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TITLE:   GCN CIRCULAR
NUMBER:  27816
SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-190523A
DATE:    20/05/25 10:36:44 GMT
FROM:    Sara Buson at GSFC/Fermi  <sara.buson@gmail.com>

S. Buson (Univ. of Wuerzburg) and S. Garrappa (DESY-Zeuthen) on behalf of the Fermi-LAT collaboration:
We report an analysis of observations of the vicinity of the high-energy IC200523A neutrino event (GCN 27787) 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 2020-05-23 02:30:07.59 UT (T0) with J2000 position RA = 338.64 (+10.77, -6.07) deg, Decl. = 1.75 (+1.84, -3.54) deg 90% PSF containment. Several cataloged >100 MeV gamma-ray sources are located within the 90% IC200523A localization error. Based on a preliminary analysis of the LAT data, none of those is significantly detected at gamma-rays over a 1-day and 1-month timescale prior T0.
We searched for the existence of intermediate (months to years) timescale emission from a new gamma-ray source. Preliminary analysis indicates no significant (>5sigma) new excess emission (>100 MeV) at the IC200523A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IceCube best-fit position, the >100 MeV flux upper limit (95% confidence) is < 1e-10 ph cm^-2 s^-1 for ~11-years (2008-08-04 / 2020-05-23 UTC), < 8e-9 (< 1e-7) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.
In the analysis of the ~11-years integrated LAT data (0.1 - 800 GeV), a 4.7 sigma new excess of gamma rays, Fermi J2231.0+0034, was detected 1.5 deg offset from the best-fit IC200523A position and within the 90% confidence localization of the direction of the neutrino. Assuming a power-law spectrum, the best-fit localization is (J2000) RA: 337.77, Dec: 0.567 (8 arcmin deg 95% containment, 5 deg 68% containment). A possible counterpart is the QSO object WISEA J223133.89+003312.8, located 7.3 arcmin away from the best-fit position of the gamma-ray excess, and within its 95% containment. The gamma-ray best-fit spectral parameters are flux = (1.4+/-0.4)e-9 ph cm^-2 s^-1 and index = 2.14+/-0.01. In a preliminary analysis of the LAT data over one day and one month prior T0, WISEA J223133.89+003312.8 is not significantly detected in the LAT data.
Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For this source the Fermi-LAT contact person 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.