30,038 research outputs found
The Causal Boundary of spacetimes revisited
We present a new development of the causal boundary of spacetimes, originally
introduced by Geroch, Kronheimer and Penrose. Given a strongly causal spacetime
(or, more generally, a chronological set), we reconsider the GKP ideas to
construct a family of completions with a chronology and topology extending the
original ones. Many of these completions present undesirable features, like
those appeared in previous approaches by other authors. However, we show that
all these deficiencies are due to the attachment of an ``excessively big''
boundary. In fact, a notion of ``completion with minimal boundary'' is then
introduced in our family such that, when we restrict to these minimal
completions, which always exist, all previous objections disappear. The optimal
character of our construction is illustrated by a number of satisfactory
properties and examples.Comment: 37 pages, 10 figures; Definition 6.1 slightly modified; multiple
minor changes; one figure added and another replace
Two Populations of Young Massive Star Clusters in Arp 220
We present new optical observations of young massive star clusters in Arp
220, the nearest ultraluminous infrared galaxy, taken in UBVI with the Hubble
Space Telescope ACS/HRC camera. We find a total of 206 probable clusters whose
spatial distribution is centrally concentrated toward the nucleus of Arp 220.
We use model star cluster tracks to determine ages, luminosities, and masses
for 14 clusters with complete UBVI indices or previously published
near-infrared data. We estimate rough masses for 24 additional clusters with I
< 24 mag from BVI indices alone. The clusters with useful ages fall into two
distinct groups: a ``young'' population (< 10 Myr) and an intermediate-age
population (~300 Myr). There are many clusters with masses clearly above 10^6
Msun and possibly even above 10^7 Msun in the most extreme instances. These
masses are high enough that the clusters being formed in the Arp 220 starburst
can be considered as genuine young globular clusters. In addition, this study
allows us to extend the observed correlation between global star formation rate
and maximum cluster luminosity by more than an order of magnitude in star
formation rate.Comment: 30 pages, 8 figures; figs 1-3 in color; accepted to Ap
Intersectionality, Race-Gender Subordination, and Education
In this chapter, we unpack intersectionality as an analytical framework. First, we cite Black Lives Matter as an impetus for discussing intersectionality’s current traction. Second, we review the genealogy of “intersectionality” beginning with Kimberlé Crenshaw’s formulation, which brought a Black Studies provocation into legal discourse in order to challenge existing antidiscrimination doctrine and single-axis theorizing. The third, and most central, task of the chapter is our account of intersectionality’s utility for social analysis. We examine some of the issues raised by the metaphor of the intersection and some of the debates surrounding the concept, such as the tension between fragmenting and universalizing perspectives mediated by the notion of “strategic essentialism.” Fourth, we review how education researchers have explained race and gender subordination in education since Ladson-Billings and Tate’s Teachers College Record article. We conclude with some remarks concerning future research on intersectionality
The Core-like Nature of HST-1 in the M87 Jet
We investigate the total flux density, spectral, polarization, and Faraday
rotation variability of HST-1 in the M87 jet during the outburst from 2003 to
2007 through multi-epoch VLA observations at 8, 15, and 22 GHz. Contrary to the
general case for blazars, the flux densities of HST-1 rise earlier at lower
frequencies from radio to X-ray, and the spectra are softening with the growth
of outburst, indicating that the newly emerging subcomponents within HST-1 have
relatively steep spectra. In particular, the intrinsic EVPA varies
monotonically by at the 3 wavebands during the period, and all
but the stationary subcomponent in the eastern end of HST-1 move downstream
superluminally deviating divergently from the overall jet direction, with the
motion of the outmost subcomponent bending from one side of the jet axis to
another. These strongly argue for the presence of helical magnetic fields
around HST-1, which is also supported by the fact that the subcomponents might
be accelerated in this region. The fractional polarization is relatively low in
the rising stage, and in the decaying stage the polarization levels are almost
comparable at the 3 wavebands. In view of the quite large RM values, Faraday
rotation is expected to occur dominantly external to HST-1 in the decaying
stage, which is well supported by the presence of diffuse emission around
HST-1, and consistent with the scenario that RM decrease gets slower with time.Comment: 5 pages, 4 figures, Accepted for publication in MNRAS Lette
Sagnac interferometry based on ultra-slow polaritons in cold atomic vapors
The advantages of light and matter-wave Sagnac interferometers -- large area
on one hand and high rotational sensitivity per unit area on the other -- can
be combined utilizing ultra-slow light in cold atomic gases. While a
group-velocity reduction alone does not affect the Sagnac phase shift, the
associated momentum transfer from light to atoms generates a coherent
matter-wave component which gives rise to a substantially enhanced rotational
signal. It is shown that matter-wave sensitivity in a large-area interferometer
can be achieved if an optically dense vapor at sub-recoil temperatures is used.
Already a noticeable enhancement of the Sagnac phase shift is possible however
with much less cooling requirements.Comment: 4 pages, 3 figure
Space-time OFDM with adaptive beamforming: Performance in spatially correlated channels
Space-time block coding (STBC) has proved to be an effective means of exploring diversity branches and improving system performance. In a previous work we proposed an extension to that where the STBC was combined with adaptive eigenbeamforming, an approach that takes an advantage of spatial channel correlation and can provide further improvement for the overall system performance. In this paper, we investigate this new transmission structure for broadband orthogonal frequency division multiplexing (OFDM) systems in frequency-selective channels. Numerical analysis showed that systems employing this scheme would not undergo any diversity losses as it maintains the maximum achievable diversity advantage of space-time (ST) codes in realistic channel conditions. Simulation results showed that the proposed structure has a significant improvement in bit/symbol error rate performance in a spatially correlated channel over systems that utilize only space-time coding
Adaptive transmit eigenbeamforming with orthogonal space-time block coding in correlated space-time channels
Conventional space-time codes can provide a significant improvement in system performance only if the signal paths are spatially uncorrelated, a condition that is hardly met in practice. In this paper, we mitigate this condition by combining a technique of eigenbeamforming, based on the channel correlation matrix, with orthogonal space-time block codes (O-STBC) at the transmitter side of the link. No feedback information from the receiver (the mobile station) is utilized in the proposed structure. Simulation results using 4-ary PSK signaling showed that this idea outperforms existing techniques in both uncorrelated and correlated channels in terms of bit-error rate and symbol-error rate
Lymphotoxin is an autocrine growth factor for Epstein-Barr virus-infected B cell lines.
Because human lymphotoxin (LT) was originally isolated from a lymphoblastoid cell line, we investigated the role of this molecule in three newly established Epstein-Barr virus (EBV)-infected human B cell lines. These lines were derived from acute lymphoblastic leukemia (Z-6), myelodysplastic syndrome (Z-43), and acute myelogenous leukemia (Z-55) patients who had a prior EBV infection. Each lymphoblastoid cell line had a karyotype that was different from that of the original parent leukemic cells, and all expressed B cell, but not T cell or myeloid surface markers. In all three lines, rearranged immunoglobulin heavy chain joining region (JH) bands were found, and the presence of EBV DNA was confirmed by Southern blotting. Z-6, Z-43, and Z-55 cell lines constitutively produced 192, 48, and 78 U/ml LT, respectively, as assessed by a cytotoxicity assay and antibody neutralization. Levels of tumor necrosis factor (TNF) were undetectable. Scatchard analysis revealed that all the cell lines expressed high-affinity TNF/LT receptors with receptor densities of 4197, 1258, and 1209 sites/cell on Z-6, Z-43, and Z-55, respectively. Furthermore, labeled TNF binding could be reversed by both unlabeled TNF, as well as by LT. Studies with p60 and p80 receptor-specific antibodies revealed that the three lines expressed primarily the p80 form of the TNF receptor. When studied in a clonogenic assay, exogenous LT stimulated proliferation of all three cell lines in a dose-dependent fashion at concentrations ranging from 25 to 500 U/ml. Similar results were obtained with [3H]TdR incorporation. Monoclonal anti-LT neutralizing antibodies at concentrations of 25-500 U/ml inhibited cellular multiplication in a dose-dependent manner. It is interesting that in spite of a common receptor, TNF (1,000 U/ml) had no direct effect on Z-55 cell growth, whereas it partially reversed the stimulatory effect of exogenous LT. In addition, TNF inhibited Z-6 and Z-43 cell proliferation, and its suppressive effect was reversed by exogenous LT. Both p80 and p60 forms of soluble TNF receptors suppressed the lymphoblastoid cell line proliferation and their inhibitory effect was partially reversed by LT. Our data suggest that (a) LT is an autocrine growth factor for EBV-transformed lymphoblastoid B cell lines; and (b) anti-LT antibodies, soluble TNF/LT receptors, and TNF itself can suppress the growth of lymphoblastoid cells, probably by modulating or competing with LT.(ABSTRACT TRUNCATED AT 400 WORDS
An efficient mobile Rayleigh fading channel simulator: A comparison with Clarke's Model
This paper presents a computer model for Rayleigh fading channels. The simulated Rayleigh fading channel model is based on Smith's method and shows ease of implementation and greater mathematical tractability. We then investigate the level-crossing rate (LCR), the average duration of fades (ADF), the probability density function (PDF), the cumulative distribution function (CDF) and the autocorrelation functions (ACF) of this proposed model. The simulated results are verified against the analytical Clarke's channel model
Performance of variable step-size dithered signed error CMA for blind equalization
Recently a dithered signed-error constant modulus algorithm (DSE-CMA) has been proposed, associated with fractionally spaced equalization, for the purpose of low complexity implementation of constant modulus algorithm (CMA). DSE-CMA has robustness properties closely resembling those of CMA under certain restrictions. As the CMA is slow in achieving its minimum mean squared error, so is the DSE-CMA. In this work, we apply an adaptive step-size instead of a fixed one and then examine the performance of few variable step-size algorithms that result in faster convergence while preserve the low computational complexity and robustness properties of the DSE-CMA algorithm. We also derive the excess mean-squared error in the case of noisy channel to examine the robustness of the algorithms
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