GCN Circular 41670
Subject
EP250827b: NOT spectroscopic observations and AT 2025wkm classification as a broad-lined type-Ic supernova
Event
Date
2025-09-03T13:24:21Z (11 days ago)
Edited On
2025-09-04T12:27:26Z (10 days ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
Web form
G. Corcoran (UCD), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud), R. A. J. Eyles-Ferris (Leicester), V. Vuolteenaho (NOT and Oulu Univ.), M. Pursiainen (Warwick), I. Worssam (Birmingham) report:
We carried out two spectroscopic observations of the optical transient AT 2025wkm (Schroeder et al., GCN 41635) likely associated with the Einstein Probe transient EP250827b, using the ALFOSC spectrograph mounted on the Nordic Optical Telescope (NOT). The observations started on 2025-09-02 at 00:31:54 and 2025-09-03 at 04:37:19 UT for the two epochs, respectively (5.76 and 6.93 days after the EP trigger). Both observations consisted of 4 x 900 s exposures and cover the wavelength range 3800-9500 AA.
Both spectra show features consistent with broad-lined type Ic supernovae at a comparable phase, though with an excess of flux on the blue end, providing a good match to the super-luminous SN 2011kl (associated with GRB 111209A; Greiner et al. 2015, doi:10.1038/nature14579) near peak. The brightness of the source, at an absolute magnitude of M_r = -18.91 and still rising (Eyles-Ferris & Starling, GCN 41669), is currently consistent with classification as both a normal Ic-BL SN and a super-luminous SN. Between the two epochs, we also identify no significant spectral evolution consistent with the constant colour observed by Eyles-Ferris & Starling (GCN 41669).
Further photometric and spectroscopic observations are planned and encouraged to better characterize this interesting source.