GCN 44107: EP260321a: Gemini-South confirmation of a supernova

Jillian Rastinejad, Gokul Srinivasaragavan (UMD), and Tomas Ahumada (NOIRLab) report on behalf of a larger collaboration:

Following the detection of the putative optical counterpart (Lee et al., GCN 44070; Tanvir et al., GCN 44082; Moran et al., GCN 44083, Ahumada et al., GCN 44084, Liu et al., GCN 44087, de Ugarte Postigo et al., GCN 44091, Pankov et al. GCN 44103) of EP 260321a (Huang et al., GCN 44068), we obtained optical spectroscopy using the Gemini Multi-Object Spectrograph (GMOS) mounted on Gemini-South, under Program GS-2026A-Q-119. The mean time of the observation was 23:50 UT on 2026 Mar 23 (3.5 days post-burst). The total exposure time was 4x600 s using the B480 grating, and covered the wavelength range 3500-7500 AA.

We clearly detect emission lines due to the underlying galaxy, which are well-matched to z = 0.0343 (Tanvir et al., GCN 44082). We also observe broad absorption features across a rest-frame wavelength range of 4200-5000 AA (assuming z=0.0343), consistent with the broad absorption lines observed in Type Ic-BL supernovae. Our observations confirm the presence of a supernova noted by Xu et al., GCN 44092 and Corcoran et al., GCN 44105.

We thank the Gemini staff for the rapid scheduling and execution of these observations.