GCN Circular 31194

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

We report an analysis of observations of the vicinity of the high-energy 
IC211208A neutrino event (GCN 31191
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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 2021-12-08 at 20:02:51.1 UT 
(T0) with J2000 position RA = 114.52 (+2.82, -2.50) deg, Decl. = 15.56 
(+1.81, -1.39) deg (90% PSF containment). Two cataloged gamma-ray (>100 
MeV) sources are located within the 90% IC211208A localization region 
(4FGL-DR2, The Fermi-LAT collaboration 2020, ApJS, 247, 33). These are 
4FGL J0738.4+1539, associated with the radio source of unknown type NVSS 
J073824+153839 at 0.13 deg and 4FGL J0743.1+1713, associated with the BL 
Lac object TXS 0740+173 at 2.05 deg. Based on a preliminary analysis of 
LAT data at timescales of 1-month and 1-day prior to T0, these objects 
are not significantly detected (>5sigma).

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 IC211208A 
best-fit position. Assuming a power-law spectrum (photon index = 2.0 
fixed) for a point source at the IC211208A best-fit position, the >100 
MeV flux upper limit (95% confidence) is < 3.0e-10 ph cm^-2 s^-1 for 
~13-years (2008-08-04 to 2021-12-08 UTC), and < 4.5e-9 (< 7.0e-8) ph 
cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

In the vicinity of the IC211208A best-fit position, outside the 90% 
localization region at 2.1 deg from the best-fit IC211208A localization, 
the cataloged >100 MeV gamma-ray source 4FGL J0738.1+1742 is 
significantly detected (> 5 sigma) integrating 1-day LAT data before T0. 
This source is associated with the BL Lac object PKS 0735+17 at z = 0.45 
(Gattano et al, 2018 A&A, 618, A80). Preliminary analysis indicates that 
on 2021-12-08 the gamma-ray source was observed with a daily averaged 
flux (E>100 MeV) of (3+/-1) x 10^-7 photons cm^-2 s^-1 (statistical 
uncertainty only), about 5 times greater than the average flux reported 
in the fourth Fermi LAT source catalog (4FGL-DR2). The source was 
significantly detected at 1-day timescale on 2021-12-01 and was observed 
on 2021-12-04 with peak daily flux value of (5+/-2) x 10^-7 photons 
cm^-2 s^-1 (statistical uncertainty only), about 10 times greater than 
the average 4FGL-DR2 flux. The source was reported to be in optical 
brightening on 2021-11-08 and is currently at its brightest state in R 
band https://vo.astro.spbu.ru/sites/default/files/optic/pks0735R.png 
(see also ATel #15021).

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 gmail.com).

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.