Observability of flashes from ejecta crashes in aspherical supernovae, with application to SN 2008D

TL;DR

This study investigates whether collisions of ejecta in aspherical supernovae can produce observable flashes, using simulations and analysis of the SN 2008D x-ray transient, suggesting such flashes are possible under specific conditions.

Contribution

The paper introduces a simplified model connecting aspherical supernova explosions to observable ejecta collision flashes, supported by hydrodynamic simulations and analysis of real transient data.

Findings

Ejecta collisions can produce photon flashes under certain asphericity conditions.

The x-ray transient SN 2008D's properties align with predictions from the aspherical collision model.

Simulations indicate a narrow range of asphericity favors observable flashes.

Abstract

A new class of transient, which has been hypothesized to accompany the explosion of an aspherical compact supernova, would arise when streams of ejecta collide outside the star. However, conditions that favour the prompt release of radiation from the collision, such as a diffuse stellar envelope, disfavour the creation of non-radial ejecta in the first place. To determine whether the collision can both occur and be visible, we simulate aspherical explosions using the HUJI-RICH moving-mesh hydrodynamics code and analyze them in terms of diffusion measures defined for individual fluid elements. While our simulations are highly idealized, they connect to realistic explosions via a single dimensionless parameter. Defining two measures of the importance of diffusivity (two versions of the inverse P\'eclet number), we find that one varies in a way that indicates colliding ejecta can release a photon flash, while the other does not. Examining the x-ray transient XT 080109 associated with supernova SN 2008D, we find that its fluence and duration are consistent with the properties of an ejecta collision in the aspherical model that is most likely to emit a flash. Our results give tentative evidence for the possibility of collision-induced flashes for a narrow and radius-dependent range of asphericity, and motivate future radiation hydrodynamics simulations.