A Fast-Evolving, Luminous Transient Discovered by K2/Kepler

TL;DR

This paper reports the discovery and analysis of KSN2015K, an extremely fast and luminous transient, showing that its light curve is best explained by shock breakout in dense circumstellar material rather than radioactive decay.

Contribution

It presents the most extreme fast-evolving luminous transient observed to date and models its light curve as resulting from shock breakout in dense external material.

Findings

KSN2015K has a rise time of 2.2 days and a peak brightness comparable to supernovae.

Its light curve is inconsistent with radioactive decay powering.

The transient is best explained by shock breakout in dense circumstellar material.

Abstract

For decades optical time-domain searches have been tuned to find ordinary supernovae, which rise and fall in brightness over a period of weeks. Recently, supernova searches have improved their cadences and a handful of fast-evolving luminous transients (FELTs) have been identified. FELTs have peak luminosities comparable to Type Ia supernovae, but rise to maximum in $<10$ days and fade from view in $<$month. Here we present the most extreme example of this class thus far, KSN2015K, with a rise time of only 2.2 days and a time above half-maximum ($t_{1/2}$) of only 6.8 days. Here we show that, unlike Type Ia supernovae, the light curve of KSN2015K was not powered by the decay of radioactive elements. We further argue that it is unlikely that it was powered by continuing energy deposition from a central remnant (a magnetar or black hole). Using numerical radiation hydrodynamical models, we show that the light curve of KSN2015K is well fit by a model where the supernova runs into external material presumably expelled in a pre-supernova mass loss episode. The rapid rise of KSN2015K therefore probes the venting of photons when a hypersonic shock wave breaks out of a dense extended medium.