GCN Circular 46131
Subject
EP261015a/SN 2026ahoa: VLT/FORS2 discovery of a pre-peak type Ic-BL supernova at a tentative spectroscopic redshift z=0.31
Event
Date
2026-11-02T21:40:20Z (10 days ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
G. Corcoran (UCD), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. J. Levan (Radboud and Warwick), A. de Ugarte Postigo (LAM), M. Fraser (UCD) report on behalf of the Stargate collaboration:
We observed the likely optical counterpart (Corcoran et al., GCN 45955; Moreno Méndez et al., GCN 45960; Fortin et al., GCN 45991; Volnova et al., GCN 45997; Pawar et al., GCN 45998; Shivaprasad et al., GCN 46000; Pankov et al., GCN 46003; Gou et al., GCN 46009; Burkhonov et al., GCN 46017; Izzo GCN 46021) of EP261015a using the ESO VLT UT1 (Antu) equipped with the FORS2 instrument. A total of 7x600 s exposures were secured using the grism 300V with the GG435 order blocker, covering the wavelength range 4450 - 8650 AA, starting at 2026-10-30 06:30:55 UT (14.72 days after the EP/WXT trigger). The position angle of the slit was set to cover both the position of the transient and that of the host galaxy nucleus. Although the observation was done under very changeable weather with seeing ~1.4”, the trace can be seen clearly in all individual exposures.
A preliminary reduction of the spectrum reveals several very low signal-to-noise emission lines ([O II] doublet, [O III] doublet, Hbeta and Halpha) consistent with a redshift solution of z = 0.31. Inspection of a FORS2 300I grism spectrum consisting of 3x600 s exposures and taken on 2026-10-23 07:34:24 UT (7.76 days after trigger), seems to confirm this solution through a low signal-to-noise detection of the Halpha emission line.
Using SNID-SAGE (Stoppa et al. 2026, doi:10.1093/mnras/stag1066), we identify clear supernova features consistent with a broad-lined type Ic (Ic-BL) supernova pre-peak. If the redshift is left free, the best matches seem to favour redshifts z ~ 0.3 supporting the tentative result found from the host emission lines analysis. When the redshift is fixed to z = 0.31, SNID-SAGE finds a satisfactory match with the archetypical SN 1998bw at 9.2 days before peak.
At this redshift, SN 2026ahoa has an absolute magnitude of M_r ~ -19.3, consistent with that seen in the GRB/FXT-SN population. These observations suggest that the identified optical counterpart is likely the afterglow of EP261015a. Further spectroscopic and photometric observations are planned and encouraged.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Luca Sbordone, Aaron Labdon, Hannah Osborne, Israel Blanchard and Gonzalo Burgos.