On the detection of shock breakouts of core-collapse supernovae with the Einstein Probe satellite

Abstract

The shock breakout process and its characteristics for various core-collapse supernovae types are described, after a brief introduction of main observational facts and theoretical hypotheses of core-collapse supernovae. A radiation-dominated shock is formed and transports outwards inside the progenitor star during a core-collapse-induced explosion, while the high-temperature radiation field is trapped behind the shock front that has a finite width corresponding to an optical depth of about 10−20. When the shock reaches close to the progenitor surface, the trapped radiation begins to leak out, mainly in the form of ultraviolet and soft X-ray photons, giving rise to a sudden huge brightening of the star which must be the first electromagnetic signal of a core-collapse supernova. The duration of a shock breakout is very short, ranging from about 10 s for the explosion of a compact Wolf-Rayet star to about 1000 s for that of an extended red supergiant. Because of this, only few questionable candidates have been discovered so far including a serendipitous detection of SN 2008D with an X-ray telescope onboard the Swift satellite. The Einstein Probe satellite will run an all-sky survey of high sensitivity and high cadence in the soft X-ray band with its wide-field telescope of a Lobster-eye type, which is very suitable for the detection of ephemeral supernova shock breakouts. Using some typical theoretical values of shock breakouts, including durations, spectrum temperatures, and peak luminosities, it is predicted that each year the satellite will be able to routinely obtain dozens of light curves of shock breakouts for type II-P supernovae, i.e., explosions of red supergiants. But the rate drops to less than few detections per year for the explosions of blue supergiants and those of Wolf-Rayet stars in total due to compactness of such stars. The shock breakout sample to be built by the Einstein Probe satellite can be used to constrain the progenitors, in particular their radii, pre-explosion mass losses, and explosion mechanisms of core-collapse supernovae.