GCN Circular 33166

J. Sinapius (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg), S. Garrappa 
(Ruhr-Universitaet Bochum) on behalf of the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the 
IceCube-Cascade-221229A high-energy neutrino event 
(https://gcn.gsfc.nasa.gov/notices_amon_icecube_cascade/137489_30229466.amon) 
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 
2022-12-29 at 22:57:12.76 UT (T0) with J2000 position RA = (27.24 +/- 
6.01) deg, Decl. = (36.80 +/- 6.01) deg (90% PSF containment). Multiple 
cataloged gamma-ray (>100 MeV) sources are located within the 90% 
IceCube-Cascade-221229A 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, one 
of these objects is significantly detected (> 5 sigma). This source is 
the BL Lac object 4FGL J0136.5+3906, associated with the TeV emitter TeV 
J0136+391, at 3.35 deg from the IC best-fit position. In a preliminary 
analysis of LAT data over 30 days before T0, the object is observed at a 
flux level of (9.75 +/- 4.48) e-9 ph cm^-2 s^-1, consistent with the 
average value reported in 4FGL-DR3.

We searched for intermediate (days to months) timescale emission from a 
new gamma-ray transient source. Preliminary analysis indicates no 
significant (> 5 sigma) new excess emission (> 100 MeV) at the 
IceCube-Cascade-221229A best-fit position. Assuming a power-law spectrum 
(photon index = 2.0 fixed) for a point source at the 
IceCube-Cascade-221229A best-fit position, the >100 MeV flux upper limit 
(95% confidence) is < 7.2e-9 (< 5.2e-8) 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 source will continue. For these observations the 
Fermi-LAT contact persons are J. Sinapius (jonas.sinapius at desy.de), 
S. Garrappa (simone.garrappa at ruhr-uni-bochum.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.