EP241213a/GRB241213a: Likely joint detection and INTEGRAL/SPI-ACS counterpart

The fast X-ray transient, EP241213a, was discovered by the Einstein Probe/WXT (GCN 38554). This transient appears to be spatially coincident with GRB 241213A (BAT: GCN 38547, XRT: GCN 38552), as the XRT coordinates are consistent with the 2.8 arcmin WXT error circle. There is a temporal delay of ~ 100 s between the BAT and WXT detections. We searched for any corresponding emission in the INTEGRAL/SPI-ACS data. In a light curve above 80 keV, we find a pulse temporally coincident with the BAT detection (thus occurring ~ 100 s after the EP/WXT detection, assuming they are the same event), with an approximate duration of 10 sec. The approximate peak count rate is 80000 cts/s over a median background rate of 62500 cts/s. This work is based on observations with INTEGRAL, an ESA project with instruments and a science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, and Spain), and with the participation of Russia and the USA. The SPI-ACS detector system has been provided by MPE Garching/Germany. ----- (a) INAF/IAPS-Rome (b) ICSC National Research Centre for High-Performance Computing