  The ASKAP-EMU survey is a deep wide-field radio continuum survey designed to
cover the entire southern sky and a significant fraction of the northern sky up
to $+30^{\circ}$. Here, we report a discovery of a radio relic in the merging
cluster SPT-CL 2023-5535 at z=0.23 from the ASKAP-EMU pilot 300 sq. deg survey
(800-1088 MHz). The deep high-resolution data reveal a $\sim2$ Mpc-scale radio
halo elongated in the east-west direction, coincident with the intracluster
gas. The radio relic is located at the western edge of this radio halo
stretched $\sim0.5$ Mpc in the north-south orientation. The integrated spectral
index of the radio relic within the narrow bandwidth is $\alpha^{\scriptstyle
\rm 1088~MHz}_{\scriptstyle \rm 800~MHz}=-0.76 \pm 0.06$. Our weak-lensing
analysis shows that the system is massive ($M_{200}=1.04\pm0.36\times 10^{15}
M_{\odot}$) and composed of at least three subclusters. We suggest a scenario,
wherein the radio features arise from the collision between the eastern and
middle subclusters. Our discovery illustrates the effectiveness of the
ASKAP-EMU survey in detecting diffuse emissions in galaxy clusters and when
completed, the survey will greatly increase the number of merging cluster
detections with diffuse radio emissions.
