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 $\times$ 2 mosaic of 4k $\times$ 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

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

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 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