40,049 research outputs found
Forecasting intermittent demand
Methods for forecasting intermittent demand are compared using a large data-set from the UK Royal Air Force (RAF). Several important results are found. First, we show that the traditional per period forecast error measures are not appropriate for intermittent demand, even though they are consistently used in the literature. Second, by comparing target service levels to achieved service levels when inventory decisions are based on demand forecasts, we show that Croston's method (and a variant) and Bootstrapping clearly outperform Moving Average and Single Exponential Smoothing. Third, we show that the performance of Croston and Bootstrapping can be significantly improved by taking into account that each lead time starts with a demand
How else can we detect Fast Radio Bursts?
We discuss possible electromagnetic signals accompanying Fast Radio Bursts
(FRBs) that are expected in the scenario where FRBs originate in neutron star
magnetospheres. For models involving Crab-like giant pulses, no appreciable
contemporaneous emission is expected at other wavelengths. Magnetar giant
flares, driven by the reconfiguration of the magnetosphere, however, can
produce both contemporaneous bursts at other wavelengths as well as
afterglow-like emission. We conclude that the best chances are: (i) prompt
short GRB-like emission; (ii) a contemporaneous optical flash that can reach
naked eye peak luminosity (but only for a few milliseconds); (iii) a high
energy afterglow emission. Case (i) could be tested by coordinated radio and
high-energy experiments. Case (ii) could be seen in a coordinated radio-optical
surveys, \eg\ by the Palomar Transient Factory in a 60-second frame as a
transient object of magnitude with an expected optical detection rate
of about 0.1~hr, an order of magnitude higher than in radio. Shallow,
but large-area sky surveys such as ASAS-SN and EVRYSCOPE could also detect
prompt optical flashes from the more powerful Lorimer-burst clones. The best
constraints on the optical-to-radio power for this kind of emission could be
provided by future observations with facilities like LSST. Case (iii) might be
seen in relatively rare cases that the relativistically ejected magnetic blob
is moving along the line of sight
An Accelerated Multiboson Algorithm for Coulomb Gases with Dynamical Dielectric Effects
A recent reformulation [1] of the problem of Coulomb gases in the presence of
a dynamical dielectric medium showed that finite temperature simulations of
such systems can be accomplished on the basis of completely local Hamiltonians
on a spatial lattice by including additional bosonic fields. For large systems,
the Monte Carlo algorithm proposed in Ref. [1] becomes inefficient due to a low
acceptance rate for particle moves in a fixed background multiboson field. We
show here how this problem can be circumvented by use of a coupled
particle-multiboson update procedure that improves acceptance rates on large
lattices by orders of magnitude. The method is tested on a one-component plasma
with neutral dielectric particles for a variety of system sizes.Comment: 13 pages, 2 figures, fixed typos, added reference
Flares from Galactic centre pulsars: a new class of X-ray transients?
Despite intensive searches, the only pulsar within 0.1 pc of the central
black hole in our Galaxy, Sgr A*, is a radio-loud magnetar. Since magnetars are
rare among the Galactic neutron star population, and a large number of massive
stars are already known in this region, the Galactic centre (GC) should harbor
a large number of neutron stars. Population syntheses suggest several thousand
neutron stars may be present in the GC. Many of these could be highly energetic
millisecond pulsars which are also proposed to be responsible for the GC
gamma-ray excess. We propose that the presence of a neutron star within 0.03~pc
from Sgr~A* can be revealed by the shock interactions with the disk around the
central black hole. As we demonstrate, these interactions result in observable
transient non-thermal X-ray and gamma-ray emission over timescales of months,
provided that the spin down luminosity of the neutron star is L_{sd}~10^{35}
erg/s. Current limits on the population of normal and millisecond pulsars in
the GC region suggest that a number of such pulsars are present with such
luminosities.Comment: 4 pages, 4 figure
Masses and Decay Constants of Heavy-Light Mesons Using the Multistate Smearing Technique
We present results for f_B and masses of low-lying heavy-light mesons.
Calculations were performed in the quenched approximation using multistate
smearing functions generated from a spinless relativistic quark model
Hamiltonian. Beta values range from 5.7 to 6.3, and light quark masses
corresponding to pion masses as low as 300 MeV are computed at each value. We
use the 1P--1S charmonium splitting to set the overall scale.Comment: 9 pages, 13 figures, and 5 tables as a single 193K compressed and
uuencoded Postscript file, FERMILAB--CONF--93/376-
A New Method for Estimating Dark Matter Halo Masses using Globular Cluster Systems
All galaxies are thought to reside within large halos of dark matter, whose
properties can only be determined from indirect observations. The formation and
assembly of galaxies is determined from the interplay between these dark matter
halos and the baryonic matter they host. Although statistical relations can be
used to approximate how massive a galaxy's halo is, very few individual
galaxies have direct measurements of their halo masses. We present a method to
directly estimate the total mass of a galaxy's dark halo using its system of
globular clusters. The link between globular cluster systems and halo masses is
independent of a galaxy's type and environment, in contrast to the relationship
between galaxy halo and stellar masses. This trend is expected in models where
globular clusters form in early, rare density peaks in the cold dark matter
density field and the epoch of reionisation was roughly coeval throughout the
Universe. We illustrate the general utility of this relation by demonstrating
that a galaxy's supermassive black hole mass and global X-ray luminosity are
directly proportional to their host dark halo masses, as inferred from our new
method.Comment: 6 pages, 4 colour figures. Accepted by MNRAS Letters. Data catalogue
available from the first autho
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