WeCAPP - The Wendelstein Calar Alto Pixellensing Project
Calar Alto Newsletter No. 3 (2001)
Capturing Dark Matter in M31
Claus A. Gössl,
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
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
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: 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.
THE WePP/WeCAPP PROJECT
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
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.
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)
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
used in our project are sensitive especially to these kind of stars, the
Johnson I RG780 (lambda=850
nm, Delta lambda 150 nm) and
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
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: 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.
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.
Figure 4: Histograms of the frames taken
during the 1999/2000 campaign at Wendelstein (left panel) and Calar Alto
Observatory (right panel).
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
Finally time coverage of the first joint campaign of Wendelstein and
Calar Alto is good, due to the often opposite weather situation in Spain
Table 2: Values given in [arcsec] for the
images taken during WeCAPP in the R and I band at Wendelstein and Calar
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: lightcurve of a nova,
upper panel: R' band, lower panel: I' band
Figure 7: lightcurve of a mira star,
upper panel: R' band, lower panel: I' band
Figure 8: lightcurve of a longperiodic variable
upper panel: R' band, lower panel: I' band
Figure 9: lightcurve of a RV tauri variable
star in the R' band
Figure 10: periodical lightcurve of a delta-cephei
variable star in the R' band
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