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

Abstract

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 $\mathrm{TS}=19.0(4.3\sigma )$, a photon index of $\Gamma =3.20\pm 0.62$, and an integrated photon flux of $(9.75\pm 2.74)\times 10^{-11}\mathrm{ph}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$, 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 $\sigma =0.4^{\circ }$ and an extension significance of ${\mathrm{TS}}_{\mathrm{ext}}=36.8(6.0\sigma )$. The adopted Gaussian model yields a source detection significance of $\mathrm{TS}=55.0(7.4\sigma )$, a photon index of $\Gamma =2.62\pm 0.26$, and an integrated photon flux of $(3.22\pm 0.50)\times 10^{-10}\mathrm{ph}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$, 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 $\sim 5$~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 $\sigma \approx 0.4^{\circ }$ and is more consistent with diffuse intracluster emission than with a single unresolved point source.

24 pages, 6 figures, 6 tables