WeCAPP - The Wendelstein Calar Alto Pixellensing Project
Capturing Dark Matter in M31

Calar Alto Newsletter No. 3 (2001)

Arno Riffeser, Jürgen Fliri, Claus A. Gössl, Ralf Bender, Ulrich Hopp, Otto Bärnbantner, Christoph Ries, Heinz Barwig, Stella Seitz, Wolfgang Mitsch


We present WeCAPP our long time project searching for microlensing events in M31. Since 1997 the bulge of M31 was monitored in two different wavebands with the Wendelstein 0.8 m telescope. In 1999 we extended our observations to the Calar Alto 1.23m telescope. Observing simultaneously on these two sites we obtained a time coverage of 53% of the visibility of M31. To check thousands of frames for variability of unresolved sources, we used the optimal image subtraction method (OIS) proposed by Alard & Lupton (1998). With this method we were able to minimize the residuals in the difference image analysis (DIA) and to detect sources with an amplitude near the photon noise level. With the expected microlensing events and their timescales, it will be possible to favour or to rule out a certain MACHO population and to derive constraints for the dark matter composition of M31.


In the last decade microlensing studies proofed to be a powerful tool for searching baryonic dark matter in the galactic halo, so called MACHOs (Massive Astrophysical Compact Halo Objects).

Several groups like the MACHO collaboration (Alcock et al. 1993), OGLE (Udalski et al. 1992), EROS (Aubourg et al. 1993) and DUO (Alard et al. 1995) followed the suggestion of Paczynski (1986) and surveyed millions of stars in the Large and Small Magellanic Clouds and in the galactic bulge for flux changes induced by gravitational microlensing. All of them discovered a large variety of gravitational lensing events, but the amount and the distribution of baryonic dark matter in the galactic halo is still unclear.

Crotts (1992) suggested to include M31 in future lensing surveys and pointed out that it should be an ideal target for these kind of experiments. Since the optical depth for galactic MACHOs would be much greater for M31 than for the LMC, SMC or the galactic bulge one would expect event rates greater than in classical lensing studies. Furthermore M31 could contribute an additional MACHO population as it possesses a dark halo of its own. Thus three populations may contribute to the optical depth along the line of sight: MACHOs in the galactic halo, MACHOs in the halo of M31 and finally stars in the bulge and the disk of M31 itself, a contribution dubbed self-lensing.

As most of the sources for possible lensing events are not resolved at the M31's distance of 770 kpc (Freedman & Madore 1990) the name `pixellensing' (Gould 1996) was adopted for these kind of microlensing studies. In the mid nineties two projects started pixellensing surveys towards M31, AGAPE (Ansari et al. 1997) and Columbia/VATT (Tomaney & Crotts 1997). First candidate events were reported (Ansari et al. 1999a, Crotts & Tomaney 1996) but could not yet be confirmed as MACHOs. This was partly due to an insufficient time coverage which didn't permit to rule out intrinsic variable stars i.e. Miras as possible sources.

1999 two new pixellensing projects, POINT-AGAPE (MNRAS, submitted) and MEGA (Crotts et al. 1999), the successor of Columbia/VATT, began their systematical observations of M31. Another project, SLOTT-AGAPE, will join them this year.

The Wendelstein Calar Alto Pixellensing Project started as WePP in autumn 1997 before it graduated after 2 campaigns to WeCAPP in summer 1999 by using in future 2 sites for the survey.


The idea of pixellensing is to check if the flux of a pixel in the images changes over time. By subtracting two images variable sources can be detected even in highly crowded fields giving isolated positive and negative sources. The most difficult part is the adjustment of the point spread function (PSF) among images. With the optimal image subtraction method by Alard & Lupton (1998) we are able to match the PSF in a way that the residuals are minimized at the photon noise level.

Figure 1
Figure 1: subtraction of two images with the same PSF and one variable source. The luminosities of the stars are indicated by the size of the circles.



In September 1997 we began our observations at the institute's Wendelstein 0.8 m telescope with a focal length of 9.9 m and an aperture ratio of f/D=12.4. In this first year we observed on 35 nights until March 1998. Since M31 is visible from beginning July until end of March, our second observational period lasted from 22nd October until 24th March 1999. We used the MONICA 1024x1024 CCD (TEK) camera covering 8.3 x 8.3 of the bulge of M31. To follow the suggestion of Tomaney & Crotts (1996) and Han & Gould (1996) we chose the field with the maximal lensing probability, looking to the far side of the M31 disk. The main fraction of the field is covered by the bulge of M31 with the nucleus of M31 visible on the upper right edge.

Figure 2
Figure 2: frame observed on 26/06/2000 with the Calar Alto 1.23 m telescope: 17.2 x 17.2 arcmin (3.8 x 3.8 kpc)

To increase the time resolution of our observations we extended our third campaign to the Calar Alto 1.23 m telescope with a focal length of 9.8 m (f/D=8.0) getting images from 2 h of service observations on 66 nights (27th June 1999 - 3th March 2000). From 1st November until 14th November we were able to observe the whole night. During the whole period we continued with our observations at the Wendelstein telescope. In this way we achieved an overall time coverage of 132 nigths (52.5%). Since most of the observations were carried out in service mode the Calar Alto data was obtained using a couple of different CCD cameras, all mounted at the Cassegrain focus. Three of these CCDs cover a field of 17.2 x 17.2 arcmin^2. A detailed overview of the properties of each CCD chip used for WeCAPP is given in table 1.

Table 1
Table 1: Properties of all CCD cameras at Calar Alto and Wendelstein Observatories respectively


Most of the sources for possible lensing events in the bulge of M31 are luminous red stars i.e. giants and supergiants. Therefore the filters used in our project are sensitive especially to these kind of stars, the Johnson I RG780 (lambda=850 nm, Delta lambda 150 nm) and Roeser R2 OG570+Cfx (lambda=650 nm, Delta lambda 150 nm). Since June 2000 we use at Calar Alto the newly installed filters Johnson R (lambda=641 nm, Delta lambda 159 nm) and Johnson I (lambda=850 nm, Delta lambda 150 nm).

It is useful to observe in 2 wavebands since an achromatic and symmetric lightcurve of a singular event can exclude variable stars as possible sources.

Despite the combination of different telescopes, CCD chips and finally filter systems we observed no systematical effect affecting our results beyond the errorlimits.


Our observation cycle comprises 5 images in the R band with an average exposure time of 150 sec and 3 images a 200 sec in the I band lasting about 45 min including read out time. After reducing the data with our reduction pipeline (Gössl & Riffeser, in prep.) we stack the images of one cycle. With an instrumental zeropoint of 23.1 mag in R2 and 21.8 mag in I we reach a magnitude limit for a signal-to-noise ratio (S/N) of 10 in over 95% of the frame between (20.8 - 22.1) mag in R2 and (19.1 - 20.4) mag in I. The cycles are repeated as often as possible during one night, usually at least 2 times. Since we have to avoid saturation of stars in the observed field we make exposure times dependent of the actual seeing value. Generally exposure times in the I filters are longer to get a (S/N) comparable to that obtained in the R band images.

During three years of WeCAPP we took at Wendelstein Observatory 1565 images in the R band and 691 images in the I band. The observations over more than one year at Calar Alto resulted in 1177 frames in the R and 608 frames in the I band. In our actual capaign 2000/2001 we are observing the 4th year at Wendelstein and the 2nd year at Calar Alto.

Quality of the Data

During the 1997/1998 campaign conditions at the Wendelstein telescope were improved significantly. A newly installed air condition system which permits to cool down the dome during daytime reduced dome seeing to a low level. Further improvements like fans just above the main mirror finally lead to a leap in the quality obtained with the telescope. Figure 3 which presents the PSF statistics of Wendelstein images from the 1997/1998 and 1998/1999 campaigns respectively underlines nicely this fact.

Figure 3
Figure 3: Histograms of the frames taken at Wendelstein Observatory during the 1997/1998 campaign (left panel) and the 1998/1999 campaign (right panel). The x-axis represents the Full Width Half Maximum (FWHM) of the point spread function (PSF) of an image. Frames in the R band are marked by the solid line, frames in the I band by a dashed line. The lower limit of the PSF is restricted by a pixel size of 0.5 arcsec.

Figure 4
Figure 4: Histograms of the frames taken during the 1999/2000 campaign at Wendelstein (left panel) and Calar Alto Observatory (right panel).

In general Wendelstein shows a slightly better PSF distribution than Calar Alto (see Figures 1 and 2). Table 2 shows the PSF median values for the images taken during WeCAPP at both sites.

Table 2
Table 2: Values given in [arcsec] for the images taken during WeCAPP in the R and I band at Wendelstein and Calar Alto Observatories.

To be able to distinguish between microlensing events and intrinsic variable sources it is necessary that the observations cover the timespan of visibility of M31 in an acceptable manner.

Figure 5 shows the time sampling we reached so far with WeCAPP. Because of time loss during the upgrades of the telescope time coverage of the 1997/1998 campaign is only fragmentary. About the same applies to the following campaign, this time due to a camera shutdown and another time consuming project.

Finally time coverage of the first joint campaign of Wendelstein and Calar Alto is good, due to the often opposite weather situation in Spain and Germany.

Figure 5
Figure 5: Illustration of the time coverage we reached so far during three years of WeCAPP. Vertical lines mark the beginning and the end of the visiblity of M31 at one of the two sites.


We present a small sample of lightcurves to show the efficiency of the method. All lightcurves were observed over more then three years from 1997 until 2000. The lightcurves are shown in flux differences in respect to the reference frame. Timespans when M31 was visible at none of the two sites are marked by two vertical lines. Because of bad dome seeing conditions and an unappropriate autoguiding system errors were largest during the first Wendelstein campaign 1997/98. During the second period 1998/99 we were able to decrease the FWHM of the PSF by a factor 2, so the photometric scatter is also smaller. During the third period 1999/2000 we observed simultaneously at Calar Alto and Wendelstein and got data points for 53% of the visibility of M31.
Figure 6 shows the lightcurve of a nova. It's the brightest variable source detected in our M31-field. Figure 7 displays a mira star and figure 8 shows a longperiodic variable source. In figure 9 we present a RV tauri star and in figure 10 a delta-Cephei variable stars.

Figure 6
Figure 6: lightcurve of a nova, 
upper panel: R' band, lower panel: I' band

Figure 7
Figure 7: lightcurve of a mira star, 
upper panel: R' band, lower panel: I' band

Figure 8
Figure 8: lightcurve of a longperiodic variable star, 
upper panel: R' band, lower panel: I' band

Figure 9
Figure 9: lightcurve of a RV tauri variable star in the R' band

Figure 10
Figure 10: periodical lightcurve of a delta-cephei variable star in the R' band


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Arno Riffeser