40,210 research outputs found
Differential evolution with an evolution path: a DEEP evolutionary algorithm
Utilizing cumulative correlation information already existing in an evolutionary process, this paper proposes a predictive approach to the reproduction mechanism of new individuals for differential evolution (DE) algorithms. DE uses a distributed model (DM) to generate new individuals, which is relatively explorative, whilst evolution strategy (ES) uses a centralized model (CM) to generate offspring, which through adaptation retains a convergence momentum. This paper adopts a key feature in the CM of a covariance matrix adaptation ES, the cumulatively learned evolution path (EP), to formulate a new evolutionary algorithm (EA) framework, termed DEEP, standing for DE with an EP. Without mechanistically combining two CM and DM based algorithms together, the DEEP framework offers advantages of both a DM and a CM and hence substantially enhances performance. Under this architecture, a self-adaptation mechanism can be built inherently in a DEEP algorithm, easing the task of predetermining algorithm control parameters. Two DEEP variants are developed and illustrated in the paper. Experiments on the CEC'13 test suites and two practical problems demonstrate that the DEEP algorithms offer promising results, compared with the original DEs and other relevant state-of-the-art EAs
Probing the X-Ray Binary Populations of the Ring Galaxy NGC 1291
We present Chandra studies of the X-ray binary (XRB) populations in the bulge
and ring regions of the ring galaxy NGC 1291. We detect 169 X-ray point sources
in the galaxy, 75 in the bulge and 71 in the ring, utilizing the four available
Chandra observations totaling an effective exposure of 179 ks. We report
photometric properties of these sources in a point-source catalog. There are
~40% of the bulge sources and ~25% of the ring sources showing >3\sigma
long-term variability in their X-ray count rate. The X-ray colors suggest that
a significant fraction of the bulge (~75%) and ring (~65%) sources are likely
low-mass X-ray binaries (LMXBs). The spectra of the nuclear source indicate
that it is a low-luminosity AGN with moderate obscuration; spectral variability
is observed between individual observations. We construct 0.3-8.0 keV X-ray
luminosity functions (XLFs) for the bulge and ring XRB populations, taking into
account the detection incompleteness and background AGN contamination. We reach
90% completeness limits of ~1.5\times10^{37} and ~2.2\times10^{37} erg/s for
the bulge and ring populations, respectively. Both XLFs can be fit with a
broken power-law model, and the shapes are consistent with those expected for
populations dominated by LMXBs. We perform detailed population synthesis
modeling of the XRB populations in NGC 1291, which suggests that the observed
combined XLF is dominated by an old LMXB population. We compare the bulge and
ring XRB populations, and argue that the ring XRBs are associated with a
younger stellar population than the bulge sources, based on the relative
overdensity of X-ray sources in the ring, the generally harder X-ray color of
the ring sources, the overabundance of luminous sources in the combined XLF,
and the flatter shape of the ring XLF.Comment: 15 pages, 11 figures. Accepted for publication in Ap
Distribution of Spectral Characteristics and the Cosmological Evolution of GRBs
We investigate the cosmological evolution of GRBs, using the total gamma ray
fluence as a measure of the burst strength. This involves an understanding of
the distributions of the spectral parameters of GRBs as well as the total
fluence distribution - both of which are subject to detector selection effects.
We present new non-parametric statistical techniques to account for these
effects, and use these methods to estimate the true distribution of the peak of
the nu F_nu spectrum, E_p, from the raw distribution. The distributions are
obtained from four channel data and therefore are rough estimates. Here, we
emphasize the methods and present qualitative results. Given its spectral
parameters, we then calculate the total fluence for each burst, and compute its
cumulative and differential distributions. We use these distributions to
estimate the cosmological rate evolution of GRBs, for three cosmological
models. Our two main conclusions are the following: 1) Given our estimates of
the spectral parameters, we find that there may exist a significant population
of high E_p bursts that are not detected by BATSE, 2) We find a GRB co-moving
rate density quite different from that of other extragalactic objects; in
particular, it is different from the recently determined star formation rate.Comment: 20 pages, including 10 postscript figures. Submitted to Ap
The low-mass Initial Mass Function in the 30 Doradus starburst cluster
We present deep Hubble Space Telescope (HST) NICMOS 2 F160W band observations
of the central 56*57" (14pc*14.25pc) region around R136 in the starburst
cluster 30 Dor (NGC 2070) located in the Large Magellanic Cloud. Our aim is to
derive the stellar Initial Mass Function (IMF) down to ~1 Msun in order to test
whether the IMF in a massive metal-poor cluster is similar to that observed in
nearby young clusters and the field in our Galaxy. We estimate the mean age of
the cluster to be 3 Myr by combining our F160W photometry with previously
obtained HST WFPC2 optical F555W and F814W band photometry and comparing the
stellar locus in the color-magnitude diagram with main sequence and pre-main
sequence isochrones. The color-magnitude diagrams show the presence of
differential extinction and possibly an age spread of a few megayears. We
convert the magnitudes into masses adopting both a single mean age of 3 Myr
isochrone and a constant star formation history from 2 to 4 Myr. We derive the
IMF after correcting for incompleteness due to crowding. The faintest stars
detected have a mass of 0.5 Msun and the data are more than 50% complete
outside a radius of 5 pc down to a mass limit of 1.1 Msun for 3 Myr old
objects. We find an IMF of dN/dlog(M) M^(-1.20+-0.2) over the mass range
1.1--20 Msun only slightly shallower than a Salpeter IMF. In particular, we
find no strong evidence for a flattening of the IMF down to 1.1 Msun at a
distance of 5 pc from the center, in contrast to a flattening at 2 Msun at a
radius of 2 pc, reported in a previous optical HST study. We examine several
possible reasons for the different results. If the IMF determined here applies
to the whole cluster, the cluster would be massive enough to remain bound and
evolve into a relatively low-mass globular cluster.Comment: Accepted in ApJ. Abstract abridge
A Fluctuation Analysis of the Bolocam 1.1mm Lockman Hole Survey
We perform a fluctuation analysis of the 1.1mm Bolocam Lockman Hole Survey,
which covers 324 square arcmin to a very uniform point source-filtered RMS
noise level of 1.4 mJy/beam. The fluctuation analysis has the significant
advantage of utilizing all of the available data. We constrain the number
counts in the 1-10 mJy range, and derive significantly tighter constraints than
in previous work: the power-law index is 2.7 (+0.18, -0.15), while the
amplitude is equal to 1595 (+85,-238) sources per mJy per square degree, or
N(>1 mJy) = 940 (+50,-140) sources/square degree (95% confidence). Our results
agree extremely well with those derived from the extracted source number counts
by Laurent et al (2005). Our derived normalization is about 2.5 times smaller
than determined by MAMBO at 1.2mm by Greve et al (2004). However, the
uncertainty in the normalization for both data sets is dominated by the
systematic (i.e., absolute flux calibration) rather than statistical errors;
within these uncertainties, our results are in agreement. We estimate that
about 7% of the 1.1mm background has been resolved at 1 mJy.Comment: To appear in the Astrophysical Journal; 22 pages, 9 figure
The Luminosity Function of the Hot and Cold Kuiper belt Populations
Abridged. We have performed an ecliptic survey of the Kuiper belt, with an
areal coverage of 8.9 square degrees to a 50% limiting magnitude of r'=24.7,
and have detected 88 Kuiper belt objects, roughly half of which received
follow-up one to two months after detection. Using this survey data alone, we
have measured the luminosity function of the Kuiper belt, thus avoiding any
biases that might come from the inclusion of other observations. We have found
that the Cold population defined as having inclinations less than 5 degrees has
a luminosity function slope alpha=0.82+-0.23, and is different from the Hot
population, which has inclinations greater than 5 degrees and a luminosity
function slope alpha=0.35+-0.21. As well, we have found that those objects
closer than 38 AU have virtually the same luminosity function slope as the Hot
population. This result, along with similar findings of past surveys
demonstrates that the dynamically cold Kuiper belt objects likely have a steep
size distribution, and are unique from all of the excited populations which
have much shallower distributions. This suggests that the dynamically excited
population underwent a different accretion history and achieved a more evolved
state of accretion than the cold population. As well, we discuss the
similarities of the Cold and Hot populations with the size distributions of
other planetesimal populations. We find that while the Jupiter family comets
and the scattered disk exhibit similar size distributions, a power-law
extrapolation to small sizes for the scattered disk cannot account for the
observed influx of comets. As well, we have found that the Jupiter Trojan and
Hot populations cannot have originated from the same parent popuation, a result
that is difficult to reconcile with scattering models similar to the NICE
model.Comment: Accepted for publication in Icarus. 27 pages, 10 figures, 4 table
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