468 research outputs found
The Cosmic Ray Precursor of Relativistic Collisionless Shocks: A Missing Link in Gamma-Ray Burst Afterglows
Collisionless shocks are commonly argued to be the sites of cosmic ray (CR)
acceleration. We study the influence of CRs on weakly magnetized relativistic
collisionless shocks and apply our results to external shocks in gamma-ray
burst (GRB) afterglows. The common view is that the transverse Weibel
instability (TWI) generates a small-scale magnetic field that facilitates
collisional coupling and thermalization in the shock transition. The TWI field
is expected to decay rapidly, over a finite number of proton plasma skin depths
from the transition. However, the synchrotron emission in GRB afterglows
suggests that a strong and persistent magnetic field is present in the plasma
that crosses the shock; the origin of this field is a key open question. Here
we suggest that the common picture involving TWI demands revision. Namely, the
CRs drive turbulence in the shock upstream on scales much larger than the skin
depth. This turbulence generates a large-scale magnetic field that quenches TWI
and produces a magnetized shock. The new field efficiently confines CRs and
enhances the acceleration efficiency. The CRs modify the shocks in GRB
afterglows at least while they remain relativistic. The origin of the magnetic
field that gives rise to the synchrotron emission is plausibly in the CR-driven
turbulence. We do not expect ultrahigh energy cosmic ray production in external
GRB shocks.Comment: 6 pages, 1 figur
A current driven instability in parallel, relativistic shocks
Recently, Bell has reanalysed the problem of wave excitation by cosmic rays
propagating in the pre-cursor region of a supernova remnant shock front. He
pointed out a strong, non-resonant, current-driven instability that had been
overlooked in the kinetic treatments, and suggested that it is responsible for
substantial amplification of the ambient magnetic field. Magnetic field
amplification is also an important issue in the problem of the formation and
structure of relativistic shock fronts, particularly in relation to models of
gamma-ray bursts. We have therefore generalised the linear analysis to apply to
this case, assuming a relativistic background plasma and a monoenergetic,
unidirectional incoming proton beam. We find essentially the same non-resonant
instability noticed by Bell, and show that also under GRB conditions, it grows
much faster than the resonant waves. We quantify the extent to which thermal
effects in the background plasma limit the maximum growth rate.Comment: 8 pages, 1 figur
Shock Vorticity Generation from Accelerated Ion Streaming in the Precursor of Ultrarelativistic Gamma-Ray Burst External Shocks
We investigate the interaction of nonthermal ions (protons and nuclei)
accelerated in an ultrarelativistic blastwave with the pre-existing magnetic
field of the medium into which the blastwave propagates. While particle
acceleration processes such as diffusive shock acceleration can accelerate ions
and electrons, the accelerated electrons suffer larger radiative losses. Under
certain conditions, the ions can attain higher energies and reach farther ahead
of the shock than the electrons, and so the nonthermal particles can be
partially charge-separated. To compensate for the charge separation, the
upstream plasma develops a return current, which, as it flows across the
magnetic field, drives transverse acceleration of the upstream plasma and a
growth of density contrast in the shock upstream. If the density contrast is
strong by the time the fluid is shocked, vorticity is generated at the shock
transition. The resulting turbulence can amplify the post-shock magnetic field
to the levels inferred from gamma-ray burst afterglow spectra and light curves.
Therefore, since the upstream inhomogeneities are induced by the ions
accelerated in the shock, they are generic even if the blastwave propagates
into a medium of uniform density. We speculate about the global structure of
the shock precursor, and delineate several distinct physical regimes that are
classified by an increasing distance from the shock and, correspondingly, a
decreasing density of nonthermal particles that reach that distance.Comment: 8 pages, no figure
The impact of Ethnic conflicts on the Quality of life and Human health: A case study of Serbian people in Kosovo and Metohija
The subject of this research is the assessment of the influence of ethnic conflicts on the quality of life and health of people. The aim of the research is to identify and highlight the influence of conflicts on human health in the environments which were exposed to those conflicts at the end of the 20th and the beginning of the 21st century. The research was conducted in Kosovo and Metohija. Participants in inter-ethnic conflicts are Serbs and Albanians. Only Serbs were included in the research, while the language barrier was an obstacle for the inclusion of Albanians in the research. The ways in which trauma, gender, age, marriage and household size affect the fear of violence were investigated. The differences between types of fear were stated, i.e. it was investigated whether respondents were more concerned about their property or their personal safety
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