A Search for Hard X-ray/Soft $\gamma$-ray Emission from SPT-CL J2012-5649 (Abell 3667) Using INTEGRAL/ISGRI

TL;DR

This study searched for hard X-ray/soft gamma-ray emission from galaxy cluster SPT-CL J2012-5649 using INTEGRAL data, found no significant emission, and set upper limits that constrain particle acceleration models in merging clusters.

Contribution

First to set stringent upper limits on IC emission in SPT-CL J2012-5649 using INTEGRAL, constraining merger-driven particle acceleration efficiency.

Findings

No significant emission detected in 30-300 keV range.

Established a 3σ upper limit of 4.4×10⁻¹⁰ erg cm⁻² s⁻¹.

Results constrain models of relativistic electron populations in the cluster.

Abstract

We present a search for hard X-ray/soft $\gamma$-ray emission from the merging galaxy cluster SPT-CL J2012-5649 (Abell~3667) using archival INTEGRAL/ISGRI observations. This cluster located at $z=0.0556$ hosts prominent radio relics associated with merger-driven shocks, suggesting the presence of relativistic electrons capable of producing inverse-Compton (IC) emission in the hard X-ray to soft $\gamma$-ray regime. We searched for emission in the 30--300~keV energy range using the INTEGRAL Off-line scientific analysis software with a total effective exposure of 2817 s. No significant emission was detected at the cluster position in the aforementioned energy interval. The extracted ISGRI spectrum is consistent with pure background, and no physically meaningful model parameters can be constrained. From the mosaic variance maps, we derive a conservative $3\sigma$ upper limit of $F_{30-100\,\mathrm{keV}} < 4.4 \times 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. This limit rules out bright IC scenarios and constrains the efficiency of merger-driven particle acceleration in SPT-CL J2012-5649. Our results provide the most stringent soft $\gamma$-ray constraint on this system to date and highlight the need for next-generation hard X-ray missions, such as HEX-P or eXTP, to probe IC emission at theoretically predicted levels in merging clusters.