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GCN Circular 32565

Fermi-LAT gamma-ray observations of IceCube-220918A and detection of a new gamma-ray source, Fermi J0502.5+0037
2022-09-19T20:54:50Z (2 years ago)
Simone Garrappa at DESY <>
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 IC220918A 
high-energy neutrino event (GCN 32562) 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 2022-09-18 at 12:46:05.32�� 
UT (T0) with J2000 position RA = 75.15 (+3.79 , -3.38) deg, Decl. = 3.58 
(+3.70 , -3.40) deg (90% PSF containment). Seven cataloged gamma-ray 
(>100 MeV) sources are located within the 90% IC220918A localization 
region (4FGL-DR3; The Fermi-LAT collaboration 2022, ApJS, 260, 53).

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 IC220918A 
best-fit position. Assuming a power-law spectrum (photon index = 2.0 
fixed) for a point source at the IC220918A best-fit position, the >100 
MeV flux upper limit (95% confidence) is < 5.9e-10 ph cm^-2 s^-1 for 
~14-years (2008-08-04 to 2022-09-18 UTC), and < 3.5e-9 (<5.8e-8) ph 
cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

Based on a preliminary analysis of the LAT data integrating over the 
time interval 1-month prior to T0, the catalogued sources 4FGL 
J0509.4+0542�� (associated with the object TXS 0506+056) and 4FGL 
J0505.3+0459 (associated with the flat-spectrum radio object PKS 
0502+049) are�� significantly detected (> 5 sigma). The observed fluxes 
are comparable to the average ones measured in the 4FGL catalog. 
Integrating over the time interval�� 1-day prior to T0, 4FGL J0509.4+0542 
is detected at a statistical significance of ~4.5 sigma, at a flux level 
comparable to the average one measured in the 4FGL catalog.

Within the 90% confidence localization of the neutrino, 3 deg offset 
from the best-fit IC220918A position, a new excess of gamma rays, Fermi 
J0502.5+0037, 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 > 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 = 75.63 deg, Decl. 
= 0.62 deg (99% containment radius = 0.1 deg) with best-fit spectral 
parameters, flux = (2.8 +/- 0.9)e-9 ph cm^-2 s^-1, index = 2.2 +/-0.1. 
In a preliminary analysis of the LAT data over one month and one day 
prior to T0, Fermi J0502.5+0037 is not significantly detected in the LAT 
data. All values include the statistical uncertainty only.

A possible counterpart of Fermi J0502.5+0037�� is the radio source PKS 
B0500+006, at RA=75.641042 deg, Dec=0.704861 deg (Condon et al. 1998 AJ, 
115, 5). This source is located about 0.08 deg from the Fermi 
J0502.5+0037 best-fit position, and within the gamma-ray 99% positional 

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 
and S. Buson (sara.buson at

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.
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