Set-Rationalizable Choice and Self-Stability

A common assumption in modern microeconomic theory is that choice should be rationalizable via a binary preference relation, which \citeauthor{Sen71a} showed to be equivalent to two consistency conditions, namely $\alpha$ (contraction) and $\gamma$ (expansion). Within the context of \emph{social} choice, however, rationalizability and similar notions of consistency have proved to be highly problematic, as witnessed by a range of impossibility results, among which Arrow's is the most prominent. Since choice functions select \emph{sets} of alternatives rather than single alternatives, we propose to rationalize choice functions by preference relations over sets (set-rationalizability). We also introduce two consistency conditions, $\hat\alpha$ and $\hat\gamma$, which are defined in analogy to $\alpha$ and $\gamma$, and find that a choice function is set-rationalizable if and only if it satisfies $\hat\alpha$. Moreover, a choice function satisfies $\hat\alpha$ and $\hat\gamma$ if and only if it is \emph{self-stable}, a new concept based on earlier work by \citeauthor{Dutt88a}. The class of self-stable social choice functions contains a number of appealing Condorcet extensions such as the minimal covering set and the essential set.Comment: 20 pages, 2 figure, changed conten

The X-ray Properties of z>4 Quasars

We report on a search for X-ray emission from quasars with redshifts greater than four using the ROSAT public database. Our search has doubled the number of z>4 quasars detected in X-rays from 6 to 12. Most of those known prior to this work were radio-loud and X-ray selected sources; our study increases the number of X-ray detected, optically selected z>4 quasars from one to seven. We present their basic X-ray properties and compare these to those of lower redshift quasars. We do not find any evidence for strong broad-band spectral differences between optically selected z>4 quasars and those at lower redshifts.Comment: 7 pages, 4 figures included, LaTeX emulateapj.sty, accepted for publication in the Astronomical Journa

On the kinematics of the neutron star low mass X-ray binary Cen X-4

We present the first determination of the proper motion of the neutron star low mass X-ray binary {Cen X-4} measured from relative astrometry of the secondary star using optical images at different epochs. We determine the Galactic space velocity components of the system and find them to be significantly different from the mean values that characterize the kinematics of stars belonging to the halo, and the thin and the thick disc of the Galaxy. The high metallicity of the secondary star of the system rules out a halo origin and indicates that the system probably originated in the Galactic disc. A statistical analysis of the galactocentric motion revealed that this binary moves in a highly eccentric ($e\simeq 0.85\pm0.1$) orbit with an inclination of $\simeq 110^\circ$ to the Galactic plane. The large Galactic space velocity components strongly support that a high natal kick as a result of a supernova explosion could have propelled the system into such an orbit from a birth place in the Galactic disc. The high Li abundance in the secondary, comparable to that of stars in star forming regions and young stellar clusters like the Pleiades, may suggest a relatively recent formation of the system. Following the orbit backwards in time, we found that the system could have been in the inner regions of the Galactic disc $\sim$100--200 Myr ago. The neutron star might have formed at that moment. However, we cannot rule out the possibility that the system formed at a much earlier time if a Li production mechanism exists in this LMXB.Comment: 6 pages, 4 figures, accepted for publication in A&

X-ray Bright Optically Inactive Galaxies (XBONG) in XMM-Newton/SDSS fields: more diluted than absorbed?

We explore the properties of X-ray Bright Optically Inactive Galaxies (XBONG) detected in the 0.5-8 keV spectral band in 20 public XMM fields overlapping with the SDSS. We constrain our sample to optically extended systems with log (f_X/f_o > -2) that have spectroscopic identifications available from the SDSS (r<19.2 mag). The resulting sample contains 12 objects with L_X (0.5-8keV)= 5 x 10^41 - 2x10^44 erg s-1 in the redshift range 0.06 < z < 0.45. The X-ray emission in four cases is extended suggesting the presence of hot gas associated with a cluster or group of galaxies. The X-ray spectral fits show that two additional sources are best fit with a thermal component emission (kT \~ 1 keV). Three sources aremost likely associated with AGN: their X-ray spectrum is described by a steep photon index ~ 1.9 typical of unobscured AGN while, they are very luminous in X-rays (L_X (0.5-8 keV) ~ 10^43 - 10^44 erg s-1.) Finally, three more sources could be associated with either normal galaxies or unobscured Low Luminosity AGN (L_X < 10^42 erg s-1). We find no evidence for significant X-ray absorbing columns in any of our XBONGs. The above suggest that XBONGs, selected in the total 0.5-8 keV band, comprise a mixed bag of objects primarily including normal elliptical galaxies and type-1 AGN whose optical nuclear spectrum is probably diluted by the strong stellar continuum. Nevertheless, as our sample is not statistically complete we cannot exclude the possibility that a fraction of optically fainter XBONG may be associated with heavily obscured AGN.Comment: 8 pages to appear in MNRA

Exploring the spectral properties of faint hard X-ray sources with XMM-Newton

We present a spectroscopic study of 41 hard X-ray sources detected serendipitously with high significance (> 5 sigma in the 2-10 keV band) in seven EPIC performance/verification phase observations. The large collecting area of EPIC allows us to explore the spectral properties of these faint hard X-ray sources with 2< F_{2-10} < 80 x 10^{-14} erg cm^{-2}s^{-1} even though the length of the exposures are modest (~ 20 ks). Optical identifications are available for 21 sources of our sample. Using a simple power law plus Galactic absorption model we find an average value of the photon index Gamma ~ 1.6-1.7, broadly consistent with recent measurements made at similar fluxes with ASCA and with Chandra stacked spectral analyses. We find that 31 out of 41 sources are well fitted by this simple model and only eight sources require absorption in excess of the Galactic value. Interestingly enough, one third of these absorbed sources are broad line objects, though with moderate column densities. Two sources in the sample are X-ray bright optically quiet galaxies and show flat X-ray spectra. Comparing our observational results with those expected from standard synthesis models of the cosmic X-ray background (CXB) we find a fraction of unabsorbed to absorbed sources larger than predicted by theoretical models at our completeness limit of F_{2-10} ~ 5 x 10^{-14} erg cm^{-2}s^{-1}. The results presented here illustrate well how wide-angle surveys performed with EPIC on board XMM-Newton allow population studies of interesting and unusual sources to be made as well as enabling constraints to be placed on some input parameters for synthesis models of the CXB.Comment: 16 pages, 11 figures. To be published in A&

Return of the lysergamides. Part IV: Analytical and pharmacological characterization of lysergic acid morpholide (LSM-775)

Lysergic acid diethylamide (LSD) is perhaps one of the best-known psychoactive substances and many structural modifications of this prototypical lysergamide have been investigated. Several lysergamides were recently encountered as “research chemicals” or new psychoactive substances (NPS). Although lysergic acid morpholide (LSM-775) appeared on the NPS market in 2013, there is disagreement in the literature regarding the potency and psychoactive properties of LSM-775 in humans. The present investigation attempts to address the gap of information that exists regarding the analytical profile and pharmacological effects of LSM-775. A powdered sample of LSM-775 was characterized by X-ray crystallography, nuclear magnetic resonance stereoscopy (NMR), gas chromatography mass spectrometry (GC-MS), high mass accuracy electrospray MS/MS, HPLC diode array detection, HPLC quadrupole MS, and GC solid-state infrared analysis. Screening for receptor affinity and functional efficacy revealed that LSM-775 acts as a nonselective agonist at 5-HT1A and 5-HT2A receptors. Head twitch studies were conducted in C57BL/6J mice to determine whether LSM-775 activates 5-HT2A receptors and produces hallucinogen-like effects in vivo. LSM-775 did not induce the head twitch response unless 5-HT1A receptors were blocked by pretreatment with the antagonist WAY-100,635 (1 mg/kg, subcutaneous). These findings suggest that 5-HT1A activation by LSM-775 masks its ability to induce the head twitch response, which is potentially consistent with reports in the literature indicating that LSM-775 is only capable of producing weak LSD-like effects in humans

Multiwavelength study of the nuclei of a volume-limited sample of galaxies I: X-ray observations

We discuss ROSAT HRI X-ray observations of 33 very nearby galaxies, sensitive to X-ray sources down to a luminosity of approximately 10^38 erg/s. The galaxies are selected from a complete, volume limited sample of 46 galaxies with d<7 Mpc for which we have extensive multi-wavelength data. For an almost complete sub-sample with M_B<-14 (29/31 objects) we have HRI images. Contour maps and source lists are presented within the central region of each galaxy, together with nuclear upper limits where no nuclear source was detected. Nuclear X-ray sources are found to be very common, occurring in ~35% of the sample. Nuclear X-ray luminosity is statistically connected to host galaxy luminosity - there is not a tight correlation, but the probability of a nuclear source being detected increases strongly with galaxy luminosity and the distribution of nuclear luminosities seems to show an upper envelope that is roughly proportional to galaxy luminosity. While these sources do seem to be a genuinely nuclear phenomenon rather than nuclear examples of the general X-ray source population, it is far from obvious that they are miniature Seyfert nuclei. The more luminous nuclei are very often spatially extended, and HII region nuclei are detected just as often as LINERs. Finally, we also note the presence of fairly common super-luminous X-ray sources in the off-nuclear population - out of 29 galaxies we find 9 sources with a luminosity larger than 10^39 erg/s. These show no particular preference for more luminous galaxies. One is already known to be a multiple SNR system, but most have no obvious optical counterpart and their nature remains a mystery.Comment: Accepted for publication in MNRAS. 26 pages, 47 figures (figures 1-29,31 and 38 as separate, low resolution files; full resolution files are available at http://www.star.le.ac.uk/~plt

The Extension of the Transition Temperature Plasma into the Lower Galactic Halo

Column densities for H I, Al III, Si IV, C IV, and O VI toward 109 stars and 30 extragalactic objects have been assembled to study the extensions of these species away from the Galactic plane into the Galactic halo. H I and Al III mostly trace the warm neutral and warm ionized medium, respectively, while Si IV, C IV and O VI trace a combination of warm photoionized and collisionally ionized plasmas. The much larger object sample compared to previous studies allows us to consider and correct for the effects of the sample bias that has affected earlier but smaller surveys of the gas distributions. We find Si IV and C IV have similar exponential scale heights of 3.2(+1.0, -0.6) and 3.6(+1.0, -0.8) kpc. The scale height of O VI is marginally smaller with h = 2.6+/- 0.6 kpc. The transition temperature gas is ~3 times more extended than the warm ionized medium traced by Al III with h = 0.90(+0.62, -0.33) kpc and ~12 times more extended than the warm neutral medium traced by H I with h = 0.24 +/- 0.06 kpc. There is a factor of 2 decrease in the dispersion of the log of the column density ratios for transition temperature gas for lines of sight in the Galactic disk compared to extragalactic lines of sight through the entire halo. The observations are compared to the predictions of the various models for the production of the transition temperature gas in the halo. The appendix presents a revision to the electron scale height proposed by Gaensler et al. (2008) based on electron dispersion measures.Comment: Accepted for publication in ApJ (63 pages including 6 figures and 5 tables

A dusty X-ray absorber in the Perseus Cluster ?

We have analyzed 0.35-7.5 keV X-ray spectra of the center of the Perseus cluster collected using the Broad Band X-Ray Telescope (BBXRT) on the Astro-1 mission. These spectra are particularly useful for examining the nature of the X-ray absorber in cooling flows because of BBXRT's sensitivity between 0.35 and 1.0 keV. We confirm that there is X-ray absorption above that expected from gas in our own galaxy. Further, the absorbing medium is deficient in helium. However, the energy of the K-edge of oxygen is consistent with neutral material (at the redshift of the cluster) and is not consistent with any ionized state of oxygen. It is not possible to completely ionize helium and have oxygen neutral so the apparent helium deficiency cannot be due to ionization. We propose that the X-ray absorption is due to dust grains that have condensed out of a medium in which helium remains ionized. This model satisfies all the observational constraints but is difficult to understand theoretically.Comment: 15 pages including 6 figures. Uses aaspp4 and psfig style

Critical Race Theory and Education: racism and anti-racism in educational theory and praxis

What is Critical Race Theory (CRT) and what does it offer educational researchers and practitioners outside the US? This paper addresses these questions by examining the recent history of antiracist research and policy in the UK. In particular, the paper argues that conventional forms of antiracism have proven unable to keep pace with the development of increasingly racist and exclusionary education polices that operate beneath a veneer of professed tolerance and diversity. In particular, contemporary antiracism lacks clear statements of principle and theory that risk reinventing the wheel with each new study; it is increasingly reduced to a meaningless slogan; and it risks appropriation within a reformist “can do” perspective dominated by the de-politicized and managerialist language of school effectiveness and improvement. In contrast, CRT offers a genuinely radical and coherent set of approaches that could revitalize critical research in education across a range of inquiries, not only in self-consciously "multicultural" studies. The paper reviews the developing terrain of CRT in education, identifying its key defining elements and the conceptual tools that characterise the work. CRT in education is a fast changing and incomplete project but it can no longer be ignored by the academy beyond North America