Gamma-ray Large Area Space Telescope

1. Gamma-ray Large Area Space Telescope IEEE Nuclear Science Symposium Wyndham El Conquistador Resort, Puerto Rico October 23 - 29, 2005 The Gamma Ray Large Area Space Telescope: an Astro-particle Mission to Explore the High Energy Sky Luca Baldini INFN - Pisa

2. GLAST GLAST: Gamma-ray Large Area Space Telescope GLAST Burst Monitor (GBM) Large Area Telescope (LAT) Launch Vehicle Delta II – 2920-10H Launch Location Kennedy Space Center Orbit Altitude 575 Km Orbit Inclination 28.5 degrees Orbit Period 95 Minutes Launch Date mid 2007 • Large Area Telescope (LAT): • Pair conversion telescope. • Converter foils + tracker + calorimeter - surrounded by an anticoincidence shield. • Will detect photons in the 20 MeV – 300 GeV range. • GLAST Burst Monitor (GBM): • Set of 14 scintillators monitoring the full sky. • Energy range: 10 keV – 25 MeV. • Optimize to detect GRBs.

3. The need for a high-energy g-ray detector • Broad spectral coverage is crucial for understanding most astrophysical sources. • Multiwavelenght campaigns: space based and ground based experiments cover complimentary energy ranges. • The improved sensitivity of GLAST will match the sensitivity of the next generation of Cherenkov telescopes filling the energy gap in between the two approaches. • Overlap for the brighter sources: cross calibration, alerts. • Predicted sensitivity to point sources: • EGRET, GLAST and MILAGRO: 1 year survey. • Cherenkov telescopes: 50 hours observation. (from Weekes, et al. 1996 – GLAST added)

4. Outline • Talk outline: • The scientific case for the GLAST experiment . • Experimental technique and design of the Large Area Telescope. • Design, construction and testing of the silicon tracker. • Conclusions

5. The sky above 100 MeV: the EGRET survey • The heritage of EGRET: • Diffuse extra-galactic background (~ 1.5 x 10-5 cm-2s-1sr-1 integral flux). • Much larger (~ 100 times) background on the galactic plane (60% of 1.4 Mg). • Few hundreds of point sources (both galactic and high latitude, 10% of the total photons). • Essential characteristic: variability in time.

6. Sky map GLAST Survey: ~300 sources (2 days) GLAST Survey: ~10,000 sources (2 years) EGRET Survey: 271 sources

7. Unidentified sources • 170 point sources of the EGRET catalog still unidentified (no know counterpart at other wavelengths). • GLAST will provide much smaller error bars on sources location (at arc-minute level). • GLAST will be able to detect typical signatures (spectral features, flares, pulsation) allowing an easier identification with know sources. • Most of the EGRET diffuse background will be resolved into point sources. • Large effective area and good angular resolution are crucial! Cygnus region: 15o x 15o, E > 1 GeV Counting stats not included.

8. Active Galactic Nuclei • AGNs phenomenology: • Vast amount of energy from a very compact central volume. • Large fluctuations in the luminosity (with ~ hour timescale). • Energetic, highly collimated, relativistic particle jets • Prevailing idea: accretion onto super-massive black holes (106 – 1010 solar masses). • AGN physics to-do-list: • Catalogue AGN classes with a large data sample (at least ~ 3000 new AGNs) • Detailed study of the high energy spectral behavior. • Track flares (t ~ minutes). • Large effective area and excellent spectral capabilities needed!

9. Gamma Ray Bursts • GRBs phenomenology: • Dramatic variations in the light curve on a very short time scale. • Isotropic distribution in the sky (basically from BATSE, on board CGRO, but little data @ energies > 50 MeV). • Non repeating (as far as we can tell…). • Spectacular energies (~ 1051 – 1052 erg). • GRBs physics: • GLAST should detect ~ 200 GRBs per year above 100 MeV (a good fraction of them localized to better than 10’ in real time). • The LAT will study the GeV energy range. • A separate instrument on the spacecraft (the GBM) will cover the 10 keV – 25 MeV energy range. • Short dead time crucial! Simulated 1 year GLAST operation (Assuming a various spectral index/flux.)

10. Experimental technique Pair conversion exploited (provides the information about the g direction/energy and a clear signature for background rejection). • Pair conversion telescope: • Tracker/converter (detection planes + high Z foils): photon conversion and reconstruction of the direction (via electron/positron track reconstruction). Main L1 trigger (three x-y planes in a row hit) for GLAST. • Calorimeter: energy measurements. • Anti-coincidence shield: background rejection (charged cosmic rays flux typically ~104 higher than g flux). Real data collected during the integration and testing activity.

11. g e+ e- Overview of the Large Area Telescope • Overall modular design: • 4x4 array of identical towers - each one including a Tracker, a Calorimeter and an Electronics Module. • Surrounded by an Anti-Coincidence shield (not shown in the picture). • Tracker/Converter (TKR): • Silicon strip detectors (single sided, each layer is rotated by 90 degrees with respect to the previous one). • W conversion foils. • ~80 m2 of silicon (total). • ~106 electronics chans. • High precision tracking, small dead time. • Anti-Coincidence (ACD): • Segmented (89 tiles). • Self-veto @ high energy limited. • 0.9997 detection efficiency (overall). • Calorimeter (CAL): • 1536 CsI crystals. • 8.5 radiation lengths. • Hodoscopic. • Shower profile reconstruction (leakage correction)

12. Tracker design • Aggressive mechanical design: • Less than 2 mm spacing between x and y layers, with front-end electronics lying on the four sides of the trays. • 90° pitch adapters from the front end chips to the silicon sensors. • 2 mm inter-tower separation in order to minimize the inactive area.

13. The Silicon Tracker performance • Construction/testing highlights: • Average detection efficiency higher than 99.5% @ the nominal threshold setting. • Single strip noise occupancy lower than 10-6. • Flight production completed in less than one year. 11500 sensors 360 trays 18 towers ~ 1M channels 83 m2 Si surface

14. LAT status Current status: • All the 16 towers (Tracker + Calorimeter + Electronics) integrated in the flight grid. • ACD ready to be integrated with the rest of the instrument. Coming soon: • Beam test of the calibration unit (2 spare TKR modules + 4 spare CAL modules). • LAT environmental tests. • Integration with the spacecraft. • Launch.

15. Summary/conclusions • GLAST has a tremendous potential of discovery. • The GLAST mission will be one of the next big NASA observatories. • The GLAST LAT tracker is the largest Si tracker ever built for a space application (80 m2 of active silicon surface, ~1M channels). • Construction is completed, integration of the LAT is now reaching its completion. • Next steps are the environmental tests of the instrument and the beam test on the calibration unit. • Launch foreseen in August 2007. RXTE launch on a DELTA II rocket.

16. Spares

17. GLAST vs. EGRET 1After background rejection. 2Single photon, 68% containment, on axis. 31s, on axis. 41s radius, high latitude source with 10-7 cm-2s-1 integral flux above 100 MeV. 51 year sky survey, high latitude, above 100 MeV.

18. Technology impact on instrument performance II

19. Technology impact on instrument performance I

20. Triggering and On-board Data Flow • Level 1 trigger: • Hardware trigger, single-tower level. • Three_in_a_row: three consecutive tracker x-y planes in a row fired. Workhorse g trigger. • CAL_LO: single log with E > 100 MeV (adjustable). Independent check on TKR trigger. • CAL_HI: single log with E > 1 GeV (adjustable). Disengage the use of the ACD. • Chargedcosmic rays in the L1T! 13 kHz peak rate. • Upon a L1T the LAT is read out within 20 ms. • On-board processing: • Identify g candidates and reduce the data volume. • Full instrument information available to the on-board processor. • Use simple and robust quantities. • Hierarchical process (first make the simple selections requiring little CPU and data unpacking). x x x • Level 3 trigger: • Final L3T rate: ~ 30 Hz on average. • Expected average g rate: ~ few Hz (g rate : cosmic rays rate = 1 : few). • On-board science analysis (flares, bursts). • Data transfer to the spacecraft.