689 research outputs found
The Monoceros very-high-energy gamma-ray source
No full textThe 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.A. Fiasson, J. A. Hinton, Y. Gallant, A. Marcowith, O. Reimer, G. Rowell, for the H.E.S.S. Collaboratio
Cosmic rays escape from their sources
No full textInternational audienceCosmic rays (CRs) are accelerated in diverse astrophysical objects like supernova remnants, massive star clusters, or pulsars. Fermi acceleration mechanisms built a power-law distribution controlled by the ratio of the acceleration to escape timescales in the acceleration site. Hence, escape is an essential mechanism to establish the particle distribution at cosmic-ray sources and to control the flux of cosmic rays injected into the galaxy. Different models have tried to account for the escape process. However, all show some limitations due to the complexity of the particle release mechanism, usually involving 3D geometry, with specific magnetic turbulence properties linked to the process itself. The escape process is also time dependent and results from the interplay of particle acceleration and injection efficiency in the astrophysical source. Once injected into the interstellar medium, freshly released particles are channelled by the ambient magnetic field, which is itself turbulent. In a simplified view, we mainly focus on the propagation of CRs along 1D magnetic flux tubes before turbulent motions start to mix them over a turbulent coherence length, and then we further question this assumption. Close to their sources, one can also expect cosmic rays to harbour higher pressure with respect to their mean value in the interstellar medium. This intermittency in the CR distribution is prone to trigger several types of kinetic and macro instabilities, among which the resonant streaming instability has been the most investigated. In this article, we review recent observational and theoretical studies treating cosmic-ray escape and propagation in the vicinity of their source. We will consider three main astrophysical contexts: association with massive star clusters, gamma-ray halos around pulsars, and, more specifically, supernova remnants. In particular, we discuss in some detail the cosmic-ray cloud (CRC) model, which has been widely used to investigate CR propagation in the environment of supernova remnants. The review also discusses recent studies on CR-induced feedback over the interstellar medium surrounding the sources associated with the release process, as well as alternative types of driven instabilitie
ALMA observations of Molecules in Supernova 1987A
No full textSupernova (SN) 1987A has provided a unique opportunity to study how SN ejecta evolve in 30 years time scale. We report our ALMA spectral observations of SN 1987A, taken in 2014, 2015 and 2016, with detections of CO, 28SiO, HCO+ and SO, with weaker lines of 29SiO.
We find a dip in the SiO line profiles, suggesting that the ejecta morphology is likely elongated. The difference of the CO and SiO line profiles is consistent with hydrodynamic simulations, which show that Rayleigh-Taylor instabilities causes mixing of gas, with heavier elements much more disturbed, making more elongated structure.
Using 28SiO and its isotopologues, Si isotope ratios were estimated for the first time in SN 1987A. The estimated ratios appear to be consistent with theoretical predictions of inefficient formation of neutron rich atoms at lower metallicity, such as observed in the Large Magellanic Cloud (about half a solar metallicity).
The deduced large HCO+ mass and small SiS mass, which are inconsistent to the predictions of chemical model, might be explained by some mixing of elements immediately after the explosion. The mixing might have made some hydrogen from the envelope to sink into carbon and oxygen-rich zone during early days after the explosion, enabling the formation of a substantial mass of HCO+. Oxygen atoms may penetrate into silicon and sulphur zone, suppressing formation of SiS.
Our ALMA observations open up a new window to investigate chemistry, dynamics and explosive-nucleosynthesis in supernovae
X- and Gamma-Ray Continuum Emission Processes
No full textCompact objects, the ultimate stage of evolution of massive stars, are strong
X- and gamma-ray emitters. The compact object (white dwarf, neutron
star, or black hole) accretes matter and electromagnetic fields from
the close environment. During this process, a part of the gravitational
potential energy is reprocessed into kinetic energy in the magnetised fluid.
The interplay of turbulence and/or shock generation allows a fraction of
this energy to be transfered to a tiny supra-thermal particle population and
ultimately to be radiated away into high energy photons. The radiation
can be produced in a limited number of ways: cyclo-synchrotron and
bremsstrahlung processes; the Compton effect; nuclear
interactions and pair creation/absorption. This lecture presents the main
properties of the aforementioned mechanisms and illustrates them in some
astrophysical situations
On the Properties of the Turbulence in Collisionless Shocks
No full textInternational audienceThe recent observations of sharp X-ray filaments in many young supernova remnants like CasA, Kepler or Tycho or in older ones like SN1006 led to the conclusion of an amplification of the magnetic field in regards to its standard interstellar medium values. This magnetic field is hardly of external origin and is certainly produced through the acceleration of cosmic-rays around the blast wave. The streaming instability driven by the upstream cosmic-ray gradient is one of the favourite way to generate the magnetic fluctuations to the required level. Recent theoretical progresses conclude that aside the usual resonant interaction an efficient non-resonant regime could be at the origin of the high magnetic fields in supernova blast waves. The present work, investigate this issue. In particular it shown that both non-resonant and resonant regimes should operate in supernova shocks. We discuss the saturation mechanisms of these regimes. We show that the turbulence spectrum is probably highly anisotropic both up- and downstream and we examine the consequences of anisotropy on the values of the magnetic field in the post shock regions as well as the cosmic-ray acceleration process
Cosmic Rays and Radiative Instabilities
No full textIn the absence of magnetic fields and cosmic rays, radiative cooling laws with a range of dependences on temperature affect the stability of interstellar gas. For about four and a half decades, astrophysicists have recognised the importance of the thermal instablity for the formation of clouds in the interstellar medium. Even in the past several years, many papers have concerned the role of the thermal instability in the production of molecular clouds. About three and a half decades ago, astrophysicists investigating radiative shocks noticed that for many cooling laws such shocks are unstable. Attempts to address the effects of cosmic rays on the stablity of radiative media that are initially uniform or that have just passed through shocks have been made. The simplest approach to such studies involves the assumption that the cosmic rays behave as a fluid. Work based on such an approach is described. Cosmic rays have no effect on the stability of initially uniform, static media with respect to isobaric perturbations, though they do affect the stability of such media with respect to isentropic perturbations. The effect of cosmic rays on the stability of radiative shocked media depends greatly on the efficiency of the conversion of energy in accelerated cosmic rays into thermal energy in the thermalized fluid. If that efficiency is low, radiative cooling makes weak shocks propagating into upstream media with low cosmic-ray pressures more likely to be cosmic-ray dominated than adiabatic shocks of comparable strength. The cosmic-ray dominated shocks do not display radiative overstability. Highly efficient conversion of cosmic-ray energy into thermal energy leads shocked media to behave as they do when cosmic rays are absent
Postshock turbulence and diffusive shock acceleration in young supernova remnants
No full textInternational audienceThin X-ray filaments are observed in the vicinity of young supernova remnants (SNR) blast waves. Identifying processes involved in the creation of such filaments would provide a direct insight of particle acceleration occurring within SNR, in particular regarding the cosmic ray yield issue. Aims. The present article investigates magnetic amplification in the upstream medium of SNR blast wave through both resonant and non-resonant regimes of the streaming instability. It aims at a better understanding of the diffusive shock acceleration (DSA) efficiency considering various relaxation processes of the magnetic fluctuations in the downstream medium. Multi-wavelength radiative signatures coming from the SNR shock wave are used in order to put to the test the different downstream turbulence relaxation models. Methods. Analytical and numerical calculations coupling stochastic differential equation schemes with 1D spherical magnetohydrodynamics simulations are used to investigate, in the context of test particles, the issues regarding the turbulence evolution in both the forshock and post-shock regions. Stochastic second order Fermi acceleration induced by resonant modes, magnetic field relaxation and amplification, turbulence compression at the shock front, are considered to model the multi-wavelength filaments produced in SNRs. -ray emission is also considered through the Inverse Compton mechanism. Results. We confirm the result of Parizot et al (2006) that the maximum CR energies should not go well beyond PeV energies in young SNRs where X-ray filaments are observed. In order to match observational data, we derive an upper limit on the magnetic field amplitude insuring that stochastic particle reacceleration remain inefficient. Considering then, various magnetic relaxation processes, we present two necessary conditions to achieve efficient acceleration and X-ray filaments in SNRs: 1/the turbulence must fulfil the inequality 2 − − d 0 where is the turbulence spectral index while d is the relaxation length energy power-law index; 2/the typical relaxation length has to be of the order the X-ray rim size. We identify that Alv'enic/fast magnetosonic mode damping does fulfil all conditions while non-linear Kolmogorov damping does not. Confronting previous relaxation processes to observational data, we deduct that among our SNR sample, the older ones (SN1006 and G347.3-0.5) fail to verify all conditions which means that their X-ray filaments are likely controlled by radiative losses. The younger SNRs, Cassiopeia A, Tycho and Kepler, do pass all tests and we infer that the downstream magnetic field amplitude is lying in the range of 200-300 μ Gaus
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