Thesis

Compact Solar Hard X-ray Space Polarimeter

• Call:

  IDPASC Portugal - PHD Programme 2015

• Academic Year:

  2015 / 2016

• Domains:

  Experimental Particle Physics | Astrophysics

• Supervisor:

  Rui Curado da Silva

• Co-Supervisor:

  João Fernandes

• Institution:

  Universidade de Coimbra

• Host Institution:

  LIP Coimbra and Observatório Geofísico e Astronómico UC

• Abstract:

  The universe has been studied in the hard X-ray domain almost exclusively through spectral and timing variability analysis as well as through imaging techniques. By measuring the polarization angle and the polarization degree of source emissions, the number of observational parameters is increased by two allowing better discrimination between different models. Polarimetric observations can provide important information about the geometry, magnetic fields, composition and emission mechanisms in a wide variety of gamma-ray sources such as: pulsars, solar flares, active galactic nuclei, galactic black holes and gamma-ray bursts [1]. In the hard X-ray domain, no dedicated polarimeters have yet been launched into space. However recent polarimetric measurements were performed using the SPI (Spectrometer on INTEGRAL) and IBIS (Imager on-Board INTEGRAL Satellite) instruments on the Crab and on other few stable and transient (GRBs) sources [2, 3]. A compact CdTe based hard X-ray polarimeter with spectro-imaging capabilities optimized for solar physics will be jointly developed, merging the solar physics expertise of CAS (Chinese Academy of Sciences) partners of the National Space Science Center with the high energy instrumentation experience of European partners (Laboratório de Instrumentação e Física Experimental de Partículas, INAF-IASF-Bologna, Bologna, Italy and CEA/DSM/Irfu/Service d'Astrophysique, France) for a common goal. Our purpose is to deploy the first dedicated space polarimeter for hard X-rays, as well as to perform the first polarimetric measurements of Sun solar flare emissions. Measuring the continuum emission polarization will allow establishing important constraints on the emission models. For example, the beaming level of charged particles which produce the Bremsstrahlung radiation could be inferred by polarization. Furthermore, pion decay models are not likely to be compatible with a high degree of polarization measured [1]. Therefore, solar polarization measurements in the 100 keV to 1 MeV energy range will be an exceptional breakthrough for solar physics, opening a new window to interpret solar flare dynamics. European partners have been developing high energy telescope focal plane prototypes with polarimetric capabilities in particular in the framework of two proposals submitted to ESA Cosmic Vision M mission calls (Gamma-Ray Imager in 2007 and DUAL in 2010) [4, 5] and at one proposal at bilateral level with Simbol-X joint CNE and ASI joint mission in 2009 [6]. From this heritage, we scaled polarimeter tested and verified concepts into the constraints of future joint CAS-ESA calls. Since the Sun is the strongest emitter of polarized hard X-rays, a small area (~ 12 cm2), ~ 2mm thick, CdTe pixelized matrix (~ 3000 sub-millimeter pixels; 500 µm size) equipped with a few hundred micron thick tantalum coded mask (allowing imaging up to ~30 keV) should provide high solar photon count rate, for a total mass of about 10 kg and a power electric consumption within 10W. This configuration is intentionally conceived to allow a multi-instrument and multipurpose small ESA and joint CAS-ESA missions. Such simple configuration together with coincidence electronics allow Compton polarimetry analysis since after undergoing Compton scattering in a matrix pixel, the polarized solar photon’s new direction depends on the orientation of its polarization before the interaction. By analysing the Compton photons distribution pattern obtained by a second interaction in the different pixel of the CdTe matrix, a modulation pattern is observed for a polarized solar flux. By both Monte Carlo simulation and prototype experimental testing we showed that a modulation Q factor of about 0.5 (or 0.8 when enough flux enable optimising parameters) is obtained with similar CdTe polarimeter configuration [7]. For a typical solar flare emission whose lifetime may vary from 20 min up to 3 hours, a minimum detectable polarization of about 2% and 1% is estimated for 10^3s and 10^4s observation times, respectively, which will allow observing the expected 10 to 25% degree of linear polarization of the whole solar flare loop. [1] F. Lei et al., “Compton Polarimetry in Gamma-Ray Astronomy”, Space Sci. R., 82, p. 309, 1997. [2] A. J. Dean, et al, “Polarized gamma ray emission from the CRAB”, Science, vol. 321, no 5893, p. 1183, 2008. [3] M. Forot, et al., “Polarization of the Crab Pulsar and Nebula as Observed by the INTEGRAL/IBIS Telescope”, Ap.J., 688, L29, 2008. [4] J. Knödlseder, et al, “GRI: focusing on the evolving violent universe”, Proc. SPIE on Optics for EUV, X-Ray, and Gamma-Ray Astronomy III, Eds, 6688, p. 668806 (2007). [5] P. von Ballmoos, et al., “A DUAL mission for nuclear astrophysics”, Experimental Astronomy vol 33, 2012, DOI: 10.1007/s10686-011-9286-6. [6] Philippe Ferrando et al., "SIMBOL-X: a new-generation hard x-ray telescope", Proc. SPIE 5168, Optics for EUV, X-Ray, and Gamma-Ray Astronomy, 65 (January 29, 2004). [7] R. M. Curado da Silva, E. Caroli, et al., “CIPHER, polarimeter telescope concept for Hard X-ray Astronomy”, Experimental Astronomy, Vol. 15, nº1, pag. 45-65, (2003).