304 research outputs found
Time delays for 11 gravitationally lensed quasars revisited
We test the robustness of published time delays for 11 lensed quasars by
using two techniques to measure time shifts in their light curves.
We chose to use two fundamentally different techniques to determine time
delays in gravitationally lensed quasars: a method based on fitting a numerical
model and another one derived from the minimum dispersion method introduced by
Pelt and collaborators. To analyse our sample in a homogeneous way and avoid
bias caused by the choice of the method used, we apply both methods to 11
different lensed systems for which delays have been published: JVAS B0218+357,
SBS 0909+523, RX J0911+0551, FBQS J0951+2635, HE 1104-1805, PG 1115+080, JVAS
B1422+231, SBS 1520+530, CLASS B1600+434, CLASS B1608+656, and HE 2149-2745
Time delays for three double lenses, JVAS B0218+357, HE 1104-1805, and CLASS
B1600+434, as well as the quadruply lensed quasar CLASS B1608+656 are confirmed
within the error bars. We correct the delay for SBS 1520+530. For PG 1115+080
and RX J0911+0551, the existence of a second solution on top of the published
delay is revealed. The time delays in four systems, SBS 0909+523, FBQS
J0951+2635, JVAS B1422+231, and HE 2149-2745 prove to be less reliable than
previously claimed.
If we wish to derive an estimate of H_0 based on time delays in
gravitationally lensed quasars, we need to obtain more robust light curves for
most of these systems in order to achieve a higher accuracy and robustness on
the time delays
The extinction curve of the lensing galaxy of B1152+199 at z=0.44
We present UBVRIz' photometry of the gravitational lens candidate CLASS
B1152+119 obtained with the Nordic Optical Telescope. The two QSO components
are resolved in the B, V, R, I and z' bands confirming the lensing nature of
the system. The z=0.44 lens galaxy is clearly detected in B, R, I and z' and
its position is found to be almost coincident with the faint QSO image which is
heavily extincted (relative to the brighter QSO image) by dust in the lens
galaxy. The extinction curve of the lens galaxy derived from the relative
photometry is well fitted by a Galactic extinction law with 1.3 < R_V < 2.0 and
E(B-V) ~ 1. From a simple model of the system we predict a time delay of ~ 60
days.Comment: 6 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Photometric monitoring of the doubly imaged quasar UM673: possible evidence for chromatic microlensing
We present the results of two-band CCD photometric monitoring of the
gravitationally lensed quasar Q 0142-100 (UM 673).The data, obtained at ESO-La
Silla with the 1.54 m Danish telescope in the Gunn i-band (October 1998 -
September 1999) and in the Johnson V-band (October 1998 to December 2001), were
analyzed using three different photometric methods. The light-curves obtained
with all methods show variations, with a peak-to-peak amplitude of 0.14
magnitude in . Although it was not possible to measure the time delay
between the two lensed QSO images, the brighter component displays possible
evidence for microlensing: it becomes bluer as it gets brighter, as expected
under the assumption of differential magnification of a quasar accretion diskComment: Accepted for publication in Astronomy & Astrophysics; 8 pages, 7
figure
A simple analysis of halo density profiles using gravitational lensing time delays
Gravitational lensing time delays depend upon the Hubble constant and the
density distribution of the lensing galaxies. This allows one to either model
the lens and estimate the Hubble constant, or to use a prior on the Hubble
constant from other studies and investigate what the preferred density
distribution is. Some studies have required compact dark matter halos (constant
M/L ratio) in order to reconcile gravitational lenses with the HST/WMAP value
of the Hubble constant (72 +/- 8 km/s /Mpc and 72 +/- 5 km/s /Mpc,
respectively). This is in direct contradiction with X-ray, stellar dynamical,
and weak lensing studies, which all point towards extended halos and isothermal
density profiles. In this work, we examine an up-to-date sample of 13 lensing
galaxies resulting in a data set consisting of 21 time delays. We select
systems in which there is a single primary lensing galaxy (e.g. excluding
systems undergoing mergers). Analysis is performed using analytic models based
upon a powerlaw density profile (rho \propto r^-n) of which the isothermal
profile is a special case (n = 2). This yields a value of n = 2.11+/-0.12
(3sigma) for the mean profile when modeling with a prior on the Hubble
constant, which is only consistent with isothermality within 3 sigma. Note that
this is a formal error from our calculations, and does not include the impact
of sample selection or simplifications in the lens modeling. We conclude that
time delays are a useful probe of density profiles, in particular as a function
of the environment in which the lens resides, when combined with a prior on the
Hubble constant.Comment: A&A accepte
Microlensing variability in time-delay quasars
We have searched for microlensing variability in the light curves of five
gravitationally lensed quasars with well-determined time delays: SBS 1520+530,
FBQ 0951+2635, RX J0911+0551, B1600+434 and HE 2149-2745. By comparing the
light curve of the leading image with a suitably time offset light curve of a
trailing image we find that two (SBS 1520+530 and FBQ 0951+2635) out of the
five quasars have significant long-term (years) and short-term (100 days)
brightness variations that may be attributed to microlensing.The short-term
variations may be due to nanolenses, relativistic hot or cold spots in the
quasar accretion disks, or coherent microlensing at large optical depth.Comment: 12 pages, 5 figures, uses natbib.sty and aa.cl
Time delay of SBS 0909+532
The time delays between the components of a lensed quasar are basic tools to
analyze the expansion of the Universe and the structure of the main lens galaxy
halo. In this paper, we focus on the variability and time delay of the double
system SBS 0909+532A,B as well as the time behaviour of the field stars. We use
VR optical observations of SBS 0909+532A,B and the field stars in 2003. The
frames were taken at Calar Alto, Maidanak and Wise observatories, and the VR
light curves of the field stars and quasar components are derived from aperture
and point-spread function fitting methods. We measure the R-band time delay of
the system from the chi-square and dispersion techniques and 1000 synthetic
light curves based on the observed records. One nearby field star (SBS
0909+532c) is found to be variable, and the other two nearby field stars are
non-variable sources. With respect to the quasar components, the R-band records
seem more reliable and are more densely populated than the V-band ones. The
observed R-band fluctuations permit a pre-conditioned measurement of the time
delay. From the chi-square minimization, if we assume that the quasar emission
is observed first in B and afterwards in A (in agreement with basic
observations of the system and the corresponding predictions), we obtain a
delay of - 45 (+ 1)/(- 11) days (95% confidence interval). The dispersion
technique leads to a similar delay range. A by-product of the analysis is the
determination of a totally corrected flux ratio in the R band (corrected by the
time delay and the contamination due to the galaxy light). Our 95% measurement
of this ratio (0.575 +/- 0.014 mag) is in excellent agreement with previous
results from contaminated fluxes at the same time of observation.Comment: 26 pages, 15 figures, Astronomy and Astrophysics (see also
http://www.astro.ulg.ac.be/RPub/Colloques/JENAM/proceedings/proceedings.html
- Quasars Section
Time-delay quasars: scales and orders of magnitudes
We can think of a lensed quasar as taking the Hubble time, shrinking it by
\~10^{-11}, and then presenting the result to us as a time delay; the shrinking
factor is of the order of fractional sky-area that the lens occupies. This cute
fact is a straightforward consequence of lensing theory, and enables a simple
rescaling of time delays. Observed time delays have a 40-fold range, but after
rescaling the range reduces to 5-fold. The latter range depends on details of
the lens and lensing configuration--for example, quads have systematically
shorter rescaled time delays than doubles--and is as expected from a simple
model. The hypothesis that observed time-delay lenses all come from a
generalized-isothermal family can be ruled out. But there is no indication of
drastically different populations either.Comment: To appear in A&
Bias and consistency in time delay estimation methods: case of the double quasar HE 1104-1805
We present a short re-evaluation of a recently published time delay estimate
for the gravitational lens system HE 1104-1805 with emphasis on important
methodological aspects: bias of the statistics, inconsistency of the methods
and use of the purposeful selection of data points(or so-called "cleaning") at
the preprocessing stage. We show how the inadequate use of simple analysis
methods can lead to too strong conclusions. Our analysis shows that there are
indications for the time delay in HE 1104-1805 to be between -0.9 and -0.7
years, but still with a large uncertainty.Comment: 5 pages, 5 figures, accepted as a Letter to the Editor in A&
Impact of distractors in item analysis of multiple choice questions
Background: Item analysis is a quality assurance of examining the performance of the individual test items that measures the validity and reliability of exams. This study was performed to evaluate the quality of the test items with respect to their performance on difficulty index (DFI), Discriminatory index (DI) and assessment of functional and non-functional distractors (FD and NFD).Methods: This study was performed on the summative examination undertaken by 113 students. The analyses include 120 one best answers (OBAs) and 360 distractors.Results: Out of the 360 distractors, 85 distractors were chosen by less than 5% with the distractor efficiency of 23.6%. About 47 (13%) items had no NFDs while 51 (14%), 30 (8.3%), and 4 (1.1%) items contained 1, 2, and 3 NFDs respectively. Majority of the items showed excellent difficulty index (50.4%, n=42) and fair discrimination (37%, n=33). The questions with excellent difficulty index and discriminatory index showed statistical significance with 1NFD and 2 NFD (p=0.03).Conclusions: The post evaluation of item performance in any exam in one of the quality assurance method of identifying the best performing item for quality question bank. The distractor efficiency gives information on the overall quality of item
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