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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17497, 17498
Message-Id: <20251118071138.30BF228808AC@server.astronomerstelegram.org>
Date: Tue, 18 Nov 2025 02:11:38 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17497							     ATEL #17497

Title:	Fermi-LAT detection of renewed gamma-ray activity from the
		FSRQ 3C 345
Author:	M. Giroletti (INAF-IRA), G. La Mura (INAF - Astronomical Observatory
		of Cagliari), C. Bartolini (University of Trento &amp; INFN Bari),
		on behalf of the Fermi Large Area Telescope Collaboration
Queries:	marcello.giroletti@inaf.it
Posted:	17 Nov 2025; 07:38 UT
Subjects:Gamma Ray, AGN, Blazar, 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 flat-spectrum radio quasar 3C
345, also known as 4FGL J1642.9+3948 (Abdollahi et al. 2022, ApJS, 260,
53), with coordinates R.A. = 250.74504 deg, Dec. = +39.81028 deg (J2000;
Xu et al. 2019, ApJS 242, 5), and redshift z=0.5934 (Albareti et al. 2015,
MNRAS 452, 4153).

Preliminary analysis indicates that this source was in an elevated gamma-ray
emission state on November 15, 2025 , with a daily averaged gamma-ray flux
(E>100MeV) of (1.2 +/- 0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty
only). This corresponds to a flux increase of a factor of 20 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). The corresponding
photon index is 1.9+/-0.1, indicating a significantly harder spectrum than
the 4FGL-DR4 value of 2.42+/-0.02. The Fermi-LAT collaboration has previously
reported gamma-ray flaring activity positionally consistent with this source
in ATels #2226, #10453, #15313, #15613, and #16274.

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 345 can be accessed via the <a href="https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J1642.9+3948">Fermi-LAT
Light-Curve Repository</a> and the <a href="https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/0FGL_J1641d4p3939">Monitored
Source List</a>.  We encourage multifrequency observations of this source.
For this source, the Fermi-LAT contact person is Janeth Valverde (valverde@llr.in2p3.fr).

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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Password Certification: Marcello Giroletti (giroletti@ira.inaf.it)
https://www.astronomerstelegram.org/?read=17497
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ATEL #17498							     ATEL #17498

Title:	The blazar 3C345 flare - the strongest one since August 2006
Author:	V. V. Vlasyuk, O. I. Spiridonova (Special Astrophysical Observatory
		of the Rus. Ac. Sci.)
Queries:	mosk@sao.ru
Posted:	17 Nov 2025; 10:17 UT
Subjects:Optical, Blazar

Our BVRI-monitoring of the bright blazars sample using by our 1-meter and
0.5-meter reflectors is in progress.

The blazar 3C 345 exhibits enhanced activity in current year: after flare
to R = 15.83 +/- 0.01 (epoch March 03/04, MJD 60738.12, see ATel #17059)
we detected set of more intense re-brightening (up to R = 15.03 +/- 0.03,
MJD 60770.94 (ATel #17134), R = 15.43 +/- 0.02, MJD 60898.84 and R = 15.31
+/- 0.02, MJD 60917.83 respectively).

The last brightening phase of 3C 345 started in last October - after some
precursor event at R = 15.88 +/- 0.01, MJD 60961.72 its brightness varied
in range 15.9-16.2 mag until November 10th.

But observations on Nov 16/17 night (MJD 60995.66) showed surprisingly
powerful flare of 3C345 - R = 14.48 +/- 0.01 mag. According to our long-term
monitoring data the blazar was brighter in this century only in August
2006 (R = 14.45 +/- 0.02, MJD 53970.81).

The monitoring of 3C 345 is continuing; the most of data presented were
taken under good weather condition (seeing (FWHM) < 2 arcsec and good transparency)
and have typical accuracy of individual flux estimates  about 0.005 mag.
We strongly encourage further multi-wavelength studies.

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Password Certification: Alexander Moskvitin (mosk@sao.ru)
https://www.astronomerstelegram.org/?read=17498
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