GCN Circular 32285
Subject
Fermi-LAT Gamma-ray Observations of IceCube-220624A and and detection of a new gamma-ray source, Fermi J1458.0+4119
Date
2022-06-28T10:12:57Z (3 years ago)
From
Simone Garrappa at DESY <simone.garrappa@desy.de>
S. Garrappa (DESY-Zeuthen) and S. Buson (Univ. of Wuerzburg) on behalf
of the Fermi-LAT collaboration:
We report an analysis of observations of the vicinity of the high-energy
IC220624A neutrino event (GCN 32260) 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 2022-06-24 at 16:13:16.41��
UT (T0) with J2000 position RA = +224.12 (+2.23, -1.95) deg, Decl. =
41.31 (+1.56, -1.56) deg (90% PSF containment). No cataloged gamma-ray
(>100 MeV) sources are located within the 90% IC220624A localization
region (4FGL-DR3; arXiv:2201.11184; The Fermi-LAT collaboration 2020,
ApJS, 247, 33).
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 IC220624A
best-fit position. Assuming a power-law spectrum (photon index = 2.0
fixed) for a point source at the IC220624A best-fit position, the >100
MeV flux upper limit (95% confidence) is < 9.3e-11 ph cm^-2 s^-1 for
~14-years (2008-08-04 to 2022-06-24 UTC), and < 3.9e-9 (< 5.4e-8) ph
cm^-2 s^-1 for a 1-month (1-day) integration time before T0.
Within the 90% confidence localization of the neutrino, 0.2 deg offset
from the best-fit IC220624A position, an excess of gamma rays, Fermi
J1458.0+4119, was detected in an analysis of the ~14-years integrated
LAT data (100 MeV - 1 TeV) prior to T0. This putative new source is
detected at a statistical significance ~4.5 sigma (calculated following
the prescription adopted in the The Fourth Fermi-LAT catalog, The
Fermi-LAT collaboration 2020, ApJS, 247, 33). Assuming a power-law
spectrum, the excess has best-fit localization of RA = 224.52 deg, Decl.
= 41.32 deg (5 arcmin 68% containment, 10 arcmin 99% containment) with
best-fit spectral parameters, flux = (5 +/- 2)e-10 ph cm^-2 s^-1, index
= 1.9 +/- 0.2. In a preliminary analysis of the LAT data over one month
prior to T0, Fermi J1458.0+4119 is not significantly detected in the LAT
data. All values include the statistical uncertainty only.
A probable counterpart of Fermi J1458.0+4119 is the high-synchrotron
peaked blazar WISEA J145820.77+412101.9 (aka 3HSP J145820.8+412102) at
RA=224.58658 deg, Dec=41.35028 deg, and redshift 0.176463 +/- 0.000027
(Sloan Digital Sky Survey Data Release 13, 2016 SDSS). It is located
about 4 arcmin from the Fermi J1458.0+4119 best-fit position, and within
the gamma-ray 68% positional uncertainty. This source has been proposed
as a promising very-high-energy candidate emitter (>100 TeV; Arsioli et
al. 2015, A&A, 579, 34).
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 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.