4,982 research outputs found
SDSS AGNs with X-ray Emission from ROSAT PSPC Pointed Observations
We present a sample of 1744 type 1 active galactic nuclei (AGNs) from the
Sloan Digital Sky Survey (SDSS DR4) spectroscopic catalog with X-ray
counterparts in the White-Giommi-Angelini Catalog (WGACAT) of ROSAT PSPC
pointed observations. Of 1744 X-ray sources, 1410 (80.9%) are new AGN
identifications. Of 4574 SDSS DR4 AGNs for which we found radio matches in the
catalog of radio sources from the FIRST catalog, 224 turned up in our sample of
SDSS X-ray AGNs. The sample objects are given in a catalog that contains
optical and X-ray parameters along with radio emission parameters where
available. We illustrate the content of our catalog and its potential for AGN
science by providing statistical relationships for the catalog data. The
potential of the morphological information is emphasized by confronting the
statistics of optically resolved and unresolved AGNs. The immediate properties
of the catalog objects include significant correlation of X-ray and optical
fluxes, which is consistent with expectations. Also expected is the decrease of
X-ray flux toward higher redshifts. The X-ray to optical flux ratio for the
unresolved AGNs exhibits a decline toward higher redshifts, in agreement with
previous results. The resolved AGNs, however, display the opposite trend. At a
given optical brightness, X-ray fluxes of radio-quiet AGNs by a factor of 2. We
caution, however, that because of the variety of selection effects present in
both the WGACAT and the SDSS, the interpretation of any relationships based on
our sample of X-ray AGNs requires a careful analysis of these effects.Comment: 34 pages, 18 figure
Natural waters in Amazonia. III. Ammonium molybdate-reactive silica
Natural waters of the Amazonian Tertiary formations along the Manaus-Itacoatjara Road were studied with respect to variations in average seasonal and yearly soluble silica content. In general the variations are small and concentrations very low, for all input fractions, i. e., rainfall, stemflow, and throughfall [about 90 percent of the total (470 analyses) lower than 1.0 mg/l]. While 90 percent of total ground water analyses (excluding well IV 3) have soluble silica concentrations lower than 1.5 mg/l, high rain forest stream waters are slightly higher (90 percent of all values lower than 2.0 mg/l). The Rio Negro waters show 90 percent of the total samples analyzed between 2.0 mg/l and 3.0 mg/l soluble silica
How Saturated are Absorption Lines in the Broad Absorption Line Quasar PG 1411+442 ?
Recently, convincing evidence was found for extremely large X-ray absorption
by column densities in broad absorption line quasars. One
consequence of this is that any soft X-ray emission from these QSOs would be
the scattered light or leaked light from partially covering absorbing material.
A detection of the unabsorbed soft X-ray and absorbed hard X-ray compo nent
will allow to determine the total column density as well as the effective
covering factor of the absorbing material, which can be hardly obtained from
the UV absorption lines. Brinkmann et al. (1999) showed that both the
unabsorbed and absorbed components are detected in the nearby very bright broad
absorption line quasar PG 1411+442. In this letter, we make a further analysis
of the broad band X-ray spectrum and the UV spectrum from HST, and demonstrate
that broad absorption lines are completely saturated at the bottom of
absorption troughs.Comment: 6 pages, 3 postscript figures. to appear in Astrophy. J. Letter
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