139 research outputs found
MDia and POTS - The Munich Difference Imaging Analysis for the pre-OmegaTranS Project
We describe the Munich Difference Imaging Analysis pipeline that we developed
and implemented in the framework of the Astro-WISE package to automatically
measure high precision light curves of a large number of stellar objects using
the difference imaging approach. Combined with programs to detect time
variability, this software can be used to search for planetary systems or
binary stars with the transit method and for variable stars of different kinds.
As a first scientific application, we discuss the data reduction and analysis
performed with Astro-WISE on the pre-OmegaTranS data set, that we collected
during a monitoring campaign of a dense stellar field with the Wide Field
Imager at the ESO 2.2m telescope.Comment: 10 pages, 5 figures, accepted for publication in topical issue of
Experimental Astronomy on Astro-WISE information syste
Microlensing events from the 11-year observations of the Wendelstein Calar Alto Pixellensing Project
We present the results of the decade-long M31 observation from the
Wendelstein Calar Alto Pixellensing Project (WeCAPP). WeCAPP has monitored M31
from 1997 till 2008 in both R- and I-filters, thus provides the longest
baseline of all M31 microlensing surveys. The data are analyzed with the
difference imaging analysis, which is most suitable to study variability in
crowded stellar fields. We extracted light curves based on each pixel, and
devised selection criteria that are optimized to identify microlensing events.
This leads to 10 new events, and sums up to a total of 12 microlensing events
from WeCAPP, for which we derive their timescales, flux excesses, and colors
from their light curves. The color of the lensed stars fall between (R-I) =
0.56 to 1.36, with a median of 1.0 mag, in agreement with our expectation that
the sources are most likely bright, red stars at post main-sequence stage. The
event FWHM timescales range from 0.5 to 14 days, with a median of 3 days, in
good agreement with predictions based on the model of Riffeser et al. (2006).Comment: 44 pages, 16 figures, 5 tables. ApJ accepte
Bias-Free Shear Estimation using Artificial Neural Networks
Bias due to imperfect shear calibration is the biggest obstacle when
constraints on cosmological parameters are to be extracted from large area weak
lensing surveys such as Pan-STARRS-3pi, DES or future satellite missions like
Euclid. We demonstrate that bias present in existing shear measurement
pipelines (e.g. KSB) can be almost entirely removed by means of neural
networks. In this way, bias correction can depend on the properties of the
individual galaxy instead on being a single global value. We present a
procedure to train neural networks for shear estimation and apply this to
subsets of simulated GREAT08 RealNoise data. We also show that circularization
of the PSF before measuring the shear reduces the scatter related to the PSF
anisotropy correction and thus leads to improved measurements, particularly on
low and medium signal-to-noise data. Our results are competitive with the best
performers in the GREAT08 competition, especially for the medium and higher
signal-to-noise sets. Expressed in terms of the quality parameter defined by
GREAT08 we achieve a Q = 40, 140 and 1300 without and 50, 200 and 1300 with
circularization for low, medium and high signal-to-noise data sets,
respectively.Comment: 19 pages, 8 figures; accepted for publication in Ap
The comet 17P/Holmes 2007 outburst: the early motion of the outburst material
Context. On October 24, 2007 the periodic comet 17P/Holmes underwent an
astonishing outburst that increased its apparent total brightness from
magnitude V\sim17 up to V\sim2.5 in roughly two days. We report on Wendelstein
0.8 m telescope (WST) photometric observations of the early evolution stages of
the outburst. Aims. We studied the evolution of the structure morphology, its
kinematic, and estimated the ejected dust mass. Methods. We analized 126 images
in the BVRI photometric bands spread between 26/10/2007 and 20/11/2007. The
bright comet core appeared well separated from that one of a quickly expanding
dust cloud in all the data, and the bulk of the latter was contained in the
field of view of our instrument. The ejected dust mass was derived on the base
of differential photometry on background stars occulted by the moving cloud.
Results. The two cores were moving apart from each other at a relative
projected constant velocity of (9.87 +/- 0.07) arcsec/day (0.135 +/-0.001
km/sec). In the inner regions of the dust cloud we observed a linear increase
in size at a mean constant velocity of (14.6+/-0.3) arcsec/day (0.200+/-0.004
km/sec). Evidence of a radial velocity gradient in the expanding cloud was also
found. Our estimate for the expanding coma's mass was of the order of 10^{-2}-1
comet's mass implying a significant disintegration event. Conclusions. We
interpreted our observations in the context of an explosive scenario which was
more probably originated by some internal instability processes, rather than an
impact with an asteroidal body. Due to the peculiar characteristics of this
event, further observations and investigations are necessary in order to
enlight the nature of the physical processes that determined it.Comment: 5 pages, 3 figures, A&A accepte
Ground-based variability surveys towards Centaurus A: worthwhile or not?
Context: Difference imaging has proven to be a powerful technique for
detecting and monitoring the variability of unresolved stellar sources in M 31.
Using this technique in surveys of galaxies outside the Local Group could have
many interesting applications. Aims: The goal of this paper is to test
difference imaging photometry on Centaurus A, the nearest giant elliptical
galaxy, at a distance of 4 Mpc. Methods: We obtained deep photometric data with
the Wide Field Imager at the ESO/MPG 2.2m at La Silla spread over almost two
months. Applying the difference imaging photometry package DIFIMPHOT, we
produced high-quality difference images and detected variable sources. The
sensitivity of the current observational setup was determined through
artificial residual tests. Results: In the resulting high-quality difference
images, we detect 271 variable stars. We find a difference flux detection limit
corresponding to m_R~24.5. Based on a simple model of the halo of Centaurus A,
we estimate that a ground-based microlensing survey would detect in the order
of 4 microlensing events per year due to lenses in the halo. Conclusions:
Difference imaging photometry works very well at the distance of Centaurus A
and promises to be a useful tool for detecting and studying variable stars in
galaxies outside the local group. For microlensing surveys, a higher
sensitivity is needed than achieved here, which would be possible with a large
ground-based telescope or space observatory with wide-field imaging
capabilities.Comment: 8 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
The Wendelstein Calar Alto Pixellensing Project (WeCAPP): the M31 Nova catalogue
We present light curves from the novae detected in the long-term, M31
monitoring WeCAPP project. The goal of WeCAPP is to constrain the compact dark
matter fraction of the M31 halo with microlensing observations. As a by product
we have detected 91 novae benefiting from the high cadence and highly sensitive
difference imaging technique required for pixellensing. We thus can now present
the largest CCD and optical filters based nova light curve sample up-to-date
towards M31. We also obtained thorough coverage of the light curve before and
after the eruption thanks to the long-term monitoring. We apply the nova
taxonomy proposed by Strope et al. (2010) to our nova candidates and found 29
S-class novae, 10 C-class novae, 2 O-class novae and 1 J-class nova. We have
investigated the universal decline law advocated by Hachichu and Kato (2006) on
the S-class novae. In addition, we correlated our catalogue with the literature
and found 4 potential recurrent novae. Part of our catalogue has been used to
search for optical counter-parts of the super soft X-ray sources detected in
M31 (Pietsch et al. 2005). Optical surveys like WeCAPP, and coordinated with
multi-wavelength observation, will continue to shed light on the underlying
physical mechanism of novae in the future.Comment: 15 pages, 15 figures, 7 tables, A&A accepted for publication. The
appendix is stored in the Data Conservanc
Detection, photometry and slitless radial velocities of 535 planetary nebulae in the flattened elliptical galaxy NGC 4697
We have detected 535 planetary nebulae (PNs) in NGC 4697, using the classic
on-band, off-band filter technique with the Focal Reducer and Spectrograph
(FORS) at the Cassegrain focus of the first 8-meter telescope unit of the ESO
Very Large Telescope. From our photometry we have built the [O III] 5007
planetary nebula luminosity function (PNLF) of NGC 4697. It indicates a
distance of 10.5 Mpc, substantially smaller than a previous estimate of 24 Mpc
used in earlier dynamical studies. The PNLF also provides an estimate of the
specific PN formation rate. Combining the information from on-band images with
PN positions on dispersed, slitless grism images, we have obtained radial
velocities for 531 of the 535 PNs. They provide kinematic information up to a
distance of almost three effective radii from the nucleus. Some rotation is
detected in the outer regions, but the rotation curve of this galaxy appears to
drop beyond one effective radius. Assuming an isotropic velocity distribution,
the velocity dispersion profile is consistent with no dark matter within three
effective radii of the nucleus (however, some dark matter can be present if the
velocity distribution is anisotropic). We obtain a blue mass-to-light ratio of
11. Earlier M/L ratios for NGC 4697 were too small, because of the too large
distance used for their derivation.Comment: 52 pages, 24 Postscript figures, ApJ 2001, in pres
The old and heavy bulge of M31 I. Kinematics and stellar populations
We present new optical long-slit data along 6 position angles of the bulge
region of M31. We derive accurate stellar and gas kinematics reaching 5 arcmin
from the center, where the disk light contribution is always less than 30%, and
out to 8 arcmin along the major axis, where the disk makes 55% of the total
light. We show that the velocity dispersions of McElroy (1983) are severely
underestimated (by up to 50 km/s) and previous dynamical models have
underestimated the stellar mass of M31's bulge by a factor 2. Moreover, the
light-weighted velocity dispersion of the galaxy grows to 166 km/s, thus
reducing the discrepancy between the predicted and measured mass of the black
hole at the center of M31. The kinematic position angle varies with distance,
pointing to triaxiality. We detect gas counterrotation near the bulge minor
axis. We measure eight emission-corrected Lick indices. They are approximately
constant on circles. We derive the age, metallicity and alpha-element
overabundance profiles. Except for the region in the inner arcsecs of the
galaxy, the bulge of M31 is uniformly old (>12 Gyr, with many best-fit ages at
the model grid limit of 15 Gyr), slightly alpha-elements overabundant
([alpha/Fe]~0.2) and at solar metallicity, in agreement with studies of the
resolved stellar components. The predicted u-g, g-r and r-i Sloan color
profiles match reasonably well the dust-corrected observations. The stellar
populations have approximately radially constant mass-to-light ratios (M/L_R ~
4-4.5 for a Kroupa IMF), in agreement with stellar dynamical estimates based on
our new velocity dispersions. In the inner arcsecs the luminosity-weighted age
drops to 4-8 Gyr, while the metallicity increases to above 3 times the solar
value.Comment: Accepted for publication in A&
Image reduction pipeline for the detection of variable sources in highly crowded fields
We present a reduction pipeline for CCD (charge-coupled device) images which
was built to search for variable sources in highly crowded fields like the M31
bulge and to handle extensive databases due to large time series. We describe
all steps of the standard reduction in detail with emphasis on the realisation
of per pixel error propagation: Bias correction, treatment of bad pixels,
flatfielding, and filtering of cosmic rays. The problems of conservation of PSF
(point spread function) and error propagation in our image alignment procedure
as well as the detection algorithm for variable sources are discussed: We build
difference images via image convolution with a technique called OIS (Alard &
Lupton, 1998), proceed with an automatic detection of variable sources in noise
dominated images and finally apply a PSF-fitting, relative photometry to the
sources found. For the WeCAPP project (Riffeser et al., 2001) we achieve 3
sigma detections for variable sources with an apparent brightness of e.g. m =
24.9 mag at their minimum and a variation of dm = 2.4 mag (or m = 21.9 mag
brightness minimum and a variation of dm = 0.6 mag) on a background signal of
18.1 mag/arcsec^2 based on a 500 s exposure with 1.5 arcsec seeing at a 1.2 m
telescope. The complete per pixel error propagation allows us to give accurate
errors for each measurement.Comment: 15 pages, 4 figures, accepted by A&
The POINT-AGAPE Survey: Comparing Automated Searches of Microlensing Events toward M31
Searching for microlensing in M31 using automated superpixel surveys raises a
number of difficulties which are not present in more conventional techniques.
Here we focus on the problem that the list of microlensing candidates is
sensitive to the selection criteria or "cuts" imposed and some subjectivity is
involved in this. Weakening the cuts will generate a longer list of
microlensing candidates but with a greater fraction of spurious ones;
strengthening the cuts will produce a shorter list but may exclude some genuine
events. We illustrate this by comparing three analyses of the same data-set
obtained from a 3-year observing run on the INT in La Palma. The results of two
of these analyses have been already reported: Belokurov et al. (2005) obtained
between 3 and 22 candidates, depending on the strength of their cuts, while
Calchi Novati et al. (2005) obtained 6 candidates. The third analysis is
presented here for the first time and reports 10 microlensing candidates, 7 of
which are new. Only two of the candidates are common to all three analyses. In
order to understand why these analyses produce different candidate lists, a
comparison is made of the cuts used by the three groups...Comment: 28 pages, 24 figures, 9 table
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