1,154 research outputs found
Predictors of disease activity during pregnancy in women with inflammatory bowel disease-a Danish cohort study
Serendipitous discovery of a projected pair of QSOs separated by 4.5 arcsec on the sky
We present the serendipitous discovery of a projected pair of quasi-stellar
objects (QSOs) with an angular separation of arcsec. The
redshifts of the two QSOs are widely different: one, our programme target, is a
QSO with a spectrum consistent with being a narrow line Seyfert 1 AGN at
. For this target we detect Lyman-, \ion{C}{4}, and
\ion{C}{3]}. The other QSO, which by chance was included on the spectroscopic
slit, is a Type 1 QSO at a redshift of , for which we detect
\ion{C}{4}, \ion{C}{3]} and \ion{Mg}{2}. We compare this system to previously
detected projected QSO pairs and find that only about a dozen previously known
pairs have smaller angular separation.Comment: 4 pages, 3 figures. Accepted for publication in A
The High A(V) Quasar Survey: Reddened quasi-stellar objects selected from optical/near-infrared photometry - II
Quasi-stellar objects (QSOs) whose spectral energy distributions (SEDs) are
reddened by dust either in their host galaxies or in intervening absorber
galaxies are to a large degree missed by optical color selection criteria like
the one used by the Sloan Digital Sky Survey (SDSS). To overcome this bias
against red QSOs, we employ a combined optical and near-infrared color
selection. In this paper, we present a spectroscopic follow-up campaign of a
sample of red candidate QSOs which were selected from the SDSS and the UKIRT
Infrared Deep Sky Survey (UKIDSS). The spectroscopic data and SDSS/UKIDSS
photometry are supplemented by mid-infrared photometry from the Wide-field
Infrared Survey Explorer. In our sample of 159 candidates, 154 (97%) are
confirmed to be QSOs. We use a statistical algorithm to identify sightlines
with plausible intervening absorption systems and identify nine such cases
assuming dust in the absorber similar to Large Magellanic Cloud sightlines. We
find absorption systems toward 30 QSOs, 2 of which are consistent with the
best-fit absorber redshift from the statistical modeling. Furthermore, we
observe a broad range in SED properties of the QSOs as probed by the rest-frame
2 {\mu}m flux. We find QSOs with a strong excess as well as QSOs with a large
deficit at rest-frame 2 {\mu}m relative to a QSO template. Potential solutions
to these discrepancies are discussed. Overall, our study demonstrates the high
efficiency of the optical/near-infrared selection of red QSOs.Comment: 64 pages, 18 figures, 16 pages of tables. Accepted to ApJ
Continuum and Emission Line Strength Relations for a large Active Galactic Nuclei Sample
We report on the analysis of a large sample of 744 type 1 Active Galactic Nuclei, including quasars and Seyfert 1 galaxies across the redshift range from 0 < z < 5 and spanning nearly 6 orders of magnitude in continuum luminosity. We discuss correlations of continuum and emission line properties in the rest-frame ultraviolet and optical spectral ranges. The well established Baldwin Effect is detected for almost all emission lines from OVI1034 to [OIII]5007. Their equivalent widths are significantly anti-correlated with the continuum strength, while they are nearly independent of redshift. This is the well known Baldwin Effect. Its slope beta, measured as log W_lambda ~ beta * log lambda * L_lambda (1450A), shows a tendency to become steeper towards higher luminosity. The slope of the Baldwin Effect also increases with the ionization energy needed to create the individual lines. In contrast to this general trend, the NV1240 equivalent width is nearly independent of continuum luminosity and remains nearly constant. The overall line behaviors are consistent with softer UV continuum shapes and perhaps increasing gas metallicity in more luminous Active Galactic Nuclei
VLT identification of the optical afterglow of the gamma-ray burst GRB 000131 at z=4.50
We report the discovery of the gamma-ray burst GRB 000131 and its optical
afterglow. The optical identification was made with the VLT 84 hours after the
burst following a BATSE detection and an Inter Planetary Network localization.
GRB 000131 was a bright, long-duration GRB, with an apparent precursor signal
62 s prior to trigger. The afterglow was detected in ESO VLT, NTT, and DK1.54m
follow-up observations. Broad-band and spectroscopic observations of the
spectral energy distribution reveals a sharp break at optical wavelengths which
is interpreted as a Ly-alpha absorption edge at 6700 A. This places GRB 000131
at a redshift of 4.500 +/- 0.015. The inferred isotropic energy release in
gamma rays alone was approximately 10^54 erg (depending on the assumed
cosmology). The rapid power-law decay of the afterglow (index alpha=2.25,
similar to bursts with a prior break in the lightcurve), however, indicates
collimated outflow, which relaxes the energy requirements by a factor of < 200.
The afterglow of GRB 000131 is the first to be identified with an 8-m class
telescope.Comment: 8 pages, 7 figures, accepted to A&A Letter
Spectral Energy Distributions and Multiwavelength Selection of Type 1 Quasars
We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z ~ 3, with predictions to z = 7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGNs) than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA, and ROSAT data, where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to errors as large as 50% for individual quasars when inferring a bolometric luminosity from an optical luminosity. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened/obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio
Jet and accretion power in the most powerful Fermi blazars
Among the blazars detected by the Fermi satellite, we have selected the 23
blazars that in the three months of survey had an average gamma-ray luminosity
above 1e48 erg/s. For 17 out of the 23 sources we found and analysed X-ray and
optical-UV data taken by the Swift satellite. With these data, implemented by
archival and not simultaneous data, we construct the spectral energy
distributions, and interpreted them with a simple one-zone, leptonic,
synchrotron and inverse Compton model. When possible, we also compare different
high energy states of single sources, like 0528+134 and 3C 454.3, for which
multiple good sets of multi-wavelength data are available. In our powerful
blazars the high energy emission always dominates the electromagnetic output,
and the relatively low level of the synchrotron radiation often does not hide
the accretion disk emission. We can then constrain the black hole mass and the
disk luminosity. Both are large (i.e. masses equal or greater than 1e9 solar
masses and disk luminosities above 0.1 Eddington). By modelling the non-thermal
continuum we derive the power that the jet carries in the form of bulk motion
of particles and fields. On average, the jet power is found to be slightly
larger than the disk luminosity, and proportional to the mass accretion rate.Comment: 15 pages, 14 figures, accepted for publication in MNRA
X-ray observational signature of a black hole accretion disc in an active galactic nucleus RXJ1633+4718
We report the discovery of a luminous ultra-soft X-ray excess in a radio-loud
narrow-line Seyfert1 galaxy, RXJ1633+4718, from archival ROSAT observations.
The thermal temperature of this emission, when fitted with a blackbody, is as
low as 32.5(+8.0,-6.0)eV. This is in remarkable contrast to the canonical
temperatures of ~0.1-0.2keV found hitherto for the soft X-ray excess in active
galactic nuclei (AGN), and is interestingly close to the maximum temperature
predicted for a postulated accretion disc in this object. If this emission is
indeed blackbody in nature, the derived luminosity
[3.5(+3.3,-1.5)x10^(44)ergs/s] infers a compact emitting area with a size
[~5x10^(12)cm or 0.33AU in radius] that is comparable to several times the
Schwarzschild radius of a black hole at the mass estimated for this AGN
(3x10^6Msun). In fact, this ultra-steep X-ray emission can be well fitted as
the (Compton scattered) Wien tail of the multi-temperature blackbody emission
from an optically thick accretion disc, whose parameters inferred (black hole
mass and accretion rate) are in good agreement with independent estimates using
optical emission line spectrum. We thus consider this feature as a signature of
the long-sought X-ray radiation directly from a disc around a super-massive
black hole, presenting observational evidence for a black hole accretion disc
in AGN. Future observations with better data quality, together with improved
independent measurements of the black hole mass, may constrain the spin of the
black hole.Comment: 8 pages, 4 figures, ApJ in pres
Previous glucocorticoid treatment in childhood and adolescence is associated with long-term differences in subcortical grey matter volume and microstructure
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