3,491 research outputs found
A Robust Determination of the size of quasar accretion disks using gravitational microlensing
Using microlensing measurements from a sample of 27 image-pairs of 19 lensed
quasars we determine a maximum likelihood estimate for the accretion disk size
of an {{\em}average} quasar of light days at rest
frame \AA\ for microlenses with a mean mass of
. This value, in good agreement with previous results from
smaller samples, is roughly a factor of 5 greater than the predictions of the
standard thin disk model. The individual size estimates for the 19 quasars in
our sample are also in excellent agreement with the results of the joint
maximum likelihood analysis.Comment: 6 pages, 3 figures, submitted to Ap
A GPU-Enabled, High-Resolution Cosmological Microlensing Parameter Survey
In the era of synoptic surveys, the number of known gravitationally lensed
quasars is set to increase by over an order of magnitude. These new discoveries
will enable a move from single-quasar studies to investigations of statistical
samples, presenting new opportunities to test theoretical models for the
structure of quasar accretion discs and broad emission line regions (BELRs). As
one crucial step in preparing for this influx of new lensed systems, a
large-scale exploration of microlensing convergence-shear parameter space is
warranted, requiring the computation of O(10^5) high resolution magnification
maps. Based on properties of known lensed quasars, and expectations from
accretion disc/BELR modelling, we identify regions of convergence-shear
parameter space, map sizes, smooth matter fractions, and pixel resolutions that
should be covered. We describe how the computationally time-consuming task of
producing ~290000 magnification maps with sufficient resolution (10000^2
pixel/map) to probe scales from the inner edge of the accretion disc to the
BELR can be achieved in ~400 days on a 100 teraflop/s high performance
computing facility, where the processing performance is achieved with graphics
processing units. We illustrate a use-case for the parameter survey by
investigating the effects of varying the lens macro-model on accretion disc
constraints in the lensed quasar Q2237+0305. We find that although all
constraints are consistent within their current error bars, models with more
densely packed microlenses tend to predict shallower accretion disc radial
temperature profiles. With a large parameter survey such as the one described
here, such systematics on microlensing measurements could be fully explored.Comment: 30 pages, 3 figures, accepted for publication in Ap
Detection of chromatic microlensing in Q 2237+0305 A
We present narrowband images of the gravitational lens system Q~2237+0305
made with the Nordic Optical Telescope in eight different filters covering the
wavelength interval 3510-8130 \AA. Using point-spread function photometry
fitting we have derived the difference in magnitude versus wavelength between
the four images of Q~2237+0305. At \AA, the wavelength range
covered by the Str\"omgren-v filter coincides with the position and width of
the CIV emission line. This allows us to determine the existence of
microlensing in the continuum and not in the emission lines for two images of
the quasar. Moreover, the brightness of image A shows a significant variation
with wavelength which can only be explained as consequence of chromatic
microlensing. To perform a complete analysis of this chromatic event our
observations were used together with Optical Gravitational Lensing Experiment
light curves. Both data sets cannot be reproduced by the simple phenomenology
described under the caustic crossing approximation; using more realistic
representations of microlensing at high optical depth, we found solutions
consistent with simple thin disk models ();
however, other accretion disk size-wavelength relationships also lead to good
solutions. New chromatic events from the ongoing narrow band photometric
monitoring of Q~2237+0305 are needed to accurately constrain the physical
properties of the accretion disk for this system.Comment: 9 pages, 9 figures, 2 tables. Matches ApJ published version. Some
references adde
Measuring Microlensing using Spectra of Multiply Lensed Quasars
We report on a program of spectroscopic observations of
gravitationally-lensed QSOs with multiple images. We seek to establish whether
microlensing is occurring in each QSO image using only single-epoch
observations. We calculate flux ratios for the cores of emission lines in image
pairs to set a baseline for no microlensing. The offset of the continuum flux
ratios relative to this baseline yields the microlensing magnification free
from extinction, as extinction affects the continuum and the lines equally.
When we find chromatic microlensing, we attempt to constrain the size of the
QSO accretion disk. SDSSJ1004+4112 and HE1104-1805 show chromatic microlensing
with amplitudes and mag,
respectively. Modeling the accretion disk with a Gaussian source () of size and using magnification maps
to simulate microlensing we find ) and for SDSS1004+4112, and
) and
for HE1104-1805. For SDSSJ1029+2623 we find strong chromaticity
of mag in the continuum flux ratio, which probably arises from
microlensing although not all the available data fit within this explanation.
For Q0957+561 we measure B-A magnitude differences of 0.4 mag, much greater
than the 0.05 mag amplitude usually inferred from lightcurve variability.
It may substantially modify the current interpretations of microlensing in this
system, likely favoring the hypothesis of smaller sources and/or larger
microdeflectors. For HS0818+1227, our data yield posible evidence of
microlensing.Comment: 45pp, 17figs, ApJ accepted (june 4th 2012
Evaluating the impact of public subsidies on a firm's performance: A quasi-experimental approach
Many regional governments in developed countries design programs to improve the competitiveness of local firms. In this paper, we evaluate the effectiveness of public programs whose aim is to enhance the performance of firms located in Catalonia (Spain). We compare the performance of publicly subsidised companies (treated) with that of similar, but unsubsidised companies (non-treated). We use the Propensity Score Matching (PSM) methodology to construct a control group which, with respect to its observable characteristics, is as similar as possible to the treated group, and that allows us to identify firms which retain the same propensity to receive public subsidies. Once a valid comparison group has been established, we compare the respective performance of each firm. As a result, we find that recipient firms, on average, change their business practices, improve their performance, and increase their value added as a direct result of public subsidy programs.Public policy, evaluation studies, firm performance, propensity Score Matching.
Are scholarships effective in Spain? An impact assessment of the upper secondary non-tertiary level
España tiene un gran reto a futuro en la gran cantidad de individuos que no finalizan sus estudios más allá de la escolarización obligatoria y que se traduce en una alta tasa de abandono escolar prematuro y una baja tasa de graduación en secundaria post-obligatoria. En este marco, el objetivo de este trabajo es identificar si la política de becas, llevada a cabo durante el período 2004-2005 en España, ha sido eficaz en su objetivo de incrementar el porcentaje de alumnos que finalizan con éxito el nivel secundario post-obligatorio. Para ello se aplicará una evaluación de impacto (ex-post) con información individual para conocer si hubo cambios en las tasas de finalización del nivel educativo estudiado y si los mismos son atribuibles a la intervención evaluada. La fuente de información es la Encuesta de Condiciones de Vida (ECV-2006). La interpretación de los resultados indica que las becas incrementan en más de un 40% las posibilidades de finalizar con éxito el nivel secundario post-obligatorio, para el caso de individuos con similares características. Este resultado es relevante teniendo en cuenta que el valor medio recibido por cada beneficiario de una beca o ayuda al estudio, durante este período fue de, aproximadamente, 1.000€ por año. En este sentido, si bien está claro que existen muchos elementos que acaban determinando el nivel educativo alcanzado por el individuo, se comprueba que el Estado mediante sus políticas públicas puede incorporar un elemento favorecedor del éxito escolar en un contexto de alto abandono escolar prematuro.The large number of individuals who do not complete their studies beyond compulsory schooling is an important challenge in Spain. This situation can be seen in high rates of early school leaving and low graduation rates in upper secondary education. In this context, the aim of this work is to identify whether the scholarship policy carried out in Spain during the 2004-2005 period has been effective in its goal of increasing the percentage of students who successfully complete the upper secondary level. A quasi-experimental design has been performed applying the Propensity Score Matching technique, the control group consisting of a representative sample of the target population who did not participate in the program. The information source was the Survey of Living Conditions (EUSILC-2006). Results indicate that scholarships increase in more than 40% the chances of completing post compulsory secondary level, in the case of individuals with similar characteristics. This result is relevant, taking into account that the average amount received by each recipient during this period was approximately € 1,000 by year. In this sense, although there are many elements determining the educational attainment of individuals, it is found that the State, through its public policies, can incorporate an element which enables of school success in a context of high early school leaving rates
Asymmetrical structure of ionization and kinematics in the Seyfert galaxy NGC 5033
We present integral field spectroscopy of NGC 5033, a low luminosity Seyfert
galaxy. The observations were made with INTEGRAL, a fiber based system
operating at the WHT. The intensity map of the H emission line
represents a spiral or ring-like pattern of HII regions. On the contrary, the
[OIII] intensity map morphology is markedly anisotropic. The strong
morphological differences imply that the [OIII] emitters represent highly
ionized gas illuminated by the central source. The [OIII] map morphology is
compatible with a biconical structure of ionization induced by strong
extinction in the galaxy disc that also obscures half of the spheroidal stellar
bulge. We identify the spectrum corresponding to the Seyfert 1 nucleus from the
presence of H broad emission lines. This spectrum is located in a region
where strong extinction is expected but exhibits the bluest spectral energy
distribution. The Seyfert 1 nucleus seems to be offcenter with respect to the
stellar rotation center. This result has been also found in other Seyfert
galaxies and interpreted in terms of a past merger. The offcentering could
indicate the presence of nonsymmetric departures in the gravitational potential
which could be fueling the active nucleus. The kinematics of the [OIII]
emitters show important deviations at a kpc scale with respect to the stellar
velocity field and show features related to the asymmetrical morphology of the
high ionization region.Comment: 9 pages, accepted for publication in Astronomy and Astrophysics.
Figures 1 and 7 are attached as .gif file
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