Dario Grasso on the Fermi Gamma-ray Space Telescope
New Hot Paper Commentary, July 2010
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ArticleOn possible interpretations of the high energy electron-positron spectrum measured by the Fermi Large Area Telescope
Authors: Grasso, D, et al. |
Dario Grasso talks with ScienceWatch.com and answers a few questions about this month's New Hot Papers paper in the field of Space Science.
Why do you think your paper is highly
cited?
One year after the launch of the Fermi Gamma-ray Space Telescope, which took place in June 2008, the Fermi Large Area Telescope (LAT) collaboration released the spectrum of the Cosmic Ray Electrons (CRE) between 20 GeV and 1 TeV. Our work was one of the very first, and still one of the most comprehensive, attempts to interpret that extraordinary measurement.
Several related observations came just before Fermi-LAT to warm up the interest in this subject. Most noticeably, the ATIC balloon-borne experiment claimed a pronounced bump in the electron + positron (e- + e+) spectrum at ~ 500 GeV, and the PAMELA orbital observatory found a rising behavior of the e+/(e- + e+) fraction above 10 GeV. Both features are not expected in the standard scenario in which cosmic ray electrons are accelerated by Galactic supernova remnants.
While Fermi-LAT, and subsequently the H.E.S.S. atmospheric Cherenkov telescope, did not confirm the ATIC spectral bump, it found the CRE spectrum to be significantly flatter than that inferred on the basis of previous experimental data.
We pointed out that while the CRE spectrum measured by Fermi-LAT may still fit in the standard scenario, a consistent interpretation of most available data sets, including those provided by H.E.S.S. and PAMELA, calls for the presence of a new electron and positron component peaked at about 1 TeV.
Furthermore, we showed as such extra e± component may be produced either by intense e± astrophysical emitters within few hundred parsecs from the solar system (one or a few pulsars, most likely) or by dark matter annihilation or decay. Our results, therefore, contributed significantly to scientific debate on the interpretation of those very important experimental results.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
Taking Fermi-LAT, H.E.S.S., and PAMELA experimental data on the same ground, the results of our analysis point to the presence of a previously unknown component in the electron and positron cosmic flux above 100 GeV. As mentioned above, we provided two viable interpretations for its origin.
Concerning the methodology of our work, we developed a new approach which, in order to properly model the different components of the CRE spectrum in different energy ranges, consistently combines dedicated numerical and semi-analytical tools. To compute the electron Galactic large scale component up to few hundred GeV, which, according to the common wisdom, we assumed to be generated by standard supernova remnants, we used the GALPROP package.
Figure 1:
Pictorial view of the Fermi spacecraft in orbit above the Earth.
Figure 2:
Cosmic ray electron spectrum data points measured by Fermi-LAT..
View larger images & complete descriptions in tabs
below.
This has become a standard analysis tool in cosmic ray and diffuse gamma-ray research (it is a tool adopted for the interpretation of Galactic emission for the Large Area Telescope on Fermi). At larger energies the CRE spectrum is expected to be dominated by a few nearby astrophysical sources or from dark matter annihilation or decay. We estimated the possible contribution of those sources by means of dedicated semi-analytical routines which we developed for that purpose.
Such routines also allow a prediction the CRE anisotropy which, when compared with high-statistics data being taken by Fermi-LAT, may help to discriminate among the different viable interpretations.
Would you summarize the significance of your paper
in layman's terms?
The TeV (tera-electronvolt) energy scale is currently in the spotlight of both astrophysics and particle physics communities. The Large Hadron Collider (LHC) which was built at the European Center for Nuclear Physics (CERN) has just started operating to look for possible signatures of physics beyond the standard model which are expected to emerge around that energy.
Several well-motivated theories predict the dark matter pervading our Galaxy and the Universe to be composed of particles that can be discovered at LHC. An indirect evidence of such new physics may also be found in space if dark matter particles in the Milky Way halo annihilate/decay into electrons and positrons (a positron is the antiparticle of an electron).
Among the aims of Fermi-LAT and PAMELA (as well as other planned space experiments) there is the search of an electron and positron flux excess respect to that expected from astrophysical sources.
Even if the signature of dark matter weren't found in the cosmic ray electron spectrum, such measurements and their interpretation would provide valuable information about the nature of cosmic rays electron sources (e.g. supernova remnants or pulsars) and of the properties of the interstellar medium.
Good measurements, however, are not enough. The signals expected to be generated by dark matter and astrophysical sources need to be properly modeled on the basis of theories and complementary data in order to permit any meaningful interpretation of those measurements. This is what we tried to do in our paper.
How did you become involved in this research and
were any particular problems encountered along the way?
I was lucky joining the Fermi-LAT collaboration, as an affiliated member in the INFN (the Italian National Institute for Nuclear and Particle Physics) Pisa group directed by Prof. R. Bellazzini, just few months before the launch of the observatory. Our group is very active in the analysis of the cosmic electron data and that gave me the opportunity to closely follow the measurement of the CRE spectrum.
At the same time I was intrigued by the positron-to-electron fraction measurement performed by PAMELA satellite experiment, in which INFN is also considerably involved. As soon as Fermi-LAT results came, I therefore tried to look for a consistent interpretation of the results of both those experiments.
To work within a big collaboration as Fermi-LAT is an enriching, though not always easy experience for me, as I was used to working within small groups as a particle astrophysics theoretician.
Where do you see your research leading in the
future?
In the near future, Fermi-LAT, PAMELA, and the AMS-02 cosmic ray observatory—which will be installed on board the International Space Station at the end of this year—will provide new, and more accurate, measurements of the cosmic ray electron and positron spectra and flux isotropy, as well as complementary observations of other cosmic rays species and especially of the diffuse gamma-ray emission of the Galaxy.
Taken together, these observations should considerably reduce the degeneration which still exists among the different models proposed in our paper and many others. We will continue working to look for consistent interpretations of all those measurements.
Do you foresee any social or political
implications for your research?
Particle astrophysics, which was born in the early days of cosmic-ray physics about a century ago, is nowadays flourishing with the launch of several balloons and satellites as well as the construction of new type of atmospheric gamma-ray telescopes and underground, deep sea or deep ice, neutrino observatories.
The realization of those instruments and the interpretation of their data require the synergy of forefront astrophysics and particle physics experimental and theoretical techniques and methods. Some of those developments may have several applications in modern technology.
Furthermore, especially if supported by a valid public outreach, particle astrophysics discoveries and debate attract a wide audience which help to increase the general interest in science and, hopefully, political decisions in favor of education and scientific research.
I hope that our work and results will contribute, even a little, making this interest to grow.
Acknowledgements
I thank Stefano Profumo (Santa Cruz Institute for Particle Physics,
University of California, USA) and Andrew W. Strong (Max Planck Institute
für extraterrestrische Physics, Garching, Germany), who are
corresponding co-authors, as well as all other co-authors and especially my
graduate student Daniele Gaggero, for their support and help writing our
paper and these answers. A special thanks also goes to Seth Digel and David
Thomson, of the Fermi-LAT collaboration, for reading the draft of these
answers and giving me several valuable comments.
Dario Grasso, Ph.D.
Contract Researcher
Istituto Nazionale di Fisica Nucleare (INFN)
Sezione di Pisa
Pisa, Italy
KEYWORDS: COSMIC RAY ELECTRONS AND POSITRONS, PULSARS, DARK MATTER, FERMI-LAT, SUPERNOVA REMNANTS, GAMMA RAYS, PROPAGATION, ORIGIN, CRE SPECTRUM.
Image 1:
Image 1:
Pictorial view of the Fermi spacecraft in orbit above the Earth. From the NASA, Fermi mission web site.
Image 2:
Image 2:
In this figure the cosmic ray electron spectrum data points measured by Fermi-LAT (in red) are pictorially compared with the prediction of our model (white line). This is supposed to be the sum of a conventional component (blue line), which is expected to be produced by Galactic Supernova Remnants, and a new one (red line) which may be originated by nearby pulsars or dark matter annihilation.