1,917 research outputs found
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&
Century scale persistence in longitude distribution: in the Sun and in silico
Using Greenwich sunspot data for 120 years it was recently observed that
activity regions on the Sun's surface tend to lie along smoothly changing
longitude strips 180 degrees apart from each other. However, numerical
experiments with random input data show that most, if not all, of the observed
longitude discrimination can be looked upon as an artifact of the analysis
method.Comment: 4 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
The Rewards of Patience: An 822 Day Time Delay in the Gravitational Lens SDSS J1004+4112
We present 107 new epochs of optical monitoring data for the four brightest
images of the gravitational lens SDSS J1004+4112 observed between October 2006
and June 2007. Combining this data with the previously obtained light curves,
we determine the time delays between images A, B and C. We confirm our previous
measurement finding that A leads B by dt_BA=40.6+-1.8 days, and find that image
C leads image A by dt_CA=821.6+-2.1 days. The lower limit on the remaining
delay is that image D lags image A by dt_AD>1250 days. Based on the
microlensing of images A and B we estimate that the accretion disk size at a
rest wavelength of 2300 angstrom is 10^{14.8+-0.3} cm for a disk inclination of
cos{i}=1/2, which is consistent with the microlensing disk size-black hole mass
correlation function given our estimate of the black hole mass from the MgII
line width of logM_BH/M_sun=8.44+-0.14. The long delays allow us to fill in the
seasonal gaps and assemble a continuous, densely sampled light curve spanning
5.7 years whose variability implies a structure function with a logarithmic
slope of gamma = 0.35+-0.02. As C is the leading image, sharp features in the C
light curve can be intensively studied 2.3 years later in the A/B pair,
potentially allowing detailed reverberation mapping studies of a quasar at
minimal cost.Comment: Submitted to ApJ, 12 pages, 3 figure
Hourly Variability in Q0957+561
We have continued our effort to re-reduce archival Q0957+561 brightness
monitoring data and present results for 1629 R-band images using the methods
for galaxy subtraction and seeing correction reported previously. The new
dataset comes from 4 observing runs, several nights apiece, with sampling of
typically 5 minutes, which allows the first measurement of the structure
function for variations in the R-band from timescales of hours to years.
Comparison of our reductions to previous reductions of the same data, and to
r-band photometry produced at Apache Point Observatory shows good overall
agreement. Two of the data runs, separated by 417 days, permit a sharpened
value for the time delay of 417.4 days, valid only if the time delay is close
to the now-fashionable 417-day value; our data do not constrain a delay if it
is more than three days from this 417-day estimate. Our present results show no
unambiguous signature of the daily microlensing, though a suggestive feature is
found in the data. Both time delay measurement and microlensing searches suffer
from from the lack of sampling at half-day offsets, inevitable at a single
observatory, hence the need for round-the-clock monitoring with participation
by multiple observatories.Comment: AASTeX 4.0 preprint style, 21 pages, 8 EPS figure
Kinematic frames and "active longitudes": does the Sun have a face?
It has recently been claimed that analysis of Greenwich sunspot data over 120
years reveals that sunspot activity clusters around two longitudes separated by
180 degrees (``active longitudes'') with clearly defined differential rotation
during activity cycles.In the present work we extend this critical examination
of methodology to the actual Greenwich sunspot data and also consider newly
proposed methods of analysis claiming to confirm the original identification of
active longitudes. Our analysis revealed that values obtained for the
parameters of differential rotation are not stable across different methods of
analysis proposed to track persistent active longitudes. Also, despite a very
thorough search in parameter space, we were unable to reproduce results
claiming to reveal the century-persistent active longitudes. We can therefore
say that strong and well substantiated evidence for an essential and
century-scale persistent nonaxisymmetry in the sunspot distribution does not
exist.Comment: 14 pages, 1 table, 21 figures, accepted in A&
Precision Photometry for Q0957+561 Images A and B
Since the persuasive determination of the time-delay in Q0957+561, much
interest has centered around shifting and subtracting the A and B light-curves
to look for residuals due to microlensing. Solar mass objects in the lens
galaxy produce variations on timescales of decades, with amplitudes of a few
tenths of a magnitude, but MACHO's (with masses of order to
) produce variations at only the 5% level. To detect such small
variations, highly precise photometry is required.
To that end, we have used 200 observations over three nights to examine the
effects of seeing on the light-curves. We have determined that seeing itself
can be responsible for correlated 5% variations in the light-curves of A and B.
We have found, however, that these effects can be accurately removed, by
subtracting the light from the lens galaxy, and by correcting for cross
contamination of light between the closely juxtaposed A and B images. We find
that these corrections improve the variations due to seeing from 5% to a level
only marginally detectable over photon shot noise (0.5%).Comment: 21 Pages with 9 PostScript figures, AASTeX 4 (preprint style
Galaxy filaments as pearl necklaces
Context. Galaxies in the Universe form chains (filaments) that connect groups
and clusters of galaxies. The filamentary network includes nearly half of the
galaxies and is visually the most striking feature in cosmological maps.
Aims. We study the distribution of galaxies along the filamentary network,
trying to find specific patterns and regularities.
Methods. Galaxy filaments are defined by the Bisous model, a marked point
process with interactions. We use the two-point correlation function and the
Rayleigh Z-squared statistic to study how galaxies and galaxy groups are
distributed along the filaments.
Results. We show that galaxies and groups are not uniformly distributed along
filaments, but tend to form a regular pattern. The characteristic length of the
pattern is around 7 Mpc/h. A slightly smaller characteristic length 4 Mpc/h can
also be found, using the Z-squared statistic.
Conclusions. We find that galaxy filaments in the Universe are like pearl
necklaces, where the pearls are galaxy groups distributed more or less
regularly along the filaments. We propose that this well defined characteristic
scale could be used to test various cosmological models and to probe
environmental effects on the formation and evolution of galaxies.Comment: 8 pages, 9 figures, 1 table, accepted for publication in A&
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
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