  The type IIA superstring partition function Z_IIA on the euclidean attractor
geometry AdS_2 x S^2 x CY_3, computes the modified elliptic genus Z_BH of the
associated black hole. The hybrid formalism of superstrings defined as a
conformally invariant sigma model on the coset supermanifold
PSU(1,1|2)/U(1)xU(1), together with Calabi-Yau and chiral boson CFTs, is used
to calculate Z_IIA. The sigma model action on AdS_2 x S^2 is explicitly written
in U(1)xU(1) invariant variables. The N=2 generators of AdS_2 x S^2 x CY_3 are
enlarged and embedded in an N=4 topological algebra. The world sheet
superconformal invariance is then used to construct a nilpotent BRST operator,
in contrast to the kappa symmetry analysis used by Beasely et. al. in
hep-th/0608021. The sigma model action is explicitly shown to be closed under
this BRST operator. Localization arguments are then used to deform the world
sheet path integral with the addition of a BRST exact term, where,
contributions arise only from the center of AdS_2 and, the north and south
poles of S^2. This leads to the OSV result Z_BH = Z_IIA = |Z_top|^2, where
|Z_top|^2 is the square of the topological string partition function.
