT
Subjects:Gamma Ray, >GeV, 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,
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 <a href="https:// 
fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?

source_name=4FGL_J0521.2+1637">4FGL_J0521.2+1637</a>. 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.

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