650 research outputs found
Computer simulations of cosmic-ray diffusion near supernova remnant shock waves
A plasma simulation model was used to study the resonant interactions between streaming cosmic-ray ions and a self-consistent spectrum of Alfven waves, such as might exist in the interstellar medium upstream of a supernova remnant shock wave. The computational model is a hybrid one, in which the background interstellar medium is an MHD fluid and the cosmic-rays are discrete kinetic particles. The particle sources for the electromagnetic fields are obtained by averaging over the fast cyclotron motions. When the perturbed magnetic field is larger than 10 percent of the background field, the macro- and microphysics are no longer correctly predicted by quasi-linear theory. The particles are trapped by the waves and show sharp jumps in their pitch-angles relative to the background magnetic field, and the effective ninety-degree scattering time for diffusion parallel to the background magnetic field is reduced to between 5 and 30 cyclotron periods. Simulation results suggest that Type 1 supernova remnants may be the principal sites of cosmic ray acceleration
Relativistic Jets and Long-Duration Gamma-ray Bursts from the Birth of Magnetars
We present time-dependent axisymmetric magnetohydrodynamic simulations of the
interaction of a relativistic magnetized wind produced by a proto-magnetar with
a surrounding stellar envelope, in the first seconds after core
collapse. We inject a super-magnetosonic wind with ergs
s into a cavity created by an outgoing supernova shock. A strong
toroidal magnetic field builds up in the bubble of plasma and magnetic field
that is at first inertially confined by the progenitor star. This drives a jet
out along the polar axis of the star, even though the star and the magnetar
wind are each spherically symmetric. The jet has the properties needed to
produce a long-duration gamma-ray burst (GRB). At s after core bounce,
the jet has escaped the host star and the Lorentz factor of the material in the
jet at large radii cm is similar to that in the magnetar wind
near the source. Most of the spindown power of the central magnetar escapes via
the relativistic jet. There are fluctuations in the Lorentz factor and energy
flux in the jet on second timescale. These may contribute to
variability in GRB emission (e.g., via internal shocks).Comment: 5 pages, 3 figures, accepted in MNRAS letter, presented at the
conference "Astrophysics of Compact Objects", 1-7 July, Huangshan, Chin
Evidence for "Propeller" Effects In X-ray Pulsars GX 1+4 And GROJ1744-28
We present observational evidence for "propeller" effects in two X-ray
pulsars, GX 1+4 and GROJ1744-28. Both sources were monitored regularly by the
Rossi X-ray Timing Explorer (RXTE) throughout a decaying period in the X-ray
brightness. Quite remarkably, strong X-ray pulsation became unmeasurable when
total X-ray flux had dropped below a certain threshold. Such a phenomenon is a
clear indication of the propeller effects which take place when pulsar
magnetosphere grows beyond the co-rotation radius as a result of the decrease
in mass accretion rate and centrifugal force prevents accreting matter from
reaching the magnetic poles. The entire process should simply reverse as the
accretion rate increases. Indeed, steady X-ray pulsation was reestablished as
the sources emerged from the non-pulsating faint state. These data allow us to
directly derive the surface polar magnetic field strength for both pulsars:
3.1E+13 G for GX 1+4 and 2.4E+11 G for GROJ1744-28. The results are likely to
be accurate to within a factor of 2, with the total uncertainty dominated by
the uncertainty in estimating the distances to the sources. Possible mechanisms
for the persistent emission observed in the faint state are discussed in light
of the extreme magnetic properties of the sources.Comment: 12 pages including 3 ps figures. To appear in ApJ Letters Vol. 48
Arrival processes in port modeling: insights from a case study
This paper investigates the impact of arrival processes on the ship handling process. Two types of arrival processes are considered: controlled and uncontrolled. Simulation results show that uncontrolled arrivals of ships perform worst in terms of both ship delays and required storage capacity. Stock-controlled arrivals perform best with regard to large vessel delays and storage capacity. The combination of stock-controlled arrivals for large vessels and equidistant arrivals for barges also performs better than the uncontrolled process. Careful allocation of ships to the mooring points of a jetty further improves the efficiency.supply chain management;logistics;simulation;transportation;case study
Relativistic Particle Acceleration in a Folded Current Sheet
Two-dimensional particle simulations of a relativistic Harris current sheet
of pair plasmashave demonstrated that the system is unstable to the
relativistic drift kink instability (RDKI) and that a new kind of acceleration
process takes place in the deformed current sheet. This process contributes to
the generation of non-thermal particles and contributes to the fast magnetic
dissipation in the current sheet structure. The acceleration mechanism and a
brief comparison with relativistic magnetic reconnection are presented.Comment: 11 preprint pages, including 3 .eps figure
Thermal Cyclotron Reprocessing of Gammy-Ray Bursts - Theory and Model Spectra
We examine the generation of infrared, optical, and ultraviolet flashes from single, magnetized neutron stars are experiencing of gamma-ray burst. Cyclotron reprocessing of energetic gamma-ray burst photons in the neutron star magnetosphere is assumed to be the underlying mechanism reponsible for the display at longer wavelengths, and thermal equilibrium is assumed in order to calculate electron distribution function. It is shown that thesea good approximations for a wide range of conditions expected in neutron star magnetospheres. The thermal cycoltron model proves capable fo generating ptical outbursts similar to bright historical events. althrough opitcal transients most likely would be much fainter. For a wide range of conditions the model predicts bright, nondelayed flashes, extending in some cases even beyond the ultraviolet. Since the emission at long wavelengths is correlated with the gammar-rays down to time scales small compared with the burst duration, time-averaged spectra are calculated corresponding to the time-averaged gamma-ray burst spectrum. For flashes that do not exhibit a spectral turnover in the optical region, Lopt α Bas with α ~ 3/4, so that optical transients could be used to constrain the magnetic field strength and distance of gamma-ray burst sources. The long-wavelength fluxes for the recently discovered soft repearing source SGR 1806-20 are also estimated
Time Variability in the X-ray Nebula Powered by Pulsar B1509-58
We use new and archival Chandra and ROSAT data to study the time variability
of the X-ray emission from the pulsar wind nebula (PWN) powered by PSR B1509-58
on timescales of one week to twelve years. There is variability in the size,
number, and brightness of compact knots appearing within 20" of the pulsar,
with at least one knot showing a possible outflow velocity of ~0.6c (assuming a
distance to the source of 5.2 kpc). The transient nature of these knots may
indicate that they are produced by turbulence in the flows surrounding the
pulsar. A previously identified prominent jet extending 12 pc to the southeast
of the pulsar increased in brightness by 30% over 9 years; apparent outflow of
material along this jet is observed with a velocity of ~0.5c. However, outflow
alone cannot account for the changes in the jet on such short timescales.
Magnetohydrodynamic sausage or kink instabilities are feasible explanations for
the jet variability with timescale of ~1.3-2 years. An arc structure, located
30"-45" north of the pulsar, shows transverse structural variations and appears
to have moved inward with a velocity of ~0.03c over three years. The overall
structure and brightness of the diffuse PWN exterior to this arc and excluding
the jet has remained the same over the twelve year span. The photon indices of
the diffuse PWN and possibly the jet steepen with increasing radius, likely
indicating synchrotron cooling at X-ray energies.Comment: accepted to ApJ, 14 pages, 8 figure
- …