20,574 research outputs found
Electron and Ion Acceleration in Relativistic Shocks with Applications to GRB Afterglows
We have modeled the simultaneous first-order Fermi shock acceleration of
protons, electrons, and helium nuclei by relativistic shocks. By parameterizing
the particle diffusion, our steady-state Monte Carlo simulation allows us to
follow particles from particle injection at nonthermal thermal energies to
above PeV energies, including the nonlinear smoothing of the shock structure
due to cosmic-ray (CR) backpressure. We observe the mass-to-charge (A/Z)
enhancement effect believed to occur in efficient Fermi acceleration in
non-relativistic shocks and we parameterize the transfer of ion energy to
electrons seen in particle-in-cell (PIC) simulations. For a given set of
environmental and model parameters, the Monte Carlo simulation determines the
absolute normalization of the particle distributions and the resulting
synchrotron, inverse-Compton, and pion-decay emission in a largely
self-consistent manner. The simulation is flexible and can be readily used with
a wide range of parameters typical of gamma-ray burst (GRB) afterglows. We
describe some preliminary results for photon emission from shocks of different
Lorentz factors and outline how the Monte Carlo simulation can be generalized
and coupled to hydrodynamic simulations of GRB blast waves. We assume Bohm
diffusion for simplicity but emphasize that the nonlinear effects we describe
stem mainly from an extended shock precursor where higher energy particles
diffuse further upstream. Quantitative differences will occur with different
diffusion models, particularly for the maximum CR energy and photon emission,
but these nonlinear effects should be qualitatively similar as long as the
scattering mean free path is an increasing function of momentum.Comment: Accepted for publication in MNRA
The psychology of computer displays in the modern mission control center
Work at NASA's Western Aeronautical Test Range (WATR) has demonstrated the need for increased consideration of psychological factors in the design of computer displays for the WATR mission control center. These factors include color perception, memory load, and cognitive processing abilities. A review of relevant work in the human factors psychology area is provided to demonstrate the need for this awareness. The information provided should be relevant in control room settings where computerized displays are being used
Variable angle photoelectron spectrometer
The design, construction, and performance of a spectrometer for measuring the angular and energy distributions of electrons photoejected by rare gas resonance light is described. Results using 584.4‐Å photons from a helium lamp are reported. Flexibility of instrumental design allows for the use of other light sources. A 180° hemispherical electrostatic electron energy analyzer is rotatable about the center of an enclosed sample chamber. The instrument is highly automated, with an on‐line computer used to control the detector angle and the data acquisition and reduction. This automation is required by the long and continuous data acquisition
Space VLBI Observations Show in the Quasar NRAO 530
We present here space-based VLBI observations with VSOP and a southern
hemisphere ground array of the gamma-ray blazar NRAO 530 at 1.6 GHz and 5 GHz.
The brightness temperature of the core at 1.6 GHz is K. The
size is near the minimum observable value in the direction of NRAO~530 due to
interstellar scattering. The 5 GHz data show a single component with a
brightness temperature of K, significantly in excess of
the inverse Compton limit and of the equipartition brightness temperature limit
(Readhead 1994). This is strong evidence for relativistic motion in a jet
requiring model-dependent Doppler boosting factors in the range 6 to 60. We
show that a simple homogeneous sphere probably does not model the emission
region accurately. We favor instead an inhomogeneous jet model with a Doppler
boosting factor of 15.Comment: 12 pages, 2 figures. Accepted for publication in ApJ Letter
Slowly modulated oscillations in nonlinear diffusion processes
It is shown here that certain systems of nonlinear (parabolic) reaction-diffusion equations have solutions which are approximated by oscillatory functions in the form R(ξ - cτ)P(t^*) where P(t^*) represents a sinusoidal oscillation on a fast time scale t* and R(ξ - cτ) represents a slowly-varying modulating amplitude on slow space (ξ) and slow time (τ) scales. Such solutions describe phenomena in chemical reactors, chemical and biological reactions, and in other media where a stable oscillation at each point (or site) undergoes a slow amplitude change due to diffusion
The Binary Black Hole Model for Mrk 231 Bites the Dust
Mrk 231 is a nearby quasar with an unusually red near-UV-to-optical
continuum, generally explained as heavy reddening by dust (e.g., Leighly et al.
2014). Yan et al. 2015 proposed that Mrk~231 is a milli-parsec black-hole
binary with little intrinsic reddening. We show that if the observed FUV
continuum is intrinsic, as assumed by Yan et al. 2015, it fails by a factor of
about 100 in powering the observed strength of the near-infrared emission
lines, and the thermal near and mid-infrared continuum. In contrast, the line
and continuum strengths are typical for a reddened AGN spectral energy
distribution. We find that the HeI*/Pbeta ratio is sensitive to the spectral
energy distribution for a one-zone model. If this sensitivity is maintained in
general broad-line region models, then this ratio may prove a useful diagnostic
for heavily reddened quasars. Analysis of archival HST STIS and FOC data
revealed evidence that the far-UV continuum emission is resolved on size scales
of ~40 parsecs. The lack of broad absorption lines in the far-UV continuum
might be explained if it were not coincident with the central engine. One
possibility is that it is the central engine continuum reflected from the
receding wind on the far side of the quasar.Comment: Consistent with the accepted ApJ pape
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