56 research outputs found
Second class particles and cube root asymptotics for Hammersley's process
We show that, for a stationary version of Hammersley's process, with Poisson
sources on the positive x-axis and Poisson sinks on the positive y-axis, the
variance of the length of a longest weakly North--East path from
to is equal to , where is the
location of a second class particle at time . This implies that both
and the variance of are of order .
Proofs are based on the relation between the flux and the path of a second
class particle, continuing the approach of Cator and Groeneboom [Ann. Probab.
33 (2005) 879--903].Comment: Published at http://dx.doi.org/10.1214/009117906000000089 in the
Annals of Probability (http://www.imstat.org/aop/) by the Institute of
Mathematical Statistics (http://www.imstat.org
A new look at distances and velocities of neutron stars
We take a fresh look at the determination of distances and velocities of
neutron stars. The conversion of a parallax measurement into a distance, or
distance probability distribution, has led to a debate quite similar to the one
involving Cepheids, centering on the question whether priors can be used when
discussing a single system. With the example of PSRJ0218+4232 we show that a
prior is necessary to determine the probability distribution for the distance.
The distance of this pulsar implies a gamma-ray luminosity larger than 10% of
its spindown luminosity. For velocities the debate is whether a single
Maxwellian describes the distribution for young pulsars. By limiting our
discussion to accurate (VLBI) measurements we argue that a description with two
Maxwellians, with distribution parameters sigma1=77 and sigma2=320 km/s, is
significantly better. Corrections for galactic rotation, to derive velocities
with respect to the local standards of rest, are insignificant.Comment: Has appeared in Journal of Astrophysics and Astronomy special issue
on 'Physics of Neutron Stars and Related Objects', celebrating the 75th
birth-year of G. Srinivasan. Ten pages, nine figure
Hammersley's process with sources and sinks
We show that, for a stationary version of Hammersley's process, with Poisson
``sources'' on the positive x-axis, and Poisson ``sinks'' on the positive
y-axis, an isolated second-class particle, located at the origin at time zero,
moves asymptotically, with probability 1, along the characteristic of a
conservation equation for Hammersley's process. This allows us to show that
Hammersley's process without sinks or sources, as defined by Aldous and
Diaconis [Probab. Theory Related Fields 10 (1995) 199-213] converges locally in
distribution to a Poisson process, a result first proved in Aldous and Diaconis
(1995) by using the ergodic decomposition theorem and a construction of
Hammersley's process as a one-dimensional point process, developing as a
function of (continuous) time on the whole real line. As a corollary we get the
result that EL(t,t)/t converges to 2, as t\to\infty, where L(t,t) is the length
of a longest North-East path from (0,0) to (t,t). The proofs of these facts
need neither the ergodic decomposition theorem nor the subadditive ergodic
theorem. We also prove a version of Burke's theorem for the stationary process
with sources and sinks and briefly discuss the relation of these results with
the theory of longest increasing subsequences of random permutations.Comment: Published at http://dx.doi.org/10.1214/009117905000000053 in the
Annals of Probability (http://www.imstat.org/aop/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Space-time stationary solutions for the Burgers equation
We construct space-time stationary solutions of the 1D Burgers equation with
random forcing in the absence of periodicity or any other compactness
assumptions. More precisely, for the forcing given by a homogeneous Poissonian
point field in space-time we prove that there is a unique global solution with
any prescribed average velocity. These global solutions serve as one-point
random attractors for the infinite-dimensional dynamical system associated to
solutions to the Cauchy problem. The probability distribution of the global
solutions defines a stationary distribution for the corresponding Markov
process. We describe a broad class of initial Cauchy data for which the
distribution of the Markov process converges to the above stationary
distribution.
Our construction of the global solutions is based on a study of the field of
action-minimizing curves. We prove that for an arbitrary value of the average
velocity, with probability 1 there exists a unique field of action-minimizing
curves initiated at all of the Poissonian points. Moreover action-minimizing
curves corresponding to different starting points merge with each other in
finite time.Comment: 50 pages. In this version: small technical corrections in Lemmas 6.1
and 6.
Cube root fluctuations for the corner growth model associated to the exclusion process
We study the last-passage growth model on the planar integer lattice with
exponential weights. With boundary conditions that represent the equilibrium
exclusion process as seen from a particle right after its jump we prove that
the variance of the last-passage time in a characteristic direction is of order
t^{2/3}. With more general boundary conditions that include the rarefaction fan
case we show that the last-passage time fluctuations are still of order
t^{1/3}, and also that the transversal fluctuations of the maximal path have
order t^{2/3}. We adapt and then build on a recent study of Hammersley's
process by Cator and Groeneboom, and also utilize the competition interface
introduced by Ferrari, Martin and Pimentel. The arguments are entirely
probabilistic, and no use is made of the combinatorics of Young tableaux or
methods of asymptotic analysis.Comment: 41 pages, 4 figure
The observed velocity distribution of young pulsars
We argue that comparison with observations of theoretical models for the
velocity distribution of pulsars must be done directly with the observed
quantities, i.e. parallax and the two components of proper motion. We develop a
formalism to do so, and apply it to pulsars with accurate VLBI measurements. We
find that a distribution with two maxwellians improves significantly on a
single maxwellian. The `mixed' model takes into account that pulsars move away
from their place of birth, a narrow region around the galactic plane. The best
model has 42% of the pulsars in a maxwellian with average velocity sigma
sqrt{8/pi}=120 km/s, and 58% in a maxwellian with average velocity 540 km/s.
About 5% of the pulsars has a velocity at birth less than 60\,km/s. For the
youngest pulsars (tau_c<10 Myr), these numbers are 32% with 130 km/s, 68% with
520 km/s, and 3%, with appreciable uncertainties.Comment: submitted to A&A; 14 pages, 10 figure
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