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 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 ) 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.