arXiv subjects

A. Marcowith

Publications and source records attributed to A. Marcowith.

62 records · Page 4Linked to original sources

arXiv2007

The Monoceros very-high-energy gamma-ray source

The H.E.S.S. telescope array has observed the complex Monoceros Loop SNR/Rosette Nebula region which contains unidentified high energy EGRET sources and potential very-high-energy (VHE) gamma-ray source. We announce the discovery of a new point-like VHE gamma-ray sources, HESS J0632+057. It is located close to the rim of the Monoceros SNR and has no clear counterpart at other wavelengths. Data from the NANTEN telescope have been used to investigate hadronic interactions with nearby molecular clouds. We found no evidence for a clear association. The VHE gamma-ray emission is possibly associated with the lower energy gamma-ray source 3EG J0634+0521, a weak X-ray source 1RXS J063258.3+054857 and the Be-star MWC 148.

astro-ph↗

arXiv2007

Transport of positrons in the interstellar medium

This work investigates some aspects of the transport of low-energy positrons in the interstellar medium (ISM). We consider resonance interactions with magnetohydrodynamic waves above the resonance threshold. Below the threshold, collisions take over and deflect positrons in their motion parallel to magnetic-field lines. Using Monte-Carlo simulations, we model the propagation and energy losses of positrons in the different phases of the ISM until they annihilate. We suggest that positrons produced in the disk by an old population of stars, with initial kinetic energies below 1 MeV, and propagating in the spiral magnetic field of the disk, can probably not penetrate the Galactic bulge.

astro-ph↗

arXiv2006

Gravitational effects on the high energy emission of accreting black holes

We extend the investigation of general relativistic effects on the observed X-ray continuum of Kerr black holes in the context of the light bending model (Miniutti & Fabian, 2004). Assuming a ring-like illuminating source, co-rotating with the underlying accretion disk, we study the shape and normalisation of the primary and disc reflected continuum as well as the dependence of the observed spectrum on the line of sight for various source heights and radii. These calculations are performed using Monte-Carlo methods to compute the angle dependent reflection spectrum from the disc. The effects of general relativity are illustrated by a comparison with Newtonian and Special Relativity calculations. We find that relativistic distortions can strongly affect the shape of the reflected spectrum. Light bending can dramatically increase the observable reflected flux and reduce the primary emission. In addition, multiple reflections due to the reflected photons deflected toward the disc can alter significantly the shape of the spectrum above 10 keV. Large variations of the ring radius at constant height can lead to an (unobserved) anti-correlation between primary and reflected flux. In another side, the variability behaviour of several sources can be reproduced if the ring source radius is small (<5 r_g), and its height varies by a large factor. In particular, a non-linear flux-flux relation, similar to that observed in several sources, can be produced. We compare our model with the flux-flux plot of NGC4051, and find an agreement for low inclination angles (<20 deg.), ring source radius <3 r_g and a height varying between 0.5 to 10 r_g.

astro-ph↗

arXiv2006

Observational constraints on energetic particle diffusion in young SNRs: amplified magnetic field and maximum energy

Constraints on the diffusion and acceleration parameters in five young supernova remnants (SNRs) are derived from the observed thickness of their X-ray rims, as limited by the synchrotron losses of the highest energy electrons, assuming uniform and isotropic turbulence. It is shown that the magnetic field must be amplified up to values between 250 and 500 microGauss in the case of Cas A, Kepler, and Tycho, or ~ 100 microGauss in the case of SN 1006 and G347.3-0.5. The diffusion coefficient at the highest electron energy can also be derived from the data, by relating the X-ray energy cutoff to the acceleration timescale. Values typically between 1 and 10 times the Bohm diffusion coefficient are found to be required. We also find interesting constraints on the energy dependence of the diffusion coefficient. This favours diffusion regimes between the Kraichnan and the Bohm regime, and rejects turbulence spectrum indices larger than ~ 3/2. Finally, the maximum energy of the accelerated particles is found to lay between 10^13 and 5 10^13 eV for electrons, and around Z \times 8 10^14 eV at most for nuclei (or ~ 2.5 times less if a Bohm diffusion regime is assumed), roughly independently of the compression ratio assumed at the shock. Even by taking advantage of the uncertainties on the measured parameters, it appears very difficult for the considered SNRs in their current stage of evolution to produce protons up to the knee of the cosmic-ray spectrum, at ~ 3 10^15 eV, and essentially impossible to accelerate Fe nuclei up to either the ankle at ~ 3 10^18 eV or the second knee at ~ 5 10^17 eV.

astro-ph↗

arXiv2005

Spectral analysis of the Galactic e+e- annihilation emission

We present a spectral analysis of the e+e- annihilation emission from the Galactic Centre region based on the first year of measurements made with the spectrometer SPI of the INTEGRAL mission. We have found that the annihilation spectrum can be modelled by the sum of a narrow and a broad 511 keV line plus an ortho-Ps continuum. The broad line is detected with a flux of (0.35+/-0.11)e-3 s-1 cm-2. The measured width of 5.4+/-1.2 keV FWHM is in agreement with the expected broadening of 511 keV photons emitted in the annihilation of Ps that are formed by the charge exchange process of slowing down positrons with H atoms. The flux of the narrow line is (0.72+/-0.12)e-3 s-1 cm-2 and its width is 1.3+/-0.4 keV FWHM. The measured ortho-Ps continuum flux yields a fraction of Ps of (96.7+/-2.2)%. To derive in what phase of the interstellar medium positrons annihilate, we have fitted annihilation models calculated for each phase to the data. We have found that 49(+2,-23)% of the annihilation emission comes from the warm neutral phase and 51(+3,-2)% from the warm ionized phase. While we may not exclude that less than 23% of the emission might come from cold gas, we have constrained the fraction of annihilation emission from molecular clouds and hot gas to be less than 8% and 0.5%, respectively. We have compared our knowledge of the interstellar medium in the bulge and the propagation of positrons with our results and found that they are in good agreement if the sources are diffusively distributed and if the initial kinetic energy of positrons is lower than a few MeV. Despite its large filling factor, the lack of annihilation emission from the hot gas is due to its low density, which allows positrons to escape this phase.

astro-ph↗

arXiv2004

Superbubbles and Energetic Particles in the Galaxy. I: Collective effects of particle acceleration

Observations indicate that most massive stars in the Galaxy appear in groups, called OB associations, where their strong wind activity generates large structures known as superbubbles, inside which the subsequent supernovae (SNe) explode, in tight space and time correlation. Acknowledging this fact, we investigate four main questions: 1) does the clustering of massive stars and SN explosions influence the particle acceleration process usually associated with SNe, and induce collective effects which would not manifest around isolated supernova remnants?; 2) does it make a difference for the general phenomenology of Galactic Cosmic Rays (GCRs), notably for their energy spectrum and composition?; 3) can this help alleviate some of the problems encountered within the standard GCR source model?; and 4) Is the link between superbubbles and energetic particles supported by observational data, and can it be further tested and constrained? We argue for a positive answer to all these questions. Theoretical, phenomenological and observational aspects are treated in separate papers. Here, we discuss the interaction of massive stellar winds and SN shocks inside superbubbles and indicate how this leads to specific acceleration effects. We also show that due to the high SN explosion rate and low diffusion coefficient, low-energy particles experience repeated shock acceleration inside superbubbles.

astro-ph↗

arXiv2003

Relativistic particle transport in extragalactic jets: I. Coupling MHD and kinetic theory

Multidimensional magneto-hydrodynamical (MHD) simulations coupled with stochastic differential equations (SDEs) adapted to test particle acceleration and transport in complex astrophysical flows are presented. The numerical scheme allows the investigation of shock acceleration, adiabatic and radiative losses as well as diffusive spatial transport in various diffusion regimes. The applicability of SDEs to astrophysics is first discussed in regards to the different regimes and the MHD code spatial resolution. The procedure is then applied to 2.5D MHD-SDE simulations of kilo-parsec scale extragalactic jets. The ability of SDE to reproduce analytical solutions of the diffusion-convection equation for electrons is tested through the incorporation of an increasing number of effects: shock acceleration, spatially dependent diffusion coefficients and synchrotron losses. The SDEs prove to be efficient in various shock configuration occurring in the inner jet during the development of the Kelvin-Helmholtz instability. The particle acceleration in snapshots of strong single and multiple shock acceleration including realistic spatial transport is treated. In chaotic magnetic diffusion regime, turbulence levels $η_T=<δB^2>/(B^2+<δB^2>)$ around $0.2-0.3$ are found to be the most efficient to enable particles to reach the highest energies. The spectrum, extending from 100 MeV to few TeV (or even 100 TeV for fast flows), does not exhibit a power-law shape due to transverse momentum dependent escapes. Out of this range, the confinement is not so efficient and the spectrum cut-off above few hundreds of GeV, questioning the Chandra observations of X-ray knots as being synchrotron radiation. The extension to full time dependent simulations to X-ray extragalactic jets is discussed.

astro-ph↗

arXiv1999

Computation of diffusive shock acceleration using stochastic differential equations

The present work considers diffusive shock acceleration at non-relativistic shocks using a system of stochastic differential equations (SDE) equivalent to the Fokker-Planck equation. We compute approximate solutions of the transport of cosmic particles at shock fronts with a SDE numerical scheme. The momentum gain is given by implicit calculations of the fluid velocity gradients using a linear interpolation between two consecutive time steps. We validate our procedure in the case of single shock acceleration with different shock thickness, with or without synchrotron losses. A comparative discussion of implicit and explicit schemes for different shock thickness shows that implicit calculations extend the range of applicability of SDE schemes to infinitely thin 1D shocks. The method is then applied to multiple shock acceleration for a system of identical shocks which free parameters are the distance between two consecutive shocks, the synchrotron losses time and the escape time of the particles. The stationary distribution reproduces quite well the flat differential logarithm energy distribution produced by multiple shock effect, and also the piling-up effect due synchrotron losses at a momentum where they equilibrate the acceleration rate. At higher momenta particle losses dominate and the spectrum drops. The competition between acceleration and loss effects leads to a pile-up shaped distribution which appears to be effective only in a restrict range of inter-shock distances of 10-100 diffusion lengths. We finally compute the optically thin synchrotron spectrum produced such periodic pattern which can explain flat and/or inverted spectra observed in Flat Radio spectrum Quasars and in the galactic centre.

astro-ph↗