Fermi-LAT detection of gamma-ray transient emission in the LMC field
ATel #17572; G. Marti-Devesa (University and INFN Trieste), C. C. Cheung (NRL) on behalf of the Fermi Large Area Telescope Collaboration
on 2 Jan 2026; 16:30 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Guillem Marti-Devesa (guillem.marti-devesa@ts.infn.it)
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.
