1. 2001/9/111 X-ray polarimetry with balloon borne gas proportional counters K.Hayashida, H. Tsunemi, T. Horikawa, Y. Nakashima (Osaka University, Japan), F. Makino (NASDA, Japan), & B. Paul (TIFR, India) Contact: hayasida@ess.sci.osaka-u.ac.jp

2. 2001/9/11 2 ABSTRACT X-ray polarimetry has been an unexploited field in X-ray astronomy. Detection of the X-ray polarization was succeeded only in a few sources including Crab nebulae at 2.6keV and 5.2keV. In this paper, we present our plan of a balloon experiment for X-ray polarimetry. We will employ conventional Xe gas proportional counters with collimators. We will measure the signal rise time of each event, which reflects polarization direction of the incident X-rays relative to the anode wire direction. Ground experiments revealed that the modulation contrast (M) of this method amounts to about 0.1 at 20keV and 0.35 at 40keV (K. Hayashida et al., 1999, NIMA421, p.241). Different from scattering methods, the large effective area of gas proportional counters is exploited as it is in this method. We show that a pair of 300 cm 2 PCs is enough for detecting X- ray polarization of Crab nebulae at 20-60keV range, where no polarization measurement has been done so far.

3. 2001/9/11 3 Anisotropic Emission of Photo-electrons K-shell photo-electrons are emitted preferentially toward the direction parallel to the electric vector of incident photons. We have been developing polarization detectors employing this principle. CCD (Tsunemi et al.,1992; Hayashida et al., 1999). Gas proportional counter risetime (Hayashida et al., 1999). Electric vector of incident photon Emission direction of photo-electron

4. 2001/9/11 4 PC risetime polarimeter : Polarization and Anode Direction Anode Electron Cloud Rise Time = Short Rise Time = Long Anode X-ray Photon

5. 2001/9/11 5 PC risetime polarimeter : Experiment on the Ground Polarized X-ray beam was irradiated to the Xe gas proportional counter of which anode direction was set at angle  from the E vector of the incident beam. Recently, we have developed a facility for polarized X- ray beam in our laboratory, in which electron impact type X-ray source and double crystal spectrometer are employed. (Tanaka et al., 1997, Koike et al. 2000). Hayashida et al., 1999

6. 2001/9/11 6 Sampling the rise time of the output pulse from proportional counters Risetime was sampled from the digitized signal of the pre- amplifier output of the proportional counters. We have started developing an equivalent digital circuit using an ADC and an FPGA instead of using the oscilloscope and the computer. Hayashida et al., 1999

7. 2001/9/11 7 PC risetime polarimeter: Results from the Ground Experiment Average of the risetime shows the modulation depending on the angle  between the beam E vector and the anode (top panel). Modulation contrast (M) of this PC risetime polarimeter for 10-40keV incidence is shown in the bottom panel. Hayashida et al., 1999

8. 2001/9/11 8 Polarization Detector for a Balloon Experiment Merit of the PC risetime polarimeter for a balloon experiment The system will be realized with conventional gas proportional counters by adding some electronics which enable the risetime measurement. Large effective area of gas proportional counters is utilized easily without mirror system. On this point, CCD method and Thomson-scattering method are more suitable for the focal-plane polarization detector.

9. 2001/9/11 9 Plan of the observation A pair of Xe-gas proportional counters (PC) with risetime measurement will be used. The counters will be set orthogonally so that the anode direction of one PC is parallel to the E vector of the target and another perpendicular. Target is Crab nebulae, in which P=19.2+-1.0%(2.6keV) 19.5+-2.8%(5.2keV) were detected (Weisskopf et al., 1978), but no positive measurements have been done above 10keV.

10. 2001/9/11 10 Feasibility Study: Assumptions Exposure Time=20ksec (5.6hr) Gas proportional counters Xe 1.5atm, Depth 144mm Geometrical Area of one counter in the pair Plan-A: 1200 cm 2 Plan-B: 300cm 2 M: measured values are used for 20-40keV, the value 0.35 is used above 40keV. BGD counts, escape events, air attenuation are taken into account.

11. 2001/9/11 11 Feasibility Study: Pmin Feasibility of the observation is evaluated in terms of Pmin (minimum delectable polarization degree) for each 5 keV band. Since Crab nebulae has P=19-20% below 10keV, we expect to get positive detection for each band even with plan-B(a pair of 300cm 2 PCs)

12. 2001/9/11 12 Calibration Facility at Osaka Polarized X-ray Beam-Line at Osaka

13. 2001/9/11 13 Polarization Degree of the X-ray Beam with various Ex/HV We had measured the polarization degree of the X-ray beam in Koike et al., 2000. We did further experiment with higher accuracy. When we set Ex/HV~95%, we obtain P=0.4-0.5 as shown in the figure.

14. 2001/9/11 14 Experiments at TIFR Polarized X-ray beam sources were developed at TIFR, in which X-rays with high Ex/HV are selected with absorption filters. Another system using scatters was also constructed. We have been testing a large area Xe PC counter on its polarization sensitivity through the rise time method. Small Xe PC specialized for the similar experiment is developed.

15. 2001/9/11 15 References A precision measurement of the X-ray polarization of the Crab Nebula without pulsar contamination, Weisskopf, M.C., Silver, E.H., Kestenbaum, H.L., Long, K.S., Novick, R., 1978, ApJ, Vol.220, L117. Detection of X-ray Polarization with a Charge Coupled Device, H.Tsunemi, K.Hayashida, K.Tamura, S.Nomoto, M.Wada, A.Hirano and E.Miyata,1992, Nuclear Instruments & Methods, A Vol.321(3), p.629-p.631. How Linearly Polarized is the X-ray Beam from the X-ray Generator ?, S. Tanaka, H. Tsunemi, K. Hayashida, 1997, Jpn. J. Appl. Phys., Vol.36, p.5770-5773. X-ray Polarimetory with a conventional gas proportional counter through rise- time analysis, K. Hayashida, N. Miura, H. Tsunemi, K. Torii, K., H. Murakami, Y. Ohno, K. Tamura, 1999, Nucl.Instr, and Methods A, Vol. 421, p.85-90. Optimization of polarimetry sensitivity for X-ray CCD, K. Hayashida, S. Tanaka, H. Tsunemi. Y. Hashimoto, M. Ohtani, 1999, Nucl.Instr, and Methods A, Vol. 436, p.96- 101. Polarized Monochromatic X-ray Beam Extracted from Laboratory Electron Impact Source, T. Koike, K. Hayashida, Y. Hashimoto, D. Akutsu, M. Ohtani, H. Tsunemi, 2000, Proc. of SPIE, Vol.4012, p.414-421.