Mark Pearce

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A High Sensitivity Balloon-borne Soft Gamma-ray
Polarimeter (PoGOLite)
Mark Pearce Dept. of Physics, KTH, Stockholm,
Sweden ECRS 2006, Lisbon, 2006-09-05
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Overview

• Measuring soft gamma-ray polarisation
• PoGOLite instrument design
• PoGOLite science goals and expected performance
• Tests of a PoGOLite prototype with g / particle
  beams
• 2009 maiden flight
• Summary

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P o G OLite
g
SLAC / KIPAC (PI Tune Kamae)
KTH, Stockholm University
Tokyo Institute of Technology, Hiroshima
University, ISAS.
25 100 keV
g

• Photons can be characterised by their energy,
  direction, time of detection and polarisation
• Polarisation is not measured (OSO-8, 1976)
• Measuring the polarisation of gamma-rays
  provides a powerful diagnostic for source
  emission mechanisms
• Polarisation can occur through scattering /
  synchrotron processes, interactions with a strong
  magnetic field
• ? sensitive to the history of the photon

G L A S T
10 keV 300 GeV
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Compton scattering

• Incident g deposits little energy at Compton
  site
• Large energy deposited at photoelectric
  absorption site
• ? large energy difference
• Can be distinguished by simple plastic
  scintillators (despite intrinsic poor energy
  resolution)

Photoelectric absorption
Array of plastic scintillators
g
Compton scatter
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Measuring polarisation
0 when f90o
Compton scattering Klein-Nishina formula
Max when f90o

• g from a polarised source undergo Compton
  scattering in a suitable detector material
• Higher probability of being scattered
  perpendicular to the electric field vector
  (polarisation direction)
• Observed azimuthal scattering angles are
  therefore modulated by polarisation

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PoGOLite instrument schematic
BGO
BGO
NB simplified! 217 wells in reality
BGO anticoincidence
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Key design features
Charged particle anticoincidence. Active g
collimation
Modulation factor difference/average
Active g detector
Lower anticoincidence
Side anticoincidence
MDP (6 h) 10 _at_ 100 mCrab
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Crab Pulsar emission models
Outer gap
Slot gap caustic
Polar cap
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Testing emission models with PoGOLite
(OSO-8 assumed)
Slot gap caustic
Polar cap
Outer gap
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Background reduction
Dominant backgroundCXB / atmospheric gamma-ray
(down, up)
10 mCrab

• Excellent background suppression with narrow
  aperture well-type phoswich design
• GLAST-BFEM (CSBF) data used to provide
  background model
• Charged cosmic ray background rejection by BGO
  shields and active collimators
• Neutron background reduced with Compton
  kinematics

Low (10 mCrab) background Large (115-250 cm2)
effective area ? PoGOLite can detect 10 plane
polarised signal from 100 mCrab source in a
single 6 hour balloon flight
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PoGOLite prototype
Solid fast plastic scintillator (20 cm)
Bottom BGO (4 cm)
BaSO4

• Tested in Dec. 2005 at KEK-PF
• 30, 50, 70 keV
• Vertically plane polarised via 2 Si(553)
  crystals (911)

VM2000
Lead foil
(50 mm)
Tin foil
Hollow slow plastic scintillator (60 cm)
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Selecting fast scintillator events

• Pulse shape discrimination

fast scintillator
BGO / slow scintillator

• Clear separation between signals from fast
  scintillator and BGO/slow scintillator
• Fast scintillator branch is chosen for analysis

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Selecting Compton scatter events
Central unit
(Events where one peripheral scintillator is hit
are plotted)
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Results

• Average events in opposite scintillator pairs

Channel 25 Channel 36 Channel 47

• Agreement with GEANT4 simulations within 10
• work in progress (calibrations, scint.
  linearity, )

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Charged particle background rejection
FWHM
(421)
(441)
(421)
241Am (59.5keV)
90Sr (e-, lt2.3 MeV, 10 kHz) NB x10 expected!
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Charged particle background rejection
Preliminary
Total Fast BGO Slow
Beam off
392 MeV p
4.9 kHz
241Am (59.5keV)
Proton beam test at RCNP Osaka, July 2006
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PoGOLite payload
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Maiden flight 2009
Primary Northern-sky targets

• Proposed location NASA Columbia Scientific
  Balloon Facility, Palestine, Texas
• Nominal 6 hour long maiden flight
• Total payload weight 1000 kg
• 1.11x106 m3 balloon
• Target altitude 40 km
• Long duration Sweden to Canada also planned

Accreting X-ray pulsar
High-mass X-ray binary
Pulsar / SNR
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Summary

• PoGOLite stands to open a new observation window
  on sources such as rotation-powered pulsars and
  accreting black holes through a measurement of
  the polarisation of soft gamma rays (25-100 keV).
  
• Well-type Phoswich detectors are used to
  significantly reduce aperture and cosmic ray
  backgrounds.
• A prototype Phoswich system has been tested with
  photon and proton beams and the design and
  simulation validated.
• Construction of flight hardware is currently in
  progress
• Maiden balloon flight scheduled for 2009.
• Long duration flights and flights of opportunity
  (GLAST, SWIFT) will extend the rich scientific
  program.

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• S PA R E

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Potential targets

• Super-massive black holes where matter accretion
  powers relativistic jets,
• accelerates particles, and emits photons via
  synchrotron and inverse-Compton mechanisms
• Galactic X-ray binaries where matter accretes
  onto a black hole or a neutron star and emits
  hard X-rays. Inverse-Compton reflection off the
  accretion disk polarizes hard X-rays.
  Micro-quasars belong to this category, where the
  accretion is likely to be powering stellar-scale
  relativistic jets
• Active galaxies where isotropic emission is
  scattered toward the Earth by inverse-Compton
  scattering
• Accreting neutron stars with strong cyclotron
  line features
• Hard X-ray emission from Soft Gamma-ray Repeaters
  with super-critical magnetic fields
• Isolated pulsars with strong magnetic field
• Ordinary galaxies (including our own) with
  extended inverse Compton halo
• Solar flares and coronae
• Gamma-ray bursts (with luck)

Crab pulsar a primary target
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OSO-8 (1976)

• Crab Nebula viewed at 2.6 keV, 5.2 keV
• Polarisation measured using Bragg diffraction
  (16.11.4)
• No measurements since then!
• At higher (soft g-ray) energies, non-thermal
  processes are expected to increase polarisation

M. Weisskopf et al, ApJ 208 L125 (1976), ApJ 220
L117 (1978)
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Lateral Anticoincidence
BGO
EpoTek-301
BaSO4 MOS-7 epoxy
Rubber pad
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Instrument characteristics
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• HEFT High Energy Focusing Telescope

May 2005
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Performance of HEFT pointing system

• Day-time star trackers (8th mag)
• Differential GPS
• Gyroscopes, accelerometers, magnetometers

lt 3 arcminute (3/60)o absolute pointing lt 0.2
arcminute in attitude lt5 of total field-of-view
? maximises effective area