1,721,081 research outputs found
Pixel lensing: Microlensing towards M31
No full textPixel lensing is gravitational microlensing of unresolved stars. The main target explored up to now has been the nearby galaxy of Andromeda, M31. The scientific issues of interest are the search for dark matter in form of compact halo objects, the study of the characteristics of the luminous lens and source populations and the possibility of detecting extra-solar (and extra-galactic) planets. In the present work we intend to give an updated overview of the observational status in this field
Microlensing towards the SMC: a new analysis of OGLE and EROS results
Full text linkWe present a new analysis of the results of the EROS-2, OGLE-II, and OGLE-III microlensing campaigns towards the Small Magellanic Cloud (SMC). Through a statistical analysis we address the issue of the \emph{nature} of the reported microlensing candidate events, whether to be attributed to lenses belonging to known population (the SMC luminous components or the Milky Way disc, to which we broadly refer to as "self lensing") or to the would be population of dark matter compact halo objects (MACHOs). To this purpose, we present profiles of the optical depth and, comparing to the observed quantities, we carry out analyses of the events position and duration. Finally, we evaluate and study the microlensing rate. Overall, we consider five reported microlensing events towards the SMC (one by EROS and four by OGLE). The analysis shows that in terms of number of events the expected self lensing signal may indeed explain the observed rate. However, the characteristics of the events, spatial distribution and duration (and for one event, the projected velocity) rather suggest a non-self lensing origin for a few of them. In particular we evaluate, through a likelihood analysis, the resulting upper limit for the halo mass fraction in form of MACHOs given the expected self-lensing and MACHO lensing signal. At 95% CL, the tighter upper limit, about 10%, is found for MACHO mass of 10−2M⊙, upper limit that reduces to above 20% for 0.5M⊙ MACHOs
Microlensing towards the LMC: self lensing for OGLE-II and OGLE-III
No full textWe 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 upper limit for f, the halo mass fraction in form of MACHOs, in the range 10-20% for (0.01-0.5) Msun and f=24%$ for 1 solar mass MACHOs (and below 10% in this full range, and in particular below 5% for (0.01-0.1) Msun) 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 about f=50% (95% CL) for 100 Msun, 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
Microlensing towards the Large Magellanic Cloud: a study of the LMC halo contribution
No full textWe carry out 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 characterising 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 Galactic 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 Galaxy and the LMC halos are equal, and indeed we show that, for a MACHO mass in the range 0.1–0.3 M, the LMC halo fraction can be significantly larger than the MilkyWay’s. 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
LMC self lensing for OGLE-II microlensing observations
No full textIn the framework of microlensing searches towards the Large Magellanic Cloud (LMC), we discuss the results presented by the Optical Gravitational Lensing Experiment (OGLE) collaboration for their OGLE-II campaign (Wyrzykowski et al). We evaluate the optical depth, the duration and the expected rate of events for the different possible lens populations: both luminous, dominated by the LMC self-lensing, and `dark', the would be compact halo objects (massive compact halo objects) belonging to either the Galactic or the LMC halo. The OGLE-II observational results, two microlensing candidate events located in the LMC bar region with duration of 24.2 and 57.2 days, compare well with the expected signal from the luminous lens populations: n<SUB>exp</SUB> = 1.5, with typical duration, for LMC self-lensing, of about 50 days. Because of the small statistics at disposal, however, the conclusions that can be drawn as for the halo mass fraction, f, in the form of compact halo objects are not too severe. By means of a likelihood analysis we find an upper limit for f, at 95 per cent confidence level, of about 15 per cent in the mass range (10<SUP>-2</SUP>-10<SUP>-1</SUP>)M<SUB>solar</SUB> and 26 per cent for 0.5M<SUB>solar</SUB>
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