PS15cey and PS17cke: prospective candidates from the Pan-STARRS Search for Kilonovae

TL;DR

This paper analyzes two fast, faint transient candidates from Pan-STARRS, exploring their nature and potential as kilonovae or merger events, contributing to the understanding of fast transients in the universe.

Contribution

It presents detailed analysis of two candidate kilonovae, proposing possible models and scenarios, including merger events and stellar outbursts, expanding knowledge of fast transient phenomena.

Findings

PS15cey was a luminous, fast transient possibly from a BH-NS merger or similar event.

PS17cke is a faint, fast transient plausibly identified as a kilonova candidate.

Both candidates exhibit properties consistent with fast, faint transients, with PS17cke being a strong kilonova candidate.

Abstract

Time domain astronomy was revolutionised with the discovery of the first kilonova, AT2017gfo, in August 2017 which was associated with the gravitational wave signal GW170817. Since this event, numerous wide-field surveys have been optimising search strategies to maximise their efficiency of detecting these fast and faint transients. With the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), we have been conducting a volume limited survey for intrinsically faint and fast fading events to a distance of $D\simeq200$ Mpc. Two promising candidates have been identified from this archival search, with sparse data - PS15cey and PS17cke. Here we present more detailed analysis and discussion of their nature. We observe that PS15cey was a luminous, fast declining transient at 320 Mpc. Models of BH-NS mergers with a very stiff equation of state could possibly reproduce the luminosity and decline but the physical parameters are extreme. A more likely scenario is that this was a SN2018kzr-like merger event. PS17cke was a faint and fast declining event at 15 Mpc. We explore several explosion scenarios of this transient including models of it as a NS-NS and BH-NS merger, the outburst of a massive luminous star, and compare it against other known fast fading transients. Although there is uncertainty in the explosion scenario due to difficulty in measuring the explosion epoch, we find PS17cke to be a plausible kilonova candidate from the model comparisons.