[vsnet-grb-info 26651] Fermi-LAT Gamma-ray Observations of IceCube-201021A and detection of a new gamma-ray source, Fermi J1725.5+1312

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28751
SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-201021A and detection of a new gamma-ray source, Fermi J1725.5+1312
DATE:    20/10/22 19:14:08 GMT
FROM:    Simone Garrappa at DESY  <simone.garrappa at desy.de>

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

We report an analysis of observations of the vicinity of the high-energy 
IC201021A neutrino event (GCN 28715) 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 2020-10-21 at 
06:37:47.48 UT (T0) with J2000 position RA = 260.82 (+1.73, -1.68) deg, 
Decl. = 14.55 (+1.35, -0.74) deg (90% PSF containment). No cataloged 
 >100 MeV gamma-ray sources (The Fourth Fermi-LAT catalog, 4FGL; The 
Fermi-LAT collaboration 2020, ApJS, 247, 33) are located within the 90% 
IC201021A localization error.

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 
IC201021A best-fit position. Assuming a power-law spectrum (photon index 
= 2.0 fixed) for a point source at the IC201021A best-fit position, the 
 >100 MeV flux upper limit (95% confidence) is < 4.2e-10 ph cm^-2 s^-1 
for ~12-years (2008-08-04 to 2020-10-21 UTC), and < 1.3e-8 (< 4.3e-8) ph 
cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

Within the 90% confidence localization of the neutrino, 10 arcmin offset 
from the best-fit IC201021A position, an excess of gamma rays, Fermi 
J1725.5+1312, was detected in an analysis of the integrated LAT data 
(0.1 - 300 GeV) between 2008-08-04 and 2020-10-21. This putative new 
source is detected at a statistical significance >3 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 
= 260.76 deg, Decl. = 14.39 deg (5 arcmin 68% containment, 11 arcmin 99% 
containment) with best-fit spectral parameters, flux = (1.8 +/- 0.5)e-9 
ph cm^-2 s^-1, index = 2.2 +/- 0.2. In a preliminary analysis of the LAT 
data over one day and one month prior T0, Fermi J1725.5+1312 is not 
significantly detected in the LAT data. All values include the 
statistical uncertainty only. The highest-energy photon likely 
associated to the source is a ~52 GeV event (90% prob), detected on 2009 
November 20.

A possible counterpart of Fermi J1725.5+1312 is 1RXS J172314.4+142103 
(RA = 260.812500 deg, Decl. = 14.350556 deg; Voges et al. 1999, A&A, 
349, 389) of unknown redshift. It is located 4 arcmin from the Fermi 
J1725.5+1312 best-fit position, and within the gamma-ray 68% positional 
uncertainty. This X-ray source has been previously reported by 
XMM-Newton, as XMMSL1 J172315.0+142102 (observed on 2006-09-04; Saxton 
et al. 2008 A&A, 480, 611). A Swift follow up observation of the field 
of IC201021A was performed on 2020-10-21 and detected 1RXS 
J172314.4+142103 at a flux consistent with catalogued values (GCN 
28724). A possible infrared counterpart of 1RXS J172314.4+142103 is 
WISEA J172314.13+142101.5 (Cutri et al. 2013 wise.rept, 1). Coincident 
with this WISE source there is a faint radio source (~1.1 mJy peak at 3 
GHz), as seen in the the NRAO VLASS quick-look 
image (https://science.nrao.edu/vlass/data-access/vlass-epoch-1-quick-look-users-guide) 
from data obtained on 2019-03-30.

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 
<http://desy.de/>) and S. Buson (sara.buson at uni-wuerzburg.de 
<http://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.