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From: atel@astronomerstelegram.org
Subject: ATel  17572
Message-Id: <20260103070551.4A09528808F2@server.astronomerstelegram.org>
Date: Sat,  3 Jan 2026 02:05:51 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17572							     ATEL #17572

Title:	Fermi-LAT detection of gamma-ray transient emission in the
		LMC field
Author:	G. Marti-Devesa (University and INFN Trieste), C. C. Cheung
		(NRL) on behalf of the Fermi Large Area Telescope Collaboration
Queries:	guillem.marti-devesa@ts.infn.it
Posted:	2 Jan 2026; 16:30 UT
Subjects:Gamma Ray, >GeV, Transient, Pulsar

The Large Area Telescope (LAT), one of the two instruments on the Fermi
Gamma-ray Space Telescope, has observed transient gamma-ray emission from
a source with coordinates R.A. = 85.09 deg, Decl. = -69.41 deg (95% confidence
error radius of 0.21 deg), in the Large Magellanic Cloud field. The source
is spatially consistent with the gamma-ray bright pulsar PSR J0540-6919
(4FGL J0540.3-6920) in the Large Magellanic Cloud.

Preliminary analysis indicates that this source was significantly detected
(>8 sigma) in a high gamma-ray state in the week 22-29 December 2025, with
a weekly averaged gamma-ray flux (E>100MeV) of (2.3+/-0.4) X 10^-7 photons
cm^-2 s^-1 and a power-law photon index of 2.5+/-0.1 (statistical uncertainties
only). This corresponds to a flux increase by a factor of more than 4 relative
to the average flux of 4FGL J0540.3-6920 reported in the fourth Fermi-LAT
catalog Data-Release 4 (4FGL-DR4, Ballet et al. 2023, arXiv, arXiv:2307.12546).
A spectral cut-off is identified at 2+/-1 GeV. The source is detected at
>4 sigma in daily bins for December 23, 24, 27, and 29. 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).

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 Guillem
Marti-Devesa (guillem.marti-devesa@ts.infn.it).

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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