TITLE: GCN CIRCULAR NUMBER: 32602 SUBJECT: Fermi-LAT gamma-ray observations of IceCube-220928A DATE: 22/09/29 21:21:24 GMT FROM: Simone Garrappa at DESY simone.garrappa@desy.de
S. Garrappa (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg) and J. Sinapius (DESY-Zeuthen) on behalf of the Fermi-LAT collaboration:
We report an analysis of observations of the vicinity of the IC220928A high-energy neutrino event (GCN 32599) with all-sky survey data from the Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope. The IceCube event was detected on 2022-09-28 at 12:32:38.30 UT (T0) with J2000 position RA = 207.42 (+1.41 , -2.52) deg, Decl. = 10.43 (+0.98 , -0.98) deg (90% PSF containment). Three cataloged gamma-ray (>100 MeV) sources are located within the 90% IC220928A localization region (4FGL-DR3; The Fermi-LAT collaboration 2022, ApJS, 260, 53). Based on a preliminary analysis of the LAT data over the timescales of 1-day and 1-month prior to T0, these objects are not significantly detected (> 5 sigma).
We searched for intermediate (days to years) timescale emission from a new gamma-ray transient source. Preliminary analysis indicates no significant (> 5 sigma) new excess emission (> 100 MeV) at the IC220928A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC220928A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 6.5e-10 ph cm^-2 s^-1 for ~14-years (2008-08-04 to 2022-09-28 UTC), and < 9e-9 (<1.1e-7) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.
Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this region will continue. For these observations the Fermi-LAT contact persons are S. Garrappa (simone.garrappa at desy.de) and S. Buson (sara.buson at uni-wuerzburg.de).
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.