GCN Circular 31653

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

We report an analysis of observations of the vicinity of the high-energy 
neutrino event IC220225A (GCN 31650
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 ) 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 2022-02-25 14:12:00.7 UTC 
(T0) with J2000 position RA = 34.7 (+3.1, -2.6) deg, Dec = 0.0 (+1.8, 
-1.5) deg (90% PSF containment).

Two cataloged >100 MeV gamma-ray sources are located within the 90% 
IC220225A localization error (The Fourth Fermi-LAT catalog, 4FGL-DR3; 
The Fermi-LAT collaboration 2022, arXiv:2201.11184
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 ). These are 4FGL 
J0217.8+0144, located ~1.75 deg from the neutrino best-fit position and 
associated with the flat-spectrum radio quasar PKS 0215+015 located at z 
= 1.715 (Foltz & Chaffee 1987, AJ, 93, 529) and 4FGL J0208.5-0046, 
located ~2.7 deg from the neutrino best-fit position and associated with 
the BL Lac object PKS 0205-010 with an uncertain SDSS-based redshift 
(Richards et al. 2004, ApJS, 155, 258; Richards et al. 2009, ApJS, 180, 
67). Based on a preliminary analysis of the LAT data over a 1-day and 
1-month integration time before T0, 4FGL J0208.5-0046 is not 
significantly detected at gamma rays.

4FGL J0217.8+0144 has been in an enhanced gamma-ray activity state since 
mid-2021.�� Preliminary analysis of the 1-day interval preceding T0 
indicates that the source is in a high state with flux (E>100 MeV) of 
(3+/-1) x 10^-7 photons cm^-2 s^-1 (integrated before T0, statistical 
uncertainty only) and power-law index 2.3 +/- 0.3. This flux is more 
than 7 times greater than the average flux reported in the 4FGL-DR3 
catalog. Averaged over the month preceding T0, the flux (E>100 MeV) is 
(1.6+/-0.2) x 10^-7 photons cm^-2 s^-1 (statistical uncertainty only), 
more than 3 times greater than the average flux reported in the 4FGL-DR3 
catalog, and the power-law index is 2.1 +/- 0.1 . The gamma-ray spectral 
indices of the flaring state are consistent with the value reported in 
the latest 4FGL-DR3 catalog (index = 2.21 +/- 0.02). A preliminary light 
curve can be accessed via the Fermi-LAT Light-Curve Repository at 
https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.php?source_name=4FGL_J0217.8+0144.

We searched for the existence of intermediate (months to years) 
timescale emission from a new gamma-ray transient source. Preliminary 
analysis indicates no significant (>5sigma) new excess emission (> 100 
MeV) at the the IC220225A 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 < 
2.5 x 10^-10 ph cm^-2 s^-1 for ~13-years (2008-08-04 / 2022-02-25 UTC), 
< 1.0x 10^-8 (< 1.1 x 10^-7) 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 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). We encourage 
multifrequency observations of these sources.

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.