6,259 research outputs found
Which acceleration process for UHE-Cosmic Rays in Gamma Ray Bursts ?
In this paper, we have made an accurate investigation of proton acceleration
in GRBs and we have predicted a possible signature of cosmic rays, in a
sufficiently baryon-loaded fireball, via GeV -ray emission produced by
-meson decay. If two ungrounded assumptions are removed, namely,
Bohm's scaling and a slow magnetic field decrease, the usual Fermi processes
are unable to generate ultra high energy cosmic rays (UHECRs) in GRBs. We
propose to develop another scenario of relativistic Fermi acceleration in the
internal shock stage. We present the results of a realistic Monte-Carlo
simulation of a multi-front acceleration which clearly shows the possible
generation of UHECR. The amount of energy converted into UHECRs turns out to be
a sizeable fraction of the magnetic energy.Comment: 22 pages, accepted for publication in A&
Jig protects transistors from heat while tinning leads
In tinning transistor leads, an aluminum jig is used to dip the leads into the molten tin. The jigs mass shunts excess heat given off by the molten tin before it reaches and damages the transistor body
3-d resistive MHD simulations of magnetic reconnection and the tearing mode instability in current sheets
Magnetic reconnection plays a critical role in many astrophysical processes
where high energy emission is observed, e.g. particle acceleration,
relativistic accretion powered outflows, pulsar winds and probably in
dissipation of Poynting flux in GRBs. The magnetic field acts as a reservoir of
energy and can dissipate its energy to thermal and kinetic energy via the
tearing mode instability. We have performed 3d nonlinear MHD simulations of the
tearing mode instability in a current sheet. Results from a temporal stability
analysis in both the linear regime and weakly nonlinear (Rutherford) regime are
compared to the numerical simulations. We observe magnetic island formation,
island merging and oscillation once the instability has saturated. The growth
in the linear regime is exponential in agreement with linear theory. In the
second, Rutherford regime the island width grows linearly with time. We find
that thermal energy produced in the current sheet strongly dominates the
kinetic energy. Finally preliminary analysis indicates a P(k) 4.8 power law for
the power spectral density which suggests that the tearing mode vortices play a
role in setting up an energy cascade.Comment: 4 pages, 8 figures, accepted for publication in the International
Journal of Modern Physics D, proceedings of HEPRO meeting, held in Dublin, in
September 200
Current-driven filamentation upstream of magnetized relativistic collisionless shocks
The physics of instabilities in the precursor of relativistic collisionless
shocks is of broad importance in high energy astrophysics, because these
instabilities build up the shock, control the particle acceleration process and
generate the magnetic fields in which the accelerated particles radiate. Two
crucial parameters control the micro-physics of these shocks: the magnetization
of the ambient medium and the Lorentz factor of the shock front; as of today,
much of this parameter space remains to be explored. In the present paper, we
report on a new instability upstream of electron-positron relativistic shocks
and we argue that this instability shapes the micro-physics at moderate
magnetization levels and/or large Lorentz factors. This instability is seeded
by the electric current carried by the accelerated particles in the shock
precursor as they gyrate around the background magnetic field. The compensation
current induced in the background plasma leads to an unstable configuration,
with the appearance of charge neutral filaments carrying a current of the same
polarity, oriented along the perpendicular current. This ``current-driven
filamentation'' instability grows faster than any other instability studied so
far upstream of relativistic shocks, with a growth rate comparable to the
plasma frequency. Furthermore, the compensation of the current is associated
with a slow-down of the ambient plasma as it penetrates the shock precursor (as
viewed in the shock rest frame). This slow-down of the plasma implies that the
``current driven filamentation'' instability can grow for any value of the
shock Lorentz factor, provided the magnetization \sigma <~ 10^{-2}. We argue
that this instability explains the results of recent particle-in-cell
simulations in the mildly magnetized regime.Comment: 14 pages, 8 figures; to appear in MNRA
Synchrotron Emissions in GRB Prompt Phase Using a Semi Leptonic and Hadronic Model
In this communication devoted to the prompt emission of GRBs, we claim that
some important parameters associated to the magnetic field, such as its index
profile, the index of its turbulence spectrum and its level of irregularities,
will be measurable with GLAST. In particular the law relating the peak energy
Epeak with the total energy E (like Amati's law) constrains the turbulence
spectrum index and, among all existing theories of MHD turbulence, is
compatible with the Kolmogorov scaling only. Thus, these data will allow a much
better determination of the performances of GRBs as particle accelerators. This
opens the possibility to characterize both electron and proton acceleration
more seriously. We discuss the possible generation of UHECRs and of its
signature through GeV-TeV synchrotron emission.Comment: 30th International Cosmic Ray Conference (ICRC2007) - Proceeding
#107
A multi-flow model for microquasars
We present a new picture for the central regions of Black Hole X-ray
Binaries. In our view, these central regions have a multi-flow configuration
which consists in (1) an outer standard accretion disc down to a transition
radius r_J, (2) an inner magnetized accretion disc below r_J driving (3) a non
relativistic self-collimated electron-proton jet surrounding, when adequate
conditions for pair creation are met, (4) a ultra relativistic
electron-positron beam. This accretion-ejection paradigm provides a simple
explanation to the canonical spectral states, from radio to X/gamma-rays, by
varying the transition radius r_J and disc accretion rate independently. Large
values of r_J and low accretion rate correspond to Quiescent and Hard states.
These states are characterized by the presence of a steady electron-proton MHD
jet emitted by the disc below r_J. The hard X-ray component is expect to form
at the jet basis. When r_J becomes smaller than the marginally stable orbit
r_i, the whole disc resembles a standard accretion disc with no jet,
characteristic of the Soft state. Intermediate states correspond to situations
where r_J ~ r_i. At large accretion rate, an unsteady pair cascade process is
triggered within the jet axis, giving birth to flares and ejection of
relativistic pair blobs. This would correspond to the luminous intermediate
state, with its associated superluminal motions.Comment: 12 pages, 3 figures. Proceedings of ``High Energies in the
Highlands'', Fort-William, 27 June-1 July 200
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