50 research outputs found
Q2237+0305 source structure and dimensions from light curves simulation
Assuming a two-component quasar structure model consisting of a central
compact source and an extended outer feature, we produce microlensing
simulations for a population of star-like objects in the lens galaxy. Such a
model is a simplified version of that adopted to explain the brightness
variations observed in Q0957 (Schild & Vakulik 2003). The microlensing light
curves generated for a range of source parameters were compared to the light
curves obtained in the framework of the OGLE program. With a large number of
trials we built, in the domain of the source structure parameters, probability
distributions to find "good" realizations of light curves. The values of the
source parameters which provide the maximum of the joint probability
distribution calculated for all the image components, have been accepted as
estimates for the source structure parameters. The results favour the
two-component model of the quasar brightness structure over a single compact
central source model, and in general the simulations confirm the Schild-Vakulik
model that previously described successfully the microlensing and other
properties of Q0957. Adopting 3300 km/s for the transverse velocity of the
source, the effective size of the central source was determined to be about
2x10^15 cm, and Epsilon =2 was obtained for the ratio of the integral
luminosity of the outer feature to that of the central source.Comment: 7 pages, 4 figures, LaTe
Gravitational potential of a homogeneous circular torus: new approach
The integral expression for gravitational potential of a homogeneous circular
torus composed of infinitely thin rings is obtained. Approximate expressions
for torus potential in the outer and inner regions are found. In the outer
region a torus potential is shown to be approximately equal to that of an
infinitely thin ring of the same mass; it is valid up to the surface of the
torus. It is shown in a first approximation, that the inner potential of the
torus (inside a torus body) is a quadratic function of coordinates. The method
of sewing together the inner and outer potentials is proposed. This method
provided a continuous approximate solution for the potential and its
derivatives, working throughout the region.Comment: 10 pages, 9 figures, 1 table; some misprints in formulae were
correcte
Time delays in PG1115+080: new estimates
We report new estimates of the time delays in the quadruple gravitationally
lensed quasar PG1115+080, obtained from the monitoring data in filter R with
the 1.5-m telescope at the Maidanak Mountain (Uzbekistan, Central Asia) in
2004-2006. The time delays are 16.4 days between images C and B, and 12 days
between C and A1+A2, with image C being leading for both pairs. The only known
estimates of the time delays in PG1115 are those based on observations by
Schechter et al. (1997) -- 23.7 and 9.4 days between images C and B, C and
A1+A2, respectively, as calculated by Schechter et al., and 25 and 13.3 days as
revised by Barkana (1997) for the same image components with the use of another
method. The new values of time delays in PG 1115+080 may be expected to provide
larger estimates of the Hubble constant thus decreasing a diversity between the
H_0 estimates taken from gravitationally lensed quasars and with other methods.Comment: 5 pages, 2 figures, Accepted for publication in MNRAS Letter
Asymptotic Expansions and Amplification of a Gravitational Lens Near a Fold Caustic
We propose two methods that enable us to obtain approximate solutions of the
lens equation near a fold caustic with an arbitrary degree of accuracy. We
obtain "post-linear" corrections to the well-known formula in the linear
caustic approximation for the total amplification of two critical images of a
point source. In this case, in order to obtain the nontrivial corrections we
had to go beyond the approximation orders earlier used by Keeton et al. and to
take into account the Taylor expansion of the lens equation near caustic up to
the fourth order. Corresponding analytical expressions are derived for the
amplification in cases of the Gaussian and power-law extended source models;
the amplifications depend on three additional fitting parameters. Conditions of
neglecting the correction terms are analysed. The modified formula for the
amplification is applied to the fitting of light curves of the Q2237+0305
gravitational lens system in a vicinity of the high amplification events
(HAEs). We show that the introduction of some "post-linear" corrections reduces
chi^2 by 30% in the case of known HAE on the light curve of image C (1999).
These corrections can be important for a precise comparison of different source
models with regard for observational data. Key words: gravitational lensing:
micro - quasars: individual (Q2237+0305) - gravitational lensing: strong -
methods: analyticalComment: 16 pages, 3 figure
Q2237+0305 in X-rays: spectra and variability with XMM-Newton
X-ray observations of gravitationally lensed quasars may allow us to probe
the inner structure of the central engine of a quasar. Observations of
Q2237+0305 (Einstein Cross) in X-rays may be used to constrain the inner
structure of the X-ray emitting source. Here we analyze the XMM-Newton
observation of the quasar in the gravitational lens system Q2237+0305 taken
during 2002. Combined spectra of the four images of the quasar in this system
were extracted and modelled with a power-law model. Statistical analysis was
used to test the variability of the total flux. The total X-ray flux from all
the images of this quadruple gravitational lens system is 6 x 10^{-13}
erg/cm2/s in the range 0.2-10 keV, showing no significant X-ray spectral
variability during almost 42 ks of the observation time. Fitting of the cleaned
source spectrum yields a photon power-law index of Gamma=1.82+0.07/-0.08. The
X-ray lightcurves obtained after background subtraction are compatible with the
hypothesis of a stationary flux from the source.Comment: 6 pages, 7 figures, 4 tables, accepted by A&
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 multiple quasar Q2237+0305 under a microlensing caustic
We use the high magnification event seen in the 1999 OGLE campaign light
curve of image C of the quadruply imaged gravitational lens Q2237+0305 to study
the structure of the quasar engine. We have obtained g'- and r'-band photometry
at the Apache Point Observatory 3.5m telescope where we find that the event has
a smaller amplitude in the r'-band than in the g'- and OGLE V-bands. By
comparing the light curves with microlensing simulations we obtain constraints
on the sizes of the quasar regions contributing to the g'- and r'-band flux.
Assuming that most of the surface mass density in the central kiloparsec of the
lensing galaxy is due to stars and by modeling the source with a Gaussian
profile, we obtain for the Gaussian width 1.20 x 10^15 sqrt(M/0.1M_sun)cm <
sigma_g' < 7.96 x 10^15 sqrt(M/0.1Msun) cm, where M is the mean microlensing
mass, and a ratio sigma_r'/sigma_g'=1.25^{+0.45}_{-0.15}. With the limits on
the velocity of the lensing galaxy from Gil-Merino et al. (2005) as our only
prior, we obtain 0.60 x 10^15 sqrt(M/0.1Msun) cm < sigma_g' < 1.57 x 10^15
sqrt(M/0.1Msun) cm and a ratio sigma_r'/sigma_g'=1.45^{+0.90}_{-0.25} (all
values at 68 percent confidence). Additionally, from our microlensing
simulations we find that, during the chromatic microlensing event observed, the
continuum emitting region of the quasar crossed a caustic at >72 percent
confidence.Comment: Accepted for publication in A&A, 8 pages, 4 figures. Slightly
modified compared to the original version: qualitative results unchanged,
constraints on the r'/g' source size ratio now tighter due to correction of
an error in the numerical treatment of the simulated light curve