11,953 research outputs found
Impulse approximation in the n p --> d pi^0 reaction reexamined
The impulse approximation (one-body operator) in the n p --> d pi^0 reaction
is reexamined with emphasis on the issues of reducibility and recoil
corrections. An inconsistency when one pion exchange is included in the
production operator is demonstrated and then resolved via the introduction of
"wave function corrections" which nearly vanish for static nucleon propagators.
Inclusion of the recoil corrections to the nucleon propagators is found to
change the magnitude and sign of the impulse production amplitude, worsening
agreement with the experimental cross section by approximately 30%. A cutoff is
used to account for the phenomenological nature of the external wave functions,
and is found to have a significant impact up to approximately 2.5 GeV.Comment: Published versio
Fake View Analytics in Online Video Services
Online video-on-demand(VoD) services invariably maintain a view count for
each video they serve, and it has become an important currency for various
stakeholders, from viewers, to content owners, advertizers, and the online
service providers themselves. There is often significant financial incentive to
use a robot (or a botnet) to artificially create fake views. How can we detect
the fake views? Can we detect them (and stop them) using online algorithms as
they occur? What is the extent of fake views with current VoD service
providers? These are the questions we study in the paper. We develop some
algorithms and show that they are quite effective for this problem.Comment: 25 pages, 15 figure
Can dry merging explain the size evolution of early-type galaxies?
The characteristic size of early-type galaxies (ETGs) of given stellar mass
is observed to increase significantly with cosmic time, from redshift z>2 to
the present. A popular explanation for this size evolution is that ETGs grow
through dissipationless ("dry") mergers, thus becoming less compact. Combining
N-body simulations with up-to-date scaling relations of local ETGs, we show
that such an explanation is problematic, because dry mergers do not decrease
the galaxy stellar-mass surface-density enough to explain the observed size
evolution, and also introduce substantial scatter in the scaling relations.
Based on our set of simulations, we estimate that major and minor dry mergers
increase half-light radius and projected velocity dispersion with stellar mass
(M) as M^(1.09+/-0.29) and M^(0.07+/-0.11), respectively. This implies that: 1)
if the high-z ETGs are indeed as dense as estimated, they cannot evolve into
present-day ETGs via dry mergers; 2) present-day ETGs cannot have assembled
more than ~45% of their stellar mass via dry mergers. Alternatively, dry
mergers could be reconciled with the observations if there was extreme fine
tuning between merger history and galaxy properties, at variance with our
assumptions. Full cosmological simulations will be needed to evaluate whether
this fine-tuned solution is acceptable.Comment: 5 pages, 2 figures. Accepted for publication in ApJ Letter
The Structure & Dynamics of Massive Early-type Galaxies: On Homology, Isothermality and Isotropy inside one Effective Radius
Based on 58 SLACS strong-lens early-type galaxies with direct total-mass and
stellar-velocity dispersion measurements, we find that inside one effective
radius massive elliptical galaxies with M_eff >= 3x10^10 M_sun are
well-approximated by a power-law ellipsoid with an average logaritmic density
slope of = -dlog(rho_tot)/dlog(r)=2.085^{+0.025}_{-0.018} (random
error on mean) for isotropic orbits with beta_r=0, +-0.1 (syst.) and
sigma_gamma' <= 0.20^{+0.04}_{-0.02} intrinsic scatter (all errors indicate the
68 percent CL). We find no correlation of gamma'_LD with galaxy mass (M_eff),
rescaled radius (i.e. R_einst/R_eff) or redshift, despite intrinsic differences
in density-slope between galaxies. Based on scaling relations, the average
logarithmic density slope can be derived in an alternative manner, fully
independent from dynamics, yielding =1.959 +- 0.077. Agreement
between the two values is reached for =0.45 +- 0.25, consistent with
mild radial anisotropy. This agreement supports the robustness of our results,
despite the increase in mass-to-light ratio with total galaxy mass: M_eff ~
L_{V,eff}^(1.363+-0.056). We conclude that massive early-type galaxies are
structurally close-to homologous with close-to isothermal total density
profiles (<=10 percent intrinsic scatter) and have at most some mild radial
anisotropy. Our results provide new observational limits on galaxy formation
and evolution scenarios, covering four Gyr look-back time.Comment: Accepted for publication by ApJL; 4 pages, 2 figure
Ultraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state
(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus
X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features
show strong, broad absorption components when the X-ray source is behind the
companion star and noticeably weaker absorption when the X-ray source is
between us and the companion star. We fit the P Cygni profiles using the SEI
method applied to a spherically symmetric stellar wind subject to X-ray
photoionization from the black hole. The Si IV doublet provides the most
reliable estimates of the parameters of the wind and X-ray illumination. The
velocity increases with radius according to
, with and
km s.The microturbulent velocity was
km s. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate
of L, measured at = 4.8. Our
models determine parameters that may be used to estimate the accretion rate
onto the black hole and independently predict the X-ray luminosity. Our
predicted L matches that determined by contemporaneous RXTE ASM remarkably
well, but is a factor of 3 lower than the rate according to
Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the
energy of accretion may go into powering a jet.Comment: 34 pages, 21 figures, 4 tables, accepted for publication in Ap
The Sloan Lens ACS Survey. IX. Colors, Lensing and Stellar Masses of Early-type Galaxies
We present the current photometric dataset for the Sloan Lens ACS (SLACS)
Survey, including HST photometry from ACS, WFPC2, and NICMOS. These data have
enabled the confirmation of an additional 15 grade `A' (certain) lens systems,
bringing the number of SLACS grade `A' lenses to 85; including 13 grade `B'
(likely) systems, SLACS has identified nearly 100 lenses and lens candidates.
Approximately 80% of the grade `A' systems have elliptical morphologies while
~10% show spiral structure; the remaining lenses have lenticular morphologies.
Spectroscopic redshifts for the lens and source are available for every system,
making SLACS the largest homogeneous dataset of galaxy-scale lenses to date. We
have developed a novel Bayesian stellar population analysis code to determine
robust stellar masses with accurate error estimates. We apply this code to
deep, high-resolution HST imaging and determine stellar masses with typical
statistical errors of 0.1 dex; we find that these stellar masses are unbiased
compared to estimates obtained using SDSS photometry, provided that informative
priors are used. The stellar masses range from 10^10.5 to 10^11.8 M and
the typical stellar mass fraction within the Einstein radius is 0.4, assuming a
Chabrier IMF. The ensemble properties of the SLACS lens galaxies, e.g. stellar
masses and projected ellipticities, appear to be indistinguishable from other
SDSS galaxies with similar stellar velocity dispersions. This further supports
that SLACS lenses are representative of the overall population of massive
early-type galaxies with M* >~ 10^11 M, and are therefore an ideal
dataset to investigate the kpc-scale distribution of luminous and dark matter
in galaxies out to z ~ 0.5.Comment: 20 pages, 18 figures, 5 tables, published in Ap
Possible evidence for an inverted temperature-density relation in the intergalactic medium from the flux distribution of the Lyman-alpha forest
We compare the improved measurement of the Lya forest flux probability
distribution at 1.7<z<3.2 presented by Kim et al. (2007) to a large set of
hydrodynamical simulations of the Lya forest with different cosmological
parameters and thermal histories. The simulations are in good agreement with
the observational data if the temperature-density relation for the low density
intergalactic medium (IGM), T=T_0 Delta^{gamma-1}, is either close to
isothermal or inverted (gamma<1). Our results suggest that the voids in the IGM
may be significantly hotter and the thermal state of the low density IGM may be
substantially more complex than is usually assumed at these redshifts. We
discuss radiative transfer effects which alter the spectral shape of ionising
radiation during the epoch of HeII reionisation as a possible physical
mechanism for achieving an inverted temperature-density relation at z~3.Comment: 16 pages, 6 figures, accepted for publication in MNRAS following
minor revision. The accepted version includes an expanded discussion of the
flux power spectru
Inference of the Cold Dark Matter substructure mass function at z=0.2 using strong gravitational lenses
We present the results of a search for galaxy substructures in a sample of 11
gravitational lens galaxies from the Sloan Lens ACS Survey. We find no
significant detection of mass clumps, except for a luminous satellite in the
system SDSS J0956+5110. We use these non-detections, in combination with a
previous detection in the system SDSS J0946+1006, to derive constraints on the
substructure mass function in massive early-type host galaxies with an average
redshift z ~ 0.2 and an average velocity dispersion of 270 km/s. We perform a
Bayesian inference on the substructure mass function, within a median region of
about 32 kpc squared around the Einstein radius (~4.2 kpc). We infer a mean
projected substructure mass fraction at the 68
percent confidence level and a substructure mass function slope < 2.93
at the 95 percent confidence level for a uniform prior probability density on
alpha. For a Gaussian prior based on Cold Dark Matter (CDM) simulations, we
infer and a slope of =
1.90 at the 68 percent confidence level. Since only one
substructure was detected in the full sample, we have little information on the
mass function slope, which is therefore poorly constrained (i.e. the Bayes
factor shows no positive preference for any of the two models).The inferred
fraction is consistent with the expectations from CDM simulations and with
inference from flux ratio anomalies at the 68 percent confidence level.Comment: Accepted for publication on MNRAS, some typos corrected and some
important references adde
The ontogeny, palaeobiology and systematic palaeontology of some lower liassic belemnitida
The Belemnite Marls of the Dorset coast constitute the jamesoni and ibex Zones of that area. This study is essentially a revision of the total belemnite fauna of these beds, with a preliminary appraisal of the belemnites from the overlying basal Green Ammonite Beds (davoei Zone). Twelve morphospecies are described which are defined by their rostral ontogeny and other morphometric data. Probable sexual dimorphism into relatively stout and slender rostra is demonstrated for Four species (Belemnites longissimus Miller, B. charmouthensis Mayer, B. imus (Lang) and B. cricki (Lissajous)). It is recognised that, at different localities and horizons, single species may be represented by assemblages that vary with respect to age and sex of individuals, and it is concluded that such belemnite assemblages are at least partly due to local population structures analagous to those that are found in modern cephalopods. Early ontogeny, rostral form and function, epirostra and surface ornamentation are briefly discussed with reference to the species described
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