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Register a new userOn photohadronic processes in astrophysical environments
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A. Muecke
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We discuss the first applications of our newly developed Monte Carlo event generator SOPHIA to multiparticle photoproduction of relativistic protons with thermal and power law radiation fields. The measured total cross section is reproduced in terms of excitation and decay of baryon resonances, direct pion production, diffractive scattering, and non-diffractive multiparticle production. Non--diffractive multiparticle production is described using a string fragmentation model. We demonstrate that the widely used `$Delta$--approximation for the photoproduction cross section is reasonable only for a restricted set of astrophysical applications. The relevance of this result for cosmic ray propagation through the microwave background and hadronic models of active galactic nuclei and gamma-ray bursts is briefly discussed.
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Astrophysical neutrino fluxes are often modeled as power laws of the energy. This is reasonable in the case of hadronic sources, but it does not capture the behavior in photohadronic sources, where the spectrum depends on the properties of the target photons on which protons collide. This limits the possibility of a unified treatment of different sources. In order to overcome this difficulty, we model the target photons by a blackbody spectrum. This model is sufficiently flexible to reproduce neutrino fluxes from known photohadronic sources; we apply it to study the sensitivity of Dense Neutrino Arrays, Neutrino Telescopes and Neutrino Radio Arrays to photohadronic sources. We also classify the flavor composition of the neutrino spectrum in terms of the parameter space. We discuss the interplay with the experiments, studying the changes in the track-to-shower ratio induced by different flavor compositions, both within and outside the region of the Glashow resonance.
Magnetic reconnection is a basic plasma process of dramatic rearrangement of magnetic topology, often leading to a violent release of magnetic energy. It is important in magnetic fusion and in space and solar physics --- areas that have so far provided the context for most of reconnection research. Importantly, these environments consist just of electrons and ions and the dissipated energy always stays with the plasma. In contrast, in this paper I introduce a new direction of research, motivated by several important problems in high-energy astrophysics --- reconnection in high energy density (HED) radiative plasmas, where radiation pressure and radiative cooling become dominant factors in the pressure and energy balance. I identify the key processes distinguishing HED reconnection: special-relativistic effects; radiative effects (radiative cooling, radiation pressure, and Compton resistivity); and, at the most extreme end, QED effects, including pair creation. I then discuss the main astrophysical applications --- situations with magnetar-strength fields (exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares and magnetically-powered central engines and jets of GRBs. Here, magnetic energy density is so high that its dissipation heats the plasma to MeV temperatures. Electron-positron pairs are then copiously produced, making the reconnection layer highly collisional and dressing it in a thick pair coat that traps radiation. The pressure is dominated by radiation and pairs. Yet, radiation diffusion across the layer may be faster than the global Alfven transit time; then, radiative cooling governs the thermodynamics and reconnection becomes a radiative transfer problem, greatly affected by the ultra-strong magnetic field. This overall picture is very different from our traditional picture of reconnection and thus represents a new frontier in reconnection research.
Certain classes of astrophysical objects, namely magnetars and central engines of supernovae and gamma-ray bursts (GRBs), are characterized by extreme physical conditions not encountered elsewhere in the Universe. In particular, they possess magnetic fields that exceed the critical quantum field of 44 teragauss. Figuring out how these complex ultra-magnetized systems work requires understanding various plasma processes, both small-scale kinetic and large-scale magnetohydrodynamic (MHD). However, an ultra-strong magnetic field modifies the underlying physics to such an extent that many relevant plasma-physical problems call for building QED-based relativistic quantum plasma physics. In this review, after describing the extreme astrophysical systems of interest and identifying the key relevant plasma-physical problems, we survey the recent progress in the development of such a theory. We discuss how a super-critical field modifies the properties of vacuum and matter and outline the basic theoretical framework for describing both non-relativistic and relativistic quantum plasmas. We then turn to astrophysical applications of relativistic QED plasma physics relevant to magnetar magnetospheres and central engines of supernovae and long GRBs. Specifically, we discuss propagation of light through a magnetar magnetosphere; large-scale MHD processes driving magnetar activity and GRB jet launching and propagation; energy-transport processes governing the thermodynamics of extreme plasma environments; micro-scale kinetic plasma processes important in the interaction of intense magnetospheric electric currents with a magnetars surface; and magnetic reconnection of ultra-strong magnetic fields. Finally, we point out that future progress will require the development of numerical modeling capabilities.
Several models for type Ia-like supernovae events rely on the production of a self-sustained detonation powered by nuclear reactions. In the absence of hydrogen, the fuel that powers these detonations typically consists of either pure helium (He) or a mixture of carbon and oxygen (C/O). Studies that systematically determine the conditions required to initiate detonations in C/O material exist, but until now no analogous investigation of He matter has been conducted. We perform one-dimensional reactive hydrodynamical simulations at a variety of initial density and temperature combinations and find critical length scales for the initiation of He detonations that range between 1 - $10^{10}$ cm. A simple estimate of the length scales over which the total consumption of fuel will occur for steady-state detonations is provided by the Chapman-Jouguet (CJ) formalism. Our initiation lengths are consistently smaller than the corresponding CJ length scales by a factor of $Sim 100$, providing opportunities for thermonuclear explosions in a wider range of low-mass white dwarfs (WDs) than previously thought possible. We find that virialized WDs with as little mass as 0.24 $M_odot$ can be detonated, and that even less massive WDs can be detonated if a sizable fraction of their mass is raised to a higher adiabat. That the initiation length is exceeded by the CJ length implies that certain systems may not reach nuclear statistical equilibrium within the time it takes a detonation to traverse the object. In support of this hypothesis, we demonstrate that incomplete burning will occur in the majority of He WD detonations and that $^{40}$Ca, $^{44}$Ti, or $^{48}$Cr, rather than $^{56}$Ni, is the predominant burning product for many of these events. We anticipate that ...
We examine whether the newly derived neutrino spin coherence could lead to large-scale coherent neutrino-antineutrino conversion. In a linear analysis we find that such transformation is largely suppressed, but demonstrate that nonlinear feedback can enhance it. We point out that conditions which favor this feedback may exist in core collapse supernovae and in binary neutron star mergers.
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