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Paper text:
the structure and composition of our galaxy is one of the outstanding problems in contemporary astrophysics .
microlensing is a powerful tool to learn about massive objects in the galaxy .
the amount of matter between the source and observer is typically described in terms of the microlensing optical depth , which is defined as the probability that a source flux will be gravitationally magnified by more than a factor of 1.34 .
early analyses ( udalski et al .  1994 ;
alcock et al .
1997 ) of the lines of sight toward the galactic center produced two unexpected results : 1 ) a very high optical depth of @xmath3 inconsistent with galactic models and observations , 2 ) an overabundance of long events . here
we analyze a new set of events to probe these controversial issues .
blending is a major problem in any analysis of the microlensing data involving point spread function photometry .
the bulge fields are crowded , so that the objects observed at a certain atmospheric seeing are blends of several stars , of which only one is typically lensed .
this complicates a determination of an event s parameters and the analysis of the detection efficiency of microlensing events . if the sources are bright one can avoid these problems .
red clump giants are among the brightest and most numerous stars in the bulge .
therefore , this analysis concentrates on the events where the lensed stars are clump giants .
the macho project observations were performed with @xmath4-meter telescope at mount stromlo observatory , australia . in total , we collected 7 seasons ( 1993 - 1999 ) of data in the 94 galactic bulge fields .
the data that are currently available for the analysis consist of 5 seasons ( 1993 - 1997 ) in 77 fields , and contain the photometry of about 30 million stars , including 2.1 million clump giants .
the events with clump giants as sources have been selected from the sample of all events which contains about @xmath5 candidates .
the determination of which of these sources are clump giants , is investigated through the analysis of the global properties of the color - magnitude diagram in the galactic bulge .
using the accurately measured extinction towards baade s window allows one to locate _
clump giants on the reddening - free color - magnitude diagram .
this defines the parallelogram - shaped box in the upper left corner of the left panel of figure 1 . with the assumption that the clump populations in the whole bulge have the same properties as the ones in the baade s window
, the parallelogram described above can be shifted by the reddening vector to mark the expected locations of clump giants in different fields .
the solid lines are the boundaries of the region where one could find the clump giants in fields with different extinctions . using this approach
, we identified 52 clump events .
there is a high concentration of long - duration events in macho field 104 at @xmath0 as 5 out of 10 clump events longer than 50 days are in 104 .
the analysis of event durations _ uncorrected _ for efficiencies provides a lower limit on the difference between field 104 and all the other fields .
we use the wilcoxon s test on two samples : events in field 104 and all the remaining ones , and find that the events in 104 differ ( are longer ) at the level of @xmath6 .
in addition , field 104 also has the highest optical depth ( see figure 1 ) .
both of these features can be explained by the concentration of mass along this particular line of sight .
first , we report the average optical depth of : @xmath7 which is 1.5 - 2 times lower than the previously obtained values .
we caution that this result is only preliminary , because possible systematic errors may be a fair fraction of the statistical error .
we also note that about 40% of the optical depth is in the events longer than 50 days .
this is at odds with standard models of the galactic structure and kinematics .
second , we plot in the right panel of figure 1 the spatial distribution of the optical depth .
the variation of the optical depth is dominated by the poisson noise .
the gradient of the optical depth is stronger in @xmath8 than in @xmath9 direction .
again , we note the anomalous character of field 104 . it is marked with a black square and has an optical depth of @xmath10 .
we presented first results from the analysis of five years of microlensing data toward the galactic bulge collected by the macho collaboration .
it is possible to select an unbiased sample of clump events based only on an event s position on the color - magnitude diagram .
ten out of 52 clump events have durations @xmath11 days , which implies that @xmath12% of the optical depth is in the long events .
this is surprising , because long events are the most likely a result of the disk - disk lensing ( kiraga & paczyski 1994 ) , while clump giants trace the bar rather than the inner disk ( stanek et al .
field 104 centered on @xmath13 is anomalous : it has longer events and substantially higher optical depth . both effects can be explained simultaneously by a concentration of mass along this line of sight .
the optical depth averaged over the clump giants in 77 fields is @xmath14 at @xmath15 .
when anomalous field 104 is removed , the optical depth drops to @xmath2 , which is fully consistent with infrared - based models of the galactic bar .
we present preliminary results of the analysis of 5 years of macho data on the galactic bulge microlensing events with clump giants as sources . 
 this class of events allows one to obtain robust conclusions because relatively bright clump stars are not strongly affected by blending . 
 we discuss : 1 ) the selection of ` giant ' events , 2 ) the distribution of event durations , 3 ) the anomalous character of event durations and optical depth in the macho field 104 centered on @xmath0 . 
 we report the preliminary average optical depth of ( internal ) at @xmath1 , and present a map of the spatial distribution of the optical depth . 
 when field 104 is removed from the sample , the optical depth drops to @xmath2 , which is in excellent agreement with infrared - based models of the central galactic region .