[vsnet-grb-info 26381] Fermi-LAT Gamma-ray Observations of IceCube-200921A and detection of a new gamma-ray source, Fermi J1256.9+2630

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28481
SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-200921A and detection of a new gamma-ray source, Fermi J1256.9+2630
DATE:    20/09/23 07:56:04 GMT
FROM:    Simone Garrappa at DESY  <simone.garrappa at 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 
IC200921A neutrino event (GCN 28468) 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 2020-09-21 19:07:12.89 UT 
(T0) with J2000 position RA = 195.29 (+2.35, -1.73) deg, Decl. = 26.24 
(+1.51, -1.77) deg 90% PSF containment. Three cataloged >100 MeV 
gamma-ray sources are located within the 90% IC200921A localization 
error (The Fourth Fermi-LAT catalog; The Fermi-LAT collaboration 2020, 
ApJS, 247, 33). These are 4FGL J1303.0+2434 (associated with the BL Lac 
object MG2 J130304+2434), 4FGL J1256.9+2736 (associated with the AGN 
  NGC 4839) and 4FGL J1310.6+2449 (associated with the BL Lac object 
CRATES J131038.52+244822.1). Based on a preliminary analysis of LAT data 
at timescales of 1-month and 1-day prior to T0, these objects are not 
significantly detected (>5sigma).

We searched for the existence of intermediate (days to years) timescale 
emission from a new gamma-ray transient source. Preliminary analysis 
indicates no significant (>5sigma) new excess emission (>100 MeV) at the 
IC200921A best-fit position. Assuming a power-law spectrum (photon index 
= 2.0 fixed) for a point source at the IceCube best-fit position, the 
 >100 MeV flux upper limit (95% confidence) is < 2.8e-10 ph cm^-2 s^-1 
for ~12-years (2008-08-04 / 2020-09-21 UTC), < 3.7e-9 (< 6.1e-8) ph 
cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

In the analysis of the ~12-years integrated LAT data (0.1 - 800 GeV), a 
4.7 sigma new excess of gamma rays, Fermi J1256.9+2630, was detected 1.0 
deg offset from the best-fit IC200921A position and within the 90% 
confidence localization of the direction of the neutrino. Assuming a 
power-law spectrum, the best-fit localization is (J2000) RA: 194.23 deg, 
Dec: 26.51 deg (7.4 arcmin 99% containment, 3.6 arcmin 68% containment). 
The gamma-ray best-fit spectral parameters are flux = (4+/-3)e-10 ph 
cm^-2 s^-1 and index = 1.8+/-0.2. In a preliminary analysis of the LAT 
data over 1-day and 1-month prior T0, Fermi J1256.9+2630 is not 
significantly detected in the LAT data. All values include 
the statistical uncertainty only.

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.