The Dark Energy Camera is a new imager with a 2°. 2 diameter field of view mounted at the prime focus of the Victor M. Blanco 4m telescope on Cerro Tololo near La Serena, Chile. The camera was designed and constructed by the Dark Energy Survey Collaborationand meets or exceeds the stringent requirements designed for the widefield and supernova surveys for which the collaboration uses it. The camera consists of a five-element optical corrector, seven filters, a shutter with a 60 cm aperture, and a charge-coupled device (CCD) focal plane of 250 μm thick fully depleted CCDs cooled inside a vacuum Dewar. The 570 megapixel focal plane comprises 62 2k × 4k CCDs for imaging and 12 2k × 2k CCDs for guiding and focus. The CCDs have 15 μm × 15 μm pixels with a plate scale of 0 263 pixel −1. A hexapod system provides state-of-the-art focus and alignment capability. The camera is read out in 20 s with 6-9 electronreadout noise. This paper provides a technical description of the cameraʼs engineering, construction, installation, and current status.

We search for excess γ-ray emission coincident with the positions of confirmed and candidate Milky Way satellite galaxies using six years of data from the Fermi Large Area Telescope (LAT). Our sample of 45 stellar systems includes 28 kinematically confirmed dark-matter-dominated dwarf spheroidal galaxies (dSphs) and 17 recently discovered systems that have photometric characteristics consistent with the population of known dSphs. For each of these targets, the relative predicted γ-ray flux due to dark matter annihilation is taken from kinematic analysis if available, and estimated from a distance-based scaling relation otherwise, assuming that the stellar systems are DM-dominated dSphs. LAT data coincident with four of the newly discovered targets show a slight preference (each 2σ local) for γ-ray emission in excess of the background. However, the ensemble of derived γ-ray flux upper limits for individual targets is consistent with the expectation from analyzing random blank-sky regions, and a combined analysis of the population of stellar systems yields no globally significant excess (global significance ). Our analysis has increased sensitivity compared to the analysis of 15 confirmed dSphs by Ackermann et al. The observed constraints on the DM annihilation cross section are statistically consistent with the background expectation, improving by a factor of ∼2 for large DM masses ( and ) and weakening by a factor of ∼1.5 at lower masses relative to previously observed limits.

We report the discovery of eight new Milky Way companions in~1800 deg 2 of optical imaging data collected during the first year of the Dark Energy Survey (DES). Each system is identified as a statistically significant overdensity of individual stars consistent with the expected isochrone and luminosity function of an old and metal-poor stellar population. The objects span a wide range of absolute magnitudes (M V from -2.2 to -7.4 mag), physical

On 2017 August 17, gravitational waves were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst,GRB170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical and infrared light curves of SSS17a extending from 10.9 hours to 18 days post-merger. We constrain the radioactively-powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve in-2 dicates that SSS17a produced at least ∼0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in r-process nucleosynthesis in the Universe.The discovery of gravitational waves (GWs) from coalescing binary black holes by the Laser Interferometer Gravitational Wave Observatory (LIGO) has transformed the study of compact objects in the Universe (1, 2). Unlike black holes, merging neutron stars are expected to produce electromagnetic radiation. The electromagnetic signature of such an event can provide more information than the GW signal alone: constraining location of the source, reducing the degeneracies in GW parameter estimation (3), probing the expansion rate of the Universe (4,5), and producing a more complete picture of the merger process (6, 7).Short gamma-ray bursts (GRBs) have long been expected to result from neutron star mergers (8, 9), and therefore would be a natural electromagnetic counterpart to GWs (10). Unfortunately, their emission is beamed, so that it may not intersect our line of sight (11). The possibility that only a small fraction of GRBs may be detectable has motivated theoretical and observational searches for more-isotropic electromagnetic signatures, such as an astronomical transient powered by the radioactive decay of neutron-rich ejecta from the merger. (12)(13)(14)(15)(16)(17). Referred to as a macronova or kilonova, the detection of these events would provide information on the origin of many of the heaviest elements in the periodic table (18).It has long been realized that approximately half of the elements heavier than iron are created via r-process nucleosynthesis-the capture of neutrons onto lighter seed nuclei on a timescale more rapid than β-decay pathways (19,20). However, it is less clear where the r-process predominantly occurs, namely whether the primary sources of these elements are core-collapse supernovae or compact binary mergers (black hole-neutron star or neutron starneutron star) (21,22). For supernovae, direct detection of the electromagnetic signatures from r-process nucleosynthesis is obscured by the much larger luminosity originating from hydrogen 3 recombination (for hydrogen-rich supernovae) or nickel-56 and cobalt-56 decay (for hydrogenpoor supernovae). By contrast, it may be possible to measure the r-process nucleosynthesis after a compact ob...