405 research outputs found
Estimating Time Delay in Gravitationally Lensed Fluxes
We study the problem of estimating the time delay between two signals representing delayed, irregularly sampled and noisy versions of the same underlying pattern. We propose a kernel-based technique in the context of an astronomical problem, namely estimating the time delay between
two gravitationally lensed signals from a distant quasar.
We test the algorithm on several artificial data sets, and
also on real astronomical observations. By carrying out a statistical analysis of the results we present a detailed comparison of our method with the most popular methods for time delay estimation in astrophysics. Our method yields more accurate and more stable time delay estimates. Our methodology can be readily applied to current state-of-the-art optical monitoring data in astronomy, but can also be applied in other disciplines involving similar time series data
Kernel regression estimates of time delays between gravitationally lensed fluxes
Strongly lensed variable quasars can serve as precise cosmological probes,
provided that time delays between the image fluxes can be accurately measured.
A number of methods have been proposed to address this problem. In this paper,
we explore in detail a new approach based on kernel regression estimates, which
is able to estimate a single time delay given several datasets for the same
quasar. We develop realistic artificial data sets in order to carry out
controlled experiments to test of performance of this new approach. We also
test our method on real data from strongly lensed quasar Q0957+561 and compare
our estimates against existing results.Comment: Updated to match published versio
X-ray Observations of Gravitationally Lensed Quasars; Evidence for a Hidden Quasar Population
The large magnification factors of gravitationally lensed (GL) systems allow
us to investigate the properties of quasars with X-ray luminosities that are
substantially lower than those of unlensed ones and also provide an independent
means of estimating the contribution of faint quasars to the hard X-ray
component of the cosmic X-ray background. Our spectral analysis indicate a
flattening of the spectral index in the hard band (4-20keV restframe) for 2
radio-loud quasars in the GL quasar sample for which the data have moderate
signal-to-noise ratio. We have identified a large fraction of Broad Absorption
Line (BAL) quasars amongst the GL quasar population. We find that approximately
35% of radio-quiet GL quasars contain BAL features which is significantly
larger than the 10% fraction of BAL quasars presently found in optically
selected flux limited quasar samples. We present a simple model that estimates
the effects of attenuation and lens magnification on the luminosity function of
quasars and that explains the observed fraction of GL BAL quasars. These
observations suggest that a large fraction of BAL quasars are missed from flux
limited optical surveys. Modeling of several X-ray observations of the GL BAL
quasar PG1115+080 suggests that the observed large X-ray variability may be
caused in part by a variable intrinsic absorber consistent with previously
observed variability of the BAL troughs in the UV band. The observed large
X-ray flux variations in PG1115+080 offer the prospect of considerably reducing
errors in determining the time delay with future X-ray monitoring of this
system and hence constraining the Hubble constant H.Comment: 19 pages, 10 figures, 9 Tables, accepted for publication in Ap
Gravitational Lenses as High-Resolution Telescopes
The inner regions of active galaxies host the most extreme and energetic
phenomena in the universe including, relativistic jets, supermassive black hole
binaries, and recoiling supermassive black holes. However, many of these
sources cannot be resolved with direct observations. I review how strong
gravitational lensing can be used to elucidate the structures of these sources
from radio frequencies up to very high energy gamma rays. The deep
gravitational potentials surrounding galaxies act as natural gravitational
lenses. These gravitational lenses split background sources into multiple
images, each with a gravitationally-induced time delay. These time delays and
positions of lensed images depend on the source location, and thus, can be used
to infer the spatial origins of the emission. For example, using
gravitationally-induced time delays improves angular resolution of modern
gamma-ray instruments by six orders of magnitude, and provides evidence that
gamma-ray outbursts can be produced at even thousands of light years from a
supermassive black hole, and that the compact radio emission does not always
trace the position of the supermassive black hole. These findings provide
unique physical information about the central structure of active galaxies,
force us to revise our models of operating particle acceleration mechanisms,
and challenge our assumptions about the origin of compact radio emission.
Future surveys, including LSST, SKA, and Euclid, will provide observations for
hundreds of thousands of gravitationally lensed sources, which will allow us to
apply strong gravitational lensing to study the multi-wavelength structure for
large ensembles of sources. This large ensemble of gravitationally lensed
active galaxies will allow us to elucidate the physical origins of
multi-wavelength emissions, their connections to supermassive black holes, and
their cosmic evolution.Comment: Invited (Accepted) review for Physics Report
PG 1115+080: variations of the A2/A1 flux ratio and new values of the time delays
We report the results of our multicolor observations of PG 1115+080 with the
1.5-m telescope of the Maidanak Observatory (Uzbekistan, Central Asia) in
2001-2006. Monitoring data in filter R spanning the 2004, 2005 and 2006 seasons
(76 data points) demonstrate distinct brightness variations of the source
quasar with the total amplitude of almost 0.4 mag. Our R light curves have
shown image C leading B by 16.4d and image (A1+A2) by 12d that is inconsistent
with the previous estimates obtained by Schechter et al. in 1997 - 24.7d
between B and C and 9.4d between (A1+A2) and C. The new values of time delays
in PG 1115+080 must result in larger values for the Hubble constant, thus
reducing difference between its estimates taken from the gravitational lenses
and with other methods. Also, we analyzed variability of the A2/A1 flux ratio,
as well as color changes in the archetypal "fold" lens PG 1115+080. We found
the A1/A2 flux ratio to grow during 2001-2006 and to be larger at longer
wavelengths. In particular, the A2/A1 flux ratio reached 0.85 in filter I in
2006. We also present evidence that both the A1 and A2 images might have
undergone microlensing during 2001-2006, with the descending phase for A1 and
initial phase for A2. We find that the A2/A1 flux ratio anomaly in PG 1115 can
be well explained both by microlensing and by finite distance of the source
quasar from the caustic fold.Comment: 14 pages, 7 figures, 8 tables, Accepted for publication in MNRA
The quasar-galaxy cross SDSS J1320+1644: A probable large-separation lensed quasar
We report the discovery of a pair of quasars at , with a separation
of 8\farcs585\pm0\farcs002. Subaru Telescope infrared imaging reveals the
presence of an elliptical and a disk-like galaxy located almost symmetrically
between the quasars, creating a cross-like configuration. Based on absorption
lines in the quasar spectra and the colors of the galaxies, we estimate that
both galaxies are located at redshift . This, as well as the
similarity of the quasar spectra, suggests that the system is a single quasar
multiply imaged by a galaxy group or cluster acting as a gravitational lens,
although the possibility of a binary quasar cannot be fully excluded. We show
that the gravitational lensing hypothesis implies these galaxies are not
isolated, but must be embedded in a dark matter halo of virial mass assuming an NFW model with a
concentration parameter of , or a singular isothermal sphere profile
with a velocity dispersion of km s. We place constraints on
the location of the dark matter halo, as well as the velocity dispersions of
the galaxies. In addition, we discuss the influence of differential reddening,
microlensing and intrinsic variability on the quasar spectra and broadband
photometry.Comment: Published in The Astrophysical Journa
COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses XIII: Time delays and 9-yr optical monitoring of the lensed quasar RX J1131-1231
We present the results from nine years of optically monitoring the
gravitationally lensed z=0.658 quasar RX J1131-1231. The R-band light curves of
the four individual images of the quasar were obtained using deconvolution
photometry for a total of 707 epochs. Several sharp quasar variability features
strongly constrain the time delays between the quasar images. Using three
different numerical techniques, we measure these delays for all possible pairs
of quasar images while always processing the four light curves simultaneously.
For all three methods, the delays between the three close images A, B, and C
are compatible with being 0, while we measure the delay of image D to be 91
days, with a fractional uncertainty of 1.5% (1 sigma), including systematic
errors. Our analysis of random and systematic errors accounts in a realistic
way for the observed quasar variability, fluctuating microlensing magnification
over a broad range of temporal scales, noise properties, and seasonal gaps.
Finally, we find that our time-delay measurement methods yield compatible
results when applied to subsets of the data.Comment: 11 pages, 9 figures, minor additions to the text only, techniques and
results remain unchanged, A&A in pres
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