  We present the results of simulations done with the code HARM-COOL developed
in the CTP PAS Warsaw research group over the years 2017-2019. It is based in
the original GR MHD scheme proposed by Gammie et al. (2003) for the simulation
of Active Galactic Nucleus, but now it has been suited for the engine of a
short Gamma Ray Burst event.
  We compute time-dependent evolution of a black hole accretion disk, in
two-dimensional, axisymmetric scheme. The code includes neutrino cooling and
accounts for nuclear structure of dense, degenerate matter. Free protons,
neutrons, and electron-positron pairs form a neutron-rich, magnetically driven
outflow that provides site for subsequent r-process nucleosynthesis.
  Here the heavy elements up to the Uranium and Gold are synthesized and may
contribute to the chemical enrichment of the circum-burst medium. Their
radio-active decay will give signal in lower energies in a timescale of
weeks-months after the GRB prompt phase.
  In addition, the magnetic fields are responsible for the launching of
ultra-relativistic jets along the rotation axis of the central black hole,
according to the well-known Blandford-Znajek mechanism. These jets are sites of
variable high energy emission in gamma rays. We find that the magnetic field
and the black hole spin account for the observed variability timescales and jet
energetics.
