1,046 research outputs found
A Synoptic, Multiwavelength Analysis of a Large Quasar Sample
We present variability and multi-wavelength photometric information for the
933 known quasars in the QUEST Variability Survey. These quasars are grouped
into variable and non-variable populations based on measured variability
confidence levels. In a time-limited synoptic survey, we detect an
anti-correlation between redshift and the likelihood of variability. Our
comparison of variability likelihood to radio, IR, and X-ray data is consistent
with earlier quasar studies. Using already-known quasars as a template, we
introduce a light curve morphology algorithm that provides an efficient method
for discriminating variable quasars from periodic variable objects in the
absence of spectroscopic information. The establishment of statistically robust
trends and efficient, non-spectroscopic selection algorithms will aid in quasar
identification and categorization in upcoming massive synoptic surveys.
Finally, we report on three interesting variable quasars, including variability
confirmation of the BL Lac candidate PKS 1222+037.Comment: AJ, accepted for publication 15 Dec 200
Existence and approximation of probability measure solutions to models of collective behaviors
In this paper we consider first order differential models of collective
behaviors of groups of agents based on the mass conservation equation. Models
are formulated taking the spatial distribution of the agents as the main
unknown, expressed in terms of a probability measure evolving in time. We
develop an existence and approximation theory of the solutions to such models
and we show that some recently proposed models of crowd and swarm dynamics fit
our theoretic paradigm.Comment: 31 pages, 1 figur
The dusty environment of Quasars. Far-IR properties of Optical Quasars
We present the ISO far-IR photometry of a complete sub-sample of optically
selected bright quasars belonging to two complete surveys selected through
multicolour (U,B,V,R,I) techniques. The ISOPHOT camera on board of the ISO
Satellite was used to target these quasars at wavelengths of 7.3, 11.5, 60, 100
and 160 micron. Almost two thirds of the objects were detected at least in one
ISOPHOT band. The detection rate is independent of the source redshift, very
likely due to the negative K-correction of the far-IR thermal emission. More
than a half of the optically selected QSOs show significant emission between 4
and 100 micron in the quasar rest-frame. These fluxes have a very likely
thermal origin, although in a few objects an additional contribution from a
non-thermal component is plausible in the long wavelength bands. In a
colour-colour diagram these objects span a wide range of properties from
AGN-dominated to ULIRG-like. The far-IR composite spectrum of the quasar
population presents a broad far-IR bump between 10 and 30 micron and a sharp
drop at wavelengths greater than 100 micron in the quasar restframe. The amount
of energy emitted in the far-IR, is on average a few times larger than that
emitted in the blue and the ratio L(FIR)/L(B) increases with the bolometric
luminosity. Objects with fainter blue magnitudes have larger ratios between the
far-IR (wavelengths > 60 micron) fluxes and the blue band flux, which is
attributed to extinction by dust around the central source. No relation between
the blue absolute magnitude and the dust colour temperature is seen, suggesting
that the dominant source of FIR energy could be linked to a concurrent
starburst rather than to gravitational energy produced by the central engine.Comment: Astronomical Journal, in pres
Apparatus for timeâresolved measurements of acoustic birefringence in particle dispersions
An apparatus for timeâresolved measurements of the birefringence induced in a particle suspension by an acoustic wave pulse is described. Efficient acoustic coupling is obtained by operating near the transducer resonant frequency and by matching the acoustic impedances of the cell constituents. An almostâoverdamped acoustic configuration can alternatively be employed whenever a faster response is needed. Careful design of the optical setup and of the detection unit minimize diffraction and stressâbirefringence parasitic effects and yields a good responsivity at fairly low acoustic intensities. A test of the apparatus on a colloidal suspension of PTFE rodlike particles is presented and discussed
Time Dilation and Quasar Variability
The timescale of quasar variability is widely expected to show the effects of
time dilation. In this paper we analyse the Fourier power spectra of a large
sample of quasar light curves to look for such an effect. We find that the
timescale of quasar variation does not increase with redshift as required by
time dilation. Possible explanations of this result all conflict with widely
held consensus in the scientific community.Comment: 6 pages including 3 figures. Accepted for publication in ApJ Letter
The power spectrum of the flux distribution in the Lyman-alpha forest of a Large sample of UVES QSO Absorption Spectra (LUQAS)
The flux power spectra of the Lyman-alpha forest from a sample of 27 QSOs
taken with the high resolution echelle spectrograph UVES on VLT are presented.
We find a similar fluctuation amplitude at the peak of the ``3D'' flux power
spectrum at k ~ 0.03 (km/sec)^(-1) as the study by Croft et al. (2002), in the
same redshift range. The amplitude of the flux power spectrum increases with
decreasing redshift if corrected for the increase in the mean flux level as
expected if the evolution of the flux power spectrum is sensitive to the
gravitational growth of matter density fluctuations. This is in agreement with
the findings of McDonald et al. (2000) at larger redshift. The logarithmic
slope of the "3D" flux power spectrum, P_F(k), at large scales k < 0.03
(km/sec)^(-1), is 1.4 +- 0.3, i.e. 0.3 shallower than that found by Croft et
al. (2002) but consistent within the errors.Comment: 18 pages, 9 PS figures, 6 tables. Note that the k-values of the 1D
flux power spectrum had been erroneously shifted by half a bin size (in log
k) in the previous version. All the other results are unaffected. New tables
can be found at http://www.ast.cam.ac.uk/~rtnigm/luqas.ht
Photometric Monitoring of the Gravitationally Lensed Ultraluminous BAL Quasar APM08279+5255
We report on one year of photometric monitoring of the ultraluminous BAL
quasar APM 08279+5255. The temporal sampling reveals that this gravitationally
lensed system has brightened by ~0.2 mag in 100 days. Two potential causes
present themselves; either the variability is intrinsic to the quasar, or it is
the result of microlensing by stars in a foreground system. The data is
consistent with both hypotheses and further monitoring is required before
either case can be conclusively confirmed. We demonstrate, however, that
gravitational microlensing can not play a dominant role in explaining the
phenomenal properties exhibited by APM 08279+5255. The identification of
intrinsic variability, coupled with the simple gravitational lensing
configuration, would suggest that APM 08279+5255 is a potential golden lens
from which the cosmological parameters can be derived and is worthy of a
monitoring program at high spatial resolution.Comment: 17 pages, with 2 figures. Accepted for publication in P.A.S.
HST Astrometry of M4 and the Galactic Constant V_0/R_0
From multi-epoch WFPC2/HST observations we present astrometric measurements
of stars in the Galactic globular cluster M4 (NGC 6121) and in the
foreground/background. The presence of an extragalactic point source allows us
to determine the absolute proper motion of the cluster, and, through use of the
field stars in this region only 18 degree from the Galactic center, to measure
the difference between the Oort constants, A-B. We find: (mu_alpha cos dec,
mu_dec)_J2000 = (-13.21 +/- 0.35, -19.28 +/- 0.35) mas/yr, and A-B = V_0/R_0 =
27.6 +/- 1.7 km / s / kpc.Comment: 20 pages, 6 figures, A.J.__ACCEPTED__, 1 April, 2003, (...!
The Spectral Slope and Escape Fraction of Bright Quasars at : the Contribution to the Cosmic UV Background
We use a sample of 1669 QSOs (, ) from the BOSS survey to
study the intrinsic shape of their continuum and the Lyman continuum photon
escape fraction (f), estimated as the ratio between the observed flux
and the expected intrinsic flux (corrected for the intergalactic medium
absorption) in the wavelength range 865-885 \AA\ rest-frame. Modelling the
intrinsic QSO continuum shape with a power-law,
, we find a median (with a
dispersion of , no dependence on the redshift and a mild intrinsic
luminosity dependence) and a mean f (independent of the QSO
luminosity and/or redshift). The f distribution shows a peak around
zero and a long tail of higher values, with a resulting dispersion of . If
we assume for the QSO continuum a double power-law shape (also compatible with
the data) with a break located at \AA\ and a softening
at wavelengths shorter than , the mean
f rises to . Combining our and f estimates with
the observed evolution of the AGN luminosity function (LF) we compute the AGN
contribution to the UV ionizing background (UVB) as a function of redshift. AGN
brighter than one tenth of the characteristic luminosity of the LF are able to
produce most of it up , if the present sample is representative of
their properties. At higher redshifts a contribution of the galaxy population
is required. Assuming an escape fraction of Lyman continuum photons from
galaxies between and , independent of the galaxy luminosity and/or
redshift, a remarkably good fit to the observational UVB data up to
is obtained. At lower redshift the extrapolation of our empirical estimate
agrees well with recent UVB observations, dispelling the so-called Photon
Underproduction Crisis.Comment: 8 pages, 9 figures, MNRAS accepte
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