An analytic approximation to the isothermal sphere

We present a useful analytic approximation to the solution of the Lane-Emden equation for infinite polytropic index - the isothermal sphere. The optimized expression obtained for the density profile can be accurate to within 0.04% within 5 core-radii and to 0.1% within 10 core-radii.Comment: 6 pages, figures included, revised version, accepted for publication in MNRA