Roy R. Gal

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

The optical counterpart to the γ-ray burst GRB970508

Understanding the nature of the γ-ray burst phenomenon is one of the outstanding problems of modern astrophysics. The identification of counterparts at optical wavelengths is considered a crucial factor for determining the origin of these events. Here we report the detection and temporal properties of a variable optical source, which has been identified, as the counterpart of the X-ray transient associated with the γ-ray burst GRB970508 (ref. 3). The initial optical images were obtained only 5.8 hours after the initial γ-ray burst, after which the optical source was observed to brighten over the next two days before declining in luminosity with a t−1 power law. The decline in brightness follows a form predicted by many relativistic fireball models for γ-ray bursts, although the initial rise does not appear to be compatible with the simplest of these models. The observed fluence of the source at visible wavelengths over the period spanned by our observations is ⩾4.6 × 10−8 erg cm−2, about 3% of the fluence of the γ-ray burst itself.

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 Butcher-Oemler Effect in 295 Clusters: Strong Redshift Evolution and Cluster Richness Dependence

We examine the Butcher-Oemler effect and its cluster richness dependence in the largest sample studied to date: 295 Abell clusters. We find a strong correlation between cluster richness and the fraction of blue galaxies, , at every redshift. The slope of the relation is similar for all richnesses, but at a given redshift, is ff (z) f BB B systematically higher for poor clusters. This is the chief cause of scatter in the versus z diagram; the spread fB caused by the richness dependence is comparable to the trend in over a typical redshift baseline so that fB conclusions drawn from smaller samples have varied widely. The two parameters, z and a consistently defined projected galaxy number density N, together account for all of the observed variation in within the measurement fB errors. The redshift evolution of is real and occurs at approximately the same rate for clusters of all richness fB classes. Subject headings: galaxies: clusters: general — galaxies: evolution

The Host Galaxy of GRB 990123

We present deep images of the field of GRB 990123 obtained in a broadband UV/visible bandpass with the Hubble Space Telescope (HST) and deep near-infrared images obtained with the Keck I 10 m telescope. The HST image reveals that the optical transient (OT) is offset by 0farcs67 (5.8 kpc in projection) from an extended, apparently interacting galaxy. This galaxy, which we conclude is the host galaxy of GRB 990123, is the most likely source of the absorption lines of metals at a redshift of z=1.6 seen in the spectrum of the OT. With magnitudes of Gunn-r = 24.5 ± 0.2 and K = 22.1±0.3 mag, this corresponds to an L ~ 0.5L

Luminosity function of clusters of galaxies

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

galaxy, assuming that it is located at z = 1.6. The estimated unobscured star formation rate is ≈4 M_☉ yr^(−1), which is typical for normal galaxies at comparable redshifts. There is no evidence for strong gravitational lensing magnification of this burst, and some alternative explanation for its remarkable energetics (such as beaming) may therefore be required. The observed offset of the OT from the nominal host center, the absence of broad absorption lines in the afterglow spectrum, and the relatively blue continuum of the host do not support the notion that gamma-ray bursts (GRBs) originate from active galactic nuclei or massive black holes. Rather, the data are consistent with models of GRBs that involve the death and/or merger of massive stars. Indeed, the HST image suggests an intimate connection between GRB 990123 and a star-forming region.

The Spitzer Extragalactic Representative Volume Survey (SERVS): Survey Definition and Goals

The composite galaxy luminosity function (hereafter LF) of 39 Abell clusters of galaxies is derived by computing the statistical excess of galaxy counts in the cluster direction with respect to control elds. Due to the wide eld coverage of the digitised POSS-II plates, we can measure eld counts around each cluster in a fully homogeneous way. Furthermore, the availability of virtually unlimited sky coverage allows us to directly compute the LF errors without having to rely on the estimated variance of the background. The wide eld coverage also allows us to derive the LF of the whole cluster, including galaxies located in the cluster outskirts. The global composite LF has a slope 1:1 0:2 with minor variations from blue to red lters, and M 21:7; 22:2; 22: 4m ag (H0 =5 0 km s 1 Mpc 1 )i ng;r and i lters, respectively (errors are detailed in the text). These results are in quite good agreement with several previous determinations and in particular with the LF determined for the inner region of a largely overlapping set of clusters, but derived making use of a completely dierent method for background subtraction. The similarity of the two LFs suggests the existence of minor dierences between the LF in the cluster outskirts and in the central region, or a negligible contribution of galaxies in the cluster outskirts to the global LF.