Pan-STARRS Follow-up of the Gravitational-wave Event S250818k and the Light Curve of SN2025ulz

Gillanders, JH; Huber, ME; Nicholl, M; Smartt, SJ; Smith, KW; Chambers, KC; Young, DR; Tweddle, JW; Srivastav, S; Fulton, MD; Stoppa, F; Paek, GSH; Aamer, A; Alarcon, MR; Andersson, A; Aryan, A; Auchettl, K; Chen, T; de Boer, T; Kong, AKH; Licandro, J; Lowe, T; Magill, D; Magnier, EA

Journal article

Pan-STARRS Follow-up of the Gravitational-wave Event S250818k and the Light Curve of SN2025ulz

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 multiwavelength (gamma-ray) and multimessenger (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 ± 62 Mpc. Pan-STARRS tiled 286 and 318 deg2 (32% and 34% of the 90% sky localisation region) within 3 and 7 days of the GW signal, respectively. ATLAS covered 65% of the sky map 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 AT2025ulz, the purported counterpart to S250818k. The griz-band light curve, combined with our redshift measurement (z = 0.0849 ± 0.0003), all indicate that SN2025ulz is a type IIb supernova and thus not the counterpart to S250818k. We rule out the presence of an AT2017gfo-like kilonova within ≈27% of the distance posterior sampled by our Pan-STARRS pointings (≈9.1% across the total 90% 3D sky localisation). We demonstrate that early observations are optimal for probing the distance posterior of the 3D gravitational-wave sky map, and that SN2025ulz was a plausible kilonova candidate for ≲5 days, before ultimately being ruled out.

Publication status:: Published

Peer review status:: Peer reviewed

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APA Style

Gillanders, J. H., Huber, M. E., Nicholl, M., Smartt, S. J., Smith, K. W., Chambers, K. C., Young, D. R., Tweddle, J. W., Srivastav, S., Fulton, M. D., Stoppa, F., Paek, G. S. H., Aamer, A., Alarcon, M. R., Andersson, A., Aryan, A., Auchettl, K., Chen, T., de Boer, T., … Magnier, E. A. (2025). Pan-STARRS Follow-up of the Gravitational-wave Event S250818k and the Light Curve of SN2025ulz. The Astrophysical Journal Letters, 995(1).

MLA Style

Gillanders, JH, et al. “Pan-STARRS Follow-up of the Gravitational-Wave Event S250818k and the Light Curve of SN2025ulz.” The Astrophysical Journal Letters, vol. 995, no. 1, 2025.

Chicago Style

Gillanders, JH, ME Huber, M Nicholl, et al. 2025. “Pan-STARRS Follow-up of the Gravitational-Wave Event S250818k and the Light Curve of SN2025ulz.” The Astrophysical Journal Letters 995 (1).
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