Meaning of unique_ptr

Why unique_ptr can take owership of a const ptr? What is the meaning of unique_ptr < const T>?

  std::unique_ptr<int> a(new int);
  // Only this is compile error.                                 
  std::unique_ptr<int> b((const int*)new int);
  std::unique_ptr<const int> c(new int);
  // Why unique_ptr can take a const ptr, and delete it when deconstruct?
  std::unique_ptr<const int> d((const int*)new int);

Tested with g++ 4.8