The POINT-AGAPE Survey

The POINT-AGAPE (Pixel-lensing Observations with the Isaac Newton Telescope-Andromeda Galaxy Amplified Pixels Experiment) survey is an optical search for gravitational microlensing events towards the Andromeda galaxy (M31). As well as microlensing, the survey is sensitive to many different classes of variable stars and transients.

The POINT-AGAPE collaboration monitored M31 for three seasons (1999-2001) with the Wide Field Camera on the INT. In each season, data were taken for one hour per night for roughly 60 nights during the six months that M31 was visible. The two 33×33 arcmin2 fields of view straddle the central bulge, northwards and southwards.

The galactic dark matter may be partly composed of compact objects (e.g., black holes, faint stars, brown dwarfs, jupiters) that reside in halos and are popularly called MACHOs ("MAssive Compact Halo Objects''). Observations toward the Magellanic Clouds by the first generation of microlensing surveys yielded important constraints on the Milky Way (MW) halo. The EROS collaboration obtained an upper limit to the contribution by MACHOs to a standard MW halo. Also, according to the MACHO collaboration, the optical depth toward the Large Magellanic Cloud is too large by a factor ~5, to be accounted for by known populations of stars. This excess is attributed to MACHOs of mass ~0.4 solar masses or in the mid-range of normal stars.

M 31, being both nearby and similar to the MW, is a suitable target for such a search. It allows observers to explore the MW halo along a different line of sight. It has its own halo that can be studied globally, and its high inclination is expected to give a strong gradient in the spatial distribution of microlensing events. However, the latter feature, which was at first believed to provide an unmistakable signature for M 31 microlensing halo events, seems to be shared, at least to some extent, by the variable star population within M 31.

A high-threshold analysis of the 3 years of data yielded 6 bright, short-duration microlensing events, which are confronted to a simulation of the observations and the analysis. The observed signal is much larger than expected from self lensing alone and the astronoemrs conclude, at the 95% confidence level, that at least 20% of the halo mass in the direction of M 31 must be in the form of MACHOs if their average mass lies in the range 0.5-1 solar masses. This lower bound drops to 8% for MACHOs with masses ~0.01 solar masses.

In addition, they found a likely binary microlensing candidate with caustic crossing. Its location, some 32 arcmin away from the centre of M31, supported the conclusion that they were detecting a MACHO signal in the direction of M31.

The boundaries of the observed fields are marked as red lines, and the centre of M 31 is a cross. Circles mark the positions of the 6 microlensing events detected. The open circle (S4) corresponds to and event seen toward M32. The star (S5) indicates the position of a binary event candidate.(Extracted from Calchi Novati et al., 2005, A&A, 443, 911). [ JPEG | TIFF ]

The POINT-AGAPE survey also yielded the identification of 20 classical novae (CNe) candidates observed over three seasons in M31. CNe were detected both in the bulge region as well as over a wide area of the M31 disc. Nine of the CNe were caught during the final rise phase and all are well sampled in at least two colours. The excellent light-curve coverage allowed the astronomers to detect and classify CNe over a wide range of speed class, from very fast to very slow. Among the light curves is a moderately fast CN exhibiting entry into a deep transition minimum, followed by its final decline. The CN catalogue constitutes a uniquely well-sampled and objectively-selected data set with which to study the statistical properties of CNe in M31, such as the global nova rate, the reliability of novae as standard-candle distance indicators and the dependence of the nova population on stellar environment.

A catalogue with the locations, periods and brightness of 35,414 variable stars in M31 was produced as a by-product of the microlensing search. The variables were classified according to their period and brightness. Rough correspondences with classical types of variable star (such as Population I and II Cepheids, Miras and semiregular long-period variables) were established. The spatial distribution of Population I Cepheids is clearly associated with the spiral arms, while the central concentration of the Miras and long-period variables varies noticeably, the brighter and the shorter period Miras being much more centrally concentrated.

A crucial role in the microlensing experiment is played by the asymmetry signal – the excess of events expected in the southern or more distant fields as measured against those in the northern or nearer fields. It was initially assumed that the variable star populations in M31 would be symmetric with respect to the major axis, and thus variable stars would not be a serious contaminant for measuring the microlensing asymmetry signal. However, it was found that all the variable star distributions are asymmetric primarily because of the effects of differential extinction associated with the dust lanes.

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