We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the solar system, exploring the transient optical sky, and mapping the Milky Way. LSST will be a large, wide-field ground-based system designed to obtain repeated images covering the sky visible from Cerro Pachón in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg 2 field of view, a 3.2-gigapixel camera, and six filters (ugrizy) covering the wavelength range 320-1050 nm. The project is in the construction phase and will begin regular survey operations by 2022. About 90% of the observing time will be devoted to a deep-wide-fast survey mode that will uniformly observe a 18,000 deg 2 region about 800 times (summed over all six bands) during the anticipated 10 yr of operations and will yield a co-added map to r∼27.5. These data will result in databases including about 32 trillion observations of 20 billion galaxies and a similar number of stars, and they will serve the majority of the primary science programs. The remaining 10% of the observing time will be allocated to special projects such as Very Deep and Very Fast time domain surveys, whose details are currently under discussion. We illustrate how the LSST science drivers led to these choices of system parameters, and we describe the expected data products and their characteristics.

No abstract

We present results of our Chandra observation with ACIS-I centered on the position of Sagittarius A * (Sgr A * ), the compact nonthermal radio source associated with the massive black hole (MBH) at the dynamical center of the Milky Way Galaxy. We have obtained the first high spatial resolution (≈ 1 ′′ ), hard X-ray (0.5-7 keV) image of the central 40 pc (17 ′ ) of the Galaxy.We have discovered an X-ray source, CXOGC J174540.0−290027, coincident with the radio position of Sgr A * to within 0. ′′ 35, corresponding to a maximum projected distance of 16 light-days for an assumed distance to the center of the Galaxy of 8.0 kpc. We received 222 ± 17 (1σ) net counts from the source in 40.3 ks. The source is detected with high significance, S/N ≃ 37σ, despite the highly elevated diffuse X-ray background in the central parsec of the Galaxy. Due to the low number of counts, the spectrum is well fit either by an absorbed power-law model with photon index Γ = 2.7 +1.3 −0.9 (N (E) ∝ E −Γ photons cm −2 s −1 keV −1 ) and column density N H = (9.8 +4.4 −3.0 ) × 10 22 cm −2 (90% confidence interval) or by an absorbed optically thin thermal plasma model with kT = 1.9 +0.9 −0.5 keV and N H = (11.5 +4.4 −3.1 ) × 10 22 cm −2 . Using the power-law model, the measured (absorbed) flux in the 2-10 keV band is (1.3 +0.4 −0.2 ) × 10 −13 ergs cm −2 s −1 , and the absorption-corrected luminosity is (2.4 +3.0 −0.6 ) × 10 33 ergs s −1 . The X-ray source coincident with Sgr A * is resolved, with an apparent diameter of ≈ 1 ′′ . We report the possible detection, at the 2.7σ significance level, of rapid continuum variability on a timescale of several hours. We also report the possible detection of an Fe Kα line at the ≃ 2σ level. The long-term variability of Sgr A * is constrained via comparison with the ROSAT /PSPC observation in 1992. The origin of the X-ray emission (MBH vs. stellar) and the implications of our observation for the various proposed MBH emission mechanisms are discussed. The current observations, while of limited signalto-noise, are consistent with the presence of both thermal and nonthermal emission components in the Sgr A * spectrum.We also briefly discuss the complex structure of the X-ray emission from the Sgr A radio complex and along the Galactic plane and present morphological evidence that Sgr A * and Sgr A West lie within the hot plasma in the central cavity of Sgr A East. Over 150 point sources are detected in the 17 ′ × 17 ′ field of view. Our survey of X-ray sources is complete down to a limiting 2-10 keV absorbed flux of F X ≈ 1.7 × 10 −14 ergs cm −2 s −1 . For sources at the distance of the Galactic Center, the corresponding absorption-corrected luminosity is L X ≈ 2.5 × 10 32 ergs s −1 . The complete flux-limited sample contains 85 sources. Finally, we present an analysis of the integrated emission from the detected point sources and the diffuse emission within the central 0.4 pc (10 ′′ ) of the Galaxy.

We present source catalogs for the 4 Ms Chandra Deep Field-South (CDF-S), which is the deepest Chandra survey to date and covers an area of 464.5 arcmin 2 . We provide a main Chandra source catalog, which contains 740 X-ray sources that are detected with WAVDETECT at a false-positive probability threshold of 10 −5 in at least one of three X-ray bands (0.5-8 keV, full band; 0.5-2 keV, soft band; and 2-8 keV, hard band) and also satisfy a binomial-probability source-selection criterion of P < 0.004 (i.e., the probability of sources not being real is less than 0.004); this approach is designed to maximize the number of reliable sources detected. A total of 300 main-catalog sources are new compared to the previous 2 Ms CDF-S main-catalog sources. We determine X-ray source positions using centroid and matched-filter techniques and obtain a median positional uncertainty of ≈ 0.42 ′′ . We also provide a supplementary catalog, which consists of 36 sources that are detected with WAVDETECT at a false-positive probability threshold of 10 −5 , satisfy the condition of 0.004 < P < 0.1, and have an optical counterpart with R < 24. Multiwavelength identifications, basic optical/infrared/radio photometry, and spectroscopic/photometric redshifts are provided for the X-ray sources in the main and supplementary catalogs. 716 (≈ 97%) of the 740 main-catalog sources have multiwavelength counterparts, with 673 (≈ 94% of 716) having either spectroscopic or photometric redshifts. The 740 main-catalog sources span broad ranges of full-band flux and 0.5-8 keV luminosity; the 300 new main-catalog sources span similar ranges although they tend to be systematically lower. Basic analyses of the X-ray and multiwavelength properties of the sources indicate that > 75% of the main-catalog sources are AGNs; of the 300 new main-catalog sources, about 35% are likely normal and starburst galaxies, reflecting the rise of normal and starburst galaxies at the very faint flux levels uniquely accessible to the 4 Ms CDF-S. Near the center of the 4 Ms CDF-S (i.e., within an off-axis angle of 3 ′ ), the observed AGN and galaxy source densities have reached 9800 +1300 −1100 deg −2 and 6900 +1100 −900 deg −2 , respectively. Simulations show that our main catalog is highly reliable and is reasonably complete. The mean backgrounds (corrected for vignetting and exposure-time variations) are 0.063 and 0.178 count Ms −1 pixel −1 (for a pixel size of 0.492 ′′ ) for the soft and hard bands, respectively; the majority of the pixels have zero background counts. The 4 Ms CDF-S reaches on-axis flux limits of ≈ 3.2 × 10 −17 , 9.1 × 10 −18 , and 5.5 × 10 −17 erg cm −2 s −1 for the full, soft, and hard bands, respectively. An increase in the CDF-S exposure time by a factor of ≈ 2-2.5 would provide further significant gains and probe key unexplored discovery space.

We describe the Sloan