GCN Circular 35329

L. Pfeiffer (Univ. of Wuerzburg), S. Buson (Univ. of Wuerzburg), S. Garrappa (Weizmann Institute of Science), C. Bartolini (INFN Bari) and J. Sinapius (DESY) on behalf of the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the IC231211A high-energy neutrino event (GCN 35328
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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 23-12-11 at 00:47:31.79 UT (T0) with J2000 position RA = 311.48 (+1.15, -2.33) deg, Decl. = +10.28 (+0.67, -0.68) deg (90% PSF containment). There are three gamma-ray sources (>100 MeV; 4FGL-DR4, The Fermi-LAT collaboration 2023, arXiv:2307.12546
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) located within the 90% IC231211A localization region. These are the blazar of uncertain type 4FGL J2044.0+1036 (a.k.a. NVSS J204351+103406) at a distance of roughly 0.57 degree from the best-fit localization and the pulsar 4FGL J2047.3+1051 at a distance of roughly 0.68 degree.  Based on a preliminary analysis of the LAT data over the timescales of 1-day and 1-month prior to T0, these objects are not significantly detected (> 5 sigma).

The third gamma-ray catalogued source found within the 90% event localisation is the BL Lac object 4FGL J2049.9+1002 (a.k.a. PKS 2047+098, at unknown redshift) at a distance of roughly 1.01 degree. The preliminary analysis indicates that this gamma-ray source is undergoing a prolonged, enhanced activity state. The 3-month-binned gamma-ray light curve shows activity over multiple years. Its gamma-ray flux has increased in recent years and is currently higher than what was observed in the previous 15 years of LAT monitoring. The high state began about nine months ago. During the last 3 months, i.e. 3-month integration time before T0, the observed flux (E>100 MeV) of 4FGL J2049.9+1002 is (8.40 +/- 0.80) x 10^-8 photons cm^-2 s^-1 (statistical uncertainty only), more than 3 times greater than the average flux (2.3 +/- 0.2) x 10^-8 photons cm^-2 s^-1 reported in the 4FGL-DR4. Based on a preliminary analysis of the LAT data over the timescale of 1-day prior to T0, this object is not significantly detected in gamma rays. A preliminary light curve of the object is available at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.php?source_name=4FGL_J2049.9+1002 We encourage multiwavelength observations of this source. 

Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this region will continue. For these observations the Fermi-LAT contact person is L. Pfeiffer (leonard.pfeiffer at stud-mail.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.