Search for GeV gamma-ray emission from PSZ G181.06+48.47 galaxy cluster using Fermi-LAT data

We present a search for high energy gamma rays in the energy range from 1--300 GeV from the galaxy cluster PSZ2 G181.06+48.47 using 17.9 years of Fermi-LAT data. A binned likelihood analysis employing the 16-year source catalog reveals a significant gamma-ray excess at the cluster position. Modelling the emission as a point source yields a detection with TS=19.0 (4.3 sigma), a photon index of =3.20 plus or minus 0.62, and an integrated photon flux of (9.75 plus or minus2.74) times10-11, ph,cm-2,s-1, but leaves statistically significant residual emission at the cluster center. Replacing the point-source hypothesis with a RadialGaussian spatial template significantly improves the fit, with a preferred width of sigma=0.4 and an extension significance of TS ext=36.8 (6.0 sigma). The adopted Gaussian model yields a source detection significance of TS=55.0 (7.4 sigma), a photon index of =2.62 plus or minus0.26, and an integrated photon flux of (3.22 plus or minus0.50) times10-10, ph,cm-2,s-1, while reducing the residual emission at the cluster position to a level consistent with zero. The cluster spectral energy distribution shows significant emission only in the lowest energy interval (1.0--3.13 GeV), while all higher-energy bins are consistent with upper limits, indicating a soft gamma-ray spectrum with no evidence for emission above about5 GeV. These results provide strong evidence that the gamma-ray emission associated with PSZ2 G181.06+48.47 is spatially extended on a scale of sigma about0.4 and is more consistent with diffuse intracluster emission than with a single unresolved point source.