64 research outputs found
Numerical simulations of mass loading in the tails of Bow Shock Pulsar Wind Nebulae
When a pulsar is moving through a partially ionized medium, a fraction of
neutral Hydrogen atoms penetrate inside the pulsar wind and can be
photo-ionized by the nebula UV radiation. The resulting protons remains
attached to the magnetic field of the light leptonic pulsar wind enhancing its
inertia and changing the flow dynamics of the wind. We present here the first
numerical simulations of such effect in the tails of bow shock nebulae. We
produce a set of different models representative of pulsars moving in the
interstellar medium with different velocities, from highly subsonic to
supersonic, by means of 2D hydrodynamic relativistic simulations. We compare
the different tail morphologies with results from theoretical models of mass
loading in bow shocks. As predicted by analytical models we observe a fast
sideways expansion of the tail with the formation of secondary shocks in the
ISM. This effect could be at the origin of the head-and-shoulder morphology
observed in many BSPWNe.Comment: 7 pages, 5 figures, 1 tabl
CRIME - cosmic ray interactions in molecular environments
Molecular clouds act as targets for cosmic rays (CR), revealing their
presence through either gamma-ray emission due to proton-proton interactions,
and/or through the ionization level in the cloud, produced by the CR flux. The
ionization rate is a unique tool, to some extent complementary to the gamma-ray
emission, in that it allows to constrain the CR spectrum especially for
energies below the pion production rate ( MeV). Here we study the
effect of ionization on clouds due to both CR protons and electrons,
using the fully relativistic ionization cross sections, which is important to
correctly account for the contribution due to relativistic CRs. The
contribution to ionization due to secondary electrons is also included
self-consistently. The whole calculation has been implemented into a numerical
code which is publicly accessible through a web-interface. The code also
include the calculation of gamma-ray emission once the CR spectrumComment: 8 pages 2 figures, The 34th International Cosmic Ray Conferenc
Cosmic ray transport and radiative processes in nuclei of starburst galaxies
The high rate of star formation and supernova explosions of starburst
galaxies make them interesting sources of high energy radiation. Depending upon
the level of turbulence present in their interstellar medium, the bulk of
cosmic rays produced inside starburst galaxies may lose most of their energy
before escaping, thereby making these sources behave as calorimeters, at least
up to some maximum energy. Contrary to previous studies, here we investigate in
detail the conditions under which cosmic ray confinement may be effective for
electrons and nuclei and we study the implications of cosmic ray confinement in
terms of multifrequency emission from starburst nuclei and production of high
energy neutrinos. The general predictions are then specialized to three cases
of active starbursts, namely M82, NGC253 and Arp220. Both primary and secondary
electrons, as well as electron-positron pairs produced by gamma ray absorption
inside starburst galaxies are taken into account. Electrons and positrons
produced as secondary products of hadronic interactions are found to be
responsible for most of the emission of leptonic origin. In particular,
synchrotron emission of very high energy secondary electrons produces an
extended emission of hard X-rays that represent a very interesting signature of
hadronic process in starburst galaxies, potentially accessible to current and
future observations in the X-ray band. A careful understanding of both the
production and absorption of gamma rays in starburst galaxies is instrumental
to the assessment of the role of these astrophysical sources as sources of high
energy astrophysical neutrinos.Comment: Version accepted for publication in MNRA
Impact of transport modelling on the Fe abundance inside Galactic cosmic ray sources
The ACE-CRIS collaboration has recently released the measurement of
radioactive Fe nuclei abundance in Galactic Cosmic Rays, in the energy
range MeV per nucleon. We model Cosmic Ray propagation and
derive from this measurement the Fe/Fe ratio that is expected in
the sources of Galactic Cosmic Rays. We describe Cosmic Ray origin and
transport within the framework of the disk/halo diffusion model, namely a
scenario in which the matter and the Cosmic Ray sources in our Galaxy are
confined to a thin disk, while Cosmic Ray propagation occurs in a much larger
halo with negligible matter density. We solve the Cosmic Ray transport equation
accounting for spallation reactions, decay and ionization losses as well as
advection. We find that the Fe/Fe ratio at the source must be
very close to the value detected in the local Cosmic Ray spectrum at Earth, due
to the fact that spallation reactions are more effective for Fe than for
Fe. Such a result could help identify the sources of Galactic Cosmic
Rays.Comment: 11 pages, 5 figures. Accepted for publication in PR
Different spectra of cosmic ray H, He and heavier nuclei escaping compact star clusters
Cosmic ray acceleration at the termination shock of compact star clusters has
recently received much attention, mainly because of the detection of gamma ray
emission from some of such astrophysical sources. Here we focus on the
acceleration of nuclei at the termination shock and we investigate the role
played by proton energy losses and spallation reactions of nuclei, especially
downstream of the shock. We show that for a rather generic choice of the mean
gas density in the cavity excavated by the cluster wind, the spectrum of He
nuclei is systematically harder than the spectrum of hydrogen, in a manner that
appears to be qualitatively consistent with the observed and yet unexplained
phenomenon of discrepant hardening. We also find that the spallation reactions
of heavier nuclei are likely to be so severe that their spectra become very
hard and with a low normalization, meaning that it is unlikely that heavy
nuclei escaping star clusters can provide a sizeable contribution to the
spectrum of cosmic rays at the Earth.Comment: 9 Pages, Submitted to MNRA
Interplay between Physics and Geometry in Balmer filaments: the Case of SN 1006
The analysis of Balmer-dominated emission in supernova remnants is
potentially a very powerful way to derive information on the shock structure,
on the physical conditions of the ambient medium and on the cosmic-ray
acceleration efficiency. However, the outcome of models developed in
plane-parallel geometry is usually not easily comparable with the data, since
they often come from regions with rather a complex geometry. We present here a
general scheme to disentangle physical and geometrical effects in the data
interpretation, which is especially powerful when the transition zone of the
shock is spatially resolved and the spectral resolution is high enough to allow
a detailed investigation of spatial changes of the line profile. We then apply
this technique to re-analyze very high quality data of a region along the
northwestern limb of the remnant of SN~1006. We show how some observed
features, previously interpreted only in terms of spatial variations of
physical quantities, naturally arise from geometrical effects. With these
effects under control, we derive new constraints on physical quantities in the
analyzed region, like the ambient density (in the range 0.03-), the upstream neutral fraction (more likely in the range 0.01-0.1),
the level of face-on surface brightness variations (with factors up to ) and the typical scale lengths related to such variations (, corresponding to angular scales ).Comment: 22 pages, 24 figures, 3 tables, accepted on 2018 November 21 for
publication on MNRA
Non-linear Cosmic Ray propagation close to the acceleration site
Recent advances on gamma-ray observations from SuperNova Remnants and
Molecular Clouds offer the possibility to study in detail the properties of the
propagation of escaping Cosmic Rays (CR). However, a complete theory for CR
transport outside the acceleration site has not been developed yet. Two
physical processes are thought to be relevant to regulate the transport: the
growth of waves caused by streaming instability, and possible wave damping
mechanisms that reduce the growth of the turbulence. Only a few attempts have
been made so far to incorporate these mechanisms in the theory of CR diffusion.
In this work we present recent advances in this subject. In particular, we show
results obtained by solving the coupled equations for the diffusion of CRs and
the evolution of Alfven waves. We discuss the importance of streaming
instabilities and wave damping in different ISM phases.Comment: Contribution to the Proceedings of the 34th International Cosmic Ray
Conference (ICRC 2015), The Hague, The Netherland
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