def _dens(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _dens
        PURPOSE:
           evaluate the density for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the density
        HISTORY:
           2010-08-08 - Written - Bovy (NYU)
        """
        r= numpy.sqrt(R**2.+z**2.)
        return (self.a/r)**self.alpha/(1.+r/self.a)**(self.beta-self.alpha)/4./m.pi/self.a**3.