TITLE:   GCN CIRCULAR
NUMBER:  30156
SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-210608A
DATE:    21/06/08 21:17:17 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 
IC210608A neutrino event (GCN 30153) 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 2021-06-08 at 03:41:00.98 
UT (T0) with J2000 position RA = 337.41 (+4.89, -11.64) deg, Decl. = 
18.37 (+3.75, -3.24) deg (90% PSF containment). Fifteen cataloged 
gamma-ray (>100 MeV) sources are located within the 90% IC210608A 
localization region (4FGL-DR2, The Fermi-LAT collaboration 2020, ApJS, 
247, 33). Based on a preliminary analysis of the LAT data over the 
timescale of 1-day prior to T0, none of these objects is significantly 
detected (> 5 sigma).

We searched for intermediate (days to month) timescale emission from a 
new gamma-ray transient source. Preliminary analysis indicates no 
significant (> 5 sigma) new excess emission (> 100 MeV) at the IC210608A 
best-fit position.  Assuming a power-law spectrum (photon index = 2.0 
fixed) for a point source at the IC210608A best-fit position, the >100 
MeV flux upper limit (95% confidence) is < 5.4e-9 (< 9.6e-8) ph cm^-2 
s^-1 for a 1-month (1-day) integration time before T0.

Based on a preliminary analysis, the cataloged object 4FGL J2243.9+2021 
is significantly (> 5 sigma) detected in a 1-month integration time 
before T0. The source is associated with the RGB J2243+203 
(Laurent-Muehleisen, S. A. et al. 1999, ApJ, 525, 127), a 
very-high-energy (>100GeV) BL Lac object located at unknown redshift 
(Abeysekara, A. U. et al. 2017, ApJS, 233, 7). It is detected with flux 
(>100 MeV) = (4 +/- 1)e-08 ph cm^-2 s^-1, consistent with the 4FGL 
catalog value, and power-law index = 1.9 +/- 0.2, consistent with the 
4FGL value of 1.85 +/- 0.02. The source has not shown enhanced activity 
within the past year, and it is not significantly detected on a 
timescale of 1-day 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 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.