Fermi J1820-1648: extended GeV duration and improved LAT localization
ATel #17699; C. C. Cheung (NRL), P. Jean (IRAP, Toulouse), on behalf of the Fermi Large Area Telescope Collaboration
on 25 Feb 2026; 19:36 UT
Credential Certification: Teddy Cheung (Teddy.Cheung@nrl.navy.mil)
Subjects: Gamma Ray, >GeV, Transient
Following the Large Area Telescope (LAT) detection of Fermi J1820-1648 in the 1-week interval from 2026 February 9.0-16.0 (ATel #17688), we searched for additional GeV emission over an extended time window from February 2.0-23.0.
Preliminary analysis in 1-day time bins indicates that this source was detected (>3 sigma) on February 6, 7, 8, and 16, giving a total gamma-ray duration of 11 days. Taking the LAT data from 2026 February 6.0-17.0, the average gamma-ray flux (E>100 MeV) is (1.5+/-0.2) X 10^-6 photons cm^-2 s^-1 with a single power-law photon index of 2.3+/-0.1 (statistical uncertainties only). The duration and spectrum are consistent with those of previous gamma-ray novae detected by Fermi-LAT. The best-fit location is RA = 275.09 deg, Dec. = -16.83 deg (J2000) with a 95% containment radius, r95 = 0.08 deg (errors are statistical only) that is smaller than that originally reported (r95 = 0.11 deg).
The highly-reddened optical transient detected on 2026 February 22.5087 by Sokolovsky et al.) ATel #17698) is within the improved LAT localization.
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.
