A MINOT-based Study of Gamma-ray emission from SPT-CL J2012-5649/Abell 3667

Abstract

We present an analysis of the non-thermal properties of the merging galaxy cluster SPT-CL J2012-5649/Abell~3667 ($z = 0.0556$, $M_{500} = 7.16 \times 10^{14}\ M_\odot$) using the MINOT non-thermal emission modeling framework. The predicted hadronic gamma-ray flux from $pp$ interactions in the $1$--$300\ \mathrm{GeV}$ band is $2.82 \times 10^{-11}\ \mathrm{cm^{-2}\ s^{-1}}$ within $R_{500}$, rising to $1.15 \times 10^{-10}\ \mathrm{cm^{-2}\ s^{-1}}$ at the truncation radius ($3.7\,R_{500}$), in broad agreement with the Fermi-LAT reported flux of $1.3 \times 10^{-10}\ \mathrm{cm^{-2}\ s^{-1}}$. Approximately $76\%$ of the predicted hadronic flux originates from beyond $R_{500}$. The Inverse Compton contribution from cosmic-ray electrons is subdominant relative to the hadronic $\pi^0$-decay gamma-ray component by a factor of ${\sim}20$ in the $1$--$300\ \mathrm{GeV}$ energy band, and therefore does not contribute significantly to the observable signal. The best fit of the observed Fermi-LAT data is achieved with a hadronic emission assuming a proton spectrum $\propto E^{-3.5}$ and a proton energy density almost in equipartition with the thermal energy density. Even though those parameters may look extreme, they could well be explained by the particle acceleration by low Mach number accretion shocks.