Using 7 years of MACHO survey data , we present a new determination of the optical depth to microlensing towards the Galactic bulge .
We select the sample of 62 microlensing events ( 60 unique ) on clump giant sources and perform a detailed efficiency analysis .
We use only the clump giant sources because these are bright bulge stars and are not as strongly affected by blending as other events .
Using a subsample of 42 clump events concentrated in an area of 4.5 deg ^ { 2 } with 739000 clump giant stars , we find \tau = 2.17 ^ { +0.47 } _ { -0.38 } \times 10 ^ { -6 } at ( l,b ) = ( 1 \hbox { $ . ^ { \circ } $ } 50 , -2 \hbox { $ . ^ { \circ } $ } 68 ) , somewhat smaller than found in most previous MACHO studies , but in excellent agreement with recent theoretical predictions .
We also present the optical depth in each of the 19 fields in which we detected events , and find limits on optical depth for fields with no events .
The errors in optical depth in individual fields are dominated by Poisson noise .
We measure optical depth gradients of ( 1.06 \pm 0.71 ) \times 10 ^ { -6 } { deg } ^ { -1 } and ( 0.29 \pm 0.43 ) \times 10 ^ { -6 } { deg } ^ { -1 } in the galactic latitude b and longitude l directions , respectively .
Finally , we discuss the possibility of anomalous duration distribution of events in the field 104 centered on ( l,b ) = ( 3 \hbox { $ . ^ { \circ } $ } 11 , -3 \hbox { $ . ^ { \circ } $ } 01 ) as well as investigate spatial clustering of events in all fields .