The C4 Clustering Algorithm: Clusters of Galaxies in the Sloan Digital Sky Survey
We present the C4 Cluster Catalog, a new sample of 748 clusters of galaxies identified in the spectroscopic sample of the Second Data Release (DR2) of the Sloan Digital Sky Survey (SDSS). The C4 cluster-finding algorithm identifies clusters as overdensities in a seven-dimensional position and color space, thus minimizing projection effects that have plagued previous optical cluster selection. The present C4 catalog covers ~2600 deg2 of sky and ranges in redshift from z = 0.02 to 0.17. The mean cluster membership is 36 galaxies (with measured redshifts) brighter than r = 17.7, but the catalog includes a range of systems, from groups containing 10 members to massive clusters with over 200 cluster members with measured redshifts. The catalog provides a large number of measured cluster properties including sky location, mean redshift, galaxy membership, summed r-band optical luminosity (Lr), and velocity dispersion, as well as quantitative measures of substructure and the surrounding large-scale environment. We use new, multicolor mock SDSS galaxy catalogs, empirically constructed from the ΛCDM Hubble Volume (HV) Sky Survey output, to investigate the sensitivity of the C4 catalog to the various algorithm parameters (detection threshold, choice of passbands, and search aperture), as well as to quantify the purity and completeness of the C4 cluster catalog. These mock catalogs indicate that the C4 catalog is 90% complete and 95% pure above M200 = 1 × 1014 h-1 M⊙ and within 0.03 ≤ z ≤ 0.12. Using the SDSS DR2 data, we show that the C4 algorithm finds 98% of X-ray–identified clusters and 90% of Abell clusters within 0.03 ≤ z ≤ 0.12. Using the mock galaxy catalogs and the full HV dark matter simulations, we show that the Lr of a cluster is a more robust estimator of the halo mass (M200) than the galaxy line-of-sight velocity dispersion or the richness of the cluster. However, if we exclude clusters embedded in complex large-scale environments, we find that the velocity dispersion of the remaining clusters is as good an estimator of M200 as Lr. The final C4 catalog will contain 2500 clusters using the full SDSS data set and will represent one of the largest and most homogeneous samples of local clusters.

title: Evidence that gamma-ray bursts repeat and are Galactic in origin
abstract: We describe work which has provided evidence that there are two distinct classes of γ-ray bursts, that the bursts repeat, and that they are Galactic in origin.

title: High-energy observations of XRF 030723: Evidence for an off-axis gamma-ray burst?
abstract: We report High Energy Transient Explorer 2 (HETE-2) Wide Field X-ray Monitor (WXM) and French Gamma Telescope observations of XRF 030723 along with observations of the XRF afterglow made using the 6.5 m Magellan Clay telescope and Chandra. The observed peak energy E of the νFν burst spectrum is found to lie within (or below) the WXM 2-25 keV passband at 98.5% confidence, and no counts are detected above 30 keV. Our best-fit value is E = 8.4 keV. The ratio of X-ray to γ-ray flux for the burst follows a correlation found for GRBs observed with HETE-2, and the duration of the burst is similar to that typical of long-duration GRBs. If we require that the burst isotropic equivalent energy Eiso and Epk satisfy the relation discovered by Amati et al. (2002), a redshift of z = 0.38 can be determined, in agreement with constraints determined from optical observations. We are able to fit the X-ray afterglow spectrum and to measure its temporal fade. Although the best-fit fade is shallower than the concurrent fade in the optical, the spectral similarity between the two bands indicates that the X-ray fade may actually trace the optical fade. If this is the case, the late-time rebrightening observed in the optical cannot be due to a supernova bump. We interpret the prompt and afterglow X-ray emission as arising from a jetted GRB observed off-axis and possibly viewed through a complex circumburst medium that is due to a progenitor wind.

title: Likelihood methods and classical burster repetition
abstract: We develop a likelihood methodology which can be used to search for evidence of burst repetition in the BATSE catalog, and to study the properties of the repetition signal. We use a simplified model of burst repetition in which a number Nr of sources which repeat a fixed number of times Nrep are superposed upon a number Nnr of non‐repeating sources. The instrument exposure is explicitly taken into account. By computing the likelihood for the data, we construct a probability distribution in parameter space that may be used to infer the probability that a repetition signal is present, and to estimate the values of the repetition parameters. The likelihood function contains contributions from all the bursts, irrespective of the size of their positional errors—the more uncertain a burst’s position is, the less constraining is its contribution. Thus this approach makes maximal use of the data, and avoids the ambiguities of sample selection associated with data cuts on error circle size. We present the results ...

title: The Extinction toward the GRB 970228 Field
abstract: We determine the local Galactic extinction toward the field of gamma-ray burst GRB 970228 using a variety of methods. We develop a maximum-likelihood method for measuring the extinction that compares galaxy counts in different bands in the field of interest to those in a field of known extinction, and we apply this method to the GRB 970228 field. We also measure the extinction by comparing the observed broadband colors of stars in the GRB 970228 field to the colors of library spectra of the same spectral type. Finally, we estimate the extinction using the neutral hydrogen column density and the amount of infrared dust emission toward this field. Combining the results of these methods, we find a best-fit Galactic extinction in the optical of AV = 1.09, which implies that the intrinsic afterglow of GRB 970228 is significantly brighter and bluer than it appears.

title: Evidence for reionization at z ∼ 6: Detection of a gunn-peterson trough in a z = 6.28 quasar
abstract: We present moderate-resolution Keck spectroscopy of quasars at z = 5.82, 5.99, and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the Ly? absorption in the spectra of these quasars evolves strongly with redshift. To z ~ 5.7, the Ly? absorption evolves as expected from an extrapolation from lower redshifts. However, in the highest-redshift object, SDSSp J103027.10+052455.0 (z = 6.28), the average transmitted flux is 0.0038 ? 0.0026 times that of the continuum level over 8450 ? 20, on the optical depth to Ly? absorption at z = 6. This is a clear detection of a complete Gunn-Peterson trough, caused by neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen fraction in the intergalactic medium would result in an undetectable flux in the Ly? forest region. Therefore, the existence of the Gunn-Peterson trough by itself does not indicate that the quasar is observed prior to the reionization epoch. However, the fast evolution of the mean absorption in these high-redshift quasars suggests that the mean ionizing background along the line of sight to this quasar has declined significantly from z ~ 5 to 6, and the universe is approaching the reionization epoch at z ~ 6.

title: Measurement of open beauty production at HERA
abstract: The first observation of open b production in ep collisions is reported. An event sample containing muons and jets has been selected which is enriched in semileptonic b quark decays. The visible cross section σ(ep → b b X → μ X′) for Q 2 GeV 2 , 0.1 0.176 ± 0.016 (stat.) +0.026 −0.017 (syst.) nb for the muons to be detected in the range 35° p ⊥ μ >2.0 GeV in the laboratory frame. The expected visible cross section based on a NLO QCD calculation is 0.104 ± 0.017 nb . The cross sections for electroproduction with Q 2 GeV 2 and photoproduction are derived from the data and found to be σ(ep→eb b X)=7.1±0.6 (stat.) +1.5 −1.3 (syst.) nb and σ(γp→b b X)=111 ± 10 (stat.) +23 −20 (syst.) nb at an average 〈W γp 〉∼180 GeV , respectively.

title: Measurement of dijet electroproduction at small jet separation
abstract: Deep-inelastic scattering data in the range 150 < Q^2 < 35000 GeV^2 are used to investigate the minimum jet separation necessary to allow accurate description of the rate of dijet production using next-to-leading order perturbative QCD calculations. The required jet separation is found to be small, allowing about 1/3 of DIS data to be classified as dijet, as opposed to approximately 1/10 with more typical jet analyses. A number of precision measurements made using this dijet sample are well described by the calculations. The data are also described by the combination of leading order matrix elements and parton showers, as implemented in the QCD based Monte Carlo model RAPGAP.

title: Inelastic photoproduction of J/psi mesons at HERA
abstract: An analysis of inelastic photoproduction of \(J/\psi\) mesons is presented using data collected at the ep collider HERA corresponding to an integrated luminosity of above 80 pb-1. Differential and double differential cross sections are measured in a wide kinematic region: \(60 < W_{\gamma p} < 260\) GeV,\(1 < p_{t, \psi}^2 < 60 {\rm GeV}^2\) and 0.05<z<0.9, where z is the fraction of the energy of the exchanged photon transferred to the \(J/\psi\) meson in the rest frame of the target proton. Cross sections at \(z\lesssim 0.3\) are presented for the first time. Theoretical calculations within the Colour Singlet Model at NLO for direct photon processes are shown to give a good description of the data in the medium z region (0.3<z<0.9) up to the highest \(p_{t, \psi}^2\) values. A calculation using a kt factorisation approach in LO in the Colour Singlet Model is also able to describe these data. The data in the full z range are also compared to LO calculations within a non-relativistic QCD framework including colour octet and colour singlet contributions for direct and resolved photons. It seems possible to reconcile data and theory with modest contributions from colour octet processes. The polarisation of the \(J/\psi\) meson is measured as a function of z and \(p_{t, \psi}\) and is reasonably described by the theoretical predictions.

title: Measurement of di-jet cross-sections in photoproduction and photon structure
abstract: The production of hard di-jet events in photoproduction at HERA is dominated by resolved photon processes in which a parton in the photon with momentum fraction x γ is scattered from a parton in the proton. These processes are sensitive to the quark and gluon content of the photon. The differential di-jet cross-section d σ /dlog( x γ ) is presented here, measured in tagged photoproduction at HERA using data taken with the H1 detector, corresponding to an integrated luminosity of 7.2 pb −1 . Using a restricted data sample at high transverse jet energy, E T,jet >6 GeV, the effective parton density f γ ,eff (x γ )= q(x γ )+ q (x γ )+9/4 g(x γ ) in the photon in leading order QCD is measured down to x γ = 0.05 from which the gluon density in the photon is derived.

title: On the rise of the proton structure function F2 towards Low x
abstract: A measurement of the derivative ( ∂ ln F 2 / ∂ ln x ) Q 2 ≡− λ ( x , Q 2 ) of the proton structure function F 2 is presented in the low x domain of deeply inelastic positron–proton scattering. For 5×10 −5 ⩽ x ⩽0.01 and Q 2 ⩾1.5 GeV 2 , λ ( x , Q 2 ) is found to be independent of  x and to increase linearly with ln Q 2 .