GCN Circular 41705
Subject
EP250827b: Gemini Spectroscopic Confirmation of AT2025wkm as a Ic-BL Supernova
Event
Date
2025-09-04T17:53:57Z (3 months ago)
Edited On
2025-09-05T13:21:23Z (3 months ago)
From
Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>
Via
Web form
Brendan O’Connor (CMU), Xander J. Hall (CMU), Antonella Palmese (CMU), Daniel Gruen (LMU), Gokul Srinivasaragavan (UMD), Anna Y. Q. Ho (Cornell):
We observed the optical transient AT2025wkm, associated with EP250827b (Schroeder et al., GCN 41635), using GMOS mounted on Gemini-North (PI: O’Connor). Observations started on 2025-09-02 at 13:36:55 UT (6.3 d) and 2025-09-04 at 10:58:00 UT (8.2 d). At each epoch, we obtained 2x600 s exposures using the B480 grating covering 3950-8210 AA.
The spectra show a steep blueward slope (likely due to shock cooling) and display broad features that are consistent with ejecta moving at a high velocity (see also Corcoran et al., GCN 41670 and Ho et al., GCN 41639). We identify blueshifted Fe II absorption features in both spectra, indicative of ejecta velocities ~ 40,000 km/s. The Fe II absorption features in combination with the lack of broad Hydrogen and Helium features associated with the transient itself allow us to confirm the classification of a Ic-BL supernova as initially reported by Corcoran et al. (GCN 41670). In addition, we identify numerous narrow emission lines, including the [O II] doublet, multiple [Ne III] lines, the entire Balmer series between Heta and Halpha, He I lines, [N II], [S II], and [Ar III], that identify the host galaxy as highly star forming.
Further observations are planned and encouraged.
We thank the staff of the Gemini Observatory, in particular Brian Lemaux, Eunchong Kim, Teo Mocnik, and Aleksandar Cikota for their assistance in planning and obtaining these observations.