  We study the topological susceptibility in 2+1 flavor QCD above the chiral
crossover transition temperature using Highly Improved Staggered Quark action
and several lattice spacings, corresponding to temporal extent of the lattice,
$N_\tau=6,8,10$ and $12$. We observe very distinct temperature dependences of
the topological susceptibility in the ranges above and below $250$ MeV. While
for temperatures above $250$ MeV, the dependence is found to be consistent with
dilute instanton gas approximation, at lower temperatures the fall-off of
topological susceptibility is milder. We discuss the consequence of our results
for cosmology wherein we estimate the bounds on the axion decay constant and
the oscillation temperature if indeed the QCD axion is a possible dark matter
candidate.
