Dear Einstein Toolkit user
about three months ago Hannah Klion found and fixed a bug in the GRHydro code in the current release of the Einstein Toolkit.
You can see the commit here
https://bitbucket.org/einsteintoolkit/einsteinevolve/commits/c79140d
c79140d - Fixed bug in calculation of conserved S vector in MHD Prim2Con
This bug has since been fixed and the fix backported to the current release branch (2018_02, "Tesla").
Who was affected by this? Anyone who used GRHydro with a non-zero magnetic field using the C++ code has been affected. The C++ code has been the default version used since the ET_2015_05 "Hilbert" release.
What is the effect of this bug? The bug causes the conserved momentum density Si used to compute the numerical fluxes to be incorrectly computed, missing a factor of the square root of the metric determinant in front of the magnetic field contribution. This did not affect the initial conversion from primitive variables to conserved variables, which uses the old, correct Fortran code. Only the flux computation was affected.
The effect will be strongest in regions where the magnetic field is strong and in regions of strong gravitational fields.
How do I update an existing checkout? You can use git to update to the newest version of the release or development branches:
cd repos/einsteinevolve git pull
On behalf of the Einstein Toolkit maintainers, Roland Haas