Installation of BrutusPN

Prerequisites

For Brutus, the following arbitrary precision libraries are necessary:

• gmp : https://gmplib.org/
• mpfr : http://www.mpfr.org/
• mpfr c++ : http://www.holoborodko.com/pavel/mpfr/

Also, the g++ compiler should be present.

For the Python code, there is a dependency on numpy. For plotting the output, matplotlib is necessary.

Making BrutusPN

• Adjust the path to brutuspn_bhcluster/brutuspn/mpreal/mpreal.h in makefile
• Change working directory to /brutuspn/ directory
• Type make

BrutusPN is only tested on Linux, so it is only guaranteed to work on Linux.

Instructions for starting a BrutusPN simulation for the BH cluster experiment

Step 0. set mass and distance scale, run prepare_initcond.py

The simulations can only be run with the working directory set to the brutuspn_bhcluster. The mass and distance scale are set by adjusting the MNUM and RNUM parameters in the prepare_initcond.py file, respectively. The numbers are fractions of the mass of SgrA* [Msol] and a parsec, respectively. After setting, run the python script and all initial conditions will be converted to the Nbody units. Also, the dimensionless speed of light will be set and the internal timescaling will be set. Remember, this script must be run at least ONCE on every new machine that the simulations will be run on.

Step 1. Adjust parameters in run_brutuspn.sh

Open the run_brutuspn.sh file and adjust the following to your requirements:

• INITCOND: Name of initial condition to run (without ANY extensions)
• STARTTIME: Starting time of simulation in kyr (can usually be kept at zero)
• ENDTTIME: Endtime of simulation in kyr
• DTSNAP: Snapshot interval for writing output in kyr
• EPS: Power of Bulirsch-Stoer tolerance. Note that it should be positive. For example, if a tolerance of 1e-10 is required, set EPS=10.
• RMERGE: Fraction of R_ISCO to use as merger radius (i.e., if two bodies get within this radius, a merger will take place and simulation will be terminated). The other parameters can be changed but usually dont need changing.

Step 2. Run bash run_brutuspn.sh

The output will be stored in a folder called ./outputs/, in the following format:

• <INITCOND>.out: Position, velocity, acceleration, and jerk of each body, as function of time, in physical units of m, m/s, m/s^2, and m/s^3
• <INITCOND>.energies: Total Nbody energy of the system as function of time. This file contains the timestamps
• <INITCOND>.log: This file contains a log of the simulation, which includes relevant simulation parameters. Most importantly, the last line contains the indices of the merging bodies inside the initial condition.

Step 3. Inspect the output using plot_output.py

The output can be inspected in real-time (i.e., during a simulation), by running the following command: python plot_output.py -n <INITCOND>. The script automatically plots the simulation projected in the x-y plane, but this can be changed by passing the extra -p command. For an overview of projection options, run python plot_output.py -h.