23,827 research outputs found
Three-dimensional macroporous silicon photonic crystal with large photonic band gap
Three-dimensional photonic crystals based on macroporous silicon are fabricated by photoelectrochemical etching and subsequent focused-ion-beam drilling. Reflection measurements show a high reflection in the range of the stopgap and indicate the spectral position of the complete photonic band gap. The onset of diffraction which might influence the measurement is discussed
Deconvolution of ASCA X-ray data: II. Radial temperature and metallicity profiles for 106 galaxy clusters
In Paper-I we presented a methodology to recover the spatial variations of
properties of the intracluster gas from ASCA X-ray satellite observations of
galaxy clusters. We verified the correctness of this procedure by applying it
to simulated cluster datasets which we had subjected to the various
contaminants common in ASCA data. In this paper we present the results which we
obtain when we apply this method to real galaxy cluster observations. We
determine broad-band temperature and cooling-flow mass-deposition rates for the
106 clusters in our sample, and obtain temperature, abundance and emissivity
profiles (i.e. at least two annular bins) for 98 of these clusters. We find
that 90 percent of these temperature profiles are consistent with isothermality
at the 3-sigma confidence level. This conflicts with the prevalence of
steeply-declining cluster temperature profiles found by Markevitch et al.
(1998) from a sample of 30 clusters. In Paper-III (in preparation) we utilise
our temperature and emissivity profiles to determine radial hydrostatic-mass
properties for a subsample of the clusters presented in this paper.Comment: MNRAS, accpeted. Postscript copy of paper and individual postscript
files for plots in Appendix B can be obtained from:
http://www-xray.ast.cam.ac.uk/~da
Measuring Molecular, Neutral Atomic, and Warm Ionized Galactic Gas Through X-Ray Absorption
We study the column densities of neutral atomic, molecular, and warm ionized
Galactic gas through their continuous absorption of extragalactic X-ray spectra
at |b| > 25 degrees. For N(H,21cm) < 5x10^20 cm^-2 there is an extremely tight
relationship between N(H,21cm) and the X-ray absorption column, N(xray), with a
mean ratio along 26 lines of sight of N(xray)/N(H,21cm) = 0.972 +- 0.022. This
is significantly less than the anticpated ratio of 1.23, which would occur if
He were half He I and half He II in the warm ionized component. We suggest that
the ionized component out of the plane is highly ionized, with He being mainly
He II and He III. In the limiting case that H is entirely HI, we place an upper
limit on the He abundance in the ISM of He/H <= 0.103.
At column densities N(xray) > 5x10^20 cm^-2, which occurs at our lower
latitudes, the X-ray absorption column N(xray) is nearly double N(H,21cm). This
excess column cannot be due to the warm ionized component, even if He were
entirely He I, so it must be due to a molecular component. This result implies
that for lines of sight out of the plane with |b| ~ 30 degrees, molecular gas
is common and with a column density comprable to N(H,21cm).
This work bears upon the far infrared background, since a warm ionized
component, anticorrelated with N(H,21cm), might produce such a background. Not
only is such an anticorrelation absent, but if the dust is destroyed in the
warm ionized gas, the far infrared background may be slightly larger than that
deduced by Puget et al. (1996).Comment: 1 AASTeX file, 14 PostScript figure files which are linked within the
TeX fil
Ultrafast electroabsorption dynamics in an InAs quantum dot saturable absorber at 1.3 mu m
The authors report a direct measurement of the absorption dynamics in an InAs p-i-n ridge waveguide quantum dot modulator. The carrier escape mechanisms are investigated via subpicosecond pump-probe measurements at room temperature, under reverse bias conditions. The optical pulses employed are degenerate in wavelength with the quantum dot ground state transition at 1.28 mu m. The absorption change recovers with characteristic times ranging from 62 ps (0 V) to similar to 700 fs (-10 V), showing a decrease of nearly two orders of magnitude. The authors show that at low applied fields, this recovery is attributed to thermionic emission while for higher applied fields, tunneling becomes the dominant mechanism. (c) 2006 American Institute of Physics.</p
X-ray Properties of the Abell 644 Cluster of Galaxies
We use new ASCA observations and archival ROSAT Position Sensitive
Proportional Counter (PSPC) data to determine the X-ray spectral properties of
the intracluster gas in Abell 644. From the overall spectrum, we determine the
average gas temperature to be 8.64 (+0.67,-0.56) keV, and an abundance of 0.32
(+/-0.04) . The global ASCA and ROSAT spectra imply a cooling rate
of 214 (+100,-91) yr. The PSPC X-ray surface brightness
profile and the ASCA data suggest a somewhat higher cooling rate. We determine
the gravitational mass and gas mass as a function of radius. The total
gravitating mass within 1.2 Mpc is , of which 20%
is in the form of hot gas. There is a region of elevated temperature 1.5-5
arcmin to the west of the cluster center. The south-southwest region of the
cluster also shows excess emission in the ROSAT PSPC X-ray image, aligned with
the major axis of the optical cD galaxy in the center of the cluster. We argue
that the cluster is undergoing or has recently undergone a minor merger. The
combination of a fairly strong cooling flow and evidence for a merger make this
cluster an interesting case to test the disruption of cooling flow in mergers.Comment: 26 pages LaTeX including 9 eps figures + 4 pages LaTeX tables
(landscape); accepted to ApJ, uses aaspp
Cosmic Complementarity: Joint Parameter Estimation from CMB Experiments and Redshift Surveys
We study the ability of future CMB anisotropy experiments and redshift
surveys to constrain a thirteen-dimensional parameterization of the adiabatic
cold dark matter model. Each alone is unable to determine all parameters to
high accuracy. However, considered together, one data set resolves the
difficulties of the other, allowing certain degenerate parameters to be
determined with far greater precision. We treat in detail the degeneracies
involving the classical cosmological parameters, massive neutrinos,
tensor-scalar ratio, bias, and reionization optical depth as well as how
redshift surveys can resolve them. We discuss the opportunities for internal
and external consistency checks on these measurements. Previous papers on
parameter estimation have generally treated smaller parameter spaces; in direct
comparisons to these works, we tend to find weaker constraints and suggest
numerical explanations for the discrepancies.Comment: Submitted to ApJ. LaTeX, 20 pages, emulateapj.sty and
onecolfloat.sty. Minor errors in Table 8 corrected; reference adde
Spin anisotropy effects in dimer single molecule magnets
We present a model of equal spin dimer single molecule magnets. The
spins within each dimer interact via the Heisenberg and the most general set of
four quadratic anisotropic spin interactions with respective strengths and
, and with the magnetic induction . We solve the model
exactly for , and for antiferromagnetic Heisenberg couplings
(), present curves at low for these cases. Low-
curves for and electron paramagnetic susceptibility
for are also provided. For weak anisotropy
interactions, we employ a perturbative treatment, and show that the Hartree and
extended Hartree approximations lead to reliable analytic results at low
and large for these quantities and for the inelastic neutron scattering
cross-section . Our results are discussed with
regard to existing experiments on Fe dimer
single molecule magnets, and suggest that one of them contains a substantial
amount of single-ion anisotropy, without any sizeable global spin anisotropy.
We urge further experiments of the above types on single crystals of Fe and
on some [Mn] dimers, in order to elucidate the precise values
of the various microscopic interactions.Comment: 30 pages, 25 figures, submitted to Phys. Rev.
Knowing me, knowing you: Spontaneous use of mentalistic language for self and other in autism
Recent studies on mentalizing have shown that autistic individuals who pass explicit mentalizing tasks may still have difficulties with implicit mentalizing tasks. This study explores implicit mentalizing by examining spontaneous speech that is likely to contain mentalistic expressions. The spontaneous production of meta-statements provides a clear measure for implicit mentalizing that is unlikely to be learned through experience. We examined the self- and other-descriptions of highly verbally able autistic and non-autistic adults in terms of their spontaneous use of mentalistic language and meta-representational utterances through quantitative and qualitative analysis. We devised a hierarchical coding system that allowed us to study the types of statements produced in comparable conditions for the self and for a familiar other. The descriptions of autistic participants revealed less mentalistic content relating to psychological traits and meta-statements. References to physical traits were similar between groups. Within each group, participants produced a similar pattern of types of mental utterance across ‘self’ and ‘other’ conditions. This suggests that autistic individuals show a unique pattern of mental-state-representation for both self and other. Meta-statements add a degree of complexity to self- and other-descriptions and to the understanding of mental states; their reduction in autism provides evidence for implicit mentalizing difficulties
Group-cluster merging and the formation of starburst galaxies
A significant fraction of clusters of galaxies are observed to have
substructure, which implies that merging between clusters and subclusters is a
rather common physical process of cluster formation.
It still remains unclear how cluster merging affects the evolution of cluster
member galaxies.
We report the results of numerical simulations, which show the dynamical
evolution of a gas-rich late-type spiral in a merger between a small group of
galaxies and a cluster. The simulations demonstrate that time-dependent tidal
gravitational field of the merging excites non-axisymmetric structure of the
galaxy, subsequently drives efficient transfer of gas to the central region,
and finally triggers a secondary starburst.
This result provides not only a new mechanism of starbursts but also a close
physical relationship between the emergence of starburst galaxies and the
formation of substructure in clusters. We accordingly interpret post-starburst
galaxies located near substructure of the Coma cluster as one observational
example indicating the global tidal effects of group-cluster merging.
Our numerical results furthermore suggest a causal link between the observed
excess of blue galaxies in distant clusters and cluster virialization process
through hierarchical merging of subclusters.Comment: 5 pages 3 color figures, ApJL in pres
Is attending a mental process?
The nature of attention has been the topic of a lively research programme in psychology for over a century. But there is widespread agreement that none of the theories on offer manage to fully capture the nature of attention. Recently, philosophers have become interested in the debate again after a prolonged period of neglect. This paper contributes to the project of explaining the nature of attention. It starts off by critically examining Christopher Mole’s prominent “adverbial” account of attention, which traces the failure of extant psychological theories to their assumption that attending is a kind of process. It then defends an alternative, process-based view of the metaphysics of attention, on which attention is understood as an activity and not, as psychologists seem to implicitly assume, an accomplishment. The entrenched distinction between accomplishments and activities is shown to shed new light on the metaphysics of attention. It also provides a novel diagnosis of the empirical state of play
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