Last Update: October 2000

GLAST

Collaboration:

- Boston University
- UC Santa Cruz
- CEA Saclay
- University of Chicago
- Columbia University
- INFN of Ferrara
- Goddard Space Flight Center
- Hiroshima University
- Kanagawa University
- Lockheed-Martin Palo Alto Research Laboratory
- Max Planck Institut fur Extraterrestiche Physik
- Naval Research Laboratory
- INFN of Perugia
- INFN of Pisa
- INFN of Roma 2
- Sonoma State University
- Stanford Linear Accelerator Center
- Stanford University
- University of Tokyo
- INFN of Trieste
- International Center for Theoretical Physics Trieste
- University of Utah
- University of Washington

1. Goal of the experiment

High-energy cosmic gamma rays probe the most energetic phenomena in nature. Observations of gamma rays in the GLAST energy range( ~10 MeV to 300 GeV) combined with a signicantly larger field of view, highly efficient duty cycle superior pointing and energy resolutions and long lifetime provide a unique and compelling opportunity to answer definitevily a wealth of questions and at the same time to open up major discovery opportunities. The topics GLAST will address include:

A statistically powerful characterization of Active Galactic Nuclei (AGNs).

The power output of AGNs which are currently understood to be super-massive black holes surrounded by accreting matter is enormous ( 1045 erg/s) and time-variable. Furthermore AGN are often ”blazar” class sources, that is they send most of their photon energy to us in the GeV range and above. EGRET has observed about 50 AGNs; extrapolating GLAST should see more than 5000 allowing us to learn much more on how these huge accelerators work. In addition by characterizing the energy distribution as a function of AGN distance we can probe the density of ambient IR and optical light (called the extra galactic background light or EBL) in the early universe (since higher-energy gammas will pair convert with these photons). Probing the ambient EBL density will help us to determine the era of galaxy formation.

Transients.

The high-energy gamma-ray sky is remarkably variable with fluxes changing by more than an order of magnitude in days or even hours. GLAST will be enormously important for the study of these transient phenomena. First with its large effective area, wide field of view and highly efficient duty cycle GLAST will detect transients quickly and can provide an alarm for a wide variety of detectors that either re-orient due to a limited field of view or must rely on statistical cross-correlation due to low event rates. Understanding important phenomena such us AGNs requires a coherent program of complementary measurements (optical, radio, x-ray and ground based > TeV photon detectors, neutrino telescopes etc.) and GLAST will play a key role in tying that program together.

A variety of topics related to supernova remnants and pulsars.

Fermi's seminal work on cosmic-ray acceleration eventually led to the hypothesis that supernova blasts are responsible for the flux of galactic cosmic rays (below 1015 eV). If so the surrounding medium should be swept up by these shocks producing neutral pions and hence gamma rays. Such extended sources should be imaged in gamma rays. GLAST with its substantially better pointing resolution will add valuable information to that already obtained by x-ray and ground-based Cherenkov telescopes. In addition GLAST will be able to test models of high-energy particle acceleration by neutron stars.

Probing for particle dark matter.

It is possible that galactic dark matter is composed of yet-undiscovered particles such as the lightest supersymmetric particle (LSP) in R-parity- conserving SUSY models. If so there are a number of pos-sible processes involving these particles that will yield characteristic gamma ray signature.

Gamma-ray Bursts (GRBs).

GRBs were recently in the news when the Beppo-SAX satellite detected a few transient bursts sufficiently quick to alert other telescopes, which detected fading signals at the location of the bursts, were able to measure their redshift and thereby to determine that those bursts came from a cosmological distance. GRBs may thus be the most energetic phenomena in the universe but the underlying mechanism is still a mystery. EGRET has observed high-energy emission from a few bursts and even observed an 18-GeV photon from one of them 75 minutes later. Are these delayed high-energy emissions a signature feature of gamma-ray bursts? GLAST will answer this question.

2. Physics achievements during 2000

The Italian collaboration is preparing in terms of structures, experience and manpower for the GLAST tracker assembly. The year 2000 has been devoted to the understanding of the kind of work and to the organization of the group.

Anyway, also in the italian groups a lot of activities have been going on:

- the design of the second GLAST silicon detector version for STMicroelectronics (end of 1999)
- the test on a testbeam of the first GLAST silicon detector version (August 1999)
- the definition of the final specifications of the silicon detector
- trials on the assembly procedures of the detectors in ladders and of the ladders on trays
- a detailed study of the materials used for the trays
- a detailed study of the adhesives used for gluing the detectors
- the development of the simulation code for GEANT4 (in collaboration also with CERN)

3. INFN contribution to the experiment in terms of manpower and financial support

Five INFN sections have expressed interest in GLAST, for a total of 15 FTE.

From the hardware point of view, the activity is concentrating on the tracker construction, given the experience of INFN as far as silicon detectors are concerned.

On the analysis point of view, several groups are collaborating with the American groups to write the instrument simulation in GEANT4, while, at the same time, starting to develop the analysis tools that will be used with the real data.

The financial support of INFN for the period 2000-2001 amounts to about 800 ML.

Discussions are still undergoing to fully define the financial participation of INFN to the project.

4. Number of publication in referred journals

Concerning the italian groups, none.

5. Number of talks to conferences

Concerning the italian groups, one talk

6. Number of undergraduate and doctoral thesis on the experiment

Concerning the italian groups, none.

7. Leadership role in the experiment

Cocnerning the italian group:

- Guido Barbiellini (INFN Trieste): italian spokesman
- Ronaldo Bellazzini (INFN Pisa): italian deputy chair in the Instrument Design Team

8. Innovative instruments

GLAST will be the experiment with the highest number of silicon channels on a satellite (1 million corresponding to 82 m2 of silicon).

The development and the construction of such a big detector is characterized by the problems peculiar to the high energy particle physics experiments (such as assembly lines, uniformity of products and procedures, test equipments). Moreover, it must be reminded that this detector has to fulfill all the requirements for space established by NASA.

9. Competing experiments

GLAST launch is scheduled for 2005: from then on GLAST will be the largest space g ray experiment ever existed.

AGILE will fly in the period 2003-2005 covering, even if with a smaller effective area and energy range, the period before GLAST.

No other satellite with these features is foreseen for the moment.

10. International committee which has reviewed the experiment

GLAST (with the option silicon detectors) has been approved in February for the phase B by NASA.