447 research outputs found
Alma Reville and Joan Harrison during production of SUSPICION, 1941
No full textScreenwriters Alma Reville, left, and Joan Harrison review pages during production of SUSPICION, 1941. 8x10 b&w photographic print
RADIATIVE SIGNATURES OF RELATIVISTIC SHOCKS
No full textParticle-in-cell simulations of relativistic, weakly magnetized collisionless shocks show that particles can gain energy by repeatedly crossing the shock front. This requires scattering off self-generated small length-scale magnetic fluctuations. The radiative signature of this first-order Fermi acceleration mechanism is important for models of both the prompt and afterglow emission in gamma-ray bursts and depends on the strength parameter a = lambda e/delta B/mc(2) of the fluctuations (lambda is the length scale and vertical bar delta B vertical bar is the magnitude of the fluctuations). For electrons (and positrons), acceleration saturates when the radiative losses produced by the scattering cannot be compensated by the energy gained on crossing the shock. We show that this sets an upper limit on both the electron Lorentz factor gamma <10(6) (n/1 cm(-3))(-1/6)(-1/6) and on the energy of the photons radiated during the scattering process h omega(max) <40Max(a, 1)(n/1 cm(-3))(1/6)(-1/6) eV, where n is the number density of the plasma and (gamma) over bar is the thermal Lorentz factor of the downstream plasma, provided a <a(crit) similar to 10(6). This rules out "jitter" radiation on self-excited fluctuations with a <I as a source of gamma rays, although high-energy photons might still be produced when the jitter photons are upscattered in an analog of the synchrotron self-Compton process. In fluctuations with a > 1, radiation is generated by the standard synchrotron mechanism, and the maximum photon energy rises linearly with a, until saturating at 70 MeV, when a = a(crit).</p
MAGNETIC FIELD GENERATION BY BIERMANN BATTERY AND WEIBEL INSTABILITY IN LABORATORY SHOCK WAVES
No full textMagnetic field generation in the Universe is still an open problem. Possible mechanisms involve the Weibel instability, due to anisotropic phase-space distributions, as well as the Biermann battery, due to misaligned density and temperature gradients. These mechanisms can be reproduced in scaled laboratory experiments. In this contribution we estimate the relative importance of these two processes and explore the laser-energy requirements for producing Weibel dominated shocks. © The Author(s) 2013
Supernova remnants and the effect of efficient cosmic ray acceleration
No full textIt is well known that electrons and probably cosmic rays are accelerated in supernova remnants. To maintain the observed steady galactic cosmic-ray spectrum, it is necessary that they be very efficient accelerators. The back reaction of the high-energy particles on the shock structure and environment are reviewed with a particular emphasis on the amplification of magnetic fields
The amplification of magnetic fields in parallel shocks
No full textThe amplification of magnetic fields due to plasma instabilities in various energetic environments is a crucial issue for our understanding of particle acceleration and the observed emission from these regions. This process is supported both by observations of large magnetic fields close to the outer shocks of supernova remnants and also by the theoretical motivation to explain the origin of galactic cosmic rays. The so-called non-resonant current driven instability seems to be a likely mechanism capable of driving such strong amplification. The role of this instability in various environments is reviewed, and recent results from numerical simulations are presented
Cosmic ray transport in self-excited turbulence
No full textFirst order Fermi acceleration at the outer shocks of supernova remnants is believed be an efficient process. If this is indeed true, the effect of the cosmic-ray pressure on the uid properties of the upstream plasma, can not be neglected. It is well known that the resulting pressure gradient leads to the production of an extended shock precursor. It has been suggested by Bell (2004) that cosmic rays in the precursor will also have a strong in uence on the macroscopic properties of the magnetic field. Observational evidence of bright X-ray synchrotron rims in several young supernova support this theory. Amplification of the magnetic field beyond the linear regime, via the non-resonant current driven instability is investigated. We report on numerical calculations of magnetic field growth and the resulting transport properties of relativistic particles in the amplified field
Efficient cosmic-ray acceleration in the recurrent nova RS Ophuichi revealed by H.E.S.S.
No full textRecurrent Novae (RNe) undergo episodic eruptions in the form of thermonuclear explosions, due to the accumulation of material accreted by a white dwarf from a binary companion star. The well known RN RS Ophiuchi (RS Oph) underwent its latest eruption in 2021, triggering numerous follow-up observations, including with the High Energy Stereoscopic System (H.E.S.S.), an array of Imaging Atmospheric Cherenkov Telescopes (IACTs) situated in Khomas Highland, Namibia. H.E.S.S. observes the sky in the very-high-energy (VHE) gamma-ray regime of ~100 GeV to a few tens of TeV. Non-thermal emission was observed coincident with the nova eruption within the first days and up to a month after the initial burst event, establishing novae as Galactic transients reaching TeV energies. Analysis and interpretation of the data identifies time-resolved acceleration of cosmic-rays, constraining models of particle energisation. Combining the data taken by H.E.S.S. with concurrent observations taken by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, a consistent temporal and spectral profile is observed, favouring a common origin to the emission. The detection and interpretation of the non-thermal VHE emission from the RN RS Oph by H.E.S.S. will be presented
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