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Pan-STARRS follow-up of the gravitational-wave event S250818k and the lightcurve of SN 2025ulz

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.

Pan-STARRS follow-up of the gravitational-wave event S250818k and the lightcurve of SN 2025ulz

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 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 () 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 % of the distance posterior sampled by our Pan-STARRS pointings (% 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 days, before ultimately being ruled out.

Paper Structure

This paper contains 19 sections, 4 equations, 7 figures.

Table of Contents

  1. Introduction
  2. S250818k
  3. Scanning the skymap of S250818k
  4. Pan-STARRS
  5. ATLAS
  6. Candidate counterparts
  7. Most viable candidates
  8. Alternative counterpart scenario
  9. SN 2025ulz
  10. Initial discovery and observations
  11. Historical activity search in Pan-STARRS and ATLAS
  12. Redshift estimation
  13. Observed properties of SN 2025ulz
  14. Comparison to SNe IIb
  15. Comparison to AT 2017gfo
  16. ...and 4 more sections

Figures (7)

  • Figure 1: Comparison of the GW signal properties of S250818k to those from the LVK GWTC-3 catalogue Abbott2023_GWTC3, following Nicholl2025.
  • Figure 2: Top: The bilby.fits skymap from LVKGCNDiscovery with our targeted Pan-STARRS tiling observations performed within seven days post-burst overlaid. Bottom: Same as above, but with the ATLAS observation footprints within seven days post-burst overlaid.
  • Figure 3: SNIFS spectrum of the host galaxy of SN 2025ulz. We mark the location of prominent H$\alpha$, [N ii] $\lambda6583.46$, [S ii] $\lambda \lambda 6716.44 \ \& \ 6730.81$ emission, which allows us to measure a host galaxy redshift, $z = 0.0849 \pm 0.0003$.
  • Figure 4: Lightcurve of SN 2025ulz. We include the discovery $gr$-band data from ZTF Stein2025GCN.41414 in addition to our multi-colour Pan-STARRS, ATLAS, SLT and LOT data. Hollow symbols correspond to $3 \sigma$ upper limits.
  • Figure 5: Top: Comparison of SN 2025ulz$gri$-band evolution to the early lightcurves of SNe 1993J ($BRI$), 2008ax ($gri$), 2011dh ($gri$) and 2016gkg ($gri$), all SNe IIb that possess early-time observations. The evolution of SN 2025ulz closely resembles the evolution of these transient events. Bottom: Bolometric luminosity ($L_{\rm BOL}$) lightcurve property comparisons between SN 2025ulz, SNe 1993J, 2008ax, 2011dh and 2016gkg, and the KN AT 2017gfo. The $L_{\rm BOL}$ evolution of SN 2025ulz closely mirrors the comparison sample of SNe IIb, whereas it exhibits a stark difference to that of AT 2017gfo. A similar trend exists for the temperature comparison.
  • ...and 2 more figures