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

Time domain astronomy was revolutionized 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 optimizing search strategies to maximize 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 similar or equal to 200 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 an AT2018kzr-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.

Automated summary

The discovery of the first kilonova, AT2017gfo, revolutionized time domain astronomy in August 2017, associated with the gravitational wave signal GW170817. Wide-field surveys have been optimizing search strategies to maximize efficiency in detecting fast and faint transients since then. Two promising candidates, PS15cey and PS17cke, have been identified from an archival search, with PS15cey being a luminous, fast-declining transient at 320 Mpc, and PS17cke being a faint and fast-declining event at 15 Mpc. Various explosion scenarios have been explored for PS17cke, with uncertainty in the explosion epoch but a plausible kilonova candidate based on model comparisons.