Fermi-LAT detection of a new gamma-ray transient in the Galactic plane, Fermi J1820-1648

ATel #17688; C. C. Cheung (NRL), on behalf of the Fermi Large Area Telescope Collaboration
on 18 Feb 2026; 17:05 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Teddy Cheung (Teddy.Cheung@nrl.navy.mil)

Tweet

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.