Luke O'c. Drury

The H.E.S.S. Survey of the inner galaxy in very high energy gamma rays.

We report on a survey of the inner part of the Galactic Plane in very high energy gamma-rays, with the H.E.S.S. Cherenkov telescope system. The Galactic Plane between +-30deg in longitude and +-3deg in latitude relative to the Galactic Centre was observed in 500 pointings for a total of 230 hours, reaching an average flux sensitivity of 2% of the Crab Nebula at energies above 200 GeV. Fourteen previously unknown sources were detected at a significance level greater than 4 sigma after accounting for all trials involved in the search. Initial results on the eight most significant of these sources were already reported elsewhere. Here we present detailed spectral and morphological information for all the new sources, along with a discussion on possible counterparts in other wavelength bands. The distribution in Galactic latitude of the detected sources appears to be consistent with a scale height in the Galactic disk for the parent population smaller than 100 pc, consistent with expectations for supernova remnants and/or pulsar wind nebulae.We report on a survey of the inner part of the Galactic plane in very high energy gamma rays with the H.E.S.S. Cerenkov telescope system. The Galactic plane between +/-30° in longitude and +/-3° in latitude relative to the Galactic center was observed in 500 pointings for a total of 230 hr, reaching an average flux sensitivity of 2% of the Crab Nebula at energies above 200 GeV. Fourteen previously unknown sources were detected at a significance level greater than 4 σ after accounting for all trials involved in the search. Initial results on the eight most significant of these sources were already reported elsewhere (Aharonian and coworkers). Here we present detailed spectral and morphological information for all the new sources, along with a discussion on possible counterparts in other wavelength bands. The distribution in Galactic latitude of the detected sources appears to be consistent with a scale height in the Galactic disk for the parent population smaller than 100 pc, consistent with expectations for supernova remnants and/or pulsar wind nebulae.

High-energy particle acceleration in the shell of a supernova remnant

A significant fraction of the energy density of the interstellar medium is in the form of high-energy charged particles (cosmic rays). The origin of these particles remains uncertain. Although it is generally accepted that the only sources capable of supplying the energy required to accelerate the bulk of Galactic cosmic rays are supernova explosions, and even though the mechanism of particle acceleration in expanding supernova remnant (SNR) shocks is thought to be well understood theoretically, unequivocal evidence for the production of high-energy particles in supernova shells has proven remarkably hard to find. Here we report on observations of the SNR RX J1713.7 - 3946 (G347.3 - 0.5), which was discovered by ROSAT in the X-ray spectrum and later claimed as a source of high-energy γ-rays of TeV energies (1 TeV = 1012 eV). We present a TeV γ-ray image of the SNR: the spatially resolved remnant has a shell morphology similar to that seen in X-rays, which demonstrates that very-high-energy particles are accelerated there. The energy spectrum indicates efficient acceleration of charged particles to energies beyond 100 TeV, consistent with current ideas of particle acceleration in young SNR shocks.

Galactic Cosmic Rays from Supernova Remnants. II. Shock Acceleration of Gas and Dust

We present a quantitative model of Galactic cosmic-ray (GCR) origin and acceleration, wherein a mixture of interstellar and/or circumstellar gas and dust is accelerated by a supernova remnant blast wave. The gas and dust are accelerated simultaneously, but differences in how each component is treated by the shock leave a distinctive signature, which we believe exists in the cosmic-ray composition data. A reexamination of the detailed GCR elemental composition, presented in a companion paper, has led us to abandon the long-held assumption that GCR abundances are somehow determined by first ionization potential. Instead, volatility and mass (presumably mass-to-charge ratio) seem to better organize the data: among the volatile elements, the abundance enhancements relative to solar increase with mass (except for the slightly high H/He ratio); the more refractory elements seem systematically overabundant relative to the more volatile ones in a quasi-mass-independent fashion. If this is the case, material locked in grains in the interstellar medium must be accelerated to cosmic-ray energies more efficiently than interstellar gas-phase ions. Here we present results from a nonlinear shock model that includes (1) the direct acceleration of interstellar gas-phase ions, (2) a simplified model for the direct acceleration of weakly charged grains to ~100 keV amu-1 energies, simultaneously with the acceleration of the gas ions, (3) the energy losses of grains colliding with the ambient gas, (4) the sputtering of grains, and (5) the simultaneous acceleration of the sputtered ions to GeV and TeV energies. We show that the model produces GCR source abundance enhancements of the volatile, gas-phase elements that are an increasing function of mass, as well as a net, mass-independent enhancement of the refractory, grain elements over protons, consistent with cosmic-ray observations. We also investigate the implications of the slightly high H/He ratio. The GCR22Ne excess may also be accounted for in terms of the acceleration of 22Ne-enriched presupernova Wolf-Rayet star wind material surrounding the most massive supernovae. We also show that cosmic-ray source spectra, at least below ~1014 eV, are well matched by the model.

Galactic Cosmic Rays from Supernova Remnants. I. A Cosmic-Ray Composition Controlled by Volatility and Mass-to-Charge Ratio

We show that the Galactic cosmic-ray source (GCRS) composition is best described in terms of (1) a general enhancement of the refractory elements relative to the volatile ones, and (2) among the volatile elements, an enhancement of the heavier elements relative to the lighter ones. This mass dependence most likely reflects a mass-to-charge (A/Q) dependence of the acceleration efficiency; among the refractory elements, there is no such enhancement of heavier species, or only a much weaker one. We regard as coincidental the similarity between the GCRS composition and that of the solar corona, which is biased according to first ionization potential. In a companion paper, this GCRS composition is interpreted in terms of an acceleration by supernova shock waves of interstellar and/or circumstellar (e.g.,22Ne-rich Wolf-Rayet wind) gas-phase and, especially, dust material.

Calibration of cameras of the H.E.S.S. detector

We report the detection of a point-like source of very high en ergy (VHE) γ-rays coincident within 1 ′ of Sgr A, obtained with the H.E.S.S. array of Cherenkov telescopes. The γ-rays exhibit a power-law energy spectrum with a spectral in dex of −2.2±0.09±0.15 and a flux above the 165 GeV threshold of (1 .82±0.22)·10−7m−2s−1. The measured flux and spectrum di ffer substantially from recent results reported in particular b y the CANGAROO collaboration.

Diffusive Shock Acceleration and Magnetic Field Amplification

Diffusive shock acceleration is the theory of particle acceleration through multiple shock crossings. In order for this process to proceed at a rate that can be reconciled with observations of high-energy electrons in the vicinity of the shock, and for cosmic rays protons to be accelerated to energies up to observed galactic values, significant magnetic field amplification is required. In this review we will discuss various theories on how magnetic field amplification can proceed in the presence of a cosmic ray population. On both short and long length scales, cosmic ray streaming can induce instabilities that act to amplify the magnetic field. Developments in this area that have occurred over the past decade are the main focus of this paper.

The puzzling MILAGRO hot spots

We discuss the reported detection by the MILAGRO experiment of localised hot spots in the cosmic ray arrival distribution and the difficulty of interpreting these observations. A model based on secondary neutron production in the heliotail is shown to fail. An alternative model based on loss-cone leakage through a magnetic trap from a local source region is proposed.

First-order Fermi acceleration driven by magnetic reconnection

A box model is used to study first-order Fermi acceleration driven by magnetic reconnection. It is shown, at least in this simple model, that the spectral index of the accelerated particles is related to the total compression in the same way as in diffusive shock acceleration and is not, as has been suggested, a universal E−5/2 spectrum. The acceleration time-scale is estimated and some comments made about the applicability of the process.

The Formation of a Relativistic Partially Electromagnetic Planar Plasma Shock

Relativistically colliding plasma is modeled by particle-in-cell simulations in one and two spatial dimensions, with an ion-to-electron mass ratio of 400 and a temperature of 100 keV. The energy of an initial quasi-parallel magnetic field is 1% of the plasma kinetic energy. Energy dissipation by a growing wave pulse of mixed polarity, probably an oblique whistler wave, and different densities of the colliding plasma slabs result in the formation of an energetic electromagnetic structure within milliseconds. The structure, which develops for an initial collision speed of 0.9c, accelerates electrons to Lorentz factors of several hundred. A downstream region forms, separating the forward and reverse shocks. In this region, the plasma approaches an energy equipartition between electrons, ions, and the magnetic field. The electron energy spectrum -->N(E) resembles a power law at high energies, with an exponent close to –2.7, or -->N(E) E−2.7. The magnetic field reflects upstream ions, which form a beam and drag the electrons along to preserve the plasma quasineutrality. The forward and reverse shocks are asymmetric due to the unequal slab densities. The forward shock may be representative for the internal shocks of gamma-ray bursts.

Explosion implosion duality and the laboratory simulation of astrophysical systems

The Euler equations of ideal gas dynamics possess a remarkable nonlinear involutional symmetry which allows one to factor out an arbitrary uniform expansion or contraction of the system. The nature of this symmetry (called by cosmologists the transformation to supercomoving variables) is discussed and its origin clarified. It is pointed out that this symmetry allows one to map an explosion problem to a dual implosion problem and vice versa. The application to laboratory simulations of supernova remnants is considered; in principle this duality allows the complete three-dimensional evolution of highly structured explosion ejecta to be modeled using a static target in an implosion facility.