YNOGK: A new public code for calculating null geodesics in the Kerr spacetime

Understand

Following \cite{dexagol2009} we present a new public code for the fast calculation of null geodesics in the Kerr spacetime.

• Using Weierstrass' and Jacobi's elliptic functions, we express all coordinates and affine parameters as analytical and numerical functions of a parameter $p$, which is an integral value along the geodesic.
• This is a main difference of our code compares with previous similar ones.
• The advantage of this treatment is that the information about the turning points do not need to be specified in advance by the user, and many applications such as imaging, the calculation of line profiles or the observer-emitter problem, etc become root finding problems.