X-ray Polarimetry: a new window on the high energy sky

arXiv:1004.4766v1 [astro-ph.IM] 27 Apr 2010 X-ray Polarimetry: a new window on the high energy sky R. Bellazzinia, F. Mulerib,∗ aINFN sez. Pisa, Largo B. Pontecorvo 3, I-56127 Pisa, Italy bIASF/INAF, Via del Fosso del Cavaliere 100, I-00133 Roma, Italy Abstract Polarimetry is widely considered a powerful observational technique in X-ray astronomy, useful to enhance our understanding of the emission mechanisms, geometry and magnetic field arrangement of many compact objects. However, the lack of suitable sensitive instrumentation in the X-ray energy band has been the limiting factor for its development in the last three decades. Up to now, polarization measurements have been made exclusively with Bragg diffraction at 45◦or Compton scattering at 90◦and the only unambiguous detection of X- ray polarization has been obtained for one of the brightest object in the X-ray sky, the Crab Nebula. Only recently, with the development of a new class of high sensitivity imaging detectors, the possibility to exploit the photoemission process to measure the photon polarization has become a reality. We will report on the performance of an imaging X-ray polarimeter based on photoelectric effect. The device derives the polarization information from the track of the photoelectrons imaged by a finely subdivided Gas Pixel Detector. It has a great sensitivity even with telescopes of modest area and can perform simultaneously good imaging, moderate spectroscopy and high rate timing. Being truly 2D it is non-dispersive and does not require any rotation. This device is included in the scientific payload of many proposals of satellite mission which have the potential to unveil polarimetry also in X-rays in a few years. Keywords: X-rays, Gas Detectors, Polarimetry PACS: 29.40.Cs, 07.85.Fv, 95.55.Ka, 95.75.Hi 1. Introduction X-ray polarimetry was born together with X-ray astronomy. First pioneering experiments were carried out in the seventies with polarimeters based on Bragg diffraction at 45◦or Compton scattering at 90◦on-board sounding rockets and first results were quite encouraging: already Novick et al. (Novick et al., 1972) ∗Corresponding author. Email address: fabio.muleri@iasf-roma.inaf.it (F. Muleri) Preprint submitted to Nuclear Instruments and Methods August 31, 2021 reported a marginal yet significant detection of polarization in the emission of the Crab Nebula, confirmed a few years later with high significance by the Bragg polarimeter on-board OSO-8 (Weisskopf et al., 1978). This observation was favoured by the intense flux of the source and the high degree of polarization, signature of synchrotron emission (P ≈20%), and, as a matter of fact, it has remained unique. Only upper limits were derived for other astrophysical objects because of the combined effect of a lower flux and an inferior polarization degree (Long et al., 1979; Hughes et al., 1984). Unfortunately no other tool dedicated to X-ray polarimetry has been launched after OSO-8. The Stellar X-ray Polarimeter on-board the Spectrum-X-Gamma mission, although the flight model was ready and calibrated, was never put in orbit because of the collapse of Soviet System. The proposals to include po- larimeters on-board observatories like XMM or AXAF, which were the only opportunities to have a sufficient collecting area, has never been carried out: instruments exploiting Bragg diffraction or Compton scattering didn’t look at- tractive because, while imaging and spectroscopic devices promised an enormous increase of sensitivity, polarimetry would be limited to a few bright sources even in the focus of these large telescopes. Moreover “classical” polarimeters were cumbersome because they need to be rotated around the direction of incident photons. Conversely, the lack of experimental feedback has not prevented the develop- ment of a rich literature on the basis of which we expect that almost all sources in the X-ray sky should emit partially polarized radiation (for reviews see Rees, 1975; Meszaros et al., 1988; Weisskopf et al., 2009). The study of the state of polarization would unveil the magnetic field and the geometry of the sources and it would pinpoint the emission processes at work, discriminating among competitive models otherwise equivalent from the spectral or the timing point of view. This is the case of emission geometry in pulsars (Dyks et al., 2004) or X-ray pulsars in binaries (Meszaros et al., 1988), but peculiar signatures are also expected for isolated neutron stars because of the different opacity of the two normal modes in a magnetized plasma and because of vacuum polarization (Canuto et al., 1971; Pavlov and Zavlin, 2000; Lai and Ho, 2002; Heyl et al., 2003). Moreover polarimetry is a powerful probe to investigate fundamental theories. General Relativity in the strong field regime can be tested by means of the rotation of the plane of polarization with energy expected for stellar-mass black-holes, and the amplitude of the effect would provide a measurement of the spin (

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