TITLE:   GCN CIRCULAR
NUMBER:  30046
SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-210516A
DATE:    21/05/18 13:37:54 GMT
FROM:    Sara Buson at GSFC/Fermi  <sara.buson@gmail.com>

S. Garrappa (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg) and C. C. Cheung (Naval Research Laboratory) on behalf of the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the high-energy IC210516A neutrino event (GCN 30026) 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 2021-05-16 at 14:38:20.34 UT (T0) with J2000 position RA = 91.76 (+1.05, -0.97) deg, Decl. = 9.52 (+0.50, -0.58) deg (90% PSF containment). No cataloged >100 MeV gamma-ray source is located within the 90% IC210516A localization region. 

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 IC210516A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC210516A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 6.6e-10 ph cm^-2 s^-1 for ~12-years (2008-08-04 to 2021-05-16 UTC), and < 6.5e-9 (< 7.1e-8) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.  

Within the 90% confidence localization of the neutrino, 0.8 deg offset from the best-fit IC210516A position, an excess of gamma rays, Fermi J0610.5+0946, was detected in an analysis of the ~12-years integrated LAT data (100 MeV - 1 TeV) prior to T0. This putative new source is detected at a statistical significance ~4 sigma (calculated following the prescription adopted in the 4FGL). Assuming a power-law spectrum, the excess has best-fit localization of RA = 92.644 deg, Decl. = 9.782 deg (7 arcmin 68% containment, 14 arcmin 99% containment) with best-fit spectral parameters, flux = (7.4 +/- 4.2)e-9 ph cm^-2 s^-1, index = 2.7 +/- 0.3. In a preliminary analysis of the LAT data over one month prior to T0, Fermi J0610.5+0946 is not significantly detected in the LAT data. All values include the statistical uncertainty only. 

Since Fermi normally operates in an all-sky scanning mode, regular monitoring of the IceCube event 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.