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From: atel@astronomerstelegram.org
Subject: ATel  17687, 17688
Message-Id: <20260219071140.CEC6D288093F@server.astronomerstelegram.org>
Date: Thu, 19 Feb 2026 02:11:40 -0500 (EST)

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ATEL #17687							     ATEL #17687

Title:	Einstein Probe observations of the transient gamma-ray source
		Fermi J0346.0+5405
Author:	J. W. Hu (NAO, CAS), Y.-H. I. Yin (HKU), Y.-H. Chi (NJU),
		C. C. Jin (NAO, CAS), H. Feng (IHEP, CAS), P. Zhou (NJU), J. Yang
		(ZZU), R.-Z. Li (YNAO, CAS), X. Tian (GXU), W. D. Zhang, W. Yuan (NAO,
		CAS)
Queries:	hujingwei@nao.cas.cn
Posted:	18 Feb 2026; 11:59 UT
Subjects:X-ray, Transient

We report on follow-up observations of the new GeV transient Fermi J0346.0+5405
(ATel #17679) performed with the Einstein Probe (EP).

The Wide-field X-ray Telescope (WXT) onboard EP covered the field of Fermi
J0346.0+5405 three times on 2026 February 9 and once on February 13. There
was no WXT coverage during the reported GeV flare on February 10. No X-ray
source was detected within the 95% LAT error circle in any of the four
observations. The derived 90% confidence upper limit on the 0.5-4 keV flux
for the four observations are 1.6 x 10^-11, 1.1 x 10^-11, 2.6 x 10^-11,
and 0.8 x 10^-11 ergs/s/cm^2, respectively.

A target-of-opportunity follow-up observation was performed with the Follow-up
X-ray Telescope (FXT) onboard EP starting at 2026-02-14T08:12:35 (UTC),
with an exposure of 2.2 ks. Within the LAT 95% confidence region, only
one X-ray source was detected: EP J034634.2+540101, located at R.A. = 56.6421
deg, DEC = 54.0160 deg (J2000), with an uncertainty of 10 arcsec in radius
(90% C.L. statistical and systematic). The FXT spectrum can be fitted by
an absorbed power law with a photon index of 0.86(+0.45/-0.43), with the
NH fixed at the Galactic value of 9e+21 cm^-2. The derived unabsorbed flux
in 0.5-10 keV is 2.8 (+1.3, -0.8) e-12 ergs/s/cm^2. All quoted uncertainties
are at the 90% confidence level. The FXT position is consistent with that
of the VLBI source ICRF J034634.5+540059. At this position, a historical
X-ray source, 2RXS J034636.3+540107 (count rate 1.2 x 10^-2 cts/s), has
been reported. Assuming the FXT best-fit spectral model, its estimated
unabsorbed 0.5-10 keV flux is 3.2 x 10^-12 ergs/s/cm^2, consistent with
the flux measured with FXT.

No additional new X-ray sources were detected within the LAT error circle.
We derived a 90% confidence upper limit on the 0.5-10 keV flux for any
undetected source in the FXT observation of 6 x 10^-14 ergs/s/cm^2, assuming
the same spectral model.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor
the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook
of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy
of Sciences in collaboration with ESA, MPE and CNES.

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Password Certification: Jingwei Hu (hujingwei@nao.cas.cn)
https://www.astronomerstelegram.org/?read=17687
==============================================================================
ATEL #17688							     ATEL #17688

Title:	Fermi-LAT detection of a new gamma-ray transient in the Galactic
		plane, Fermi J1820-1648
Author:	C. C. Cheung (NRL), on behalf of the Fermi Large Area Telescope
		Collaboration
Queries:	Teddy.Cheung@nrl.navy.mil
Posted:	18 Feb 2026; 17:05 UT
Subjects:Gamma Ray, >GeV, Transient

The Large Area Telescope (LAT), one of the two instruments on the Fermi
Gamma-ray Space Telescope, observed gamma-ray emission from a new transient
source, Fermi J1820-1648.  This source is not in any published LAT catalog
and was not detected by AGILE or EGRET.

Preliminary analysis indicates that this source was significantly detected
(>5 sigma) in the week of 2026 February 9.0-16.0, with a weekly averaged
gamma-ray flux (E>100MeV) of (1.4+/-0.2) X 10^-6 photons cm^-2 s^-1 and
a single power-law photon index of 2.4+/-0.1 (statistical uncertainties
only).  The best-fit location of this gamma-ray source is RA = 275.10 deg,
Dec. = -16.80 deg (J2000) with a 95% containment radius of 0.11 deg (errors
are statistical only).  This source is located in the Galactic plane (l,
b = 14.50 deg, -0.96 deg).  The transient was identified through the "Fermi
All-sky Variability Analysis" (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/FAVA/SourceReport.php?week=915&flare=3)
that searches the sky for high-energy transients on weekly time scales
(Ackermann et al. 2013, ApJ, 771, 57).

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray
monitoring of this source will continue. We encourage multifrequency observations
of this source. For this source, the Fermi-LAT contact person is C.C. Cheung
(Teddy.Cheung _at_ 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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Password Certification: Teddy Cheung (Teddy.Cheung@nrl.navy.mil)
https://www.astronomerstelegram.org/?read=17688
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