S. Buson (NASA-GSFC), S. Garrappa (DESY-Zeuthen), M. Kreter
(Univ. of W��rzburg), J. Eggen (NASA-GSFC, University of Maryland)
on behalf of the Fermi-LAT collaboration:
We report an analysis of observations of the very high-energy IC180908A neutrino event (GCN 23214) 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 2018-09-08 19:59:31.84 UTC (T0) with J2000 position, RA =144.58 deg, Decl. = -2.13 deg. There are no cataloged >100 MeV gamma-ray sources consistent with the IC180908A localization. The closest cataloged gamma-ray source is 3FGL J0948.8+0021, associated with the narrow-line Seyfert I PMN J0948+0022 (Ackermann et al. 2015 ApJS 810, 14), at a distance of roughly 3.6 deg.
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) excess emission (0.1 - 300 GeV) within the IC180908A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.2 fixed) for a point source at the IceCube position, the >100 MeV flux upper limit (95% confidence) is < 1.6e-9 ph cm^-2 s^-1 for a 1-year integration time before T0.
Within the error circle for the direction of the neutrino, a ~3.9 sigma excess of gamma rays, Fermi J0935-0227, was identified integrating the LAT data (0.1 - 300 GeV) between 2008-08-04 and 2018-03-16. Assuming a power-law spectrum, the candidate source has best-fit localization of RA: 143.85, Dec: -2.46 (0.18 deg 99% containment) with best fit spectral parameters flux = (1.0 +/- 0.6)e-9 ph cm^-2 s^-1, index = 2.0 +/- 0.2. The blazar CRATES J0935-0241 is outside the 99% LAT error region, 0.22deg away from the position of the gamma-ray excess, while within the IC180908A 90% PSF containment.
Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. During the upcoming weeks (Fermi mission week 357/359), starting on Sept. 13 2018, the Fermi-LAT will be rocking preferentially toward the northern hemisphere, enabling low but continuous exposure to the IC180908A region. For this source the Fermi-LAT contact persons are S. Buson (e-mail: sara.buson at gmail.com), S. Garrappa (simone.garrappa at desy.de) and M. Kreter (mkreter at astro.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.