S. Buson (NASA/GSFC), D. Kocevski (NASA/MSFC), S. Ciprini (ASI/SSDC & INFN) report on behalf of the Fermi-LAT collaboration
We report follow-up observations of the [very] high-energy IceCube-170321 neutrino event (GCN circular 20929) 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 2017-03-21 07:32:20.69 UTC (T0) with J2000 position, RA =98.30 deg (+/- 1.2 deg 90% PSF containment), Decl. = -15.02 deg (+/- 1.2 deg 90% PSF containment). The closest cataloged >100 MeV gamma-ray source is 3FGL J0627.9-1517, at a distance of roughly 1.29 deg. The source is classified as a blazar candidate of unknown type associated with a radio (NVSS) and X-ray (1RXS) source, and is a member of the 2WHSP catalog (a catalog of HE/VHE gamma-ray blazar candidates with high-synchrotron peak frequency). The source does not show indication of enhanced gamma-ray activity within 24 hours of the neutrino detection.
The neutrino localization region was outside the LAT field of view (FoV) at the time of the detection by IceCube. It entered the LAT FoV at ~T0 + 600 s. We searched for the existence of intermediate (hours to days) timescale emission from a new gamma-ray transient source [or excess emission from a known catalog source]. Preliminary analysis indicates no significant excess gamma-ray emission (0.1 - 300 GeV) within the IceCube-170321 90% confidence localization. Assuming a power-law spectrum (photon index = 2.2 fixed) for a point source at the IceCube position, the >100 MeV photon flux upper limits (95% confidence) are < 3.2 x 10^-7 ph cm^-2 s^-1 in 1 day of exposure beginning 12 hours prior to T0, and < 2.6 x 10^-7 ph cm^-2 s^-1 in 24 hours of exposure beginning at T0, and < 5.5 x 10^-8 ph cm^-2 s^-1 in one week of exposure prior to T0. Longer timescale analysis will be made as data become available.
Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source region will continue. For this source the Fermi LAT contact persons are D. Kocevski (e-mail: daniel.kocevski at nasa.gov) and S. Buson (email: sara.buson at nasa.gov).
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.