def _mass(self,R,z=0.,t=0.):
        """
        NAME:
           _mass
        PURPOSE:
           evaluate the mass within R for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           t - time
        OUTPUT:
           the mass enclosed
        HISTORY:
           2013-XX-XX - Written - Bovy (IAS)
        """
        if z is None: r= R
        else: r= nu.sqrt(R**2.+z**2.)
        return 2.*nu.pi*self.rc**(3.-self.alpha)*special.gammainc(1.5-self.alpha/2.,(r/self.rc)**2.)*special.gamma(1.5-self.alpha/2.)