On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

We have conducted a detailed investigation of the broadband spectral properties of the γ-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi γ-ray spectra with Swift, radio, infra-red, optical, and other hard X-ray/γ-ray data, collected within 3 months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous spectral energy distributions (SED) for 48 LBAS blazars. The SED of these γ-ray sources is similar to that of blazars discovered at other ABDO ET AL. Vol. 716 wavelengths, clearly showing, in the usual log ν-log ν F ν representation, the typical broadband spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SED to characterize the peak intensity of both the low-and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broadband colors (i.e., the radio to optical, α ro , and optical to X-ray, α ox , spectral slopes) and from the γ-ray spectral index. Our data show that the synchrotron peak frequency (ν S peak) is positioned between 10 12.5 and 10 14.5 Hz in broad-lined flat spectrum radio quasars (FSRQs) and between 10 13 and 10 17 Hz in featureless BL Lacertae objects. We find that the γ-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron-inverse Compton scenarios. However, simple homogeneous, one-zone, synchrotron self-Compton (SSC) models cannot explain most of our SED, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving external Compton radiation or multiple SSC components are required to reproduce the overall SED and the observed spectral variability. While more than 50% of known radio bright high energy peaked (HBL) BL Lacs are detected in the LBAS sample, only less than 13% of known bright FSRQs and LBL BL Lacs are included. This suggests that the latter sources, as a class, may be much fainter γ-ray emitters than LBAS blazars, and could in fact radiate close to the expectations of simple SSC models. We categorized all our sources according to a new physical classification scheme based on the generally accepted paradigm for Active Galactic Nuclei and on the results of this SED study. Since the LAT detector is more sensitive to flat spectrum γ-ray sources, the correlation between ν S peak and γ-ray spectral index strongly favors the detection of high energy peaked blazars, thus explaining the Fermi overabundance of this type of sources compared to radio and EGRET samples. This selection effect is similar to that experienced in the soft X-ray band where HBL BL Lacs are the dominant type of blazars.

We describe the results of an extremely deep, 0.28 deg 2 survey for z ¼ 3:1 Ly emission-line galaxies in the Extended Chandra Deep FieldYSouth. By using a narrowband 5000 8 filter and complementary broadband photometry from the MUSYC survey, we identify a statistically complete sample of 162 galaxies with monochromatic fluxes brighter than 1:5 ; 10 À17 ergs cm À2 s À1 and observer's frame equivalent widths greater than 80 8. We show that the equivalent width distribution of these objects follows an exponential with a rest-frame scale length of w 0 ¼ 76In addition, we show that in the emission line, the luminosity function of Ly galaxies has a faint-end power-law slope of ¼ À1:49 þ0:45 À0:34 , a bright-end cutoff of log L Ã ¼ 42:64 þ0:26 À0:15 , and a space density above our detection thresholds of (1:46 AE 0:12) ; 10 À3 h 3 70 galaxies Mpc À3 . Finally, by comparing the emission-line and continuum properties of the Ly emitters, we show that the star formation rates derived from Ly are $3 times lower than those inferred from the rest-frame UV continuum. We use this offset to deduce the existence of a small amount of internal extinction within the host galaxies. This extinction, coupled with the lack of extremely high equivalent width emitters, argues that these galaxies are not primordial Population III objects, although they are young and relatively chemically unevolved.

We present the photometric calibration of the Swift Ultraviolet/Optical Telescope (UVOT) which includes: optimum photometric and background apertures, effective area curves, colour transformations, conversion factors for count rates to flux and the photometric zero-points (which are accurate to better than 4 per cent) for each of the seven UVOT broad-band filters. The calibration was performed with observations of standard stars and standard star fields that represent a wide range of spectral star types. The calibration results include the position-dependent uniformity, and instrument response over the 1600-8000 Å operational range. Because the UVOT is a photon-counting instrument, we also discuss the effect of coincidence loss on the calibration results. We provide practical guidelines for using the calibration in UVOT data analysis. The results presented here supersede previous calibration results.

We studied the clustering properties and multiwavelength spectral energy distributions of a complete sample of 162 Ly-emitting (LAE) galaxies at z ' 3:1 discovered in deep narrowband MUSYC imaging of the Extended Chandra Deep Field-South. LAEs were selected to have observed frame equivalent widths >80 8 and emission line fluxes >1:5 ; 10 À17 ergs cm À2 s À1. Only 1% of our LAE sample appears to host AGNs. The LAEs exhibit a moderate spatial correlation length of r 0 ¼ 3:6 þ0:8 À1:0 Mpc, corresponding to a bias factor b ¼ 1:7 þ0:3 À0:4 , which implies median dark matter halo masses of log 10 M med ¼ 10:9 þ0:5 À0:9 M . Comparing the number density of LAEs, 1:5 AE 0:3 ; 10 À3 Mpc À3, with the number density of these halos finds a mean halo occupation $1%Y10%. The evolution of galaxy bias with redshift implies that most z ¼ 3:1 LAEs evolve into present-day galaxies with L < 2:5L Ã , whereas other z > 3 galaxy populations typically evolve into more massive galaxies. Halo merger trees show that z ¼ 0 descendants occupy halos with a wide range of masses, with a median descendant mass close to that of L Ã . Only 30% of LAEs have sufficient stellar mass (>$3 ;

We have obtained spectra with the 10-m Keck telescope of a sample of 24 galaxies having colors consistent with star-forming galaxies at redshifts 2 < ∼ z < ∼ 4.5 in the Hubble Deep Field (HDF). Eleven of these galaxies are confirmed to be at high redshift (z med = 3.0), one is at z = 0.5, and the other 12 have uncertain redshifts but have spectra consistent with their being at z > 2. The spectra of the confirmed high-redshift galaxies show a diversity of features, including weak Lyα emission, strong Lyα breaks or damped Lyα absorption profiles, and the stellar and interstellar rest-UV absorption lines common to local starburst galaxies and high-redshift star-forming galaxies reported recently by others. The narrow profiles and low equivalent widths of C IV, Si IV, and N V absorption lines may imply low stellar metallicities. Combined with the 5 high-redshift galaxies in the HDF previously confirmed with Keck spectra by Steidel et al. (1996b), the 16 confirmed sources yield a comoving volume density of n ≥ 2.5 × 10 −4 h 3 50 Mpc −3 for q 0 = 0.05, or n ≥ 1.2 × 10 −3 h 3 50 Mpc −3 for q 0 = 0.5. These densities are 3 − 4 times higher than the recent estimates of 1 Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the

We measure the morphology-density relation ( MDR) and morphology-radius relation (MRR) for galaxies in seven z $ 1 clusters that have been observed with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. Simulations and independent comparisons of our visually derived morphologies indicate that ACS allows one to distinguish between E, S0, and spiral morphologies down to z 850 ¼ 24, corresponding to L /L Ã ¼ 0:21 and 0.30 at z ¼ 0:83 and 1.24, respectively. We adopt density and radius estimation methods that match those used at lower redshift in order to study the evolution of the MDR and MRR. We detect a change in the MDR between 0:8 < z < 1:2 and that observed at z $ 0, consistent with recent work; specifically, the growth in the bulge-dominated galaxy fraction, f EþS0 , with increasing density proceeds less rapidly at z $ 1 than it does at z $ 0. At z $ 1 and AE ! 500 galaxies Mpc À2 , we find h f EþS0 i ¼ 0:72 AE 0:10. At z $ 0, an E+S0 population fraction of this magnitude occurs at densities about 5 times smaller. The evolution in the MDR is confined to densities AE k 40 galaxies Mpc À2 and appears to be primarily due to a deficit of S0 galaxies and an excess of Sp+Irr galaxies relative to the local galaxy population. The f E -density relation exhibits no significant evolution between z ¼ 1 and 0. We find mild evidence to suggest that the MDR is dependent on the bolometric X-ray luminosity of the intracluster medium. Implications for the evolution of the disk galaxy population in dense regions are discussed in the context of these observations.

We analyze deep multicolor Advanced Camera images of the largest known gravitational lens, A1689. Radial and tangential arcs delineate the critical curves in unprecedented detail, and many small counterimages are found near the center of mass. We construct a flexible light deflection field to predict the appearance and positions of counterimages. The model is refined as new counterimages are identified and incorporated to improve the model, yielding a total of 106 images of 30 multiply lensed background galaxies, spanning a wide redshift range, 1:0 < z < 5:5. The resulting mass map is more circular in projection than the clumpy distribution of cluster galaxies, and the light is more concentrated than the mass within r < 50 kpc h À1 . The projected mass profile flattens steadily toward the center with a shallow mean slope of dlog AE=dlog r ' À0:55 AE 0:1, over the observed range r < 250 kpc h À1 , matching well an NFW profile, but with a relatively high concentration, C vir ¼ 8:2 þ2:1 À1:8 . A softened isothermal profile (r core ¼ 20 AE 2 00 ) is not conclusively excluded, illustrating that lensing constrains only projected quantities. Regarding cosmology, we clearly detect the purely geometric increase of bend angles with redshift. The dependence on the cosmological parameters is weak owing to the proximity of A1689, z ¼ 0:18, constraining the locus, M þ Ã 1:2. This consistency with standard cosmology provides independent support for our model, because the redshift information is not required to derive an accurate mass map. Similarly, the relative fluxes of the multiple images are reproduced well by our best-fitting lens model.