Andrew W. Strong, Igor V.
Moskalenko & Olaf Reimer talk with
ScienceWatch.com and answer a few questions about
this month's Emerging Research Front Paper in the field of
Space Science.
Article: Diffuse Galactic continuum gamma rays: A
model compatible with EGRET data and cosmic-ray
measurements Authors:
Strong,
AW;Moskalenko, IV;Reimer, O Journal: ASTROPHYS
J, 613 (2): 962-976 Part 1, OCT 1 2004
Addresses: Max Planck Inst Extraterr Phys, Postfach 1603,
D-85740 Garching, Germany.
Max Planck Inst Extraterr Phys, D-85740 Garching,
Germany.
NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771
USA.
Ruhr Univ Bochum, D-44780 Bochum, Germany.
Why do you think your paper is highly
cited?
A large number of outstanding problems in physics and astrophysics are
connected with studies of cosmic rays and the associated emission (radio,
microwave, X-rays, gamma rays) produced during their propagation in
interstellar space.
Among these problems are indirect searches for dark matter, the origin and
propagation of cosmic rays, particle acceleration in putative cosmic ray
sources—such as supernova remnants—and the interstellar medium,
cosmic rays in other galaxies and the role they play in galactic evolution,
studies of our local Galactic environment, cosmic ray propagation in the
heliosphere, and the origin of extragalactic diffuse emission.
New or improved instrumentation to explore these open issues is ready or
under development. A fleet of ground-based, balloon-borne, and spacecraft
instruments measures many cosmic ray species, gamma rays, radio, and
synchrotron emission.
Our state-of-the-art model, called Galactic Propagation, or GALPROP, is the
only one which combines all cosmic-ray isotopes (from hydrogen to nickel)
and other cosmic-ray particles (such as electrons, positrons, protons, and
antiprotons) as well as photon emission mechanisms in a single
self-consistent framework.
Coauthor
Igor V. Moskalenko
Coauthor
Olaf Reimer
The paper describes the GALPROP model and the results of a study of
propagation of cosmic rays in the Milky Way Galaxy and production of
secondary particles and isotopes as well as associated diffuse gamma-ray
and synchrotron emission.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
The complex nature of current scientific goals, such as the detection of a
weak dark matter signal on top of the intense diffuse gamma-ray emission
produced by cosmic rays interacting with the interstellar medium, or the
study of electrons and positrons in cosmic rays, requires reliable and
detailed calculations. This dictates that a numerical model be used.
Many of the latest developments of astrophysics, and particle and nuclear
physics, play a role in addressing these questions—cosmic ray
acceleration and transport mechanisms, detailed maps of the
three-dimensional Galactic gas distribution, detailed studies of the
interstellar dust, radiation, and magnetic fields, and improved particle
and nuclear cross-section data and codes. Achieving these scientific goals
requires a realistic model, yet one which is simple to access and use.
As mentioned above, the paper described the best current model for
propagation of cosmic rays in the Milky Way Galaxy and production of
secondary particles and isotopes as well as associated diffuse gamma-ray
and synchrotron emission.
It combined a new methodology—a numerical approach to a problem which
is far beyond the simple analytical solutions which had dominated the
subject in the past, plus the synthesis of using all available diverse
types of data within a single model.
Because of the complex character of the model, its parts—such as
distributions of gas and radiation field as well as descriptions of various
processes—can also be used for many independent studies.
Would you summarize the significance of your paper in
layman's terms?
Exploration of the Milky Way galaxy and beyond, research in the
astrophysics of cosmic rays and gamma rays are primary elements in many
ground-based telescopes and balloon-borne and space missions. Such
information may lead to breakthroughs and discoveries in many areas of
astroparticle physics and cosmology.
Examples are the search for dark matter and antimatter, studies of the
nucleosynthesis, acceleration of nuclei and their fragmentation through
cosmic ray spectra and composition studies, the effects of heliospheric
modulation, and the origin of Galactic and extragalactic diffuse gamma-ray
emission.
The power of GALPROP to simultaneously predict many relevant observable
quantities makes this model a unique tool in the analysis of data from a
number of current and future balloon-borne and space missions.
GALPROP is a current state-of-the-art cosmic ray propagation code and has
become a standard analysis tool in cosmic ray and diffuse gamma-ray
research (e.g., it is the model adopted for the interpretation of Galactic
emission for the Large Area Telescope on Fermi).
GALPROP
logo...
A
schematic view of
cosmic
ray...
A skymap
>100
MeV...
The GALPROP code can be used to address topics as diverse as cosmic ray
isotopic composition, calculation of the background for indirect dark
matter searches and propagation of a potential dark matter signal, to
providing the gamma-ray background model for the analysis of gamma-ray
sources.
At the same time, it provides a unified framework for the interpretation of
data collected by many different kinds of experiments. This powerful
approach emphasizes the inter-relationship between different types of data,
and allows the researcher to systematically study their importance for a
global picture of the high-energy Galaxy.
How did you become involved in this research and were
any particular problems encountered along the way?
Andrew Strong:
I have been interested in high-energy astrophysics since my doctoral
thesis, and have participated in a number of space missions on this topic
from the pioneering times onward. My first attempt to construct this type
of model of cosmic ray propagation and the diffuse emission of the Galaxy
started with a research student in 1991. However, it did not develop into
the current project until December of 1996 when Igor Moskalenko, a
theoretician working in particle astrophysics, proposed we combine forces
and work on the model together.
As often happens in science, Igor and I worked in the same group in the
Max-Planck-Institute for Extraterrestrial Physics in Garching for half a
year and knew little about each other's work. It was necessary to go to a
meeting of the COMPTEL collaboration in ESA/ESTEC, in Noordwijk, The
Netherlands, to learn of each other's work and to form a long-term
collaboration.
At that time we decided that the code should be made publicly available.
The objectives to make the code public were (i) to encourage people to take
part in its development which may create a synergistic effect, and (ii) to
give people an opportunity to use it for many applications that we could
not have imagined at that time. It turned out to become a very successful
project.
The first version of GALPROP was made generally available to the public in
1998, and a dedicated website with a new advanced version went online in
2006. The project is constantly developing and has been funded by NASA
through various research grants since 1999. The current team has several
members, each making important contributions. A new version will be
released this year. Despite the progress, there is always a great deal more
to be done to make the model more realistic and to make use of new data,
etc.
Olaf Reimer, the other author on this paper, was a member of the EGRET team
of NASA's Gamma Ray Observatory, and provided the essential insight into
details of this experiment which was the main focus of this paper. Like
Andrew and Igor, Olaf is now part of the Fermi Gamma Ray Observatory
collaboration.
Do you foresee any social or political implications for
your research?
Particle astrophysics, which has recently emerged as an interdisciplinary
science, is flourishing nowadays. It was born in the early days of
cosmic-ray physics about a century ago and then reborn twice more, first
with the launch of the first X-ray telescopes, and then with the apparent
discovery that the matter in the universe is dominated by something
unknown, the dark matter.
The latter rebirth brought an army of particle physicists into
astrophysics. Particle astrophysics is now a busy intersection between
high-energy astrophysics, particle physics, and cosmology. It happens that
our research, which started before the current boom in particle
astrophysics, is currently at the forefront.
The availability of computer resources and our free, but sophisticated code
enables undergraduates, graduates, and postdocs from educational and
scientific institutions from around the world to enter the fascinating
field of astroparticle physics and conduct their own research at the
leading edge.
Particularly satisfying for us was the influence this work had on the way
this subject evolved; it goes beyond this particular paper and code, and
affects the whole strategy by which the topic is approached.
Andrew W. Strong, Ph.D.
Max-Planck-Instituts für extraterrestrische Physik (MPE)
Garching, Germany Web | Web
Igor V. Moskalenko, Ph.D.
Senior Staff Scientist
Hansen Experimental Physics Laboratory
& Kavli Institute for Particle Astrophysics and Cosmology
Stanford University
Stanford, CA, USA Web
Olaf Reimer,Ph.D.
Professor and Chair of Experimental Astro- & Particle Physics
Institut für Astro- und Teilchenphysik
Leopold-Franzens-Universität Innsbruck
Innsbruck
Austria
&
Kavli Institute for Particle Astrophysics and Cosmology
Stanford University
Stanford, CA, USA Web
KEYWORDS: SOUTHERN MILKY-WAY; SUPERNOVA-REMNANTS; CO SURVEY;
RADIAL-DISTRIBUTION; HELIUM SPECTRA; MOLECULAR CLOUDS; ENERGY-SPECTRA;
SOLAR MINIMUM; EMISSION; GALAXY.