TITLE:   GCN CIRCULAR
NUMBER:  32478
SUBJECT: Fermi-LAT gamma-ray observations of IceCube-220822A and detection of a new gamma-ray source, Fermi J1810.1+2154
DATE:    22/08/23 21:44:17 GMT
FROM:    Simone Garrappa at DESY  <simone.garrappa@desy.de>

S. Garrappa (DESY-Zeuthen) and S. Buson (Univ. of Wuerzburg) on behalf 
of the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the high-energy 
IC220822A neutrino event (GCN 32475) 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 2022-08-22 at 20:26:30.03  
UT (T0) with J2000 position RA = +273.08 (+2.47, -2.50) deg, Decl. = 
21.54 (+0.94, -1.18) deg (90% PSF containment). Two cataloged gamma-ray 
(>100 MeV) sources are located within the 90% IC220822A localization 
region (4FGL-DR3; arXiv:2201.11184; The Fermi-LAT collaboration 2020, 
ApJS, 247, 33). These are 4FGL J1809.3+2042 associated with the BL Lac 
RX J1809.3+2041 and 4FGL J1819.1+2133, associated with the BL Lac MG2 
J181902+2132 at 1.1 deg and 1.6 deg distance from the best-fit neutrino 
localization, respectively. 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 IC220822A 
best-fit position. Assuming a power-law spectrum (photon index = 2.0 
fixed) for a point source at the IC220822A best-fit position, the >100 
MeV flux upper limit (95% confidence) is < 2e-10 ph cm^-2 s^-1 for 
~14-years (2008-08-04 to 2022-08-22 UTC), and < 4.1e-9 (<8.8e-8) ph 
cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

Within the 90% confidence localization of the neutrino, 0.6 deg offset 
from the best-fit IC220822A position, an excess of gamma rays, Fermi 
J1810.1+2154, was detected in an analysis of the ~14-years integrated 
LAT data (100 MeV - 1 TeV) prior to T0. This putative new source is 
detected at a statistical significance ~4.5 sigma (calculated following 
the prescription adopted in the The Fourth Fermi-LAT catalog, The 
Fermi-LAT collaboration 2020, ApJS, 247, 33). Assuming a power-law 
spectrum, the excess has best-fit localization of RA = 272.54 deg, Decl. 
= 21.91 deg (95% containment radius = 0.14 deg) with best-fit spectral 
parameters, flux = (6.6 +/- 4.7)e-10 ph cm^-2 s^-1, index = 1.9 +/-0.2. 
In a preliminary analysis of the LAT data over one month prior to T0, 
Fermi J1810.1+2154 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 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.