High-Energy Shock Breakout from Supernovae and Gamma-ray Bursts

Abstract

Cosmic explosions play a critical role in a broad range of astrophysical fields. Although considerable progress has been made to understand the explosive engines and their progenitors, many of the details are not well understood. One of the most powerful electromagnetic probes of the explosive mechanism and the stellar progenitor is the first burst of photons emitted from this blastwave as it exits the stellar photosphere, known as shock breakout (SBO). Our understanding of SBO has evolved considerably in the past decade. Shock heating as the blastwave propagates through the star and circumstellar material can drastically alter this emission producing a much broader range of potential SBO signals than that predicted by standard analytical approaches. Here we present a semi-analytic approach to model this diverse SBO emission, focused on thermal Bremsstrahlung radiation, which more accurately captures the complexities in Nature over previous treatments. We calculate a range of signals for a range of supernova and gamma-ray burst types. Our models demonstrate how we can use these signals to place constraints on the nature of the explosive engines and better understand the role SBO can play in prompt gamma-ray bursts. We study the implications of these results to historic observations, Einstein Probe transients, and in the context of proposed missions. We find that stripped envelope events can be detected serendipitously with survey telescopes, but type Ia and II SBO detections require fast-pointing X-ray observations in response to early warning alerts from gravitational wave or neutrino detectors.

Figures (21)

• Figure 1: Spectra as a function of time from the initial shock breakout from two of our SBO models for Ibc supernovae driven by convective engines: (top) $E_{\rm in} = 10^{48} {\rm erg}$, Area $=10^{24} {\rm \, cm^2}$, $p=6$, $\Gamma_{\rm max}=2$; (bottom) $E_{\rm in} = 10^{48} {\rm erg}$, Area $=10^{24} {\rm \, cm^2}$, $p=4$, $\Gamma_{\rm max}=10$. These models initially peak in the X-rays (a few keV) but the peak photon energy quickly decreases to a few hundred eV or less. Covering the emission from 1 eV up to 10 keV is required to study all aspects of this emission.
• Figure 2: X-ray Luminosity above 0.3 keV (dashed) and above 1.5 kev (solid) for 2 model suites with total fast-ejecta energies ($\beta \Gamma >0.1$) of $10^{48}$ (top) and $10^{50} \, {\rm erg \, s^{-1}}$ (bottom) and $p=4$. For each of these energies, we study the light curves for 5 emitting areas: $10^{20},10^{21},10^{22},10^{23},10^{24} {\rm \, cm^2}$. The peak luminosity scales with total energy (although not exactly linearly). Higher emitting areas lead to brighter peaks of shorter duration. A diverse set of light-curves can be produced varying these 2 parameters.
• Figure 3: X-ray luminosity for 2 convective-engine driven Ibc model suites with emitting areas of $10^{20}{\rm \, cm^2}$ and $10^{24} {\rm \, cm^2}$ with a total energy of $10^{48}$ erg. The larger emitting area leads emission that peaks at a higher luminosity but evolves more quickly than the lower emitting area. The peak Lorentz factor is 10, the total energy with velocities $\beta \Gamma > 0.1$ and $\Gamma_{\rm max}$ are kept the same while varying the power-law index of the distribution of Lorentz factor: $p$ = 2 (solid), 4 (dashed), 6 (dotted). To study the role of this peak Lorentz factor, we include 3 additional $p$=6 models varying this peak value as shown in orange: $\Gamma_{\rm max}$ = 2 (solid), 1.2 (dashed), 1.1 (dotted).
• Figure 4: Luminosity (erg s$^{-1}$) between 1.5-100 keV versus time for strong jet ($\Gamma = 100, 200, 500$) explosions both on-axis (solid lines) and (maximally) off-axis (dashed lines). For the on-axis models, the high Lorentz factor Doppler shifts much of the emission to energies above 100 keV at early times. As such, the off-axis models are brighter in the X-ray at early times. The integrated luminosities will also be higher.
• Figure 5: Spectra for a series of snapshots in time from our $\Gamma=200$ model both on-axis (top) and off-axis (bottom). For the on-axis model, the emitted photons are initially above a few hundred keV, including some emission above 1 MeV. But the ejecta quickly cools and, within 0.5 s, the photon energy has already dropped below a few hundred keV. Without the Lorentz boost, the emission from the off-axis model is below 30 keV.