Pan-STARRS follow-up of the gravitational-wave event S250818k and the lightcurve of SN 2025ulz
Authors
J. H. Gillanders
M. E. Huber
M. Nicholl
S. J. Smartt
K. W. Smith
K. C. Chambers
D. R. Young
J. W. Tweddle
S. Srivastav
M. D. Fulton
F. Stoppa
G. S. H. Paek
A. Aamer
M. R. Alarcon
A. Andersson
A. Aryan
K. Auchettl
T. -W. Chen
T. de Boer
A. K. H. Kong
J. Licandro
T. Lowe
D. Magill
E. A. Magnier
P. Minguez
T. Moore
G. Pignata
A. Rest
M. Serra-Ricart
B. J. Shappee
I. A. Smith
M. A. Tucker
R. Wainscoat
Abstract
Kilonovae are the scientifically rich, but observationally elusive, optical transient phenomena associated with compact binary mergers. Only a handful of events have been discovered to date, all through multi-wavelength (gamma ray) and multi-messenger (gravitational wave) signals. Given their scarcity, it is important to maximise the discovery possibility of new kilonova events. To this end, we present our follow-up observations of the gravitational-wave signal, S250818k, a plausible binary neutron star merger at a distance of $237 \pm 62$ Mpc. Pan-STARRS tiled 286 and 318 square degrees (32% and 34% of the 90% sky localisation region) within 3 and 7 days of the GW signal, respectively. ATLAS covered 65% of the skymap within 3 days, but with lower sensitivity. These observations uncovered 47 new transients; however, none were deemed to be linked to S250818k. We undertook an expansive follow-up campaign of AT 2025ulz, the purported counterpart to S250818k. The griz-band lightcurve, combined with our redshift measurement ($z = 0.0849 \pm 0.0003$) all indicate that SN 2025ulz is a SN IIb, and thus not the counterpart to S250818k. We rule out the presence of a AT 2017gfo-like kilonova within $\approx 27$% of the distance posterior sampled by our Pan-STARRS pointings ($\approx 9.1$% across the total 90% three-dimensional sky localisation). We demonstrate that early observations are optimal for probing the distance posterior of the three-dimensional gravitational-wave skymap, and that SN 2025ulz was a plausible kilonova candidate for $\lesssim 5$ days, before ultimately being ruled out.
