166 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 64 Mpixel wide field imager for the Wendelstein 2m Telescope: Design and Calibration
The Wendelstein Observatory of Ludwig Maximilians University of Munich has
recently been upgraded with a modern 2m robotic telescope. One Nasmyth port of
the telescope has been equipped with a wide-field corrector which preserves the
excellent image quality (< 0.8" median seeing) of the site (Hopp et al. 2008)
over a field of view of 0.7 degrees diameter. The available field is imaged by
an optical imager (WWFI, the Wendelstein Wide Field Imager) built around a
customized 2 2 mosaic of 4k 4k 15 \mu m e2v CCDs from
Spectral Instruments. This paper provides an overview of the design and the
WWFI's performance. We summarize the system mechanics (including a structural
analysis), the electronics (and its electromagnetic interference (EMI)
protection) and the control software. We discuss in detail detector system
parameters, i.e. gain and readout noise, quantum efficiency as well as charge
transfer efficiency (CTE) and persistent charges. First on sky tests yield
overall good predictability of system throughput based on lab measurements.Comment: 38 pages 19 Figures To be published in Springer Experimental
Astronom
Gravitational Microlensing toward the Andromeda Galaxy
Aktuelle Forschungsergebnisse deuten darauf hin, dass das Universum insgesamt aus 72% Dunkler Energie, 23% nichtbaryonischer Materie und 4.5% baryonischer Materie besteht. Von dieser baryonischen Materie kann bisher nur ein neuntel sicher zugeordnet werden. Ferner laesst sich aus der Rotation von Spiralgalaxien ableiten, dass diese grosse Mengen an Dunkler Materie enthalten, die sich rein durch ihren gravitativen Einfluss auf sichtbare Objekte im Bulge und in der Scheibe der Galaxie zeigt. Dabei wird angenommen dass Bulge und Scheibe in den sog. dunklen Halo eingebettet sind, der diese unsichtbare Materie beinhaltet. Eine grundlegende Frage ist daher aus welcher Art die dunkle Materie im Halo von Spiralgalaxien besteht. Moegliche Kandidaten fuer solche Dunkle Materie sind neben schwach wechselwirkenden massiven Teilchen (WIMPs - Weakly Interacting Massive Particles) auch kompakte dunkle Objekte im Halo von Galaxien (Machos - MAssive Compact Halo Objects). Die vorliegende Doktorarbeit beschreibt die Suche nach solchen Machos im Halo unserer Nachbargalaxie Andromeda (M31). Im Falle von kompakten Objekten mit Massen im Bereich von einem milliardstel bis zum zehntausendfachen einer Sonnenmasse ermoeglicht der sogenannte Gravitationslinseneffekt deren direkten Nachweis. Dabei beeinflusst die gravitative Wirkung eines kompakten Objekts die Lichtstrahlen von im Hintergrund liegenden Sternen derart, dass das Licht durch die Relativbewegung kurzzeitig fokussiert und verstaerkt wird. Jedoch ist die Wahrscheinlichkeit, dass ein Stern eine messbare Verstaerkung aufweist, weniger als 1 : 1000000. Durch Messungen von Millionen von Sternen konnten derartige charakteristische Lichtkurven im letzten Jahrzehnt sehr zahlreich in Richtung zum Zentrum unserer Milchstrasse nachgewiesen werden. Eine noch groessere Herausforderung stellt der Nachweis von Machos in der hundertmal weiter entfernten Andromeda-Galaxie (M31) dar. Zwar erreicht uns von einzelnen Sternen von dort im Vergleich zum Milchstrassenzentrum nur ein zehntausendstel an Strahlung, jedoch lassen sich mit einer einzigen Aufnahme Millionen von Sternen gleichzeitig auf Helligkeitsaenderungen ueberpruefen. Da die Sichtlinie zum Zentrum von M31 die Halos der Milchstrasse und von M31 durchdringt, gestattet dies Rueckschluesse auf den Anteil der Machos in beiden Galaxien. Wegen der deutlich groesseren Entfernung und der damit verbundenen geringeren scheinbaren Helligkeit der Hintergrundobjekte sind die Anforderungen an die Datenanalyse ungleich hoeher. In der vorliegenden Doktorarbeit wurden daher neue Methoden entwickelt und aufgezeigt, um systematische Fehler bei der Aufnahme von Bildern zu kontrollieren und das Rauschen bei der Bildbearbeitung zu minimieren. Da die Zeitdauer eines Gravitationslinsenereignisses sehr kurz ist, mussten am Wendelstein-0.8-m-Teleskop, und waehrend einer 3-jaehrigen Phase am Calar-Alto-1.23-m-Teleskop, zehntausende Aufnahmen waehrend des Zeitraums von 1997 - 2005 gewonnen und ausgewertet werden. In dieser bezueglich der Zeitueberdeckung einmaligen Datenbasis konnten in 4 Mio. Lichtkurven insgesamt 13 Ereignisse nachgewiesen werden, die die typischen Helligkeitsaenderungen des Gravitationslinseneffekts aufweisen. Die Analyse der Lichtkurven zeigt mit den in dieser Arbeit gewonnenen theoretischen Erkenntnissen bezueglich der endlichen Groesse der Hintergrundsterne, dass alle Gravitationslinsenkandidaten mit einem Halo aus dunklen Objekten von 0.2 Sonnenmassen vereinbar sind. Waehrend die Anzahl der Detektionen im Vergleich zu frueheren theoretischen Vorhersagen deutlich geringer ausfiel, zeigten die im Rahmen dieser Doktorarbeit entwickelten theoretischen Vorhersagen eine sehr gute uebereinstimmung. Ob sich die beobachteten Gravitationslinsenereignisse wirklich durch Machos im Halo oder eventuell durch Sterne in Bulge oder Scheibe hervorgerufen wurden, soll durch weiterfuehrende Arbeiten mittels Monte-Carlo-Simulationen bezueglich der Detektionseffizienz geklaert werden. Daraus lassen sich dann quantitative Aussagen ueber die Art der dunklen Materie und den Anteil von Machos im Halo der M31 Galaxie gewinnen
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
M31 PAndromeda Cepheid sample observed in four HST bands
Using the M31 PAndromeda Cepheid sample and the HST PHAT data we obtain the
largest Cepheid sample in M31 with HST data in four bands. For our analysis we
consider three samples: A very homogeneous sample of Cepheids based on the
PAndromeda data, the mean magnitude corrected PAndromeda sample and a sample
complementing the PAndromeda sample with Cepheids from literature. The latter
results in the largest catalog with 522 fundamental mode (FM) Cepheids and 102
first overtone (FO) Cepheids with F160W and F110W data and 559 FM Cepheids and
111 FO Cepheids with F814W and F475W data. The obtained dispersion of the
Period-Luminosity relations (PLRs) is very small (e.g. 0.138 mag in the F160W
sample I PLR). We find no broken slope in the PLRs when analyzing our entire
sample, but we do identify a subsample of Cepheids that causes the broken
slope. However, this effect only shows when the number of this Cepheid type
makes up a significant fraction of the total sample. We also analyze the sample
selection effect on the Hubble constant.Comment: 32 pages, 19 figures, 9 tables, accepted for publication in ApJ,
electronic data will be available on CD
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
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
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