3 Observations and experimental setup

The data analysed in this paper have been collected on the 1.3 meter McGraw-Hill Telescope, at the MDM observatory, Kitt Peak (USA). Two fields have been observed, which lie on the two sides of the galactic bulge (see Fig. 1) and have been chosen in order to be able to study the expected gradient in the optical depth.

Figure 1: M 31 with MDM and Agape observation fields (courtesy of A. Crotts). White stars and dots give, respectively, the position of the five microlensing candidate events (labelled as in Table 2 and where candidates 1 and 2 appear to be superimposed) and of the nova.

The two fields are almost parallel to the major axis of M 31; their centers are located at $\alpha= 00^{\rm h}43^{\rm m}24^{\rm s}$, $\delta = 41^{\!\circ}12'10''$ (J2000) (named "Target''), and $\alpha= 00^{\rm h}42^{\rm m}14^{\rm s}$, $\delta =41^{\!\circ}24'20''$ (J2000) (named "Control''). The data acquired in the Target field are analysed here.

Figure 1 shows the location of the fields and for comparison also the smaller AGAPE field. The observations were taken with a CCD camera of $2048 \times 2048$ pixels with $0.\hskip-2pt ''5$ the pixel angular dimensions and therefore a total field size of $17'\times 17'$.

In order to test for achromaticity, images have been taken in two bands, a wide R and a near-standard I. The exposure time is 6 min for R, 5 min for I. The observations began in October 1998 and are still underway.

Here we analyse the data taken in the period from the beginning of October 1998 to the end of December 1999. In Fig. 2 we give the time sampling of the measurements (number of nights and images).

Figure 2: Time sampling for the observations in the "Target'' field.

For each night we have measurements in both R and I. On average, there are twice as many R images as I images. In R band we have $\sim$800 images distributed along 42 nights of observation.

Most of the observations (about $80\%$) are concentrated in the first three months, so that, unfortunately, the time distribution of the data is not optimized for the study of microlensing effects. Thus, the given time distribution allows us to select events that take place almost exclusively during the first three months of observation. Furthermore, the time coverage of about 14 months is still not long enough to test conclusively the bump uniqueness requirement for a microlensing event. Mainly for this reason, we will speak in this paper only of candidate microlensing events.

Taking into account the transmission efficiency of the filters and the catalogued magnitudes $R_{\rm c}$ and $I_{\rm c}$ (Cousins colour system), for a sample of 23 reference secondaries identified in the Target field (Magnier et al. 1993), we derive the following photometric calibrations

$$R_{\rm c}= m_R-0.13\cdot \left(m_R-m_I\right)+22.54,$$ (6)

$$I_{\rm c}=m_I-0.02\cdot \left(m_R-m_I\right)+22.21,$$ (7)

where $m_{R(I)}=-2.5\cdot\log(\phi^*_{R(I)})$ and $\phi^*_{R(I)}$is the flux of the source in ADU/s measured in the R and Ifilters respectively. The estimated error is $\simeq$0.1 mag, both for R and I.