POLAR A WIDE FIELD COMPACT DETECTOR FOR POLARIZATION MEASUREMENTS of HARD X-RAYS from GRB

1. POLAR A WIDE FIELD COMPACT DETECTOR FOR POLARIZATION MEASUREMENTS of HARD X-RAYS from GRB Giovanni Lamanna1, S.Basa6, S.M. Gierlik2, D. Haas5, W. Hajdas3, R. Hermel1, H. Hofer4, C. Lechanoine-Leluc4, R. Marcinkowski2, A. Mazure6,A. Mtchedlishvili3, S. Orsi4, M. Pohl4, N. Produit5, D. Rapin4, E. Suarez4, J.P. Vialle1 1LAPP(Laboratoire d’Annecy-le-vieux de Physique des Particules) , Annecy, FRANCE 2IPJ, Swierk/Otwock, Poland; 3PSI, Villigen, Switzerland; 4DPNC, Universite de Geneve, Switzerland; 5ISDC, Universite de Geneve, Switzerland; 6LAM (INSU/CNRS), Marseille, FRANCE;

2. Polarimetry • Electromagnetic radiation (EM) has 4 measurable quantities: • Direction • Intensity • Energy • Polarization • Measurements of polarization provide information about the radiation mechanism but they have not been enough explored • Difficult to do with actual detectors • Wrong measurements published (claim 80%+- 20%)‏ (GRB 021206 accidental coincidences) • A lot of theoretical interests ( 122 citations) POLAR • POLAR is a Compton hard X-ray GRB polarimeter using proven technologies: 40x40 homogeneous array of 6x6x200 mm3 low Z scintillator plastic bars and PM detectors • Measuring the PROMPT signal of the GRBs within a wide FoV (≈ ⅓ of the sky) • Perform first ever successful polarization measurement of hard X photons in space with high statistical significance and controlled systematic effects.

3. Transient events ranging in duration from a fraction of a second up to a few hundred seconds. Two types of GRBs: 1) Soft-spectrum Long burst (death of massive stars) 2) Hard spectrum Short burst (merger of compact star binaries: ns-ns; ns-bh..) GRB 910412, 12 April 1991 Gamma Ray Burst (GRB) • Currently about 2 to 3 GRBs detected per week. They are flashes of gamma rays, at random places in the sky and at random times. • Energetic bursts that mark the most violent, cataclysmic explosions in the universe

4. Gamma Ray Burst (GRB) • Both result in a Black Hole formation and regardless of the progenitor a “fireball” central engine would be responsible of a relativistic jet from the center of the explosion (bulk Lorentz factor G > 100.) Zhang & Meszaros Swift POLAR will use polarimetry to analyse the “PROMPT SIGNAL” : the only possible means of probing the structure of the central engine of the expanding fireball closest to the nascent black hole.

5. Gamma Ray Burst (GRB) Theories on the GRB production mechanism can be constrained by different degrees of linear polarization (P): P ~ > 80% Inverse Compton jet model 20%< P <60% synchrotron emission is the dominant source of radiation. (EM model) Low degrees of polarization: flux with a high degree of polarization experiencing partial depolarization e.g. electrons in a randomly oriented magnetic field. (IS model) • In general models fall into: • Physical: globally ordered B field and • synchrotron emission-> net polarization • -Geometrical: random B field and electrons-> • No-net polarization except…when • viewing ~ 1/G outside the jet cone (loss of emission symmetry) • (but random viewing angles make high P significantly smaller) Other POLAR scientific Goals: Solar Flares, Soft Gamma Repeaters….

6. The POLAR detector 1600 PS bars: 25 x 8x8*6mm*6mm*20cm MAPM H8500 (8*8 anode pixel) Light, fast and low Z plastic (in favor of Compton effect) BC400: rad-hard and chemically stable

7. The POLAR detector Requirements: - relies on given burst position and spectrum - or provides a crude estimate in case of unique observation Dedicated for GRB observations only thanks to: - Large area - Large modulation factor - Large field of view The scientific objectives lead to the following instrument specifications: Effective detection area 400 cm2 (at 200 keV) Incoming photon energy 50 keV to 500 keV A large modulation factor ~40% (at 200 keV) A large FOV (deep space view) 1/3 of the sky Total Mass < 30 kg Mean power Consumption < 30 W Telemetry (continuous readout) < 100 kBit/s While the exact data format has not yet been fixed, a preliminary evaluation of the event size is 128 bits per event at a mean background rate of around 500 Hz (to be compared with ~100 kHz during GRB events).

8. COMPTON POLARIMETRY: Basic Principles Geometry of the large angle Compton scattering. The strategy is to look for the two bars where interactions occur: first Compton and second Compton or Photoelectric.

9. Monte Carlo Results (GEANT4 full simulation) • Analysis uses two largest energy deposits with Ethr= 5 keV (corresponding to electron recoil energy from 50 keV photon scattered at 90o) (trigger activation: at least 2 channels) • GRB position is known • Optical insulation and thin (≈ 1 mm) carbon fiber outside shielding (stopping electrons with E< 500 keV or protons E< 13 MeV) • No active shielding; but outer layers can be used if needed for a (“topological”) trigger • Upper threshold Esum< 300+ keV (total sum) (TBC) Fit function: N=A·cos(2(h-f)+½)+B • Maximum effective area for monochromatic photons ·A = Aeff≈ 350 cm2 • Polar angular dependence varies within 15% only • Maximum Modulation Factor is 30% - 40% • Constant values kept up to more then = 30 for off-axis GRB

10. MDP 5% 10% 15% 20% 25% 30% Number of GRB in 1 year 2.9 12.7 24.4 36.8 48.6 59.4 Monte Carlo Results Background: • Cosmic rays removed by upper energy threshold • Diffuse background E,bg>10 keV Fdif=2.46 /cm2/sr/s – 430 coinc./s • Non-GRB  sources – e.g. Crab FCrab=0.7 /cm2/sr/s • S/C induced ’s – ISGRI estimated Find=0.02 /cm2/sr/s GRB signals: • MDP =E=10-5 erg/cm2→ MDP3≈ 10%(example LTC GRB060418 by RHESSI) • Statistics based on BATSE catalogue (GEANT4 full simulation)

11. Mechanics

12. The POLAR detector: Space know-how AMS-ECAL @ LAPP: Optical coupling

13. Characteristics 64 PMT channels input Variable gain 64 GTL outputs Multiplexed direct signal output 3 thresholds loaded Technology : AMS SiGe 0.35µm Area 12 mm2 Dissipation O(100 mW) MAPRA ASIC PROJECT MAPRA: Multi Anode Polar Readout Asic, based on the MAROC -LAL-IN2P3(64 channels ASIC for ATLAS (CERN) luminometer) • Lack of uniformity affects the modulation factor: Non-Uniformity of PM has a strong influence

14. Laboratory Tests

15. Laboratory Tests • Tagged -ray polarized source • 90 scattering of photons on large scintillator • Strong 137Cs source (37 MBq) • -rays (E90deg≈ 290 keV, Pav≈ 40%, Pmax≈ 60%) • Measured asymmetry up to 12% (depending on distance between plastics) • Corresponding to (0.12 / 0.3 =) 40 % polarization Measured asymmetry (~modulation factor) confirming the validity of working principle

16. SVOM as a trigger for POLAR • The POLAR project is in the middle of an hybrid “Phase A/B”: • - Demonstrator is under test on polarized beam, meanwhile an Engineer Model (EM) is under development • The EM project (e.g. Phase B) has recently produced a Preliminary Technical Design Report for all sub-projects: Mec, Ele.etc.. • The schedule of the EM is mainly driven by the FNS – Swiss funding (April 2007-April 2010) • - French funding for the MAPRA ASIC (2008-2009) + • A scientific collaboration between POLAR and SVOM… • The SVOM community strongly supports POLAR • as a low cost “natural” complementary device for an • extremely important complementary scientific measurement. • Chinese space-flight opportunity. • IHEP-proposing POLAR a Chinese experiment • and a flight onboard the Chinese SpaceLab in 2013.

17. SPACELAB • Chinese space-flight opportunity. • SpaceLab in 2013. • Space radiation environment MC studies: • Photons scattered by the SpaceLab • Polarized photons back-splashed from Earth.

18. SUMMARY • POLAR – Compton hard X-ray GRB polarimeter using low Z scintillators • 40x40 homogeneous array of 6x6x200 mm3 plastic bars • FoV ≈ ⅓ of the sky and low g energy threshold Emin < 50 keV • Aeff≈ 400 cm2 and 100≈ 40% at 200 keV • MDP3≈ 10% for GRB total energy of 10-5 erg/cm2; tens of detections/year • First asymmetry results obtained demonstrating polarimetric capability • Engineering Qualification Model under development • Demonstrator will be tested in Beam-tests • Collaboration with IHEP to include POLAR in forthcoming Chinese satellite experiments

19. BACKGROUND SOURCES • Cosmic rays removed by upper energy threshold • Diffuse background E,bg>10 keVFdif=2.46 /cm2/sr/s – 430 coinc./s • Non-GRB  sources – e.g. CrabFCrab=0.7 /cm2/s • S/C induced ’s – ISGRI estimated Find=0.02 /cm2/sr/s • Weaker GRBs at lower energies require careful background subtraction

20. Monte Carlo Results Direction and Spectrum known Implies less systematic, less MC simulated cases for cross-checks… • Analysis uses two largest energy deposits with Ethr= 5 keV (corresponding to electron recoil energy from 50 keV photon scattered at 90o) • GRB position is known (e.g. GCN) • Fit function: N=A·cos(2(h-f)+½)+B f– polarization direction • MC predicts clear modulation signal with period  • Unpolarized photons create pattern with period /2