1. POLAR: a Gamma-Ray Burst Polarimeter onboard the Chinese Spacelab
S. Orsi
Université de Genève, Switzerland
On behalf of the POLAR collaboration:
Univ. de Genéve; ISDC; PSI (Switzerland),
IHEP (China), IPJ (Poland), LAPP Annecy (France)
Image:
Image: nasa.gov

2. Gamma Ray Bursts
POLAR
GRBs: flashes of gamma rays randomly distributed in the sky and in time
Short duration
Brightest bursts in the universe since the Big Bang
~1 GRB/day observed (~682 seen by Fermi/GBM in ~3 years)
Debate about origin and classification (e.g. B.Zhang 2011):
Hard spectrum – Short bursts (<2 sec)
Merger of compact star binaries:
neutron star – neutron star or neutron star – black hole
Soft spectrum – Long bursts (> 2 sec)
Death of massive stars
Our detector:
Wide field of view
Fast read-out electronics
Measures sudden transients
Prompt GRB emission
Wide field of view to observe as many GRBs as possible
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

3. Polarization and GRBs
Electromagnetic radiation has 4 measurable quantities:
Direction, intensity, energy, polarization
GRB polarization can be produced by:
Synchrotron radiation
Cyclotron emission
Bremsstrahlung
Inverse Compton Scattering
Magnetic photon splitting
Very large interest for GRB polarization
Models + experiments
K. Toma et al., ApJ 2009
Synchrotron ‘oriented’
Synchrotron ‘random’
Compton drag
Polarization (60-500keV)
Measurements of polarization provide information about the radiation mechanism but they have not been enough explored.
Cannonball or Compton drag. --- Each point is one GRB. – interest: experimental and theoretical
In general models fall into: 1.Physical: globally ordered B field and synchrotron emission-> net polarization
2.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
T. Piran, 2004 M. Lyutikov, et al., 2003 A. Dar, et al., 2004 D.Lazzati & M. Begelman 2009
EP, obs [keV]
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

4. The POLAR Experiment
POLAR is a Compton polarimeter devoted to the measurement of polarization of X-rays from GRBs in range 50–500 keV
Compact (~30kg, ~30x30x30 cm3)
Mounted on Chinese spacelab TG-2 (atmosphere opaque for X-rays)
Field of view: ~1/3 full sky
Min. detectable polarization (MDP): < ~10%
POLAR concept: N. Produit, et al., NIM (2005)
~10 GRBs expected
per year (lifetime: 2y)
The TG-2 spacelab
POLAR onboard the TG-2 spacelab
Photo spacelab + CAD
“future” spacelab TG2; space-based since…;

5. The POLAR DETECTOR
Consists of 25 modular units, each consisting of 8x8 plastic scintillator bars (6x6x176mm) read-out by one Multi Anode PMT
Total: 1600 plastic scintillator bars + 25 MAPMT
Read-out electronics: ASIC + FPGA
The POLAR Engineering Model
CAD view of one module
One module during assembly
The Hamamatsu
H8500C MAPMT and the FE electronics
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011 5

6. When An x-ray photon interacts…
Semi-exploded view of POLAR
GEANT4-based simulations
An event simulated with GEANT4 MonteCarlo
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

7. POLAR: Compton polarimetry
Photons tend to Compton scatter at right
angles to the incident polarization direction:
(Klein-Nishina equation)
where:  is the Compton scattering angle
 is the azimuthal scattering angle
Compton scattering is the dominant process in plastic scintillators in the energy range of interest.
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

8. Modulation curve Peak amplitude Mean value m Peak amplitude
To study the response of the detector to 100% polarized photons, we have performed a beam test…
Modulation factor: m =
Peak amplitude
Mean value
Polarization: P = m
m100
where m100 is the modulation factor for 100% polarized photons
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

9. Tests with Synchrotron Radiation
High-energy 100% polarized photons at the European Synchrotron Radiation Facility (ESRF), Dec. 2009:
One POLAR module under test, placed inside an Eulerian cradle, on a translating table
Beam: 0.5x0.5 mm2 aligned on bar center
NIM/CAMAC read-out electronics

10. ESRF
>40M triggers at 4 beam energies: 200, 288, 356, 511 keV
Several polarization angles measured: 0° 30° 45° 60° 90°
After quality cuts: ~ k events in each modulation curve (~8k expected in GRB mod. curves)
Uniform illumination of the detector: offline merging of 64 data files (each with the beam directed on a different bar)
Unpolarized signal extracted from polarization at 0° and 90°
200k events: study in more detail systematic effects due to the detector
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

11. ESRF: Data Analysis Neighbor hits removed (iterative process) to:
reduce crosstalk effects
increase distance between hits (and also modulation factor)
The distribution of the angle x can be fitted with:
f(x) = A (1 + m100) cos [2(x - x0) + p] x (1 + mbkg) cos 4(x - x0,bkg)
Where x0 is the polarization angle and m100 is the modulation factor for 100% polarized incoming photons;
A term of period p/2 is inserted in f(x) due to the square geometry of POLAR
Two Analyses:
Polarized data only: fit of the modulation curve
Ratio Pol/NonPol (independent from simulations! No systematic effects)
Polarization term
Background term
Neighbors removed to improve data quality
The modulation curve can be fitted with…
2 analyses:
compatible results
no unpolarized sample in space
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

12. ESRF: modulation curves
Ebeam = 511 keV
Pol 0°, 90°; NonPol
m100 vs beam energy
Thresholds: 20keV, 30keV
As example you see here…
Results confirmed by ad hoc GEANT4 Monte Carlo simulations.
S. Orsi et al., “Response of the Compton Polarimeter POLAR to Polarized Hard X-Rays”, NIM A 648 (2011)
S. Orsi et al., Proc. 22nd ECRS, Turku, Finland, August 2010
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

13. Effective area Monte Carlo based study
Aeff is dependent on E and incoming photon angle
Figure of merit: Aeff x m100
Effective area [cm2]
Estela’s phd, fig 4.12
Energy [keV]
S. Xiong, N. Produit, B. Wu, Expected performance of a hard X-ray polarimeter (POLAR) by Monte Carlo simulation, Nucl. Instr. and Meth. A 606 (2009) 552
E. Suarez Garcia, Ph.D. Thesis, Univ. de Genève, 2010
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

14. Space Qualification Tests
A space qualification campaign (1 module) is ongoing:
Vibration test: Bern, May 2011
Thermal tests: Geneva, Spring 2011
Thermo-vacuum: Terni, June 2011
Irradiation Test of FEE: Geneva, June 2011
Tests on full POLAR detector (25 modules) in 2012
One module on the vibration table
One POLAR Module in the vacuum chamber in SERMS (Terni, Italy)
Poster by J.Sun: Qualification Tests of the Space-Based POLAR X-Ray Polarimeter (ID:1351)
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

15. Grb Localization
Using the counting rate in the 25 modules POLAR can reconstruct the position of GRB within few degrees
This localization induces a relative systematic error <10% in measuring the polarization (GRB fluence > 10-5 erg cm-2)
POLAR is able to measure polarization of GRBs not seen by other missions
On one side POLAR relies on other missions to get GRB spectra and precise localization.
The bottom line is that POLAR is able to …
E. Suarez Garcia et al., A method to localize GRBs using POLAR, NIM. A 624 (3) (2010) 624
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

16. Background during flight
‘Permanent’ background sources:
Diff.Phot.Bkg
Neutrons
Electrons
Positrons
Crab
Protons
Source related to GRB X-rays:
Earth backscattering
Satellite backscattering
Expected reduction of m100: few %
E. Suarez Garcia, X-Ray Polarization: RHESSI Results and POLAR Prospects,Ph.D. Thesis, Université de Genève, 2010
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

17. Conclusions and outlook
POLAR is a novel and compact Compton polarimeter devoted to study the prompt emission of GRBs
Successful beam test with 100% polarized synchrotron radiation in Dec 2009; new beam test in Nov 2011 (16 modules)
Construction and assembly of qualification model (QM) ongoing
Flight model under design
Launch on the Chinese Spacelab TG-2 in 2014
Lifetime: 2 years
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

18. Thank you!
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

19. Minimum detectable polarization
The Minimum Detectable Polarization (MDP) for the instrument is a function of:
Source (S) and background (B) rate and observation time (T)
Solid angle: omega = 2 pi (1 – cos theta)
Field of view: p
1.3p
1.7p
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

20. Beam test @ ESRF: Modulation Curves
Ebeam = 288 keV
Pol. Angles:
0° 30° 45°
60° 90°
Modulation:
m100~ 45%
(at 288 keV)
Geometry term:
mgeom ~ 10%
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

21. Prediction on GRBs polarization level
Introduction
Electromagnetic radiation (EM) has 4 measurable quantities:
Direction
Intensity
Energy
Polarization
Models
Prediction on GRBs polarization level
EM-driven & MHD
~ 50 %
Fireball
~ 10 – 20 %
Cannonball
0 – 100 %
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011

22. ESRF
>40M triggers at 4 beam energies: 200, 288, 356, 511 keV
Several polarization angles measured: 0° 30° 45° 60° 90°
After quality cuts: ~ k events in each modulation curve (~8k expected in GRB mod. curves)
Uniform illumination of the detector: offline merging of 64 data files (each with the beam directed on a different bar)
Unpolarized signal extracted from polarization at 0° and 90°
200k events: study in more detail systematic effects due to the detector
Create offline uniform illumination of the detector merging 64 data files
S. Orsi - Univ. Genève - 32nd ICRC - Beijing, 16 August 2011