Sub-relativistic Shock Research Articles

Particle-in-Cell simulation of cosmic ray acceleration in fast blue optical transients

Results of Particle-in-Cell modelling of cosmic ray acceleration in sub-relativistic shocks (with speed about 0.3 speed of light) are presented. Synchrotron and Inverse Compton radiation from FBOT CSS161010 are calculated using particle distributions, obtained from PIC simulation. Source parameters are evaluated via fitting modeled radiation with observations.

Thermal Electrons in Mildly Relativistic Synchrotron Blast Waves

Abstract Numerical models of collisionless shocks robustly predict an electron distribution composed of both thermal and nonthermal electrons. Here, we explore in detail the effect of thermal electrons on the emergent synchrotron emission from subrelativistic shocks. We present a complete “thermal + nonthermal” synchrotron model and derive properties of the resulting spectrum and light curves. Using these results, we delineate the relative importance of thermal and nonthermal electrons for subrelativistic shock-powered synchrotron transients. We find that thermal electrons are naturally expected to contribute significantly to the peak emission if the shock velocity is ≳0.2c, but would be mostly undetectable in nonrelativistic shocks. This helps explain the dichotomy between typical radio supernovae and the emerging class of “AT2018cow-like” events. The signpost of thermal electron synchrotron emission is a steep optically-thin spectral index and a ν 2 optically-thick spectrum. These spectral features are also predicted to correlate with a steep postpeak light-curve decline rate, broadly consistent with observed AT2018cow-like events. We expect that thermal electrons may be observable in other contexts where mildly relativistic shocks are present and briefly estimate this effect for gamma-ray burst afterglows and binary–neutron-star mergers. Our model can be used to fit spectra and light curves of events and accounts for both thermal and nonthermal electron populations with no additional physical degrees of freedom.

Plasma kinetic effects in relativistic radiation-mediated shocks.

Fast shocks that form in optically thick media are mediated by Compton scattering and, if relativistic, pair creation. Since the radiation force acts primarily on electrons and positrons, the question arises of how the force is mediated to the ions which are the dominant carriers of the shock energy. It has been widely thought that a small charge separation induced by the radiation force generates an electric field inside the shock that decelerates the ions. In this paper we argue that, while this is true in subrelativistic shocks which are devoid of positrons, in relativistic radiation mediated shocks (RRMS), which are dominated by newly created e^{+}e^{-} pairs, additional coupling is needed, owing to the opposite electric force acting on electrons and positrons. Specifically, we show that dissipation of the ions energy must involve collective plasma interactions. By constructing a multifluid model for RRMS that incorporates friction forces, we estimate that momentum transfer between electrons and positrons (and/or ions) via collective interactions on scales of tens to thousands of proton skin depths, depending on whether friction is effective only between e^{+}e^{-} pairs or also between pairs and ions, is sufficient to couple all particles and radiation inside the shock into a single fluid. This leaves open the question of whether in relativistic RMS particles can effectively accelerate to high energies by scattering off plasma turbulence. Such acceleration might have important consequences for relativistic shock breakout signals.

SN 2016coi (ASASSN-16fp): An Energetic H-stripped Core-collapse Supernova from a Massive Stellar Progenitor with Large Mass Loss

Abstract We present comprehensive observations and analysis of the energetic H-stripped SN 2016coi (a.k.a. ASASSN-16fp), spanning the γ-ray through optical and radio wavelengths, acquired within the first hours to ∼420 days post explosion. Our observational campaign confirms the identification of He in the supernova (SN) ejecta, which we interpret to be caused by a larger mixing of Ni into the outer ejecta layers. By modeling the broad bolometric light curve, we derive a large ejecta-mass-to-kinetic-energy ratio (M ej ∼ 4–7 M ⊙, E k ∼ (7–8) × 1051 erg). The small [Ca ii] λλ7291,7324 to [O i] λλ6300,6364 ratio (∼0.2) observed in our late-time optical spectra is suggestive of a large progenitor core mass at the time of collapse. We find that SN 2016coi is a luminous source of X-rays (L X > 1039 erg s−1 in the first ∼100 days post explosion) and radio emission (L 8.5 GHz ∼ 7 × 1027 erg s−1 Hz−1 at peak). These values are in line with those of relativistic SNe (2009bb, 2012ap). However, for SN 2016coi, we infer substantial pre-explosion progenitor mass loss with a rate ∼ (1–2) × and a sub-relativistic shock velocity v sh ∼ 0.15c, which is in stark contrast with relativistic SNe and similar to normal SNe. Finally, we find no evidence for a SN-associated shock breakout γ-ray pulse with energy E γ > 2 × 1046 erg. While we cannot exclude the presence of a companion in a binary system, taken together, our findings are consistent with a massive single-star progenitor that experienced large mass loss in the years leading up to core collapse, but was unable to achieve complete stripping of its outer layers before explosion.

The spectrum of a fast shock breakout from a stellar wind

The breakout of a fast ($>0.1 c$), yet sub-relativistic shock from a thick stellar wind is expected to produce a pulse of X-rays with a rise time of seconds to hours. Here, we construct a semi-analytic model for the breakout of a sub-relativistic, radiation-mediated shock from a thick stellar wind, and use it to compute the spectrum of the breakout emission. The model incorporates photon escape through the finite optical depth wind, assuming a diffusion approximation and a quasi-steady evolution of the shock structure during the breakout phase. We find that in sufficiently fast shocks, for which the breakout velocity exceeds about $0.1c$, the time-integrated spectrum of the breakout pulse is non-thermal, and the time-resolved temperature is expected to exhibit substantial decrease (roughly by one order of magnitude) during breakout, when the flux is still rising, because of the photon generation by the shock compression associated with the photon escape. We also derive a closure relation between the breakout duration, peak luminosity, and characteristic temperature that can be used to test whether an observed X-ray flare is consistent with being associated with a sub-relativistic shock breakout from a thick stellar wind or not. We also discuss implications of the spectral softening for a possible breakout event XRT 080109/SN 2008D.

The Efficiency of Electron Acceleration in Collisionless Shocks and Gamma‐Ray Burst Energetics

Afterglow observations are commonly used to determine the parameters of GRB explosions, the energy E, surrounding density n, postshock magnetic field equipartition fraction B, and electron equipartition fraction e, under the frequently made assumption that the efficiency of electron injection into relativistic shock acceleration is high, i.e., that the fraction f of electrons that undergo acceleration is f ≈ 1. We show that the value of f cannot be determined by current observations, since currently testable model predictions for a parameter choice {E' = E/f, n' = n/f, = fB, = fe} are independent of the value of f for me/mp ≤ f ≤ 1. Current observations imply that the efficiency f is similar for highly relativistic and subrelativistic shocks and plausibly suggest that f ~ 1, quite unlike the situation in the Crab Nebula. However, values me/mp ≤ f 1 cannot be ruled out, implying a factor me/mp uncertainty in determination of model parameters. We show that early, ≤10 hr, radio afterglow observations, which will be far more accessible in the Swift era, may provide constraints on f. Such observations will therefore provide a powerful diagnostic of GRB explosions and of the physics of particle acceleration in collisionless shocks.

A Subrelativistic Shock Model for the Radio Emission of SN 1998bw

SN 1998bw is the most luminous radio supernova ever observed. Previous discussions argued that its exceptional radio luminosity, ~4 × 1038 ergs s-1, must originate from a highly relativistic shock that is fully decoupled from the supernova ejecta. Here we present an alternative model in which the radio emission originates from a subrelativistic shock with a velocity 0.3c, generated in the surrounding gas by the expanding ejecta. In this model, thermal electrons heated by the shock to a relativistic temperature of ~60 MeV emit synchrotron self-absorbed radiation in the postshock magnetic field. This model provides an excellent fit to the observed spectra provided that the thermal electrons are in equipartition with the ions behind the shock. The required magnetic field is much weaker than its equipartition value and could have been carried out by the progenitor's wind prior to the supernova explosion. Our model demonstrates that the radio emission from SN 1998bw is not necessarily related to the highly relativistic blast wave that produced the γ-ray burst GRB 980425.