PoGOLite The Polarized Gammaray Observer

1
Polarization Characteristic of Multi-layer Mirror
for Hard X-ray Observation of Astrophysical
Objects
28.16
T. Mizuno1, J. Katsuta2, H. Yoshida1, H.
Takahashi1, T. Iwahara3, Y. Kano3, N. Sasaki3, Y.
Ogasaka3, T. Kamae4, T. Takahashi2, K. Hayashida5
and K. Uesugi6 1Hiroshima University 2Japan
Aerospace Exploration Agency 3Nagoya University
4Stanford Linear Accelerator Center 5Osaga
University 6Japan Synchrotron Radiation Research
Institute
Abstract
Polarization measurements above 10 keV can
provide crucial information about astrophysical
objects. Despite of its importance, X-ray
polarization has been measured only from Crab
Nebula at 2.6 and 5.2 keV by OSO-8 (Weisskopf et
al. 76 ). A hard X-ray mirror is expected to
improve the sensitivity by more than an order of
magnitude and provide a breakthrough in
high-energy astrophysics. In order to examine a
possible systematic errors in polarization
measurement, we have measured the polarization
dependence of the reflectance of multi-layer hard
X-ray mirror to 30-keV X-rays at a medium-length
beamline 20B2 in Spring-8 (Hyogo, Japan). The
dependence was less than -1, ensuring that we
are able to measure weak polarization down to a
few using hard X-ray optics.
Modivation Why polarimetory?
Multi-layer (Super) Mirror for pol. measurement
Aeff of NeXT HXT and Suzaku XRT
X-ray condensing and imaging improve the
sensitivity in spectroscopy and polarimetry in
hard X-rays.
Polarization measurement above 10 keV is a very
powerful tool to investigate source geometry and
emission mechanism
Non-thermal process where high degree pol. is
expected
X-ray

• Synchrotron emission direction of B-field.
• Pulsar wind nebular
• Binary pulsar and rotation-powered pulsar
• Jets in AGN and m-QSO
• Compton Scattering orientation of the scatterer
• Black-hole binaries (accretion disk geometry)
• Propagation in strong magnetic field
• test of quantum electrodynamics, direction of
  B-field
• Highly magnetized neutron star

X-ray mirror
Effect on polarization measurement is predicted
to be less than 1, but needs to be confirmed
experimentally.
BHB, m-QSO Mirabel 2006
Pol. angle dependence of the Intensity of
reflected beam

• Systematic errors in pol. measurement introduced
  by hard X-ray mirror
• Reflectance of multi-layer mirror (weakly)
  depends on pol. vector
• RsReflectance of s polarization (pol. vector is
  parallel to the mirror surface)
• Rp Reflectance of p polarization
• Rp/Rscos22q where q is the beam incident angle
• Artificial polarization of P(1-Rp/Rs)/(1Rp/Rs)
  (1-I90/I0)/2 will be introduced where I is the
  intensity of the reflected beam. (I0 pol. vector
  parallel to the mirror surface)

I/I0
90
270deg
180
0.03
Rot. Powered pulsar model Harding 2004
0.1
Incident angle q 0.5 deg 1 deg 2 deg
Crab nebula pulsar by CXO
0.5
pol. vector azimuthal angle
1
Experiment Precise measurement of the X-ray
reflectance
experimental setup
test on Nov. 28- Dec.1, 2007
careful examination of systematic errors
NaI scinti. PMT Reflectivity measurement

• High stability is required

Reflected beam image shifts and rotates as we
rotate the mirror. Position dependence of the
efficiency of NaI is 0.25 (1s)
Ion Chamber beam intensity monitor
Long-term stability of NaI measured using a
radioisotope (241Am)
We collimated the beam by using a pin-hole (0.5
mm diameter) to minimize the systematic errors.
Efficiency Scan profile
(image of the direct beam taken by CCD camera)
0.1 0.1
0.25 0.25
Counts
0.5mm
Efficiency a. u.
precision motorized stages rotation of mirror
piece
0.5mm
0 20 40 60
80 hr.
rotation angle b (30 degree step)
(typical size of the reflected beam image)

• fine tuning of the beam incident angle and mirror
  position

Beam incident angle a

• Systematic errors in Ion chamber and NaI
  scintillator (stability/uniformity) is lt0.3.

0.5deg (0.001 deg step)
Reflectance of Multi-layer mirror as a function
of pol. vector angle
We measured reflectance peak of each b angle

• Pol. vector dependence of the reflectance was
  -0.8
• Artificial polarization P (1-I90/I0)/2 0.8
• We are able to use hard X-ray optics to measure
  polarization down to a few .
• The dependence is larger than the theoretical
  prediction (0.03) and could affect the
  measurement of weak pol. measurement (such as the
  test of general relativity) if real.
• Not due to the stability/uniformity of Ion
  chamber/NaI scintillator (lt0.3).
• Misalignment (beam position on mirror piece,
  shape of pin-hole collimator)? gt under
  investigation

Relative reflectance b profile
Reflectance Profile for b0 deg (expanded)
Reflectance Profile for b0 deg
?????
Reflectance a.u.
Reflectance a.u.
1.6
Reflectance a.u.
0.51 0.512 deg
0.50 0.51 0.52 deg
Beam incident angle a
0 90 180
270 deg
Performance of polarimeter with hard X-ray optics
Summary

• Typical characteristics of polarimeter w/o mirror
  (e.g., PoGOLite, see poster by Madejski et al.
  28.14)
• MF1000.3
• Aeff150-200 cm2 in 25-80 keV (15 c/s for Crab
  Nebula in 4g/cm2 overburden)
• det. volume 30000cm3
• BG0.1 Crab (1.8 c/s)
• Typical characteristics of polarimeter w/ mirror
• MF1000.6
• Aeff10 cm2 in 25-70 keV (1 c/s for Crab Nebula)
• det. volume 100 cm3
• BG0.005 c/s (proportional to det. volume)

• Polarimetry will bring a breakthrough in
  high-energy astrophysics
• Pulsar model, structure of B-field and accretion
  disk, test of QED/general relativity, etc.
• Hard X-ray mirror will improve the sensitivity by
  more than an order of magnitude
• Pol. measurement of weak objects (10 mCrab) will
  be possible.
• We showed that the pol. dependence of reflectance
  of multi-layer mirror is less than 1
• The proof of the concept of Polarimeter w/ Hard
  X-ray Optics

3s Minimum Detectable Plarization, Polarimeter w/
mirror
3s Minimum Detectable Plarizaton, Polarimeter
w/o mirror
MDP
MDP
T10ks/100ks/1Ms solid and dotted w/ and w/o BG
Source Flux (Crab)
Source Flux (Crab)

• Polarimeter w/ mirror has better sensitivity for
  weak objects (lt10 mCrab)