Aspherical Supernovae: Effects on Early Light Curves

Abstract

Early light from core-collapse supernovae, now detectable in high-cadence surveys, holds clues to a star and its environment just before it explodes. However, effects that alter the early light have not been fully explored. We highlight the possibility of non-radial flows at the time of shock breakout. These develop in sufficiently non-spherical explosions if the progenitor is not too diffuse. When they do develop, non-radial flows limit ejecta speeds and cause ejecta-ejecta collisions. We explore these phenomena and their observational implications, using global, axisymmetric, non-relativistic FLASH simulations of simplified polytropic progenitors, which we scale to representative stars. We develop a method to track photon production within the ejecta, enabling us to estimate band-dependent light curves from adiabatic simulations. Immediate breakout emission becomes hidden as an oblique flow develops. Non-spherical effects lead the shock-heated ejecta to release a more constant luminosity at a higher, evolving color temperature at early times, effectively mixing breakout light with the early light curve. Collisions between non-radial ejecta thermalize a small fraction of the explosion energy; we address emission from these collisions in a subsequent paper.

Optical transient from an explosion close to the stellar surface

“…Couch et al 2008Couch et al , 2010 and even fewer accounted for non equilibrium radiation transfer (e.g. Afsariardchi & Matzner 2018). We note that the source of the asymmetry in those previous studied are bipolar jets.…”

Bolometric light curves of aspherical shock breakout

“…The light curves predicted by our model can be compared to numerical light curves calculated by, e.g., Suzuki et al (2016) (see their Figure 7) and Afsariardchi & Matzner (2018) (their Figure 13). In the case of Suzuki et al (2016), a direct comparison of their parameters and ours is difficult because they parametrize the explosion asphericity deep within the star at the inner boundary of the simulation, instead of near the edge of the star where shock acceleration begins.…”

“…The off-axis emission also stays constant for longer before rising, which appears similar to our non-instantaneous light travel time-modified case (i.e., the red curves in panel b) of our Figure 8). Afsariardchi & Matzner (2018), on the other hand, consider sig-nificantly more aspherical explosions. They use a parameter 𝜖 AM 5 to characterize the asphericity.…”

“…In a follow-up paper, Salbi et al (2014) investigated the behaviour of the oblique shock near the point of emergence through hydrodynamics simulations. Later, Afsariardchi & Matzner (2018) conducted global simulations investigating the propagation of a non-relativistic, axisymmetric shock through a simple polytropic star, and estimated the frequency-dependent light curve for several progenitor types. In addition, Suzuki et al (2016) performed coupled radiation-hydrodynamics simulations of aspherical shock breakout and computed bolometric light curves for a typical blue supergiant progenitor.…”

Relativistic Supernova Ejecta Colliding with a Circumstellar Medium: An Application to the Low-luminosity GRB 171205A

“…Although the spherical model succeeds in explaining the properties of the gamma-ray emission, both the ejecta and the CSM can be asymmetric, which could further modify the early gamma-ray and afterglow light curves. As expected in the shock emergence in massive stars, asymmetric shock fronts affect the shock breakout light curve in various ways (Suzuki & Shigeyama 2010;Matzner et al 2013;Suzuki et al 2016;Ohtani et al 2018;Afsariardchi & Matzner 2018). Although llGRBs do not require highly collimated ultra-relativistic jet owing to their low gamma-ray luminosities, deviations from spherical symmetry are naturally expected to some extent.…”