[vsnet-grb-info 27862] Fermi-LAT Gamma-ray Observations of IceCube-210503A and detection of a new gamma-ray source, Fermi J0931.9+3633

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29965
SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-210503A and detection of a new gamma-ray source, Fermi J0931.9+3633
DATE:    21/05/07 22:23:44 GMT
FROM:    Simone Garrappa at DESY  <simone.garrappa at desy.de>

S. Garrappa (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg) and C. C. 
Cheung (Naval Research Laboratory) on behalf of the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the high-energy 
IC210503A neutrino event (GCN 29951) 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 2021-05-03 at 22:19:32.96 
UT (T0) with J2000 position RA = 143.53 (+7.71, -5.16) deg, Decl. = 
41.81 (+5.02, -5.68) deg (90% PSF containment). Seventeen cataloged 
gamma-ray (>100 MeV) sources are located within the 90% IC210503A 
localization region (4FGL-DR2, The Fermi-LAT collaboration 2020, ApJS, 
247, 33). Based on a preliminary analysis of the LAT data over the 
timescale of 1-day prior to T0, none of these objects is significantly 
detected (> 5 sigma).

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 IC210503A 
best-fit position.  Assuming a power-law spectrum (photon index = 2.0 
fixed) for a point source at the IC210503A best-fit position, the >100 
MeV flux upper limit (95% confidence) is < 1e-10 ph cm^-2 s^-1 for 
~12-years (2008-08-04 to 2021-05-03 UTC), and < 6.3e-9 (< 4.6e-8) ph 
cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

Based on a preliminary analysis, the cataloged object 4FGL J0920.9+4441 
is significantly (> 5 sigma) detected in a 1-month integration time 
before T0. The source is associated with the FSRQ  S4 0917+44 
(Pauliny-Toth et al. 1978, AJ, 83, 451) at redshift z = 2.18645 (Hewett 
and Wild 2010, MNRAS, 405, 2302) and is detected with flux (>100 MeV) = 
(1.8 +/- 0.8)e-08 ph cm^-2 s^-1 (30% of the 4FGL catalog value) and 
power-law index = 2.1 +/- 0.5, consistent with the 4FGL value of 2.35 
+/- 0.01. The source has not shown enhanced activity within the last 
year, and it is not significantly detected at a timescale of 1-day 
before T0.

Within the 90% confidence localization of the neutrino, 3.76 deg offset 
from the best-fit IC210503A position, a new excess of gamma rays, Fermi 
J0931.9+3633, was detected in an analysis of the integrated LAT data 
(100 MeV - 1 TeV) between 2008-08-04 and 2021-05-03. This putative new 
source is detected at a statistical significance >5 sigma (calculated 
following the prescription adopted in the 4FGL). Assuming a power-law 
spectrum, the candidate gamma-ray source has best-fit localization of RA 
= 142.97 deg, Decl. = 36.55 deg (5 arcmin 68% containment radius, 9 
arcmin 99% containment radius) with best-fit spectral parameters, flux = 
(8.7 +/- 4.5)e-10 ph cm^-2 s^-1, index = 2.0 +/- 0.2. In a preliminary 
analysis of the LAT data over 1-day and 1-month prior to T0, Fermi 
J0931.9+3633 is not significantly detected in the LAT data. All values 
include the statistical uncertainty only.

A possible counterpart of Fermi J0931.9+3633 is the BL Lac object SDSS 
J093209.60+363002.6 at RA=143.035791 deg, Dec=36.503676 deg, and 
redshift 0.15376 +/- 0.00017 (Anderson et al. 2007 AJ, 133, 313). This 
source is located about 4 arcmin from the Fermi J0931.9+3633 best-fit 
position, and within the gamma-ray 68% positional uncertainty.

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.