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(Short version) The nature and the location of the lenses discovered in the microlensing surveys done so far towards the LMC remain unclear. This contribution is comprised of two distinct parts. In the first part, motivated by these questions, we compute the optical depth for the different intervening populations an the number of expected events for self-lensing, using a recently drawn coherent picture of the geometrical structure and dynamics of the LMC disk. In the second part (section 5), a preliminary account of the final results from the EROS-2 programme is presented. Based on the analysis of 33 million LMC and SMC stars followed during 6.7 years, strict limits on the macho content of the galactic halo are presented; they cover the range of macho masses between 0.0001 and 100 solar mass. The limits are better than 20% (resp. 5%) of the standard halo for masses between 0.0002 and 10 (resp. 0.001 to 0.1) solar mass. This is presently the data set with the largest sensitivity to halo machos.
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We present the first results of the analysis of data collected during the 1998-99 observational campaign at the 1.3 meter McGraw-Hill Telescope, towards the Andromeda galaxy (M31), aimed to the detection of gravitational microlensing effects as a probe of the presence of dark matter in our and in M31 halo. The analysis is performed using the pixel lensing technique, which consists in the study of flux variations of unresolved sources and has been proposed and implemented by the AGAPE collaboration. We carry out a shape analysis by demanding that the detected flux variations be achromatic and compatible with a Paczynski light curve. We apply the Durbin-Watson hypothesis test to the residuals. Furthermore, we consider the background of variables sources. Finally five candidate microlensing events emerge from our selection. Comparing with the predictions of a Monte Carlo simulation, assuming a standard spherical model for the M31 and Galactic haloes, and typical values for the MACHO mass, we find that our events are only marginally consistent with the distribution of observable parameters predicted by the simulation.
We report the final analysis of a search for microlensing events in the direction of the Andromeda galaxy, which aimed to probe the MACHO composition of the M31 halo using data collected during the 1998-99 observational campaign at the MDM observatory. In a previous paper, we discussed the results from a first set of observations. Here, we deal with the complete data set, and we take advantage of some INT observations in the 1999-2000 seasons. This merging of data sets taken by different instruments turns out to be very useful, the study of the longer baseline available allowing us to test the uniqueness characteristic of microlensing events. As a result, all the candidate microlensing events previously reported turn out to be variable stars. We further discuss a selection based on different criteria, aimed at the detection of short--duration events. We find three candidates whose positions are consistent with self--lensing events, although the available data do not allow us to conclude unambiguously that they are due to microlensing.
We carry on a new analysis of the sample of MACHO microlensing candidates towards the LMC. Our main purpose is to determine the lens population to which the events may belong. We give particular emphasis to the possibility of characterizing the Milky Way dark matter halo population with respect to the LMC one. Indeed, we show that only a fraction of the events have characteristics that match those expected for lenses belonging to the MACHO population of the Milky Way halo. This suggests that this component cannot explain all the candidates. Accordingly, we challenge the view that the dark matter halo fraction of both the Milky Way and the LMC halos are equal, and indeed we show that, for a MACHO mass in the range 0.1-0.3 M$_odot$, the LMC halo fraction can be significantly larger than the Milky Way one. In this perspective, our main conclusion is that up to about half of the observed events could be attributed to the LMC MACHO dark matter halo.
We report our first microlensing candidate NMS-E1 towards M31 from the data accumulated during the four years of Nainital Microlensing Survey. Cousin R and I band observations of ~13x13 field in the direction of M31 have been carried out since 1998 and data is analysed using the pixel technique proposed by the AGAPE collaboration. NMS-E1 lies in the disk of M31 at alpha = 0:43:33.3 and delta = +41:06:44, about 15.5 arcmin to the South-East direction of the center of M31. The degenerate Paczy{n}ski fit gives a half intensity duration of ~59 days. The photometric analysis of the candidate shows that it reached R~20.1 mag at the time of maximum brightness and the colour of the source star was estimated to be (R-I)_0 ~ 1.1 mag. The microlensing candidate is blended by red variable stars; consequently the light curves do not strictly follow the characteristic Paczy{n}ski shape and achromatic nature. However its long period monitoring and similar behaviour in R and I bands supports its microlensing nature.
We present an analysis of the results of the OGLE-III microlensing campaign towards the Large Magellanic Cloud (LMC). We evaluate for all the possible lens populations along the line of sight the expected microlensing quantities, number of events and duration. In particular we consider lensing by massive compact halo objects (MACHOs) in the dark matter haloes of both the Milky Way (MW) and the LMC, and self lensing by stars in the LMC bar and disc, in the MW disc and in the stellar haloes of both the LMC and the MW. As a result we find that the self-lensing signal is able to explain the 2 OGLE-III microlensing candidates. In particular, we estimate the expected MW disc signal to be almost as large as that from LMC stars and able, by itself, to explain the observed rate. We evaluate a 95% CL emph{upper} limit for $f$, the halo mass fraction in form of MACHOs, in the range 10-20% for $(10^{-2}-0.5) mathrm{M}_odot$, and $f=24%$ for $1 mathrm{M}_odot$ (below 10% in this full range, and in particular below 5% for $(10^{-2}-0.1) mathrm{M}_odot$) for the Bright (All) samples of source stars. Furthermore, we find that these limits do not rise much even if we assume the observed events emph{are} MACHOs. For the All sample we also evaluate a rather significant constraint on $f$ for larger values of the MACHO mass, in particular $fsim 50%$ (95% CL) for $100 mathrm{M}_odot$, to date the stronger bound coming from microlensing analyses in this mass range. Finally, we discuss these results in the framework of the previous observational campaigns towards the LMC, that of the MACHO and the EROS collaborations, and we present a joint analysis of the OGLE-II and the OGLE-III campaigns.
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