720 research outputs found
The OGLE View of Microlensing towards the Magellanic Clouds. III. Ruling out sub-solar MACHOs with the OGLE-III LMC data
In the third part of the series presenting the Optical Gravitational Lensing
Experiment (OGLE) microlensing studies of the dark matter halo compact objects
(MACHOs) we describe results of the OGLE-III monitoring of the Large Magellanic
Cloud (LMC). This unprecedented data set contains almost continuous photometric
coverage over 8 years of about 35 million objects spread over 40 square
degrees. We report a detection of two candidate microlensing events found with
the automated pipeline and an additional two, less probable, candidate events
found manually. The optical depth derived for the two main candidates was
calculated following a detailed blending examination and detection efficiency
determination and was found to be tau=(0.16+-0.12)10^-7.
If the microlensing signal we observe originates from MACHOs it means their
masses are around 0.2 M_Sun and they compose only f=3+-2 per cent of the mass
of the Galactic Halo. However, the more likely explanation of our detections
does not involve dark matter compact objects at all and rely on natural effect
of self-lensing of LMC stars by LMC lenses. In such a scenario we can almost
completely rule out MACHOs in the sub-solar mass range with an upper limit at
f<7 per cent reaching its minimum of f<4 per cent at M=0.1 M_Sun. For masses
around M=10 M_Sun the constraints on the MACHOs are more lenient with f ~ 20
per cent. Owing to limitations of the survey there is no reasonable limit found
for heavier masses, leaving only a tiny window of mass spectrum still available
for dark matter compact objects.Comment: Accepted for publication in MNRAS. On-line data available on OGLE
website: http://ogle.astrouw.edu.p
Repeating microlensing events in the OGLE data
Microlensing events are usually selected among single-peaked non-repeating
light curves in order to avoid confusion with variable stars. However, a
microlensing event may exhibit a second microlensing brightening episode when
the source or/and the lens is a binary system. A careful analysis of these
repeating events provides an independent way to study the statistics of wide
binary stars and to detect extrasolar planets. Previous theoretical studies
predicted that 0.5 - 2 % of events should repeat due to wide binary lenses. We
present a systematic search for such events in about 4000 light curves of
microlensing candidates detected by the Optical Gravitational Lensing
Experiment (OGLE) towards the Galactic Bulge from 1992 to 2007. The search
reveals a total of 19 repeating candidates, with 6 clearly due to a wide binary
lens. As a by-product we find that 64 events (~2% of the total OGLE-III sample)
have been miss-classified as microlensing; these miss-classified events are
mostly nova or other types of eruptive stars. The number and importance of
repeating events will increase considerably when the next-generation wide-field
microlensing experiments become fully operational in the future.Comment: 11 pages, 4 figures (+ appendix A) and 3 table
An ice giant exoplanet interpretation of the anomaly in microlensing event OGLE-2011-BLG-0173
We analyze the microlensing event OGLE-2011-BLG-0173, which shows a small
perturbation at the end of the microlensing event caused by the primary lens.
We consider both binary lens and binary source models and we explore their
degeneracies, some of which have not previously been recognized. There are two
families of binary lens solutions, one with a mass ratio
and a separation s~4.6 and the other with q~0.015 and
s~0.22, i.e, both have companions in the planetary regime. We search for
solutions by using Bayesian analysis that includes planet frequency as a prior
and find that the s~4.6 family is the preferred one with ~4 M_Uranus mass
planet on an orbit of ~10 AU. The degeneracies arise from a paucity of
information on the anomaly, demonstrating that high-cadence observations are
essential for characterizing wide-orbit microlensing planets. Hence, we predict
that the planned WFIRST microlensing survey will be less prone to these
degeneracies than the ongoing ground-based surveys. We discuss the known
low-mass, wide-orbit companions and we notice that for the largest projected
separations the mass ratios are either high (consistent with brown dwarf
companions) or low (consistent with Uranus analogs), but intermediate mass
ratios (Jupiter analogs on wide orbits) have not been detected to date, despite
the fact that the sensitivity to such planets should be higher than that of
Uranus analogs. This is therefore tentative evidence of the existence of a
massive ice giant desert at wide separations. On the other hand, given their
low intrinsic detection sensitivity, Uranus analogs may be ubiquitous.Comment: AJ accepted, 6 figures, 4 table
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