[Users] two quick question regarding parameter files and systems of coordinates

[Roland Haas]

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

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