452 research outputs found
The Double Quasar HE1104-1805: a case study for time delay determination with poorly sampled lightcurves
We present a new determination of the time delay of the gravitational lens
system HE1104-1805 ('Double Hamburger') based on a previously unpublished
dataset. We argue that the previously published value of dt_(A-B)=0.73 years
was affected by a bias of the employed method. We determine a new value of
dt_(A-B)=0.85+/-0.05 years (2 sigma confidence level), using six different
techniques based on non interpolation methods in the time domain. The result
demonstrates that even in the case of poorly sampled lightcurves, useful
information can be obtained with regard to the time delay. The error estimates
were calculated through Monte Carlo simulations. With two already existing
models for the lens and using its recently determined redshift, we infer a
range of values of the Hubble parameter: Ho=48+/-4 km/s Mpc^-1 (2 sigma) for a
singular isothermal ellipsoid (SIE) and Ho=62+/-4 km/s Mpc^-1 (2 sigma) for a
constant mass-to-light ratio plus shear model (M/L+gamma). The possibly much
larger errors due to systematic uncertainties in modeling the lens potential
are not included in this error estimate.Comment: 11 pages, 15 figures, accepted by Astronomy and Astrophysic
A CO Survey of Gravitationally Lensed Quasars with the IRAM Interferometer
We present the results of a CO survey of gravitationally lensed quasars,
conducted with the Plateau de Bure Interferometer over the last three years.
Among the 18 objects surveyed, one was detected in CO line emission, while six
were detected in the continuum at 3mm and three in the continuum at 1mm. The
low CO detection rate may at least in part be due to uncertainties in the
redshifts derived from quasar broad emission lines. The detected CO source, the
z=3.2 radio quiet quasar MG0751+2716, is quite strong in the CO(4-3) line and
in the millimeter/submillimeter continuum, the latter being emission from cool
dust. The integrated CO line flux is 5.96 +- 0.45 Jy.km/s, and the total
molecular gas mass is estimated to be in the range M(H_2) = 1.6-3.1 X 10^9
solar masses.Comment: 5 pages, 2 figures, uses aa.cls and psfig.st
Molecular gas at intermediate redshifts
We present Giant Metrewave Radio Telescope (GMRT) observations of OH
absorption in B3~1504+377 () and PKS 1413+135 ().
OH has now been detected in absorption towards four intermediate redshift
systems, viz. the lensing galaxies towards B~0218+357 (; Kanekar
et al. 2001) and 1830-211 (; Chengalur et al. 1999), in addition
to the two systems listed above. All four systems also give rise to well
studied millimetre wavelength molecular line absorption from a host of
molecules, including HCO. Comparing our OH data with these millimetre line
transitions, we find that the linear correlation between and
found in molecular clouds in the Milky Way (Liszt & Lucas 1996)
persists out to . It has been suggested (Liszt & Lucas 1999) that OH
is a good tracer of , with
under a variety of physical conditions. We use this relationship to estimate
in these absorbers. The estimated is \ga 10^{22}
in all four cases and substantially different from estimates based on CO
observations.Comment: 4 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysics Letter
Confirmation of two extended objects along the line of sight to PKS1830-211 with ESO-VLT adaptive optics imaging
We report on new high-resolution near-infrared images of the gravitationally
lensed radio source PKS1830-211, a quasar at z=2.507. These adaptive optics
observations, taken with the Very Large Telescope (VLT), are further improved
through image deconvolution. They confirm the presence of a second object along
the line of sight to the quasar, in addition to the previously known spiral
galaxy. This additional object is clearly extended in our images. However, its
faint luminosity does not allow to infer any photometric redshift. If this
galaxy is located in the foreground of PKS1830-211, it complicates the modeling
of this system and decreases the interest in using PKS1830-211 as a means to
determine H0 via the time delay between the two lensed images of the quasar.Comment: Accepted in A&A Letter
High-order combination effects and biological robustness
Biological systems are robust, in that they can maintain stable phenotypes under varying conditions or attacks. Biological systems are also complex, being organized into many functional modules that communicate through interlocking pathways and feedback mechanisms. In these systems, robustness and complexity are linked because both qualities arise from the same underlying mechanisms. When perturbed by multiple attacks, such complex systems become fragile in both theoretical and experimental studies, and this fragility depends on the number of agents applied. We explore how this relationship can be used to study the functional robustness of a biological system using systematic high-order combination experiments. This presents a promising approach toward many biomedical and bioengineering challenges. For example, high-order experiments could determine the point of fragility for pathogenic bacteria and might help identify optimal treatments against multi-drug resistance. Such studies would also reinforce the growing appreciation that biological systems are best manipulated not by targeting a single protein, but by modulating the set of many nodes that can selectively control a system's functional state
First robotic monitoring of a lensed quasar: intrinsic variability of SBS 0909+532
To go into the details about the variability of the double quasar SBS
0909+532, we designed a monitoring programme with the 2 m Liverpool Robotic
Telescope in the r Sloan filter, spanning 1.5 years from 2005 January to 2006
June. The r-band light curves of the A and B components, several
cross-correlation techniques and a large number of simulations (synthetic light
curves) lead to a robust delay of 49 +/- 6 days (1-sigma interval) that agrees
with our previous results (the B component is leading). Once the time delay and
the magnitude offset are known, the magnitude- and time-shifted light curve of
image A is subtracted from the light curve of image B. This difference light
curve of SBS 0909+532 is consistent with zero, so any possible extrinsic signal
must be very weak, i.e., the observed variability in A and B is basically due
to observational noise and intrinsic signal. We then make the combined light
curve and analyse its statistical properties (structure functions). The
structure function of the intrinsic luminosity is fitted to predictions of
simple models of two physical scenarios: accretion disc instabilities and
nuclear starbursts. Although no simple model is able to accurately reproduce
the observed trend, symmetric triangular flares in an accretion disc seems to
be the best option to account for it.Comment: 27 pages, 9 figures and 2 tables (including information on the
robotic monitoring and the final fluxes). Accepted for publication in New
Astronom
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