Hello Maria,

I have two few quick questions that are intended to facilitate a user-friendly approach to the EinsteinToolkit in order to be used for an undergraduate project. The first question concerns the availability of out-of-the-box parameter files for the evolution of a binary system. Are the parameter files in repos/einsteinexamples/par/ maintained? If not, would it be possible to have up to date parameter files for the evolution of a binary system: bbh and neutron-bh, with the current version of the toolkit?

BBH we have, you can use either the qc0-mclachlan in the examples directory which is very short or Ian, Eloisa and Barry's GW150914 parameter file from the gallery (http://einsteintoolkit.org/about/gallery/gw150914/) which also comes with example Mathematica notebooks on how to analyse it. It is quite a bit bigger though. I am not aware of a public NS-BH mixed system parameter file. For NS-NS you could try my gallery example http://einsteintoolkit.org/about/gallery/NsNsToHMNS/ which is occasionally tested (meaning it almost certainly won't work right now).

My second question is about the physical meaning of the coordinate system. My understanding about units is that you get the physical time in seconds by the formula: t[s]=t[code]*M_binary*M_sun*G/c^3 x[m]=x[code]*M_binary*M_sun*G/c^2 Will you please confirm or infirm this?

Not having exactly checked the powers of G and c, the units are such that c=1 M_sun=1 G=1 ie one mass unit is 2e33g (I think), 1 length units is 1.48km and one time unit is 4.93e-6s.

Is this measured in a plain Cartesian coordinate system centered in the center of mass of the system where x=0? Should t=0 be the time of collision or the initial time? Is this the only coordinate system used, or there is a different one in the wave zone?

This is the only coordinate system used (nowadays) and is the coordinates in the x,y,z grid functions. Llama introduces extra local coordinates which however do not show up anywhere in the output ie all tensors are w.r.t. to x,y,z. So with Llama the grid points may *not* be arranged on a Cartesian grid but each one is still labelled by its Cartesian coordinates. t=0 is usually (this *can* be changed but I have never seen it done) the beginning of the simulation. Sometimes for data analysis people will in *postprocessing* shift time so that the merger signal (max amplitude) is at t=0. Usually we try and put x=0 roughly at the center of mass though this is not necessary and certainly not done in an exact (since the center of mass is hard to define) manner.

Yours, Roland