GCN Circular 35517

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

We report an analysis of observations of the vicinity of the IC240105A high-energy neutrino event (GCN 35498
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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 2024-01-05 at 12:27:42.57 UT (T0) with J2000 coordinates RA = 72.69 (+1.92, -1.85) deg, Decl.  = +11.42 (+0.50, -0.44) deg (90% PSF containment). There are two known gamma-ray sources (>100 MeV; 4FGL-DR4, The Fermi-LAT collaboration 2023, arXiv:2307.12546
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) located within the 90% IC240105A localization region. These are 4FGL J0449.1+1121, associated with the FSRQ PKS 0446+11 at a redshift of 2.15 (Shaw et al. 2012, ApJ, 748, 49) and the blazar of uncertain type 4FGL J0458.0+1152 (a.k.a. NVSS J045804+115142). These sources are located at 0.4 deg and 1.9 deg from the neutrino best-fit position, respectively. Based on a preliminary analysis of the LAT data over the timescales of 1-day prior to T0, these objects are not significantly detected (> 5 sigma).

In a preliminary analysis of LAT data integrated over 1 month before T0, only 4FGL J0449.1+1121 is significantly detected (> 5 sigma). This gamma-ray source showed significantly increased activity in November 2023 (ATel #16332), where its flux reached 18 times the cataloged average flux reported in 4FGL-DR4. A preliminary light curve of the object is available at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J0449.1+1121.   
Preliminary analysis indicates that 4FGL J0449.1+1121 has been brighter than its average state also in the weeks before T0, and likely in the descending phase of the flare reported on November 2023. In a preliminary analysis of LAT data integrated over one month prior T0, the observed flux (E>100 MeV) is (1.9 +/- 0.3) e-7 photons cm^-2 s^-1 (statistical uncertainty only). This is more than 3 times greater than the average flux reported in the 4FGL-DR4. We encourage multiwavelength observations of this source. 

We searched for intermediate (days to years) timescale emission from a new gamma-ray source. Preliminary analysis indicates no significant (> 5 sigma) new excess emission (> 100 MeV) at the IC240105A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC240105A best-fit position, the >100 MeV flux upper limit (95% confidence) is <5.9e-10 ph cm^-2 s^-1 for ~15-years (2008-08-04 to 2023-01-05 UTC), and < 2.8e-8 (<2.8 e-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 person is J. Sinapius (jonas.sinapius at desy.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.