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Wilson-Hodge C.A.,NASA | Cherry M.L.,Louisiana State University | Case G.L.,Louisiana State University | Baumgartner W.H.,NASA | And 31 more authors.
Astrophysical Journal Letters | Year: 2011

The Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady enough to be easily used as a standard candle. As a result, it has been used as a normalization standard by most X-ray/gamma-ray telescopes. Although small-scale variations in the nebula are well known, since the start of science operations of the Fe r m i Gamma-ray Burst Monitor (GBM) in 2008 August, a ∼7% (70 mCrab) decline has been observed in the overall Crab Nebula flux in the 15-50 keV band, measured with the Earth occultation technique. This decline is independently confirmed in the ∼15-50 keV band with three other instruments: the Swift Burst Alert Telescope (Swift/BAT), the Rossi X-ray Timing Explorer Proportional Counter Array (RXTE/PCA), and the Imager on-Board the INTEGRAL Satellite (IBIS). A similar decline is also observed in the ∼3-15 keV data from the RXTE/PCA and in the 50-100 keV band with GBM, Swift/BAT, and INTEGRAL/IBIS. The pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a ∼3 year timescale are also seen independently with the PCA, BAT, and IBIS from 2005 to 2008, with a flux minimum in 2007 April. As of 2010 August, the current flux has declined below the 2007 minimum. © 2011. The American Astronomical Society.

Schneider D.P.,Pennsylvania State University | Richards G.T.,Drexel University | Hall P.B.,York University | Strauss M.A.,Princeton University | And 48 more authors.
Astronomical Journal | Year: 2010

We present the fifth edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog, which is based upon the SDSS Seventh Data Release. The catalog, which contains 105,783 spectroscopically confirmed quasars, represents the conclusion of the SDSS-I and SDSS-II quasar survey. The catalog consists of the SDSS objects that have luminosities larger than Mi = -22.0 (in a cosmology with H 0 = 70kms-1Mpc-1, ΩM = 0.3, and ΩΛ = 0.7), have at least one emission line with FWHM larger than 1000kms-1 or have interesting/complex absorption features, are fainter than i 15.0, and have highly reliable redshifts. The catalog covers an area of ≈ 9380 deg2. The quasar redshifts range from 0.065 to 5.46, with a median value of 1.49; the catalog includes 1248 quasars at redshifts greater than 4, of which 56 are at redshifts greater than 5. The catalog contains 9210 quasars with i < 18; slightly over half of the entries have i < 19. For each object the catalog presents positions accurate to better than 01rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.03mag, and information on the morphology and selection method. The catalog also contains radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large-area surveys. The calibrated digital spectra cover the wavelength region 3800-9200 at a spectral resolution of ≃ 2000; the spectra can be retrieved from the SDSS public database using the information provided in the catalog. Over 96% of the objects in the catalog were discovered by the SDSS. We also include a supplemental list of an additional 207 quasars with SDSS spectra whose archive photometric information is incomplete. © 2010. The American Astronomical Society. All rights reserved.

De Hoon A.,Leibniz Institute for Astrophysics Potsdam | Lamer G.,Leibniz Institute for Astrophysics Potsdam | Schwope A.,Leibniz Institute for Astrophysics Potsdam | Muhlegger M.,Max Planck Institute For Extraterrestische Physik | And 9 more authors.
Astronomische Nachrichten | Year: 2013

The XMM-Newton Distant Cluster Project (XDCP) aims at the identification of a well defined sample of X-ray selected clusters of galaxies at redshifts z ≥ 0.8. As part of this project, we analyse the deep archival XMM-Newton exposure of LBQS 2212-1759 to quantify the cluster content. We validate the optical follow-up strategy as well as the X-ray selection function. We base the cluster identification of the extended X-ray sources on deep imaging with the ESO-VLT and on the CFHT-LS. The confirmation of cluster candidates is done by VLT/FORS2 spectroscopy. Photometric redshifts from CFTH-LS D4 are utilised to confirm the effectiveness of the X-ray cluster selection method. The survey sensitivity is computed to have a flux limit of S0.5-2.0 keV ~ 2.5×10-15 erg s-1 for 50% completeness in an area ~0.13 deg2. We detect six clusters of galaxies above this level both in X-rays and the optical. Two newly discovered X-ray luminous clusters of galaxies in this work are at z ≥ 1.0 and one is at z = 0.41. The constructed log N -log S tends to favour a scenario where no evolution in the cluster X-ray luminosity function takes place. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

De Hoon A.,Leibniz Institute for Astrophysics Potsdam | Lamer G.,Leibniz Institute for Astrophysics Potsdam | Schwope A.,Leibniz Institute for Astrophysics Potsdam | Muhlegger M.,Max Planck Institute For Extraterrestische Physik | And 15 more authors.
Astronomy and Astrophysics | Year: 2013

Aims. The XMM-Newton distant cluster project (XDCP) aims at the identification of a well defined sample of X-ray selected clusters of galaxies at redshifts z ≥ 0.8. As part of this project, we analyse the deep XMM-Newton exposure covering one of the CFHTLS deep fields to quantify the cluster content. We validate the optical follow-up strategy as well as the X-ray selection function. Methods. We searched for extended X-ray sources in archival XMM-Newton EPIC observations. Multi-band optical imaging was performed to select high redshift cluster candidates among the extended X-ray sources. Here we present a catalogue of the extended sources in one the deepest ̃250 ks XMM-Newton fields targetting LBQS 2212-1759 covering ̃0.2 deg2. The cluster identification is based on deep imaging with the ESO VLT and from the CFHT legacy survey, among others. The confirmation of cluster candidates is done by VLT/FORS2multi-object spectroscopy. Photometric redshifts from the CFHTLS D4 were utilised to confirm the effectiveness of the X-ray cluster selection method. The survey sensitivity was computed with extensive Monte-Carlo simulations. Results. At a flux limit of S 0.5-2.0keV ̃ 2.5 × 10-15 erg s-1 we achieve a completeness level higher than 50% in an area of ̃0.13 deg2. We detect six galaxy clusters above this limit with optical counterparts, of which 5 are new spectroscopic discoveries. Two newly discovered X-ray luminous galaxy clusters are at z ≥ 1.0, another two at z = 0.41, and one at z = 0.34. For the most distant X-ray selected cluster in this field at z = 1.45, we find additional (active) member galaxies from both X-ray and spectroscopic data. Additionally, we find evidence of large-scale structures at moderate redshifts of z = 0.41 and z = 0.34. Conclusions. The quest for distant clusters in archival XMM-Newton data has led to detection of six clusters in a single field, making XMM-Newton an outstanding tool for cluster surveys. Three of these clusters are at z ≥ 1, which emphasises the valuable contribution of small, yet deep surveys to cosmology. Beta models are appropriate descriptions of the cluster surface brightness when performing cluster detection simulations to compute the X-ray selection function. The constructed log N - log S tends to favour a scenario where no evolution in the cluster X-ray luminosity function (XLF) takes place. © ESO 2013.

Wilson-Hodge C.A.,NASA | Case G.L.,Louisiana State University | Cherry M.L.,Louisiana State University | Rodi J.,Louisiana State University | And 14 more authors.
Astrophysical Journal, Supplement Series | Year: 2012

The Gamma-ray Burst Monitor (GBM) on board Fermi has been providing continuous data to the astronomical community since 2008 August 12. In this paper, we present the results of the analysis of the first three years of these continuous data using the Earth occultation technique to monitor a catalog of 209 sources. From this catalog, we detect 99 sources, including 40 low-mass X-ray binary/neutron star systems, 31 high-mass X-ray binary/neutron star systems, 12 black hole binaries, 12 active galaxies, and 2 other sources, plus the Crab Nebula, and the Sun. Nine of these sources are detected in the 100-300keV band, including seven black hole binaries, the active galaxy CenA, and the Crab. The Crab and CygX-1 are also detected in the 300-500keV band. GBM provides complementary data to other sky-monitors below 100keV and is the only all-sky monitor above 100keV. Up-to-date light curves for all of the catalog sources can be found online. © © 2012. The American Astronomical Society. All rights reserved.

Case G.L.,Louisiana State University | Cherry M.L.,Louisiana State University | Wilson-Hodge C.A.,Marshall Space Flight Center | Camero-Arranz A.,National Space Science and Technology Center | And 15 more authors.
Astrophysical Journal | Year: 2011

The NaI and BGO detectors on the Gamma-ray Burst Monitor (GBM) on Fermi are now being used for long-term monitoring of the hard X-ray/low-energy gamma-ray sky. Using the Earth occultation technique as demonstrated previously by the BATSE instrument on the Compton Gamma-Ray Observatory, GBM can be used to produce multiband light curves and spectra for known sources and transient outbursts in the 8keV to 1MeV energy range with its NaI detectors and up to 40MeV with its BGO detectors. Over 85% of the sky is viewed every orbit, and the precession of the Fermi orbit allows the entire sky to be viewed every 26 days with sensitivity exceeding that of BATSE at energies below 25keV and above 1.5MeV. We briefly describe the technique and present preliminary results using the NaI detectors after the first two years of observations at energies above 100keV. Eight sources are detected with a significance greater than 7σ: the Crab, Cyg X-1, SWIFT J1753.5-0127, 1E 1740-29, Cen A, GRS 1915+105, and the transient sources XTE J1752-223 and GX 339-4. Two of the sources, the Crab and Cyg X-1, have also been detected above 300keV. © 2011. The American Astronomical Society.

Lusso E.,Max Planck Institute for Astronomy | Hennawi J.F.,Max Planck Institute for Astronomy | Comastri A.,National institute for astrophysics | Zamorani G.,National institute for astrophysics | And 9 more authors.
Astrophysical Journal | Year: 2013

The fraction of active galactic nucleus (AGN) luminosity obscured by dust and re-emitted in the mid-IR is critical for understanding AGN evolution, unification, and parsec-scale AGN physics. For unobscured (Type 1) AGNs, where we have a direct view of the accretion disk, the dust covering factor can be measured by computing the ratio of re-processed mid-IR emission to intrinsic nuclear bolometric luminosity. We use this technique to estimate the obscured AGN fraction as a function of luminosity and redshift for 513 Type 1 AGNs from the XMM-COSMOS survey. The re-processed and intrinsic luminosities are computed by fitting the 18 band COSMOS photometry with a custom spectral energy distribution fitting code, which jointly models emission from hot dust in the AGN torus, from the accretion disk, and from the host galaxy. We find a relatively shallow decrease of the luminosity ratio as a function of L bol, which we interpret as a corresponding decrease in the obscured fraction. In the context of the receding torus model, where dust sublimation reduces the covering factor of more luminous AGNs, our measurements require a torus height that increases with luminosity as . Our obscured-fraction- luminosity relation agrees with determinations from Sloan Digital Sky Survey censuses of Type 1 and Type 2 quasars and favors a torus optically thin to mid-IR radiation. We find a much weaker dependence of the obscured fraction on 2-10 keV luminosity than previous determinations from X-ray surveys and argue that X-ray surveys miss a significant population of highly obscured Compton-thick AGNs. Our analysis shows no clear evidence for evolution of the obscured fraction with redshift. © 2013. The American Astronomical Society. All rights reserved..

Lusso E.,University of Bologna | Lusso E.,National institute for astrophysics | Comastri A.,National institute for astrophysics | Vignali C.,University of Bologna | And 18 more authors.
Astronomy and Astrophysics | Year: 2011

We present a study of the multi-wavelength properties, from the mid-infrared to the hard X-rays, of a sample of 255 spectroscopically identified X-ray selected type-2 AGN from the XMM-COSMOS survey. Most of them are obscured and the X-ray absorbing column density is determined by either X-ray spectral analyses (for 45% of the sample), or from hardness ratios. Spectral energy distributions (SEDs) are computed for all sources in the sample. The average SEDs in the optical band are dominated by the host-galaxy light, especially at low X-ray luminosities and redshifts. There is also a trend between X-ray and mid-infrared luminosity: the AGN contribution in the infrared is higher at higher X-ray luminosities. We calculate bolometric luminosities, bolometric corrections, stellar masses and star formation rates (SFRs) for these sources using a multi-component modeling to properly disentangle the emission associated to stellar light from that due to black hole accretion. For 90% of the sample we also have the morphological classifications obtained with an upgraded version of the Zurich estimator of structural types (ZEST+). We find that on average type-2 AGN have lower bolometric corrections than type-1 AGN. Moreover, we confirm that the morphologies of AGN host-galaxies indicate that there is a preference for these type-2 AGN to be hosted in bulge-dominated galaxies with stellar masses greater than 1010 solar masses. © 2011 ESO.

Ceverino D.,Hebrew University of Jerusalem | Dekel A.,Hebrew University of Jerusalem | Mandelker N.,Hebrew University of Jerusalem | Bournaud F.,University Paris Diderot | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We address the internal support against total free-fall collapse of the giant clumps that form by violent gravitational instability in high-z disc galaxies. Guidance is provided by an analytic model, where the protoclumps are cut from a rotating disc and collapse to equilibrium while preserving angular momentum. This model predicts prograde clump rotation, which dominates the support if the clump has contracted to a surface density contrast ≳10. This is confirmed in hydro adaptive mesh refinement zoom-in simulations of galaxies in a cosmological context. In most high-z clumps, the centrifugal force dominates the support, , where V rot is the rotation velocity and the circular velocity V circ measures the potential well. The clump spin indeed tends to be in the sense of the global disc angular momentum, but substantial tilts are frequent, reflecting the highly warped nature of the high-z discs. Most clumps are in Jeans equilibrium, with the rest of the support provided by turbulence, partly driven by the gravitational instability itself. The general agreement between model and simulations indicates that angular momentum loss or gain in most clumps is limited to a factor of 2. Simulations of isolated gas-rich discs that resolve the clump substructure reveal that the cosmological simulations may overestimate by ~30 per cent, but the dominance of rotational support at high z is not a resolution artefact. In turn, isolated gas-poor disc simulations produce at z= 0 smaller gaseous non-rotating transient clouds, indicating that the difference in rotational support is associated with the fraction of cold baryons in the disc. In our current cosmological simulations, the clump rotation velocity is typically more than twice the disc dispersion, V rot~ 100kms -1, but when beam smearing of ≥0.1arcsec is imposed, the rotation signal is reduced to a small gradient of ≤30kms -1kpc -1 across the clump. The velocity dispersion in the simulated clumps is comparable to the disc dispersion so it is expected to leave only a marginal signal for any beam smearing. Retrograde minor-merging galaxies could lead to massive clumps that do not show rotation even when marginally resolved. A testable prediction of the scenario as simulated is that the mean stellar age and the stellar fraction of the clumps are declining linearly with distance from the disc centre. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Bjerkeli P.,Copenhagen University | Bjerkeli P.,Chalmers University of Technology | Jorgensen J.K.,Copenhagen University | Bergin E.A.,University of Michigan | And 10 more authors.
Astronomy and Astrophysics | Year: 2016

Context. Understanding how protostars accrete their mass is one of the fundamental problems of star formation. High dust column densities and complex kinematical structures make direct observations challenging. Moreover, direct observations only provide a snapshot. Chemical tracers provide an interesting alternative to characterise the infall histories of protostars. Aims. We aim to map the distribution and kinematics of gaseous water towards the low-mass embedded protostar IRAS 15398-3359. Previous observations of H13CO+ showed a depression in the abundance towards IRAS 15398-3359. This is a sign of destruction of HCO+ by an enhanced presence of gaseous water in an extended region, possibly related to a recent burst in the accretion. Direct observations of water vapour can determine the exact extent of the emission and confirm the hypothesis that HCO+ is indeed a good tracer of the water snow-line. Methods. IRAS 15398-3359 was observed using the Atacama Large Millimeter/submillimeter Array (ALMA) at 0.5″ resolution in two setups at 390 and 460 GHz. Maps of HDO (101-000) and were taken simultaneously with observations of the CS (8-7) and N2H+ (5-4) lines and continuum at 0.65 and 0.75 mm. The maps were interpreted using dust radiative transfer calculations of the protostellar infalling envelope with an outflow cavity. Results. HDO is clearly detected and extended over the scales of the H13CO+ depression, although it is displaced by ~500 AU in the direction of the outflow. HO is tentatively detected towards the red-shifted outflow lobe, but otherwise it is absent from the mapped region, which suggests that temperatures are low. Although we cannot entirely exclude a shock origin, this indicates that another process is responsible for the water emission. Conclusions. Based on the temperature structure obtained from dust radiative transfer models, we conclude that the water was most likely released from the grains in an extended hour-glass configuration during a recent accretion burst. HDO is only detected in the region closest to the protostar, at distances of up to 500 AU. These signatures can only be explained if the luminosity has recently been increased by orders of magnitudes. Additionally, the densities in the outflow cones must be sufficiently low. © 2016 ESO.

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