1,741 research outputs found
Determining the fraction of reddened quasars in COSMOS with multiple selection techniques from X-ray to radio wavelengths
The sub-population of quasars reddened by intrinsic or intervening clouds of
dust are known to be underrepresented in optical quasar surveys. By defining a
complete parent sample of the brightest and spatially unresolved quasars in the
COSMOS field, we quantify to which extent this sub-population is fundamental to
our understanding of the true population of quasars. By using the available
multiwavelength data of various surveys in the COSMOS field, we built a parent
sample of 33 quasars brighter than mag, identified by reliable X-ray to
radio wavelength selection techniques. Spectroscopic follow-up with the
NOT/ALFOSC was carried out for four candidate quasars that had not been
targeted previously to obtain a 100\% redshift completeness of the sample. The
population of high quasars (HAQs), a specific sub-population of quasars
selected from optical/near-infrared photometry, is found to contribute
of the parent sample. The full population of bright spatially
unresolved quasars represented by our parent sample consists of
reddened quasars defined by having , and
of the sample having assuming the extinction
curve of the Small Magellanic Cloud. We show that the HAQ selection works well
for selecting reddened quasars, but some are missed because their optical
spectra are too blue to pass the color cut in the HAQ selection. This is
either due to a low degree of dust reddening or anomalous spectra. We find that
the fraction of quasars with contributing light from the host galaxy is most
dominant at . At higher redshifts the population of spatially
unresolved quasars selected by our parent sample is found to be representative
of the full population at mag. This work quantifies the bias against
reddened quasars in studies that are based solely on optical surveys.Comment: 22 pages, 10 figures, accepted for publication in A&A. The ArXiv
abstract has been shortened for it to be printabl
Observational Constraints on the Dependence of Radio-Quiet Quasar X-ray Emission on Black Hole Mass and Accretion Rate
In this work we use a sample of 318 radio-quiet quasars (RQQ) to investigate
the dependence of the ratio of optical/UV flux to X-ray flux, alpha_ox, and the
X-ray photon index, Gamma_X, on black hole mass, UV luminosity relative to
Eddington, and X-ray luminosity relative to Eddington. Our sample is drawn from
the SDSS, with X-ray data from ROSAT and Chandra, and optical data mostly from
the SDSS; 153 of these sources have estimates of Gamma_X from Chandra. We
estimate M_BH using standard estimates derived from the Hbeta, Mg II, and C IV
broad emission lines. Our sample spans a broad range in black hole mass (10^6 <
M_BH / M_Sun < 10^10) and redshift (z < 4.8). We find that alpha_ox increases
with increasing M_BH and L_UV / L_Edd, and decreases with increasing L_X /
L_Edd. In addition, we confirm the correlation seen in previous studies between
Gamma_X and M_BH and both L_UV / L_Edd and L_X / L_Edd; however, we also find
evidence that the dependence of Gamma_X of these quantities is not monotonic,
changing sign at M_BH ~ 3 x 10^8 M_Sun. We argue that the alpha_ox correlations
imply that the fraction of bolometric luminosity emitted by the accretion disk,
as compared to the corona, increases with increasing accretion rate relative to
Eddington. In addition, we argue that the Gamma_X trends are caused by a
dependence of X-ray spectral index on accretion rate. We discuss our results
within the context of accretion models with comptonizing corona, and discuss
the implications of the alpha_ox correlations for quasar feedback. To date,
this is the largest study of the dependence of RQQ X-ray parameters on black
hole mass and related quantities, and the first to attempt to correct for the
large statistical uncertainty in the broad line mass estimates.Comment: Accepted by ApJ, 23 pages, 15 figures, emulateapj styl
Touch me not
Central Poststroke Pain syndrome (CPSP) can occur due to disruption of the somatosensory pathways of the brain at any level such as the thalamus, medulla, or cerebral cortex. It is characterized by sensory abnormalities and hyperesthesia in the part of the body correlating to the central lesion. The treatment of this pain syndrome is often difficult, and it does not usually respond to traditional analgesics. The first line of treatment is drugs aimed at lowering neuronal hyperexcitability, for example, amitriptyline or lamotrigine, with gabapentin considered a second line
[OII] Emission, Eigenvector 1 and Orientation in Radio-quiet Quasars
We present supportive evidence that the Boroson and Green eigenvector 1 is
not driven by source orientation. Until recently it was generally accepted that
eigenvector 1 does not depend on orientation as it strongly correlates with
[OIII]5007 emission, thought to be an isotropic property. However, recent
studies of radio-loud AGN have questioned the isotropy of [OIII] emission and
concluded that [OII]3727 emission is isotropic. In this paper we investigate
the relation between eigenvector 1 and [OII] emission in radio-quiet BQS
(Bright Quasar Survey) quasars, and readdress the issue of orientation as the
driver of eigenvector 1. We find significant correlations between eigenvector 1
and orientation independent [OII] emission, which implies that orientation does
not drive eigenvector 1. The luminosities and equivalent widths of [OIII] and
[OII] correlate with one another, and the range in luminosities and equivalent
widths is similar. This suggests that the radio-quiet BQS quasars are largely
free of orientation dependent dust effects and ionization dependent effects in
the narrow-line region. We also conclude that neither the [OIII] emission nor
the [OII]/[OIII] ratio are dependent on orientation in our radio-quiet BQS
quasar sample, contrary to recent results found for radio-loud quasars.Comment: 24 pages, 12 figures, accepted for publication in Ap
Dust-free quasars in the early Universe
The most distant quasars known, at redshifts z=6, generally have properties
indistinguishable from those of lower-redshift quasars in the rest-frame
ultraviolet/optical and X-ray bands. This puzzling result suggests that these
distant quasars are evolved objects even though the Universe was only seven per
cent of its current age at these redshifts. Recently one z=6 quasar was shown
not to have any detectable emission from hot dust, but it was unclear whether
that indicated different hot-dust properties at high redshift or if it is
simply an outlier. Here we report the discovery of a second quasar without
hot-dust emission in a sample of 21 z=6 quasars. Such apparently hot-dust-free
quasars have no counterparts at low redshift. Moreover, we demonstrate that the
hot-dust abundance in the 21 quasars builds up in tandem with the growth of the
central black hole, whereas at low redshift it is almost independent of the
black hole mass. Thus z=6 quasars are indeed at an early evolutionary stage,
with rapid mass accretion and dust formation. The two hot-dust-free quasars are
likely to be first-generation quasars born in dust-free environments and are
too young to have formed a detectable amount of hot dust around them.Comment: To be published in Nature on the 18 March 2010
Polarization of AGN in UV Spectral Range
We present the review of some new problems in cosmology and physics of stars
in connection with future launching of WSO. We discuss three problems. UV
observations of distant z > 6 quasars allow to obtain information on the soft <
1 KeV X-ray radiation of the accretion disk around a supermassive black hole
because of its cosmological redshift. Really the region of X-ray radiation is
insufficiently investigated because of high galactic absorption. In a result
one will get important information on the reionization zone of the Universe.
Astronomers from ESO revealed the effect of alignment of electric vectors of
polarized QSOs. One of the probable mechanism of such alignment is the
conversion of QSO radiation into low mass pseudoscalar particles (axions) in
the extragalactic magnetic field. These boson like particles have been
predicted by new SUSY particle physics theory. Since the probability of such
conversion is increasing namely in UV spectral range one can expect the strong
correlation between UV spectral energy distribution of QSO radiation and
polarimetric data in the optical range. In the stellar physics one of the
interesting problems is the origin of the X-ray sources with super Eddington
luminosities. The results of UV observations of these X-ray sources will allow
to find the origin of these sources as accreting intermediate mass black holes.Comment: 6 pages, 3 figure
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
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