15,619 research outputs found
A pitfall of piecewise-polytropic equation of state inference
The only messenger radiation in the Universe which one can use to
statistically probe the Equation of State (EOS) of cold dense matter is that
originating from the near-field vicinities of compact stars. Constraining
gravitational masses and equatorial radii of rotating compact stars is a major
goal for current and future telescope missions, with a primary purpose of
constraining the EOS. From a Bayesian perspective it is necessary to carefully
discuss prior definition; in this context a complicating issue is that in
practice there exist pathologies in the general relativistic mapping between
spaces of local (interior source matter) and global (exterior spacetime)
parameters. In a companion paper, these issues were raised on a theoretical
basis. In this study we reproduce a probability transformation procedure from
the literature in order to map a joint posterior distribution of Schwarzschild
gravitational masses and radii into a joint posterior distribution of EOS
parameters. We demonstrate computationally that EOS parameter inferences are
sensitive to the choice to define a prior on a joint space of these masses and
radii, instead of on a joint space interior source matter parameters. We focus
on the piecewise-polytropic EOS model, which is currently standard in the field
of astrophysical dense matter study. We discuss the implications of this issue
for the field.Comment: 16 pages, 9 figures. Accepted for publication in MNRA
Characteristics of trapped proton anisotropy at Space Station Freedom altitudes
The ionizing radiation dose for spacecraft in low-Earth orbit (LEO) is produced mainly by protons trapped in the Earth's magnetic field. Current data bases describing this trapped radiation environment assume the protons to have an isotropic angular distribution, although the fluxes are actually highly anisotropic in LEO. The general nature of this directionality is understood theoretically and has been observed by several satellites. The anisotropy of the trapped proton exposure has not been an important practical consideration for most previous LEO missions because the random spacecraft orientation during passage through the radiation belt 'averages out' the anisotropy. Thus, in spite of the actual exposure anisotropy, cumulative radiation effects over many orbits can be predicted as if the environment were isotropic when the spacecraft orientation is variable during exposure. However, Space Station Freedom will be gravity gradient stabilized to reduce drag, and, due to this fixed orientation, the cumulative incident proton flux will remain anisotropic. The anisotropy could potentially influence several aspects of Space Station design and operation, such as the appropriate location for radiation sensitive components and experiments, location of workstations and sleeping quarters, and the design and placement of radiation monitors. Also, on-board mass could possible be utilized to counteract the anisotropy effects and reduce the dose exposure. Until recently only omnidirectional data bases for the trapped proton environment were available. However, a method to predict orbit-average, angular dependent ('vector') trapped proton flux spectra has been developed from the standard omnidirectional trapped proton data bases. This method was used to characterize the trapped proton anisotropy for the Space Station orbit (28.5 degree inclination, circular) in terms of its dependence on altitude, solar cycle modulation (solar minimum vs. solar maximum), shielding thickness, and radiation effect (silicon rad and rem dose)
Revised prediction of LDEF exposure to trapped protons
The Long Duration Exposure Facility (LDEF) spacecraft flew in a 28.5 deg inclination circular orbit with an altitude in the range from 319.4 to 478.7 km. For this orbital altitude and inclination, two components contribute most of the penetrating charge particle radiation encountered - the galactic cosmic rays and the geomagnetically trapped Van Allen protons. Where shielding is less than 1.0 g/sq cm geomagnetically trapped electrons make a significant contribution. The 'Vette' models together with the associated magnetic field models and the solar conditions were used to obtain the trapped electron and proton omnidirectional fluences reported previously. Results for directional proton spectra using the MSFC anisotropy model for solar minimum and 463 km altitude (representative for the LDEF mission) were also reported. The directional trapped proton flux as a function of mission time is presented considering altitude and solar activity variation during the mission. These additional results represent an extension of previous calculations to provide a more definitive description of the LDEF trapped proton exposure
Indications for a slow rotator in the Rapid Burster from its thermonuclear bursting behaviour
We perform time-resolved spectroscopy of all the type I bursts from the Rapid
Burster (MXB 1730-335) detected with the Rossi X-ray Timing Explorer. Type I
bursts are detected at high accretion rates, up to \sim 45% of the Eddington
luminosity. We find evidence that bursts lacking the canonical cooling in their
time-resolved spectra are, none the less, thermonuclear in nature. The type I
bursting rate keeps increasing with the persistent luminosity, well above the
threshold at which it is known to abruptly drop in other bursting low-mass
X-ray binaries. The only other known source in which the bursting rate keeps
increasing over such a large range of mass accretion rates is the 11 Hz pulsar
IGR J174802446. This may indicate a similarly slow spin for the neutron star
in the Rapid Burster
Ionizing radiation calculations and comparisons with LDEF data
In conjunction with the analysis of LDEF ionizing radiation dosimetry data, a calculational program is in progress to aid in data interpretation and to assess the accuracy of current radiation models for future mission applications. To estimate the ionizing radiation environment at the LDEF dosimeter locations, scoping calculations for a simplified (one dimensional) LDEF mass model were made of the primary and secondary radiations produced as a function of shielding thickness due to trapped proton, galactic proton, and atmospheric (neutron and proton cosmic ray albedo) exposures. Preliminary comparisons of predictions with LDEF induced radioactivity and dose measurements were made to test a recently developed model of trapped proton anisotropy
A crossing probability for critical percolation in two dimensions
Langlands et al. considered two crossing probabilities, pi_h and pi_{hv}, in
their extensive numerical investigations of critical percolation in two
dimensions. Cardy was able to find the exact form of pi_h by treating it as a
correlation function of boundary operators in the Q goes to 1 limit of the Q
state Potts model. We extend his results to find an analogous formula for
pi_{hv} which compares very well with the numerical results.Comment: 8 pages, Latex2e, 1 figure, uuencoded compressed tar file, (1 typo
changed
The Public Resource Management Game
Use of public resources for private economic gain is a longstanding, contested political issue. Public resources generate benefits beyond commodity uses, including recreation, environmental and ecological conservation and preservation, and existence and aesthetic values. We analyze this problem using a dynamic resource use game. Low use fees let commodity users capture more of the marginal benefit from private use. This increases the incentive to comply with government regulations. Optimal contracts therefore include public use fees that are lower than private rates. The optimal policy also includes random monitoring to prevent strategic learning and cheating on the use agreements and to avoid wasteful efforts to disguise noncompliant behavior. An optimal policy also includes a penalty for cheating beyond terminating the use contract. This penalty must be large enough that the commodity user who would gain the most from noncompliance experiences a negative expected net return.Renewable resources, public resources policy, optimal contracts
The Spread of Opinions and Proportional Voting
Election results are determined by numerous social factors that affect the
formation of opinion of the voters, including the network of interactions
between them and the dynamics of opinion influence. In this work we study the
result of proportional elections using an opinion dynamics model similar to
simple opinion spreading over a complex network. Erdos-Renyi, Barabasi-Albert,
regular lattices and randomly augmented lattices are considered as models of
the underlying social networks. The model reproduces the power law behavior of
number of candidates with a given number of votes found in real elections with
the correct slope, a cutoff for larger number of votes and a plateau for small
number of votes. It is found that the small world property of the underlying
network is fundamental for the emergence of the power law regime.Comment: 10 pages, 7 figure
Epidemics and percolation in small-world networks
We study some simple models of disease transmission on small-world networks,
in which either the probability of infection by a disease or the probability of
its transmission is varied, or both. The resulting models display epidemic
behavior when the infection or transmission probability rises above the
threshold for site or bond percolation on the network, and we give exact
solutions for the position of this threshold in a variety of cases. We confirm
our analytic results by numerical simulation.Comment: 6 pages, including 3 postscript figure
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