Dear Jay,

The model we have developed is based on the well-known fireball model, and
it converts energy using the internal shock mechanism. We have chosen this
model because it is a very good candidate for simulating the rapid
variability observed in the GRB signals. The model generates a series of
shells with different Lorentz factor, randomly chosen. The number of shells,
their initial separation, their thickness, and the total energy available
can be modified from the user to simulate different behaviors. The shells
evolve with time, and when a shock occurs the code calculates the main
parameters that are needed for describing the spectra. In particular, using
the equipartition of the energy between electron accelerated, protons (the
minority) and the magnetic field, one can obtain the peak of the synchrotron
spectrum, and the principal quantities that determine the shape of the flux
and its temporal evolution.

For each shock the rise time of the flux depend on the size of the resulting
shell, while the decay time depends on the cooling time (that it can be
expressed as function of the energy of the out coming photon - roughly it
goes as 1/sqrt(observed energy) - The emission mechanisms that we consider
are the synchrotron emission and the Inverse Compton Scattering (SSC).
With this simple assumption we are able to reproduce light curves and
spectrum typical of GRBs.

The next step we have done is to insert the GRB simulation in the GLAST
simulation software (the Montecarlo based code that simulates the response
of the detector).

For doing this we wrote an algorithm that extract photons in agreement with
the GRB flux (at a certain time): in this way we feed the detector
simulation with the photons produced by an astrophysical source that varies
with the time.

The code that simulates the GRB is entirely written in C++, and it is part
of the GLAST simulation software, still under development.

The main goal is to have a simulation of a rapid variable astrophysical
source, and to use it as source input for the detector simulation.  The idea
is that the user is able to choose "GRB" under the list of sources and to
have a distribution of photon that reach the detector similar to the one
expected from a GRB event (under the assumption that GRBs are fireballs!).
In this way we will be able to analyze the GLAST performance in the study of
GRBs and in the study of transient sources.

Moreover, the fact that we have developed our model using an object oriented
language, allow us to define some common "classes" that will help in the
description of different GRB scenarios, or, also, different celestial
objects. For example the concept of "shocks" or "shell" could be used to
obtain the description of GRB in the external shock phase during the
interaction with the ISM, or in the simulation of AGNs.

To have an idea of the work done, see the power point presentation at:
http://glastserver.pi.infn.it/glast/presentations/omodei180202.ppt
or, in general, the other documents in the "presentation=" area at:
http://glastserver.pi.infn.it/glast/

I'll send you any other presentation/  report/ draft concerning the GRB
simulation  model and our activity in Pisa about the study of GRB &transient
phenomena.

We are also interested in the cooperation of the preparation of the mailing
list and in helping the group activity starting up.

cheers,
Nicola Omodei