21,703 research outputs found
Accounting for the dispersion in the x ray properties of early-type galaxies
The x ray luminosities of early-type galaxies are correlated with their optical (e.g., blue) luminosities (L sub X approx. L sub B exp 1.6), but the x ray luminosities exhibit considerable scatter for a given optical luminosity L sub B. This dispersion in x ray luminosity is much greater than the dispersion of other properties of early-type galaxies (for a given L sub B), such as luminosity scale-length, velocity dispersion, color, and metallicity. Here, researchers consider several possible sources for the dispersion in x ray luminosity. Some of the scatter in x ray luminosity may result from stellar population variations between galaxies with similar L sub B. Since the x ray emitting gas is from accumulated stellar mass loss, the L sub X dispersion may be due to variations in integrated stellar mass loss rates. Another possible cause of the L sub X dispersion may be variations in the amount of cool material in the galaxies; cool gas may act as an energy sink for the hot gas. Infrared emission may be used to trace such cool material, so researchers look for a correlation between the infrared emission and the x ray emission of early-type galaxies at fixed L sub B. Velocity dispersion variations between galaxies of similar L sub B may also contribute to the L sub X dispersion. The most likely a priori source of the dispersion in L sub X is probably the varying amount of ram-pressure stripping in a range of galaxy environments. The hot gaseous halos of early-type galaxies can be stripped in encounters with other galaxies or with ambient cluster gas if the intracluster gas is sufficiently dense. Researchers find that the most likely cause of dispersion in the x ray properties of early type galaxies is probably the ram-pressure stripping of gaseous halos from galaxies. For a sample of 81 early-type galaxies with x ray luminosities or upper limits derived from Einstein Observatory observations (CFT) researchers calculated the cumulative distribution of angular distances between the x ray sample members and bright galaxies from the Revised Shapley - Ames catalog. Collectively, galaxies with low x ray luminosities (for a given L sub B) tend to be in denser environments than galaxies with higher x ray luminosities
Analysis of variability in the burst oscillations of the accreting millisecond pulsar XTE J1814-338
The accreting millisecond pulsar XTE J1814-338 exhibits oscillations at the
known spin frequency during Type I X-ray bursts. The properties of the burst
oscillations reflect the nature of the thermal asymmetry on the stellar
surface. We present an analysis of the variability of the burst oscillations of
this source, focusing on three characteristics: fractional amplitude, harmonic
content and frequency. Fractional amplitude and harmonic content constrain the
size, shape and position of the emitting region, whilst variations in frequency
indicate motion of the emitting region on the neutron star surface. We examine
both long-term variability over the course of the outburst, and short-term
variability during the bursts. For most of the bursts, fractional amplitude is
consistent with that of the accretion pulsations, implying a low degree of fuel
spread. There is however a population of bursts whose fractional amplitudes are
substantially lower, implying a higher degree of fuel spread, possibly forced
by the explosive burning front of a precursor burst. For the first harmonic,
substantial differences between the burst and accretion pulsations suggest that
hotspot geometry is not the only mechanism giving rise to harmonic content in
the latter. Fractional amplitude variability during the bursts is low; we
cannot rule out the hypothesis that the fractional amplitude remains constant
for bursts that do not exhibit photospheric radius expansion (PRE). There are
no significant variations in frequency in any of the bursts except for the one
burst that exhibits PRE. This burst exhibits a highly significant but small
(Hz) drop in frequency in the burst rise. The timescale of the
frequency shift is slower than simple burning layer expansion models predict,
suggesting that other mechanisms may be at work.Comment: 20 pages, 20 figures, accepted for publication in ApJ. Uses
emulateapj.cl
New remarks on the Cosmological Argument
We present a formal analysis of the Cosmological Argument in its two main
forms: that due to Aquinas, and the revised version of the Kalam Cosmological
Argument more recently advocated by William Lane Craig. We formulate these two
arguments in such a way that each conclusion follows in first-order logic from
the corresponding assumptions. Our analysis shows that the conclusion which
follows for Aquinas is considerably weaker than what his aims demand. With
formalizations that are logically valid in hand, we reinterpret the natural
language versions of the premises and conclusions in terms of concepts of
causality consistent with (and used in) recent work in cosmology done by
physicists. In brief: the Kalam argument commits the fallacy of equivocation in
a way that seems beyond repair; two of the premises adopted by Aquinas seem
dubious when the terms `cause' and `causality' are interpreted in the context
of contemporary empirical science. Thus, while there are no problems with
whether the conclusions follow logically from their assumptions, the Kalam
argument is not viable, and the Aquinas argument does not imply a caused
origination of the universe. The assumptions of the latter are at best less
than obvious relative to recent work in the sciences. We conclude with mention
of a new argument that makes some positive modifications to an alternative
variation on Aquinas by Le Poidevin, which nonetheless seems rather weak.Comment: 12 pages, accepted for publication in International Journal for
Philosophy of Religio
Fear of Crime, Incivilities, and Collective Efficacy in Four Miami Neighborhoods
Purpose: Extant literature indicates that individual perceptions of collective efficacy and incivilities are important in explaining fear of crime. These studies, however, often implicitly assume that the relationships between key variables do not differ between neighborhoods. The purpose of this research is to examine the relationship between perceptions of collective efficacy, perceptions of incivilities, and fear of crime and determine whether these relationships are constant between neighborhoods.
Methods: Surveys were conducted using a sample of residents from four neighborhoods within Miami-Dade County. Structural equation models were used to examine the relationships between perceptions of collective efficacy, perceptions of incivilities, and fear of crime for each neighborhood separately. Tests for invariance were conducted to determine whether the coefficients from these models differed across neighborhoods.
Results: Results from these analyses suggest that the relationship between perceptions of collective efficacy and fear of crime exhibit significant heterogeneity between neighborhoods, as do a number of other relationships. The relationships between perceptions of collective efficacy and perceptions of incivilities, and perceptions of incivilities and fear of crime do not exhibit heterogeneity.
Conclusions: These results illustrate the importance of examining perceptions of collective efficacy within the neighborhood context. Implications for policy and future research are discusse
Chandra Observations of Low Mass X-ray Binaries and Diffuse Gas in the Early-Type Galaxies NGC 4365 and NGC 4382 (M85)
(Abridged) We used the Chandra X-ray Observatory ACIS S3 to image the X-ray
faint elliptical galaxy NGC 4365 and lenticular galaxy NGC 4382. The
observations resolve much of the X-ray emission into 99 and 58 sources,
respectively, most of which are low-mass X-ray binaries (LMXBs) associated with
each of the galaxies. We identify 18 out of the 37 X-ray sources in a central
field in NGC 4365 with globular clusters. The luminosity functions of the
resolved sources for both galaxies are best fit with cutoff power-laws whose
cutoff luminosity is ergs s. These
luminosities are much larger than those previously measured for similar
galaxies; we do not find evidence for a break in the luminosity function at the
Eddington luminosity of a 1.4 neutron star. The spatial distributions
of the resolved sources for both galaxies are broader than the distribution of
optical stars. In both galaxies, a hard power-law model fits the summed
spectrum of all of the sources. The unresolved emission is best fit by the sum
of a soft mekal model representing emission from diffuse gas, and a hard
power-law, presumed to be from unresolved LMXBs. A standard beta model fits the
radial distribution of the diffuse gas in both galaxies. In the elliptical NGC
4365, the best-fit core radius is very small, while the S0 galaxy NGC 4382 has
a larger core radius. This may indicate that the gas in NGC 4382 is rotating
significantly.Comment: Astrophysical Journal, accepted: 38 pages with 20 embedded reduced
resolution Postscript figure
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