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In a six-year program started in July 2014, the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) will conduct novel cosmological observations using the BOSS spectrograph at Apache Point Observatory. These observations will be conducted simultaneously with the Time Domain Spectroscopic Survey (TDSS) designed for variability studies and the Spectroscopic Identification of eROSITA Sources (SPIDERS) program designed for studies of X-ray sources. In particular, eBOSS will measure with percent-level precision the distance-redshift relation with baryon acoustic oscillations (BAO) in the clustering of matter. eBOSS will use four different tracers of the underlying matter density field to vastly expand the volume covered by BOSS and map the large-scale-structures over the relatively unconstrained redshift range 0.6 < z < 2.2. Using more than 250,000 new, spectroscopically confirmed luminous red galaxies at a median redshift z = 0.72, we project that eBOSS will yield measurements of the angular diameter distance d A (z) to an accuracy of 1.2% and measurements of H(z) to 2.1% when combined with the z > 0.6 sample of BOSS galaxies. With ∼ 195, 000 new emission line galaxy redshifts, we expect BAO measurements of d A (z) to an accuracy of 3.1% and H(z) to 4.7% at an effective redshift of z = 0.87. A sample of more than 500,000 spectroscopically-confirmed quasars will provide the first BAO distance measurements over the redshift range 0.9 < z < 2.2, with expected precision of 2.8% and 4.2% on d A (z) and H(z), respectively. Finally, with 60,000 new quasars and reobservation of 60,000 BOSS quasars, we will obtain new Lyα forest measurements at redshifts z > 2.1; these new data will enhance the precision of d A (z) and H(z) at z > 2.1 by a factor of 1.44 relative to BOSS. Furthermore, eBOSS will provide improved tests of General Relativity on cosmological scales through redshift-space distortion (RSD) measurements, improved tests for non-Gaussianity in the primordial density field, and new constraints on the summed mass of all neutrino species. Here, we provide an overview of the cosmological goals, spectroscopic target sample, demonstration of spectral quality from early data, and projected cosmological constraints from eBOSS. eBOSS 3 confidence, where w is the ratio of pressure to energy density for dark energy. Thus, current observations are generally consistent with the simplest picture where dark energy is described completely by Einstein's cosmological constant (Λ).New precise observations can unravel the origin of the accelerating universe; specifically, to determine if cosmic acceleration is caused by deviations in General Relativity (GR) on large scales or by a new form of (dark) energy. It is possible to decouple scenarios of acceleration that require dark energy from those that require modifications to GR by independently probing both cosmic expansion history and the structure growth rate. Four primary observational techniques are generally accepted as the most powerful toward obtaining that goal (e.g. Albrech...

We present the first extensive radio to γ-ray observations of a fast-rising blue optical transient (FBOT), AT 2018cow, over its first ∼100 days. AT 2018cow rose over a few days to a peak luminosity L pk ∼ 4 × 10 44 erg s −1 exceeding those of superluminous supernovae (SNe), before declining as L ∝ t −2 . Initial spectra at δt 15 days were mostly featureless and indicated large expansion velocities v ∼ 0.1 c and temperatures arXiv:1810.10720v1 [astro-ph.HE] 25 Oct 2018 2 MARGUTTI ET AL. reaching T ∼ 3 × 10 4 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features with v ∼ 4000 km s −1 with no evidence for ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectral component at E ≥ 10 keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT 2018cow showed bright radio emission consistent with the interaction of a blastwave with v sh ∼ 0.1 c with a dense environment (Ṁ ∼ 10 −3 − 10 −4 M yr −1 for v w = 1000 km s −1 ). While these properties exclude 56 Ni-powered transients, our multi-wavelength analysis instead indicates that AT 2018cow harbored a "central engine", either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released ∼ 10 50 − 10 51.5 erg over ∼ 10 3 − 10 5 s and resides within lowmass fast-moving material with equatorial-polar density asymmetry (M ej,fast 0.3 M ). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio observations.

We present measurements of the Baryon Acoustic Oscillation (BAO) scale in redshift-space using the clustering of quasars. We consider a sample of 147,000 quasars from the extended Baryon Oscillation Spectroscopic Survey (eBOSS) distributed over 2044 square degrees with redshifts 0.8 < z < 2.2 and measure their spherically-averaged clustering in both configuration and Fourier space. Our observational dataset and the 1400 simulated realizations of the dataset allow us to detect a preference for BAO that is greater than 2.8σ. We determine the spherically averaged BAO distance to z = 1.52 to 3.8 per cent precision: D V (z = 1.52) = 3843 ± 147 (r d /r d,fid ) Mpc. This is the first time the location of the BAO feature has been measured between redshifts 1 and 2. Our result is fully consistent with the prediction obtained by extrapolating the Planck flat ΛCDM best-fit cosmology. All of our results are consistent with basic large-scale structure (LSS) theory, confirming quasars to be a reliable tracer of LSS, and provide a starting point for numerous cosmological tests to be performed with eBOSS quasar samples. We combine our result with previous, independent, BAO distance measurements to construct an updated BAO distance-ladder. Using these BAO data alone and marginalizing over the length of the standard ruler, we find Ω Λ > 0 at 6.6σ significance when testing a ΛCDM model with free curvature.

The fourth generation of the Sloan

We review the physics potential of a next generation search for solar axions: the International Axion Observatory (IAXO). Endowed with a sensitivity to discover axion-like particles (ALPs) with a coupling to photons as small as g aγ ∼ 10 −12 GeV −1 , or to electrons g ae ∼10 −13 , IAXO has the potential to find the QCD axion in the 1 meV∼1 eV mass range where it solves the strong CP problem, can account for the cold dark matter of the Universe and be responsible for the anomalous cooling observed in a number of stellar systems. At the same time, IAXO will have enough sensitivity to detect lower mass axions invoked to explain: 1) the origin of the anomalous "transparency" of the Universe to gamma-rays, 2) the observed soft X-ray excess from galaxy clusters or 3) some inflationary models. In addition, we review string theory axions with parameters accessible by IAXO and discuss their potential role in cosmology as Dark Matter and Dark Radiation as well as their connections to the above mentioned conundrums.

As part of the Sloan Digital Sky Survey IV the extended Baryon Oscillation Spectroscopic Survey (eBOSS ) will improve measurements of the cosmological distance scale by applying the Baryon Acoustic Oscillation (BAO) method to quasar samples. eBOSS will adopt two approaches to target quasars over 7500 deg 2 . First, a "CORE" quasar sample will combine optical selection in ugriz using a likelihood-based routine called XDQSOz, with a mid-IR-optical color-cut. eBOSS CORE selection (to g < 22 OR r < 22) should return ∼ 70 deg −2 quasars at redshifts 0.9 < z < 2.2 and ∼ 7 deg −2 z > 2.1 quasars. Second, a selection based on variability in multi-epoch imaging from the Palomar Transient Factory should recover an additional ∼ 3-4 deg −2 z > 2.1 quasars to g < 22.5. A linear model of how imaging systematics affect target density recovers the angular distribution of eBOSS CORE quasars over 96.7% (76.7%) of the SDSS North (South) Galactic Cap area. The eBOSS CORE quasar sample should thus be sufficiently dense and homogeneous over 0.9 < z < 2.2 to yield the first few-percent-level BAO constraint nearz ∼ 1.5. eBOSS quasars at z > 2.1 will be used to improve BAO measurements in the Lyman-α Forest. Beyond its key cosmological goals, eBOSS should be the next-generation quasar survey, comprising > 500,000 new quasars and > 500,000 uniformly selected spectroscopically confirmed 0.9 < z < 2.2 quasars. At the conclusion of eBOSS, the SDSS will have provided unique spectra of over 800,000 quasars.

Abstract. The high-latitude regions of the Northern Hemisphere are a nexus for the interaction between land surface physical properties and their exchange of carbon and energy with the atmosphere. At these latitudes, two carbon pools of planetary significance -those of the permanently frozen soils (permafrost), and of the great expanse of boreal forestare vulnerable to destabilization in the face of currently observed climatic warming, the speed and intensity of which are expected to increase with time. Improved projections of future Arctic and boreal ecosystem transformation require improved land surface models that integrate processes specific to these cold biomes. To this end, this study lays out relevant new parameterizations in the ORCHIDEE-MICT land surface model. These describe the interactions between soil carbon, soil temperature and hydrology, and their resulting feedbacks on water and CO 2 fluxes, in addition to a recently developed fire module. Outputs from ORCHIDEE-MICT, when forced by two climate input datasets, are extensively evaluated against (i) temperature gradients between the atmosphere and deep soils, (ii) the hydrological components comprising the water balance of the largest highlatitude basins, and (iii) CO 2 flux and carbon stock observations. The model performance is good with respect to empirical data, despite a simulated excessive plant water stress andPublished by Copernicus Publications on behalf of the European Geosciences Union. 122M. Guimberteau et al.: ORCHIDEE-MICT, a LSM for the high latitudes a positive land surface temperature bias. In addition, acute model sensitivity to the choice of input forcing data suggests that the calibration of model parameters is strongly forcingdependent. Overall, we suggest that this new model design is at the forefront of current efforts to reliably estimate future perturbations to the high-latitude terrestrial environment.