20,219 research outputs found
Relativistic Effects for Time-Resolved Light Transport
We present a real-time framework which allows interactive visualization of relativistic effects for time-resolved light transport. We leverage data from two different sources: real-world data acquired with an effective exposure time of less than 2 picoseconds, using an ultra-fast imaging technique termed femto-photography, and a transient renderer based on ray-tracing. We explore the effects of time dilation, light aberration, frequency shift and radiance accumulation by modifying existing models of these relativistic effects to take into account the time-resolved nature of light propagation. Unlike previous works, we do not impose limiting constraints in the visualization, allowing the virtual camera to explore freely a reconstructed 3D scene depicting dynamic illumination. Moreover, we consider not only linear motion, but also acceleration and rotation of the camera. We further introduce, for the first time, a pinhole camera model into our relativistic rendering framework, and account for subsequent changes in focal length and field of view as the camera moves through the scene
Cascades and Dissipative Anomalies in Relativistic Fluid Turbulence
We develop first-principles theory of relativistic fluid turbulence at high
Reynolds and P\'eclet numbers. We follow an exact approach pioneered by
Onsager, which we explain as a non-perturbative application of the principle of
renormalization-group invariance. We obtain results very similar to those for
non-relativistic turbulence, with hydrodynamic fields in the inertial-range
described as distributional or "coarse-grained" solutions of the relativistic
Euler equations. These solutions do not, however, satisfy the naive
conservation-laws of smooth Euler solutions but are afflicted with dissipative
anomalies in the balance equations of internal energy and entropy. The
anomalies are shown to be possible by exactly two mechanisms, local cascade and
pressure-work defect. We derive "4/5th-law"-type expressions for the anomalies,
which allow us to characterize the singularities (structure-function scaling
exponents) required for their non-vanishing. We also investigate the Lorentz
covariance of the inertial-range fluxes, which we find is broken by our
coarse-graining regularization but which is restored in the limit that the
regularization is removed, similar to relativistic lattice quantum field
theory. In the formal limit as speed of light goes to infinity, we recover the
results of previous non-relativistic theory. In particular, anomalous heat
input to relativistic internal energy coincides in that limit with anomalous
dissipation of non-relativistic kinetic energy
Tuning a Schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry
The advent of Dirac materials has made it possible to realize two dimensional
gases of relativistic fermions with unprecedented transport properties in
condensed matter. Their photoconductive control with ultrafast light pulses is
opening new perspectives for the transmission of current and information. Here
we show that the interplay of surface and bulk transient carrier dynamics in a
photoexcited topological insulator can control an essential parameter for
photoconductivity - the balance between excess electrons and holes in the Dirac
cone. This can result in a strongly out of equilibrium gas of hot relativistic
fermions, characterized by a surprisingly long lifetime of more than 50 ps, and
a simultaneous transient shift of chemical potential by as much as 100 meV. The
unique properties of this transient Dirac cone make it possible to tune with
ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that
is impossible with conventional optoelectronic materials.Comment: Nature Communications, in press (12 pages, 6 figures
Synchrotron radiation of self-collimating relativistic MHD jets
The goal of this paper is to derive signatures of synchrotron radiation from
state-of-the-art simulation models of collimating relativistic
magnetohydrodynamic (MHD) jets featuring a large-scale helical magnetic field.
We perform axisymmetric special relativistic MHD simulations of the jet
acceleration region using the PLUTO code. The computational domain extends from
the slow magnetosonic launching surface of the disk up to 6000^2 Schwarzschild
radii allowing to reach highly relativistic Lorentz factors. The Poynting
dominated disk wind develops into a jet with Lorentz factors of 8 and is
collimated to 1 degree. In addition to the disk jet, we evolve a thermally
driven spine jet, emanating from a hypothetical black hole corona. Solving the
linearly polarized synchrotron radiation transport within the jet, we derive
VLBI radio and (sub-) mm diagnostics such as core shift, polarization
structure, intensity maps, spectra and Faraday rotation measure (RM), directly
from the Stokes parameters. We also investigate depolarization and the
detectability of a lambda^2-law RM depending on beam resolution and observing
frequency. We find non-monotonic intrinsic RM profiles which could be detected
at a resolution of 100 Schwarzschild radii. In our collimating jet geometry,
the strict bi-modality in polarization direction (as predicted by Pariev et
al.) can be circumvented. Due to relativistic aberration, asymmetries in the
polarization vectors across the jet can hint to the spin direction of the
central engine.Comment: Submitted to Ap
Relativistic outflow from two thermonuclear shell flashes on neutron stars
We study the exceptionally short (32-41 ms) precursors of two
intermediate-duration thermonuclear X-ray bursts observed with RXTE from the
neutron stars in 4U 0614+09 and 2S 0918-549. They exhibit photon fluxes that
surpass those at the Eddington limit later in the burst by factors of 2.6 to
3.1. We are able to explain both the short duration and the super-Eddington
flux by mildly relativistic outflow velocities of 0.1 to 0.3 subsequent
to the thermonuclear shell flashes on the neutron stars. These are the highest
velocities ever measured from any thermonuclear flash. The precursor rise times
are also exceptionally short: about 1 ms. This is inconsistent with predictions
for nuclear flames spreading laterally as deflagrations and suggests
detonations instead. This is the first time that a detonation is suggested for
such a shallow ignition column depth ( = 10 g cm).
The detonation would possibly require a faster nuclear reaction chain, such as
bypassing the alpha-capture on C with the much faster
C(p,)N(,p)O process previously proposed.
We confirm the possibility of a detonation, albeit only in the radial
direction, through the simulation of the nuclear burning with a large nuclear
network and at the appropriate ignition depth, although it remains to be seen
whether the Zel'dovich criterion is met. A detonation would also provide the
fast flame spreading over the surface of the neutron star to allow for the
short rise times. (...) As an alternative to the detonation scenario, we
speculate on the possibility that the whole neutron star surface burns almost
instantly in the auto-ignition regime. This is motivated by the presence of 150
ms precursors with 30 ms rise times in some superexpansion bursts from 4U
1820-30 at low ignition column depths of ~10 g cm.Comment: 11 pages, 6 figures, accepted by Astronomy and Astrophysic
Magnetohydrodynamic-Particle-in-Cell Method for Coupling Cosmic Rays with a Thermal Plasma: Application to Non-relativistic Shocks
We formulate a magnetohydrodynamic-particle-in-cell (MHD-PIC) method for
describing the interaction between collisionless cosmic ray (CR) particles and
a thermal plasma. The thermal plasma is treated as a fluid, obeying equations
of ideal MHD, while CRs are treated as relativistic Lagrangian particles
subject to the Lorentz force. Backreaction from CRs to the gas is included in
the form of momentum and energy feedback. In addition, we include the
electromagnetic feedback due to CR-induced Hall effect that becomes important
when the electron-ion drift velocity of the background plasma induced by CRs
approaches the Alfv\'en velocity. Our method is applicable on scales much
larger than the ion inertial length, bypassing the microscopic scales that must
be resolved in conventional PIC methods, while retaining the full kinetic
nature of the CRs. We have implemented and tested this method in the Athena MHD
code, where the overall scheme is second-order accurate and fully conservative.
As a first application, we describe a numerical experiment to study particle
acceleration in non-relativistic shocks. Using a simplified prescription for
ion injection, we reproduce the shock structure and the CR energy spectra
obtained with more self-consistent hybrid-PIC simulations, but at substantially
reduced computational cost. We also show that the CR-induced Hall effect
reduces the growth rate of the Bell instability and affects the gas dynamics in
the vicinity of the shock front. As a step forward, we are able to capture the
transition of particle acceleration from non relativistic to relativistic
regimes, with momentum spectrum connecting smoothly through
the transition, as expected from the theory of Fermi acceleration.Comment: 24 pages, 15 figures, accepted for publication in Ap
Constraining Relativistic Bow Shock Properties in Rotation-Powered Millisecond Pulsar Binaries
Multiwavelength followup of unidentified Fermi sources has vastly expanded
the number of known galactic-field "black widow" and "redback" millisecond
pulsar binaries. Focusing on their rotation-powered state, we interpret the
radio to X-ray phenomenology in a consistent framework. We advocate the
existence of two distinct modes differing in their intrabinary shock
orientation, distinguished by the phase-centering of the double-peaked X-ray
orbital modulation originating from mildly-relativistic Doppler boosting. By
constructing a geometric model for radio eclipses, we constrain the shock
geometry as functions of binary inclination and shock stand-off . We
develop synthetic X-ray synchrotron orbital light curves and explore the model
parameter space allowed by radio eclipse constraints applied on archetypal
systems B1957+20 and J1023+0038. For B1957+20, from radio eclipses the
stand-off is -- fraction of binary separation from the
companion center, depending on the orbit inclination. Constructed X-ray light
curves for B1957+20 using these values are qualitatively consistent with those
observed, and we find occultation of the shock by the companion as a minor
influence, demanding significant Doppler factors to yield double peaks. For
J1023+0038, radio eclipses imply while X-ray light curves
suggest (from the pulsar). Degeneracies in the
model parameter space encourage further development to include transport
considerations. Generically, the spatial variation along the shock of the
underlying electron power-law index should yield energy-dependence in the shape
of light curves motivating future X-ray phase-resolved spectroscopic studies to
probe the unknown physics of pulsar winds and relativistic shock acceleration
therein.Comment: Accepted to ApJ, 36 pages, 15 figures; comments welcom
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