Abstract:

1. polarimeter 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 • Rs：Reflectance of s polarization (pol. vector is parallel to the mirror surface) • Rp: Reflectance of p polarization • Rp/Rs=cos22q where q is the beam incident angle • Artificial polarization of P’=(1-Rp/Rs)/(1+Rp/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) 90 I/I0 180 270deg 0.03% Rot. Powered pulsar model Harding 2004 0.1% Incident angleq 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 <=0.3%. ~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 (<=0.3%). • Misalignment (beam position on mirror piece, shape of pin-hole collimator)? => under investigation Relative reflectance b profile Reflectance Profile for b=0 deg (expanded) Reflectance Profile for b=0 deg 相対反射率 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) • MF100~0.3 • Aeff=150-200 cm2 in 25-80 keV (~15 c/s for Crab Nebula in 4g/cm2 overburden) • det. volume ~ 30000cm3 • BG~0.1 Crab (~1.8 c/s) • Typical characteristics of polarimeter w/ mirror • MF100~0.6 • Aeff~10 cm2 in 25-70 keV (~1 c/s for Crab Nebula) • det. volume ~100 cm3 • BG~0.005 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 [%] T=10ks/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 (<=10 mCrab)