Early supernovae light-curves following the shock-breakout

Abstract: The first light from a supernova (SN) emerges once the SN shock breaks out of the stellar surface. The first light, typically a UV or X-ray flash, is followed by a broken power-law decay of the luminosity generated by radiation that leaks out of the expanding gas sphere. Motivated by recent detection of emission from very early stages of several SNe, we revisit the theory of shock breakout and the following emission. We derive analytic light curves, paying special attention to the photon-gas coupling and deviations from thermal equilibrium. We then consider the breakout from several SNe progenitors. We find that for more compact progenitors, white dwarfs, Wolf-Rayet stars (WRs) and possibly more energetic blue-supergiant explosions, the observed radiation is out of thermal equilibrium at the breakout, during the planar phase (i.e., before the expanding gas doubles its radius), and during the early spherical phase. Therefore, during these phases we predict significantly higher temperatures than previous analysis that assumed equilibrium. When thermal equilibrium prevails, we find the location of the thermalization depth and its temporal evolution. Our results are useful for interpretation of early SN light curves. Some examples are: (i) Red supergiant SNe have an early bright peak in optical and UV flux, less than an hour after breakout. It is followed by a minimum at the end of the planar phase (about 10 hr), before it peaks again once the temperature drops to the observed frequency range. In contrast WRs show only the latter peak in optical and UV. (ii) Bright X-ray flares are expected from all core-collapse SNe types. (iii) The light curve and spectrum of the initial breakout pulse holds information on the explosion geometry and progenitor wind opacity. Its spectrum in compact progenitors shows a (non-thermal) power-law.

• The paper introduces a new analytical model for early supernova light curves that captures non-thermal emission processes after the shock breakout.
• It demonstrates that deviations from thermal equilibrium yield higher temperatures and distinct light curve features dependent on progenitor properties.
• The study outlines observational strategies to detect critical UV, X-ray, and gamma-ray signals from various supernova progenitors.

Overview of Early Supernovae Light-Curves Following the Shock-Breakout

The paper authored by Ehud Nakar and Re’em Sari presents a comprehensive analytical model for the light curves of supernovae (SNe) during their early phases—particularly following the shock breakout through the stellar surface. This study is motivated by recent observations of electromagnetic emissions from the nascent stages of several SNe, which are crucial for understanding the progenitor star properties and the underlying supernova explosion mechanisms.

Supernova Shock-Breakout and Early Emission

Upon a supernova explosion, a shock wave propagates through the stellar envelope. When this shock emerges at the star's surface—a process termed shock breakout—an observable electromagnetic pulse is produced. This initial pulse is typically a UV or X-ray flash, which subsequently transitions into a longer-lasting emission as radiation diffuses out from the expanding stellar materials.

The authors focus on analyzing the phase following the initial shock breakout, characterized by non-thermal emissions in more compact progenitors, such as white dwarfs, Wolf-Rayet stars, and potentially blue supergiants, where thermal equilibrium assumptions inadequately describe the radiation properties. They propose models that diverge from traditional thermal equilibrium analyses, predicting higher temperatures during this initial phase for certain stellar types.

Analytical Model of Early SN Light Curves

Nakar and Sari derive simplified analytic expressions for the time-evolving light curves (in terms of luminosity and temperature) post-shock breakout, catering to various progenitor stars. A key advancement in their model is the thorough treatment of photon-gas coupling, noting conditions that diverge significantly from thermal equilibrium. This contrasts with previous works that heavily relied on equilibrium assumptions without accommodating additional non-thermal processes, particularly relevant for compact progenitors.

Their results suggest that departures from thermal equilibrium can substantially alter the expected light signatures. This includes not only initially higher temperatures but also significant variations in light curves based on the progenitor's physical parameters—mass, radius, and explosion energy.

Predicted Light-Curve Features for Different Progenitor Types

1. Red Supergiant (RSG): Characterized by an early bright peak in optical and UV spectra soon after the breakout, followed by a flux minimum and then another peak, corresponding to the point when the temperature falls into the observed frequency band.
2. Blue Supergiant (BSG): The evolution tracks differ based on whether the breakout emission maintains thermal equilibrium. When non-equilibrium conditions prevail, an initially high observed temperature decreases rapidly until thermalization occurs.
3. Wolf-Rayet Stars (WRs): The study predicts persistent non-thermal conditions resulting in significant shifts in expected temperatures and spectral features, with intense X-ray and potentially gamma-ray emissions during initial breakout.
4. White Dwarfs (WDs): For Type Ia SN, the paper extends current models, highlighting that the early emission can include gamma-ray levels, challenging the capabilities of existing observational tools for SN breakout detections.

Implications for Observations and Future Research

Nakar and Sari provide a robust framework for interpreting early SN data, proposing specific observational strategies for various progenitor scenarios. Their work emphasizes the critical importance of early SN light curves in revealing the dynamics and geometry of the explosion, as well as the characteristics of progenitor winds. This marks a step forward in utilizing early SN emissions for studying progenitor systems and refining theoretical models of supernovae. Future developments in observational technologies and methodologies could leverage these findings to improve SN detection capabilities and analysis, particularly in the UV, X-ray, and gamma-ray regimes.