S. C. Odewahn

The unusual afterglow of the γ-ray burst of 26 March 1998 as evidence for a supernova connection

Cosmic γ-ray bursts have now been firmly established as one of the most powerful phenomena in the Universe, releasing almost the rest-mass energy of a neutron star within the space of a few seconds (ref. 1). The two most popular models to explain γ-ray bursts are the coalescence of two compact objects such as neutron stars or black holes, or the catastrophic collapse of a massive star in a very energetic supernova-like explosion. Here we show that, about three weeks after the γ-ray burst of 26 March 1998, the transient optical source associated with the burst brightened to about 60 times the expected flux, based upon an extrapolation of the initial light curve. Moreover, the spectrum changed dramatically, with the colour becoming extremely red. We argue that the new source is an underlying supernova. If our hypothesis is true then this provides evidence linking cosmologically located γ-ray bursts with deaths of massive stars.Palomar Observatory 105-24, Caltech, Pasadena, CA 91125, USA National Radio Astronomy Observatory, P. O. Box O, Socorro, NM 87801, USA Department of Astronomy, University of California, Berkeley, CA 94720-3411 USA National Optical Astronomy Observatories, 950 N. Cherry, Ave. Tucson, AZ 85719, USA Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, 7000 East Avenue, P. O. Box 808, L-413, Livermore, CA 94551-9900, USA Center for Particle Astrophysics, University of California, Berkeley, CA 94720 USA Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA European Southern Observatory, D-85748 Garching, Germany Istituto di Astrofisica Spaziale, CNR, via Fosso del Cavaliere, Roma I-00133, Italy

The afterglow, redshift and extreme energetics of the gamma-ray burst of 23 January 1999

Long-lived emission, known as afterglow, has now been detected from about a dozen γ-ray bursts. Distance determinations place the bursts at cosmological distances, with redshifts, z, ranging from ∼1 to 3. The energy required to produce these bright γ-ray flashes is enormous: up to ∼10 53 erg, or 10 per cent of the rest-mass energy of a neutron star, if the emission is isotropic. Here we present optical and near-infrared observations of the afterglow of GRB990123, and we determine a redshift of z ⩾ 1.6. This is to date the brightest γ-ray burst with a well-localized position and if the γ-rays were emitted isotropically, the energy release exceeds the rest-mass energy of a neutron star, so challenging current theoretical models of the sources. We argue, however, that our data may provide evidence of beamed (rather than isotropic) radiation, thereby reducing the total energy released to a level where stellar-death models are still tenable.Afterglow, or long-lived emission, has now been detected from about a dozen well-positioned gamma-ray bursts. Distance determinations made by measuring optical emission lines from the host galaxy, or absorption lines in the afterglow spectrum, place the burst sources at significant cosmological distances, with redshifts ranging from ~1--3. The energy required to produce the bright gamma-ray flashes is enormous: up to ~10^{53} erg or 10 percent of the rest mass energy of a neutron star, if the emission is isotropic. Here we present the discovery of the optical afterglow and the redshift of GRB 990123, the brightest well-localized GRB to date. With our measured redshift of >1.6, the inferred isotropic energy release exceeds the rest mass of a neutron star thereby challenging current theoretical models for the origin of GRBs. We argue that the optical and IR afterglow measurements reported here may provide the first observational evidence of beaming in a GRB, thereby reducing the required energetics to a level where stellar death models are still tenable.

The afterglow, the redshift, and the extreme energetics of the gamma-ray burst 990123

Long-lived emission, known as afterglow, has now been detected from about a dozen γ-ray bursts. Distance determinations place the bursts at cosmological distances, with redshifts, z, ranging from ∼1 to 3. The energy required to produce these bright γ-ray flashes is enormous: up to ∼10 53 erg, or 10 per cent of the rest-mass energy of a neutron star, if the emission is isotropic. Here we present optical and near-infrared observations of the afterglow of GRB990123, and we determine a redshift of z ⩾ 1.6. This is to date the brightest γ-ray burst with a well-localized position and if the γ-rays were emitted isotropically, the energy release exceeds the rest-mass energy of a neutron star, so challenging current theoretical models of the sources. We argue, however, that our data may provide evidence of beamed (rather than isotropic) radiation, thereby reducing the total energy released to a level where stellar-death models are still tenable.Afterglow, or long-lived emission, has now been detected from about a dozen well-positioned gamma-ray bursts. Distance determinations made by measuring optical emission lines from the host galaxy, or absorption lines in the afterglow spectrum, place the burst sources at significant cosmological distances, with redshifts ranging from ~1--3. The energy required to produce the bright gamma-ray flashes is enormous: up to ~10^{53} erg or 10 percent of the rest mass energy of a neutron star, if the emission is isotropic. Here we present the discovery of the optical afterglow and the redshift of GRB 990123, the brightest well-localized GRB to date. With our measured redshift of >1.6, the inferred isotropic energy release exceeds the rest mass of a neutron star thereby challenging current theoretical models for the origin of GRBs. We argue that the optical and IR afterglow measurements reported here may provide the first observational evidence of beaming in a GRB, thereby reducing the required energetics to a level where stellar death models are still tenable.

TESTS OF THE ACCELERATING UNIVERSE WITH NEAR-INFRARED OBSERVATIONS OF A HIGH-REDSHIFT TYPE Ia SUPERNOVA

We have measured the rest-frame B-, V -, and I-band light curves of a high-redshift type Ia supernova (SN Ia), SN 1999Q (z \ 0.46), using the Hubble Space Telescope (HST ) and ground-based near-infrared detectors. A goal of this study is the measurement of the color excess, a sensitive indicator of E B~I , interstellar or intergalactic dust, which could aUect recent cosmological measurements from high-redshift SNe Ia. Our observations disfavor a 30% opacity of SN Ia visual light by dust as an alternative to an accelerating universe. This statement applies to both Galactic-type dust (rejected at the 3.4 p con—dence level) and grayer dust (grain size ( 0.1 km, rejected at the 2.3¨2.6 p con—dence level) as proposed by Aguirre. The rest-frame I-band light curve shows the secondary maximum 1 month after the B maximum typical of nearby SNe Ia of normal luminosity, providing no indication of evolution as a func- tion of redshift out to z B 0.5. An expanded set of similar observations could improve the constraints on any contribution of extragalactic dust to the dimming of high-redshift SNe Ia. Subject headings: cosmology: observationsdistance scalesupernovae: general

Morphological number counts and redshift distributions to I <26 from the Hubble deep field: Implications for the evolution of ellipticals, spirals, and irregulars

We combine the photometric redshift data of Fern?ndez-Soto et al. with the morphological data of Odewahn et al. for all galaxies with I 24. The breadth of the redshift distribution at faint magnitudes implies strong clustering or an extended epoch of formation commencing at z>3. 2.?Spiral galaxies are present in numbers consistent with zero-evolution predictions to I=22. Beyond this magnitude some net positive evolution is required. Although the number counts are consistent with the passive-evolution predictions to I=26.0, the redshift distributions favor number and luminosity evolution, although few obvious mergers are seen (possibly classified as irregulars). We note that beyond z~2 very few ordered spirals are seen suggesting a formation epoch of spiral galaxies at z~1.5-2. 3.?There is no obvious explanation for the late-type/irregular class, and this category requires further subdivision. While a small fraction of the population lies at low redshift (i.e., true irregulars), the majority lie at redshifts 1 1.5 mergers are frequent and, taken in conjunction with the absence of normal spirals at z>2, the logical inference is that they represent the progenitors of normal spirals that form via hierarchical merging.

The Northern Sky Optical Cluster Survey. II. An Objective Cluster Catalog for 5800 Square Degrees

We present a new, objectively defined catalog of candidate galaxy clusters based on the galaxy catalogs from the digitized Second Palomar Observatory Sky Survey. This cluster catalog, derived from the best calibrated plates in the high-latitude (|b| > 30°) northern Galactic cap region, covers 5800 deg2 and contains 8155 candidate clusters. A simple adaptive kernel density mapping technique, combined with the SExtractor object detection algorithm, is used to detect galaxy overdensities, which we identify as clusters. Simulations of the background galaxy distribution and clusters of varying richnesses and redshifts allow us to optimize detection parameters and measure the completeness and contamination rates for our catalog. Cluster richnesses and photometric redshifts are measured, using integrated colors and magnitudes for each cluster. An extensive spectroscopic survey is used to confirm the photometric results. This catalog, with well-characterized sample properties, provides a sound basis for future studies of cluster physics and large-scale structure.

The 1990 Calán/Tololo supernova search

We have started a search for supernovae as a collaboration between the University of Chile and the Cerro Tololo Inter-American Observatory, with the aim of producing a moderately distant (0.01<z<0.10) sample of Type Ia and Type II supernovae suitable for cosmological studies. The project began in mid-1990 and continues to the present. This paper reports on the Calan/Tololo discoveries in the course of 1990, and on the spectroscopic and photometric observations gathered for these objects. All of these observations were obtained with CCDs, with the extensive collaboration of visiting astronomers. Great care was exercised in the reduction of the light curves in order to properly correct for the background light of the host galaxy of each supernova

The Northern Sky Optical Cluster Survey. IV. An Intermediate-Redshift Galaxy Cluster Catalog and the Comparison of Two Detection Algorithms

We present an optically selected galaxy cluster catalog from ~2700 deg2 of the digitized Second Palomar Observatory Sky Survey (DPOSS), spanning the redshift range 0.1 z 0.5, providing an intermediate-redshift supplement to the previous DPOSS cluster survey. This new catalog contains 9956 cluster candidates and is the largest resource of rich clusters in this redshift range to date. The candidates are detected using the best DPOSS plates based on seeing and limiting magnitude. The search is further restricted to high galactic latitude (|b| > 50°), where stellar contamination is modest and nearly uniform. We also present a performance comparison of two different detection methods applied to this data, the adaptive kernel and Voronoi tessellation techniques. In the regime where both catalogs are expected to be complete, we find excellent agreement, as well as with the most recent surveys in the literature. Extensive simulations are performed and applied to the two different methods, indicating a contamination rate of ~5%. These simulations are also used to optimize the algorithms and evaluate the selection function for the final cluster catalog. Redshift and richness estimates are also provided, making possible the selection of subsamples for future studies.

X-RAY GALAXY CLUSTERS IN NoSOCS: SUBSTRUCTURE AND THE CORRELATION OF OPTICAL AND X-RAY PROPERTIES

We present a comparison of optical and X-ray properties of galaxy clusters in the northern sky, using literature data from BAX and optically selected clusters in DPOSS. We determine the recovery rate of X-ray-detected clusters in the optical as a function of richness, redshift, and X-ray luminosity, showing that the missed clusters are typically low-contrast systems when observed optically (either poor or at high redshifts). We employ four different statistical tests to test for the presence of substructure using optical two-dimensional data. We find that approximately 35% of the clusters show strong signs of substructure in the optical. However, the results are test-dependent, with variations also due to the magnitude range and radius utilized. We have also performed a comparison of X-ray luminosity and temperature with optical galaxy counts (richness). We find that the slope and scatter of the relations between richness and the X-ray properties are heavily dependent on the density contrast of the clusters. The selection of substructure-free systems does not improve the correlation between X-ray luminosity and richness, but this comparison also shows much larger scatter than one obtained using the X-ray temperature. In the latter case, the sample is significantly reduced because temperature measurements are available only for the most massive (and thus high-contrast) systems. However, the comparison between temperature and richness is very sensitive to the exclusion of clusters showing signs of substructure. The correlation of X-ray luminosity and richness is based on the largest sample to date (~750 clusters), while tests involving temperature use a similar number of objects as previous works (≾100). The results presented here are in good agreement with existing literature.

The northern sky optical cluster survey I: detection of galaxy clusters in dposs

The Northern Sky Optical Cluster Survey is a project to create an objective catalog of galaxy clusters over the entire high–Galactic-latitude northern sky with well-understood selection criteria. We use the object catalogs generated from the Digitized Second Palomar Sky Survey as the basis for this survey. We apply a color criterion to select against field galaxies and use a simple adaptive kernel technique to create galaxy density maps, combined with the bootstrap technique, to make significance maps from which density peaks are selected. This survey attempts to eliminate some of the subjective criteria and assumptions of past surveys, including detection by eye and assumed luminosity functions and cluster profiles as in the Palomar Distant Cluster Survey. We also use more information (especially colors) than the most similar recent survey, the APM. This paper presents the details of our cluster-detection technique, as well as some initial results for two small areas totaling ~60 deg2. We find a mean surface density of ~1.5 clusters deg-2, consistent with the detection of richness class 0 and higher clusters to z ~ 0.3. In addition, we demonstrate an effective photometric redshift estimator for our clusters.