POLARIX: a small mission of x-ray polarimetry

Abstract

X-Ray Polarimetry can be now performed by using a Micro Pattern Gas Chamber in the focus of a telescope. It requires large area optics for most important scientific targets. But since the technique is additive a dedicated mission with a cluster of small telescopes can perform many important measurements and bridge the 40 year gap between OSO-8 data and future big telescopes such as XEUS. POLARIX has been conceived as such a pathfinder. It is a Small Satellite based on the optics of JET-X. Two telescopes are available in flight configuration and three more can be easily produced starting from the available superpolished mandrels. We show the capabilities of such a cluster of telescopes each equipped with a focal plane photoelectric polarimeter and discuss a few alternative solutions.Keywords: X-ray Astronomy, Polarimetry, Telescope, Detectors POLARIMETRY: BIG HOPES MEAGRE RESULTS.A long term theoretical analysis has foreseen the possibility to test models of X-ray sources and to derive relevant parameters of the models themselves by measuring the linear polarization of X-rays. In the first years the polarization deriving from the emission process was outlined. On 1985 Sunyaev and and Titarchuk demonstrated that in a hot accretion disk the scattered photons have an energy much higher than the original photons and thence the Chandrasekhar limit of 11.7 % could be exceeded. In 1988 Meszaros et al. stated "Polarimetry would add to energy and time two further observable quantities (the amount and the angle of polarization) constraining any model and interpretation: a theoretical/observational break-through". This very attractive perspective pushed some investigators to develop instrumentation aimed to perform this measurements. This was mainly based on Bragg Diffraction at 45 o and on Compton/Thomson scattering in a certain range of angles around 90 o . Bragg crystal is an excellent analyzer of polarization (100% modulation) but is effective in a very narrow interval around the energy that fulfils the Bragg condition for that particular crystal and that particular angle. A flat Bragg crystal at 45 o set before the focal plane of an X-ray telescope acts as a newtonian secondary mirror and a detector in the secondary focal plane detects the source in the thin band defined by the Bragg (plus higher orders if in the band). The whole is rotated and the source brightness is modulated depending on the amount of polarization.The scattering polarimetry is effective only when scattering exceeds photoabsorption. A well of detectors surrounds the scatterer and the polarization is measured by the angular distribution of the out-coming photons.

The Italian Polarimetry Programmes

“…PolariX (Polarimetro X, which in Italian means "X-ray polarimeter", [9]) is a small mission dedicated to X-ray polarimetry proposed to the Italian space agency (ASI, Agenzia Spaziale Italiana) and accepted for a Phase A study which ended in December 2008. PolariX is currently waiting for the downselection to two approved missions and, if selected, it will be launched in mid 2014.…”

The gas pixel detector as an x-ray photoelectric polarimeter with a large field of view

“…To reach a sufficient collecting area to perform measurements in a reasonable observing time, these devices can be efficiently employed with X-ray optics, and many proposals have been presented. [3][4][5] Two profiles of missions are emerging:…”

“…(1+β cos θ)4 , where β is the velocity of the emitted electron in units of the speed of light. Without this relativistic correction, the photoelectrons are emitted preferentially into the plane perpendicular to the direction of incidence, with a symmetry between the emission above and below…”

A very compact polarizer for an x-ray polarimeter calibration

“…Neglecting the scattered radiation, monochromatic photons at manly 2.6, 5.9 and 6.5 keV, the latter produced by Kβ manganese emission from the Fe 55 decay, are isotropically distributed. Since diffraction on graphite or lithium floride crystals can occur only at the grazing angle reported in table 4, it results that a collimator is required to stop diffraction on lithium floride of 6.5 keV photons, which occurs at an angle close to 45…”

“…In particular the very compact Gas Pixel Detector 1-3 will perform polarimetry of galactic and extragalactic sources in a few days, 4,5 allowing to study in deeper details a great variety of X-ray sources.…”