The POINT-AGAPE survey - II. An unrestricted search for microlensing events towards M31

Abstract

An automated search is carried out for microlensing events using a catalogue of 44 554 variable superpixel light curves derived from our 3‐yr monitoring programme of M31. Each step of our candidate selection is objective and reproducible by a computer. Our search is unrestricted, in the sense that it has no explicit time‐scale cut. So, it must overcome the awkward problem of distinguishing long time‐scale microlensing events from long‐period stellar variables. The basis of the selection algorithm is the fitting of the superpixel light curves to two different theoretical models, using variable star and blended microlensing templates. Only if microlensing is preferred is an event retained as a possible candidate. Further cuts are made with regard to: (i) sampling, (ii) goodness of fit of the peak to a Paczyński curve, (iii) consistency of the microlensing hypothesis with the absence of a resolved source, (iv) achromaticity, (v) position in the colour–magnitude diagram and (vi) signal‐to‐noise ratio. Our results are reported in terms of first‐level candidates, which are the most trustworthy, and second‐level candidates, which are possible microlensing events but have a lower signal‐to‐noise ratio and are more questionable. The pipeline leaves just three first‐level candidates, all of which have very short full‐width at half‐maximum time‐scales (t1/2 < 5 d) and three second‐level candidates, which have time‐scales of t1/2= 31, 36 and 51 d. We also show 16 third‐level light curves, as an illustration of the events that just fail the threshold for designation as microlensing candidates. They are almost certainly mainly variable stars. Two of the three first‐level candidates correspond to known events (PA 00‐S3 and 00‐S4) already reported by the POINT‐AGAPE project. The remaining first‐level candidate is new. This algorithm does not find short time‐scale events that are contaminated with flux from nearby variable stars (such as PA 99‐N1).

The microlensing optical depth towards the Large Magellanic Cloud: is there a puzzle?

“…Second, there are a number of microlensing experiments towards the Andromeda galaxy, such as the POINT-AGAPE (pixel-lensing observations with INT-Andromeda galaxy amplified pixels experiment) and MEGA (microlensing exploration of our Galaxy and Andromeda) experiments (e.g. Paulin-Henriksson et al 2003;De Jong et al 2004;Belokurov et al 2005), that are now reporting results. These experiments are motivated by the suggestion of Crotts (1992) that the event rate to sources in the near and far discs in M31 is different.…”

Planetary nebula velocities in the disc and bulge of M31

“…Knowledge of the total mass of M31 is vital for the interpretation of the latest generation of pixel microlensing experiments, which concentrate on that galaxy (Crotts 1992;Kerins et al 2001Kerins et al , 2003Riffeser et al 2003;de Jong et al 2004;Belokurov et al 2005;Cseresnjes et al 2005). have found that there is no kinematic evidence to support the widely held belief that M31 is more massive than the Milky Way, in contradiction to previous studies.…”

List of Figures. List of Tables

“…Positions (α and δ ) and other parameters (projected separation from the center of M31, fullwidth-half-maximum time t FWHM , flux excess, colors, time of event, quality of the Paczynski fit) of microlensing events in M31. AGAPE Z1 was reported by Ansari et al (1999), the POINT-AGAPE events (PA-) by Auriere et al (2001);Paulin-Henriksson et al (2003Calchi Novati et al (2005); Belokurov et al (2005), MEGA events (ML) by de Jong et al (2004de Jong et al ( , 2006. Note that some the MEGA candidates have been withdrawn meanwhile, and some of the Belokurov-POINT-AGAPE candidates are doubtful (Calchi Novati, priv.…”