The Physics of Flash (Supernova) Spectroscopy

TL;DR

This paper investigates the physics behind flash spectroscopy of supernova circumstellar media, revealing how emission features constrain CSM properties and explaining observed spectra without requiring pre-supernova eruptions.

Contribution

It introduces a model linking flash spectroscopy features to CSM location, ionization, and acceleration, providing explanations for observations of different supernova types.

Findings

CSM location constrained by Halpha emission timescales

Broad line wings caused by shock acceleration, not pre-SN winds

Observed spectra explained by normal stellar winds in binaries

Abstract

We examine flash spectroscopy of a circumstellar medium (CSM) ionized by the hard radiation pulse produced by the emerging shock of a supernova (SN). We first find that the rise and fall times of the Halpha emission constrains the location of the CSM with a peak at tpeak=Rstar sqrt(2/c vshock) for a star of radius Rstar and a shock velocity of vshock. The dropping temperature of the transient emission naturally reproduces the evolution of lines with different ionization energies. Second, for red supergiants (RSGs), the shock break out radiatively accelerates the CSM to produce broad, early-time line wings independent of the Thomson optical depth of the CSM. Finally, the CSM recombination rates in binaries can be dominated by a dense, cool, wind collision interface like those seen in Wolf-Rayet binaries rather than the individual stellar winds. Combining these three results, the flash spectroscopy observations of the normal Type IIP iPTF13dqy (SN 2013fs) are naturally explained by an RSG with a normal, Thomson optically thin wind in a binary with a separation of ~10^4 Rsun without any need for a pre-SN eruption. Similarly, the broad line wings seen for the Type IIb iPTF13ast (SN 2013cu), whose progenitors are generally yellow supergiants in binaries, are likely due to radiative acceleration of the CSM rather than pre-existing, Wolf-Rayet-like wind.