<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _xy_idx(self):
 """ 1D arrays of x and y indices of unmasked pixels in the cutout reference frame. """


        yidx, xidx = np.indices(self._data.shape)
        return xidx[~self.mask].ravel(), yidx[~self.mask].ravel()