GCN Circular 46231
Subject
EP261105a: ePESSTO+ host galaxy spectroscopic redshift measurement of z=0.0765
Event
Date
2026-11-11T22:11:17Z (14 hours ago)
From
James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>
Via
Web form
J. H. Gillanders, S. Srivastav (Oxford), S. J. Smartt (Oxford/QUB), A. J. Smith, C. Angus (QUB), L. Eastman (Oxford), D. Magill (QUB), A. Martin-Carrillo (UCD), M. Bulla (Ferrara), K. W. Smith (Oxford/QUB), M. Nicholl (QUB), M. Pursiainen (Warwick), G. Dimitriadis (Lancaster), J. Anderson (ESO), T.-W. Chen (NCU), M. Gromadzki (Warsaw), C. Inserra (Cardiff), E. Kankare (Turku), T. Müller Bravo (Trinity), P. Pessi (Warsaw), O. Yaron (Weizmann), D. R. Young (QUB), A. Badash (Weizmann).
ePESSTO+, the advanced Public ESO Spectroscopic Survey for Transient Objects (Smartt et al., 2015, 2015A&A...579A..40S) observed the potential host galaxy of the optical counterpart (Ma et al., GCN 46160; see also Waseem et al., GCNs 46170 & 46191) to the fast X-ray transient EP261105a (Guo et al., GCNs 46155 & 46168).
Our observation was carried out with ESO’s New Technology Telescope (NTT) at La Silla, employing EFOSC2 and Grism #13 (sampling 365-925 nm). Starting at 2026-11-11 06:51 UTC, it consisted of a single 2700s exposure employing a 1.0 arcsec slit.
The reduced spectrum shows a number of prominent galaxy emission lines. From jointly fitting H-alpha, H-beta, [O II] 3726, [O III] 4959, 5007, and [N II] 6548, 6583, we estimate a spectroscopic redshift, z_spec = 0.0765 +/- 0.0002. This is larger than the photometric redshift measurement from Legacy Surveys DR9 (z_phot = 0.055 +/- 0.014). This z_spec value corresponds to a distance modulus of 37.7 mag, and luminosity distance of 347 Mpc (assuming H0 = 70 km/s/Mpc, and flat lambdaCDM cosmology).
With this more precise redshift estimate, and assuming this galaxy is indeed the host of the optical counterpart to EP261105a, the FORS2 R-band detection of ~24.3 mag at 18.0 hours post-X-ray detection (Waseem et al., GCN 46170) corresponds to M_R ~ -13.6 (accounting for Milky Way extinction; Schlafly and Finkbeiner, 2011, 2011ApJ...737..103S). This, and the HAWKI J-band detection of ~24 mag at 46 hours (Waseem et al., GCN 46191) lie at the faint end of model kilonova predictions (Bulla, 2023, 2023MNRAS.520.2558B; Ahumada et al., 2026, 2026PASP..138c4101A). However, the two reported R-band detections (Ma et al., GCN 46160; Waseem et al., GCN 46170) and one r-band upper limit (van Hoof et al., GCN 46162) are also entirely consistent with a power-law decay. At present, the available data cannot be used to distinguish between the possibility of a faint kilonova in this nearby host, or a higher-redshift transient event in chance coincidence.