68 research outputs found
Turbulent amplification of magnetic field driven by dynamo effect at rippled shocks
We derive analytically the vorticity generated downstream of a
two-dimensional rippled hydromagnetic shock neglecting fluid viscosity and
resistivity. The growth of the turbulent component of the downstream magnetic
field is driven by the vortical eddies motion. We determine an analytic
time-evolution of the magnetic field amplification at shocks, so far described
only numerically, until saturation occurs due to seed-field reaction to field
lines whirling. The explicit expression of the amplification growth rate and of
the non-linear field back-reaction in terms of the parameters of shock and
interstellar density fluctuations is derived from MHD jump conditions at
rippled shocks. A magnetic field saturation up to the order of milligauss and a
short-time variability in the -ray observations of supernova remnants can be
obtained by using reasonable parameters for the interstellar turbulence.Comment: 9 pages, 4 figures, The Astrophyical Journal in pres
Bi-directional streaming of particles accelerated at the STEREO-A shock on 9th March 2008
We present an interpretation of anisotropy and intensity of supra-thermal
ions near a fast quasi-perpendicular reverse shock measured by Solar
Terrestrial Relations Observatory Ahead (ST-A) on 2008 March 9th. The measured
intensity profiles of the supra-thermal particles exhibit an enhancement, or
"spike", at the time of the shock arrival and pitch-angle anisotropies before
the shock arrival are bi-modal, jointly suggesting trapping of
near-scatter-free ions along magnetic field lines that intersect the shock at
two locations. We run test-particle simulations with pre-existing upstream
magnetostatic fluctuations advected across the shock. The measured bi-modal
upstream anisotropy, the nearly field-aligned anisotropies up to ~15 minutes
upstream of the shock, as well as the "pancake-like" anisotropies up to ~10
minutes downstream of the shock are well reproduced by the simulations. These
results, in agreement with earlier works, suggest a dominant role of the
large-scale structure (100s of supra-thermal proton gyroradii) of the magnetic
field in forging the early-on particle acceleration at shocks.Comment: 7 pages, 4 figures, MNRAS in pres
GRB 980425, SN1998bw and the EMBH model
The EMBH model, previously developed using GRB 991216 as a prototype, is here
applied to GRB 980425. We fit the luminosity observed in the 40-700 keV, 2-26
keV and 2-10 keV bands by the BeppoSAX satellite. In addition we present a
novel scenario in which the supernova SN1998bw is the outcome of an ``induced
gravitational collapse'' triggered by GRB 980425, in agreement with the
GRB-Supernova Time Sequence (GSTS) paradigm (Ruffini et al. 2001c). A further
outcome of this astrophysically exceptional sequence of events is the formation
of a young neutron star generated by the SN1998bw event. A coordinated
observational activity is recommended to further enlighten the underlying
scenario of this most unique astrophysical system.Comment: 10 pages, 3 figures, in the Proceedings of the 34th COSPAR scientific
assembly, Elsevier. Fixed some typos in this new versio
The Stellar CME-flare relation: What do historic observations reveal?
Solar CMEs and flares have a statistically well defined relation, with more
energetic X-ray flares corresponding to faster and more massive CMEs. How this
relation extends to more magnetically active stars is a subject of open
research. Here, we study the most probable stellar CME candidates associated
with flares captured in the literature to date, all of which were observed on
magnetically active stars. We use a simple CME model to derive masses and
kinetic energies from observed quantities, and transform associated flare data
to the GOES 1--8~\AA\ band. Derived CME masses range from to
~g. Associated flare X-ray energies range from to
~erg. Stellar CME masses as a function of associated flare energy
generally lie along or below the extrapolated mean for solar events. In
contrast, CME kinetic energies lie below the analogous solar extrapolation by
roughly two orders of magnitude, indicating approximate parity between flare
X-ray and CME kinetic energies. These results suggest that the CMEs associated
with very energetic flares on active stars are more limited in terms of the
ejecta velocity than the ejecta mass, possibly because of the restraining
influence of strong overlying magnetic fields and stellar wind drag. Lower CME
kinetic energies and velocities present a more optimistic scenario for the
effects of CME impacts on exoplanets in close proximity to active stellar
hosts.Comment: 23 pages, 3 tables, 4 figures, accepted by Ap
X-ray Measurements of the Particle Acceleration Properties at Inward Shocks in Cassiopeia A
We present new evidence that the bright non-thermal X-ray emission features
in the interior of the Cassiopeia A supernova remnant (SNR) are caused by
inward moving shocks based on Chandra and NuSTAR observations. Several bright
inward-moving filaments were identified using monitoring data taken by Chandra
in 2000-2014. These inward-moving shock locations are nearly coincident with
hard X-ray (15-40 keV) hot spots seen by NuSTAR. From proper motion
measurements, the transverse velocities were estimated to be in the range
2,100-3,800 km s for a distance of 3.4 kpc. The shock velocities
in the frame of the expanding ejecta reach values of 5,100-8,700 km
s, slightly higher than the typical speed of the forward shock.
Additionally, we find flux variations (both increasing and decreasing) on
timescales of a few years in some of the inward-moving shock filaments. The
rapid variability timescales are consistent with an amplified magnetic field of
0.5-1 mG. The high speed and low photon cut-off energy of the
inward-moving shocks are shown to imply a particle diffusion coefficient that
departs from the Bohm regime ( 3-8) for the few
simple physical configurations we consider in this study. The maximum electron
energy at these shocks is estimated to be 8-11 TeV, smaller than the
values of 15-34 TeV inferred for the forward shock. Cassiopeia A is
dynamically too young for its reverse shock to appear to be moving inward in
the observer frame. We propose instead that the inward-moving shocks are a
consequence of the forward shock encountering a density jump of 5-8
in the surrounding material.Comment: 16 pages, 8 figures, accepted for publication in Ap
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