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Register a new userRecent Developments in Gravitational Microlensing and the Latest MACHO Results: Microlensing Towards the Galactic Bulge
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We review recent gravitational microlensing results from the EROS, MACHO, and OGLE collaborations, and present some details of the very latest MACHO results toward the Galactic Bulge. The MACHO collaboration has now discovered in excess of 40 microlensing events toward the Galactic Bulge during the 1993 observing season. A preliminary analysis of this data suggests a much higher microlensing optical depth than predicted by standard galactic models suggesting that these models will have to be revised. This may have important implications for the structure of the Galaxy and its dark halo. Also shown are MACHO data of the first microlensing event ever detected substantially before peak amplification, the first detection of parallax effects in a microlensing event, and the first caustic crossing to be resolved in a microlensing event.
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We present the microlensing optical depth towards the Galactic bulge based on the detection of 99 events found in our Difference Image Analysis (DIA) survey. This analysis encompasses three years of data, covering ~ 17 million stars in ~ 4 deg^2, to a source star baseline magnitude limit of V = 23. The DIA technique improves the quality of photometry in crowded fields, and allows us to detect more microlensing events with faint source stars. We find this method increases the number of detection events by 85% compared with the standard analysis technique. DIA light curves of the events are presented and the microlensing fit parameters are given. The total microlensing optical depth is estimated to be tau_(total)= 2.43^(+0.39/-0.38) x 10^(-6) averaged over 8 fields centered at l=2.68 and b=-3.35. For the bulge component we find tau_(bulge)=3.23^(+0.52/-0.50) x 10^(-6) assuming a 25% stellar contribution from disk sources. These optical depths are in good agreement with the past determinations of the MACHO Alcock et al. (1997) and OGLE Udalski et al. (1994) groups, and are higher than predicted by contemporary Galactic models. We show that our observed event timescale distribution is consistent with the distribution expected from normal mass stars, if we adopt the stellar mass function of Scalo (1986) as our lens mass function. However, we note that as there is still disagreement about the exact form of the stellar mass function, there is uncertainty in this conclusion. Based on our event timescale distribution we find no evidence for the existence of a large population of brown dwarfs in the direction of the Galactic bulge.
The MACHO project carries out regular photometric monitoring of millions of stars in the Magellanic Clouds and Galactic Bulge, to search for very rare gravitational microlensing events due to compact objects in the galactic halo and disk. A preliminary analysis of one field in the Galactic Bulge, containing {$sim430,000$} stars observed for 190 days, reveals four stars which show clear evidence for brightenings which are time-symmetric, achromatic in our two passbands, and have shapes consistent with gravitational microlensing. This is significantly higher than the $sim 1$ event expected from microlensing by known stars in the disk. If all four events are due to microlensing, a 95% confidence lower limit on the optical depth towards our bulge field is $1.3 times 10^{-6}$, and a ``best fit value is $tau approx 1.6 times 10^{-6}/epsilon$,where $epsilon$ is the detection efficiency of the experiment, and $epsilon < 0.4$. If the true optical depth is close to the ``best fit value, possible explanations include a ``maximal disk which accounts for most of the galactic circular velocity at the solar radius, a halo which is centrally concentrated, or bulge-bulge microlensing.
Recent work by the MOA gravitational microlensing group is briefly described, including (i) the current observing strategy, (ii) use of a high-speed parallel computer for analysis of results by inverse ray shooting, (iii) analysis of the light curve of event OGLE-2000-BUL12 in terms of extra-solar planets, and (iv) the MOA alert system using difference imaging.
Perhaps as many as 30 parallax microlensing events are known, thanks to the efforts of the MACHO, OGLE, EROS and MOA experiments monitoring the bulge. Using Galactic models, we construct mock catalogues of microlensing light curves towards the bulge, allowing for the uneven sampling and observational error bars of the OGLE-II experiment. The fraction of parallax events with delta chi^2 > 50 in the OGLE-II database is around ~1%, though higher fractions are reported by some other surveys. This is in accord with expectations from standard Galactic models. The fraction of parallax events depends strongly on the Einstein crossing time (t_E), being less than 5% at t_E = 50 days but rising to 50% at t_E > 1 yr. We find that the existence of parallax signatures is essentially controlled by the acceleration of the observer normalised to the projected Einstein radius on the observer plane divided by t_E^2. The properties of the parallax events - time-scales, projected velocities, source and lens locations - in our mock catalogues are analysed. Typically, ~38% of parallax events are caused by a disk star microlensing a bulge source, while ~33% are caused by a disk star microlensing a disk source (of these disk sources, one sixth are at a distance of 5 kpc or less). There is a significant shift in mean time-scale from 32 d for all events to ~130d for our parallax events. There are corresponding shifts for other parameters, such as the lens-source velocity projected onto the observer plane (~1110 km/s for all events versus ~80 km/s for parallax events) and the lens distance (6.7 kpc versus 3.7 kpc). We also assess the performance of parallax mass estimators and investigate whether our mock catalogue can reproduce events with features similar to a number of conjectured `black hole lens candidates.
We report the detection of 45 candidate microlensing events in fields toward the Galactic bulge. These come from the analysis of 24 fields containing 12.6 million stars observed for 190 days in 1993. Many of these events are of extremely high signal to noise and are remarkable examples of gravitational microlensing. The distribution of peak magnifications is shown to be consistent with the microlensing interpretation of these events. Using a sub-sample of 1.3 million ``Clump Giant stars whose distance and detection efficiency are well known, we find 13 events and estimate the microlensing optical depth toward the Galactic Bulge as $tau_{rm bulge} = 3.9 {+ 1.8 atop - 1.2} times 10^{-6}$ averaged over an area of $sim 12$ square degrees centered at Galactic coordinates $ell = 2.55^circ$ and $b = -3.64^circ$. This is similar to the value reported by the OGLE collaboration, and is marginally higher than current theoretical models for $tau_{rm bulge}$. The optical depth is also seen to increase significantly for decreasing $vert bvert$. These results demonstrate that obtaining large numbers of microlensing events toward the Galactic bulge is feasible, and that the study of such events will have important consequences for the structure of the Galaxy and its dark halo.
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