Evolution of the Cluster X-ray Luminosity Function

We report measurements of the cluster X-ray luminosity function out to z=0.8 based on the final sample of 201 galaxy systems from the 160 Square Degree ROSAT Cluster Survey. There is little evidence for any measurable change in cluster abundance out to z~0.6 at luminosities less than a few times 10^44 ergs/s (0.5-2.0 keV). However, between 0.6 < z < 0.8 and at luminosities above 10^44 ergs/s, the observed volume densities are significantly lower than those of the present-day population. We quantify this cluster deficit using integrated number counts and a maximum-likelihood analysis of the observed luminosity-redshift distribution fit with a model luminosity function. The negative evolution signal is >3 sigma regardless of the adopted local luminosity function or cosmological framework. Our results and those from several other surveys independently confirm the presence of evolution. Whereas the bulk of the cluster population does not evolve, the most luminous and presumably most massive structures evolve appreciably between z=0.8 and the present. Interpreted in the context of hierarchical structure formation, we are probing sufficiently large mass aggregations at sufficiently early times in cosmological history where the Universe has yet to assemble these clusters to present-day volume densities.Comment: 15 pages, 10 figures, accepted for publication in Ap

Accurate multimodal probabilistic prediction of conversion to Alzheimer's disease in patients with mild cognitive impairment

Accurately identifying the patients that have mild cognitive impairment (MCI) who will go on to develop Alzheimer's disease (AD) will become essential as new treatments will require identification of AD patients at earlier stages in the disease process. Most previous work in this area has centred around the same automated techniques used to diagnose AD patients from healthy controls, by coupling high dimensional brain image data or other relevant biomarker data to modern machine learning techniques. Such studies can now distinguish between AD patients and controls as accurately as an experienced clinician. Models trained on patients with AD and control subjects can also distinguish between MCI patients that will convert to AD within a given timeframe (MCI-c) and those that remain stable (MCI-s), although differences between these groups are smaller and thus, the corresponding accuracy is lower. The most common type of classifier used in these studies is the support vector machine, which gives categorical class decisions. In this paper, we introduce Gaussian process (GP) classification to the problem. This fully Bayesian method produces naturally probabilistic predictions, which we show correlate well with the actual chances of converting to AD within 3 years in a population of 96 MCI-s and 47 MCI-c subjects. Furthermore, we show that GPs can integrate multimodal data (in this study volumetric MRI, FDG-PET, cerebrospinal fluid, and APOE genotype with the classification process through the use of a mixed kernel). The GP approach aids combination of different data sources by learning parameters automatically from training data via type-II maximum likelihood, which we compare to a more conventional method based on cross validation and an SVM classifier. When the resulting probabilities from the GP are dichotomised to produce a binary classification, the results for predicting MCI conversion based on the combination of all three types of data show a balanced accuracy of 74%. This is a substantially higher accuracy than could be obtained using any individual modality or using a multikernel SVM, and is competitive with the highest accuracy yet achieved for predicting conversion within three years on the widely used ADNI dataset

Appearance, happiness and success: ‘what is beautiful is good’

In our beauty-obsessed world, where incessant dissatisfaction with appearance is considered to be ‘normal’, it can seem as though those with ‘good looks’ are the ones who have it all. But is it the case that being beautiful can get you further in life? Nicola Stock explores the human fascination with keeping up appearances and highlights the impact of attempting to meet modern unrealistic ideals of beauty

The Discovery of Argon in Comet C/1995 O1 (Hale-Bopp)

On 30.14 March 1997 we observed the EUV spectrum of the bright comet C/1995 O1 (Hale-Bopp) at the time of its perihelion, using our EUVS sounding rocket telescope/spectrometer. The spectra reveal the presence H Ly beta, O+, and, most notably, Argon. Modelling of the retrieved Ar production rates indicates that comet Hale-Bopp is enriched in Ar relative to cosmogonic expectations. This in turn indicates that Hale-Bopp's deep interior has never been exposed to the 35-40 K temperatures necessary to deplete the comet's primordial argon supply.Comment: 9 pages, 2 figures. ApJ, 545, in press (2000

Spatial Correlation Function of X-ray Selected AGN

We present a detailed description of the first direct measurement of the spatial correlation function of X-ray selected AGN. This result is based on an X-ray flux-limited sample of 219 AGN discovered in the contiguous 80.7 deg^2 region of the ROSAT North Ecliptic Pole (NEP) Survey. Clustering is detected at the 4 sigma level at comoving scales in the interval r = 5-60 h^-1 Mpc. Fitting the data with a power law of slope gamma=1.8, we find a correlation length of r_0 = 7.4 (+1.8, -1.9) h^-1 Mpc (Omega_M=0.3, Omega_Lambda=0.7). The median redshift of the AGN contributing to the signal is z_xi=0.22. This clustering amplitude implies that X-ray selected AGN are spatially distributed in a manner similar to that of optically selected AGN. Furthermore, the ROSAT NEP determination establishes the local behavior of AGN clustering, a regime which is poorly sampled in general. Combined with high-redshift measures from optical studies, the ROSAT NEP results argue that the AGN correlation strength essentially does not evolve with redshift, at least out to z~2.2. In the local Universe, X-ray selected AGN appear to be unbiased relative to galaxies and the inferred X-ray bias parameter is near unity, b_X~1. Hence X-ray selected AGN closely trace the underlying mass distribution. The ROSAT NEP AGN catalog, presented here, features complete optical identifications and spectroscopic redshifts. The median redshift, X-ray flux, and X-ray luminosity are z=0.41, f_X=1.1*10^-13 cgs, and L_X=9.2*10^43 h_70^-2 cgs (0.5-2.0 keV), respectively. Unobscured, type 1 AGN are the dominant constituents (90%) of this soft X-ray selected sample of AGN.Comment: 17 pages, 8 figures, accepted for publication in ApJ, a version with high-resolution figures is available at http://www.eso.org/~cmullis/papers/Mullis_et_al_2004b.ps.gz, a machine-readable version of the ROSAT NEP AGN catalog is available at http://www.eso.org/~cmullis/research/nep-catalog.htm

Statistical methods in cosmology

The advent of large data-set in cosmology has meant that in the past 10 or 20 years our knowledge and understanding of the Universe has changed not only quantitatively but also, and most importantly, qualitatively. Cosmologists rely on data where a host of useful information is enclosed, but is encoded in a non-trivial way. The challenges in extracting this information must be overcome to make the most of a large experimental effort. Even after having converged to a standard cosmological model (the LCDM model) we should keep in mind that this model is described by 10 or more physical parameters and if we want to study deviations from it, the number of parameters is even larger. Dealing with such a high dimensional parameter space and finding parameters constraints is a challenge on itself. Cosmologists want to be able to compare and combine different data sets both for testing for possible disagreements (which could indicate new physics) and for improving parameter determinations. Finally, cosmologists in many cases want to find out, before actually doing the experiment, how much one would be able to learn from it. For all these reasons, sophisiticated statistical techniques are being employed in cosmology, and it has become crucial to know some statistical background to understand recent literature in the field. I will introduce some statistical tools that any cosmologist should know about in order to be able to understand recently published results from the analysis of cosmological data sets. I will not present a complete and rigorous introduction to statistics as there are several good books which are reported in the references. The reader should refer to those.Comment: 31, pages, 6 figures, notes from 2nd Trans-Regio Winter school in Passo del Tonale. To appear in Lectures Notes in Physics, "Lectures on cosmology: Accelerated expansion of the universe" Feb 201

Chandra and RXTE spectroscopy of the accreting msec pulsar IGR J00291+5934

We report on an observation of the recently discovered accreting millisecond X-ray pulsar IGR J00291+5934 performed with the RXTE-Proportional Counter Array (PCA) and Chandra-High Energy Transmission Grating Spectrometer (HETGS). The RXTE data are from a twoweek follow-up of the source, while the Chandra observation took place around the end of the follow-up, about 12 days after the discovery of the source, when the source flux had decreased already by a factor of ten. The analysis of the Chandra data allowed us to extract the most precise X-ray position of IGR J00291+5934, RA = 00h 29m 03.08s, and Dec =+59◦ 34 19.2 (0.6 error), compatible with the optical and radio ones. We find that the spectra of IGR J00291+5934 can be described by a combination of a thermal component and a power-law. Along the outburst detected by PCA, the power-law photon index showed no particular trend, while the thermal component (∼1 keV, interpreted as a hot spot on the neutron star surface) became weaker until non-detection. In the simultaneous observation of the weak Chandra /RXTE spectrum, there was no longer any indication of the ∼1 keV thermal component, while we detected a colder thermal component (∼0.4 keV) that we interpret as the emission from the cold disc. A hint of a 6.4 keV iron line was detected, together with an excess around 6.8 keV and absorption feature around 7.1 keV. The last two features have never been detected in the spectra of accretion-driven millisecond pulsars before and, if confirmed, would suggest the presence of an expanding hot corona with high outflow velocities

Swift follow-up observations of 17 INTEGRAL sources of uncertain or unknown nature

(abridged) We analysed data from observations of 17 INTEGRAL sources made with the Swift satellite. We refine the position of the hard X-ray sources to an accuracy of a few arcsec. We then browsed the online catalogs (e.g., NED, SIMBAD, 2MASS, 2MASX, USNO) to search for counterparts at other wavelengths. We also made use of the X-ray spectral parameters to try to identify the nature of those sources. We provide the X-ray position with arcsec accuracy, identify possible infrared and optical counterparts (when found), give the magnitudes in those bands and in the optical and UV as seen with the Swift/UVOT telescope when observations are available. We confirm the previously suggested associations and source types for IGR J03532-6829, J05346-5759, J10101-5654, J13000+2529, J13020-6359, J15479-4529, J18214-1318, and J23206+6431. We identify IGR J09025-6814 as an AGN for the first time, and we suggest that it may be a Seyfert 2. We suggest that IGR J05319-6601, J16287-5021, J17353-3539 and J17476-2253 are X-ray binaries, with J05319-6601 being located in the LMC and the other three possibly being HMXBs in our Galaxy. For IGR J15161-3827 and J20286+2544, we find several possible X-ray counterparts in the IBIS error region, and we discuss which, if any, are the likely counterparts. Both are likely AGNs, although the latter could be a blend of two AGNs. For IGR J03184-0014 and J19267+1325, we find X-ray sources slightly outside the IBIS error circle. In the former, we do not favour an association of the Swift and INTEGRAL source, while it is very likely that IGR J19267+1325 and the Swift source are the same.Comment: 12 pages, 3 figures, accepted for publication in A&

The Lick Planet Search: Detectability and Mass Thresholds

We analyse 11 years of precise radial velocities for 76 solar type stars from the Lick survey. Eight stars in this sample have previously reported planetary-mass companions, all with mass (m sin i) less than 8 Jupiter masses (MJ). For the stars without a detected companion, we place upper limits on possible companion mass. For most stars, we can exclude companions with m sin i > 0.7 MJ (a/AU)^1/2 for orbital radii a < 5 AU. We use our results to interpret the observed masses and orbital radii of planetary-mass companions. For example, we show that the finite duration of the observations makes detection of Jupiter mass companions more and more difficult for orbital radii beyond 3 AU. Thus it is possible that the majority of solar type stars harbor Jupiter-mass companions much like our own, and if so these companions should be detectable in a few years. To search for periodicities, we adopt a "floating-mean" periodogram, which improves on the traditional Lomb-Scargle periodogram by accounting for statistical fluctuations in the mean of a sampled sinusoid. We discuss in detail the normalization of the periodogram, an issue which has been of some debate in the literature.Comment: To appear in the Astrophysical Journal (50 pages, LaTeX, including 11 figures