Active galactic nuclei (AGN) are definitely not spherical objects, but rather have an axisymmetric morphology. This, together with the fact that the primary X-ray emission is due to Comptonization, and that there are several circumnuclear regions which may scatter the primary radiation, implies that most, if not all, of the X-ray spectral components observed in radio-quiet AGN should be significantly polarized. Moreover, the polarization properties of the radiation emitted very close to the black hole are modified by Special and General Relativity effects, which therefore can be probed with X-ray polarimetry.

Programs of X-ray polarimetry in Italy arise from the convergence of a long experience of X-ray astronomy missions with an outstanding tradition in development of radiation detectors. The gas pixel detector in the focus of X-ray optics can perform angular-resolved polarimetry with a breakthrough improvement in sensitivity, even with a moderate collecting surface. POLARIX makes a large use of already existing items and, in particular, of the three telescopes from the JET-X program. It can extend the X-ray polarimetry from one positive detection only, to tens of sources, including a few brighter extragalactics: an ambitious pathfinder on a very limited budget. Phase A study of POLARIX, and of four other missions, was performed in 2008 and ASI should select two missions to fly. Another pathfinder is under study: two short telescopes, designed with modern tight packing techniques, mounted as piggy-back on the Chinese mission HXMT.

Hard X-ray / soft gamma-ray polarimetric analysis can be performed efficiently by the study of Compton scattering anisotropy in a detector composed of fine pixels. But in the energy range above 100 keV where source fluxes are extremely weak and instrumental background very strong, such delicate measurement is actually very difficult to perform. The Laue lens is an emerging technology based on diffraction in crystals allowing the concentration of soft gamma-rays. This kind of optics can be applied to realize an efficient high-sensitivity and highangular-resolution telescope, though at the cost of a field of view reduced to a few arcmin. A 20-m focal-length telescope concept focusing in the 100–600 keV energy range is taken as example here to show that recent progresses in the domain of high-reflectivity crystals can lead to very appealing performance. The Laue lens being fully transparent to polarization, this kind of telescope would be well suited to perform polarimetric studies since the ideal focal plane is a stack of finely pixelated planar detectors – in order to reconstruct the point spread function – which is also ideal to perform Compton tracking of events.

Neutron stars generate powerful winds of relativistic particles that form bright synchrotron nebulae around them. Polarimetry provides a unique insight into the geometry and magnetic configuration of the wind, but high-energy measurements have failed until recently. The INTEGRAL-IBIS telescope has been used in its Compton mode to search for linearly polarized emission for energies above 200 keV from the Crab nebula. The asymmetries in the instrument response are small and we obtain evidence for a strongly polarized signal at an angle parallel to the pulsar rotation axis. This result confirms the detection recently reported by, and extends the polarization measure for all the pulsar's phases. We also report the recent observation of a variable polarization signal from the long GRB 041219A. The achieved sensitivity opens a new window for polarimetric studies at energies above 200 keV.

Polarization measurements of rotation-powered pulsars have been a very powerful diagnostic in the radio band and promise to be at least as useful a diagnostic in the X-ray band. Since the relativistic particles that radiate pulsar high-energy emission tightly beam the radiation along the pulsar magnetic field, phase-resolved polarimetry has the potential to map the emission patterns. Fermi observations of young pulsars at gamma-ray energies have disfavored polar-cap models where emission takes place near the neutron star surface, and strongly favor outer-magnetosphere models where emission takes place close to the light cylinder or beyond. Since the different outer-magnetosphere models predict similar gamma-ray light curves, it is difficult to discriminate between them at gamma-ray energies. But X-ray polarization has the potential to provide this discrimination, since the models predict distinct polarization signatures and optical detections will only be possible for a small number of pulsars.

We present X-ray and radio polarimetric observations of selected Virgo Cluster spiral galaxies. The X-ray extended emission traces hot-gas filaments in galactic halos and is sensitive to the environmental effects exerted by interactions inside the cluster, like ram-pressure stripping. The radio polarization studies provide clues about alignment, distortion, compression and strength of detected magnetic fields. When used together, the two types of observations constitute an excellent tool for examining disturbances in galactic disks and halos caused by interactions of galaxies with the intracluster medium or between the galaxies themselves. The coming of age of X-ray polarimetry could provide us with unprecedented tools to explore further the evolution of galaxies in a cluster environment.

The radiative transfer code STOKES was extended to allow for X-ray polarimetry modelling. The physical mechanisms of Compton scattering, photo-absorption, and the production of iron K lines were added and are illustrated by modelling the X-ray polarization spectrum of irradiated, cold matter disks. These models confirm that the orientation of the polarization position angle is related to the size of the disk. Although strongly diminishing the spectral flux, an obscuring torus around a small irradiated disk significantly increases the polarization at intermediate viewing angles. Our modelling shows that the polarization can be very sensitive to the radiative coupling between different reprocessing regions.

The advent of a new generation of X-ray polarimeters based on the photoelectric effect poses the problem of their calibration. We devised and built a calibration facility aimed at the study of the performances of photoelectric X-ray polarimeters such as the Gas Pixel Detector (GPD). The calibration facility exploits the 45° Bragg diffraction from crystals of both X-ray lines characteristic of X-ray tubes and from continuum. A set of linear and rotary stages allows the GPD to be calibrated on its whole surface. We successfully tested the GPD filled with a mixture of He-DME 30-70 at one atmosphere. We measured the modulation factor at 2.69 keV and 4.51 keV. We also studied the homogeneity of the modulation factor, of the angular phase and of the position reconstruction capability on the surface of the GPD.

We developed an instrument design capable of measuring linear X-ray polarization over a broad band using conventional spectroscopic optics. A set of multilayer-coated flats reflects the dispersed X-rays to the instrument detectors. The intensity variation with position angle is measured to determine three Stokes parameters: I, Q, and U – all as a function of energy. By laterally grading the multilayer optics and matching the dispersion of the gratings, one may take advantage of high multilayer reflectivities and achieve modulation factors >50% over the entire 0.2–0.8 keV band. This instrument could be used in a small orbiting mission or scaled up for the International X-ray Observatory. Laboratory work has begun that would demonstrate the capabilities of key components.

We describe the current status of the design and development of a Thomson X-ray polarimeter suitable for a small satellite mission. Currently we are considering two detector geometries, one using rectangular detectors placed on four sides of a scattering element and the other using a single cylindrical detector with the scattering element at the center. The rectangular detector configuration has been fabricated and tested. The cylindrical detector is currently under fabrication. In order to compensate any pointing offset of the satellite, a collimator with a flat-topped response has been developed that provides a constant effective area over an angular range. We have also developed a double crystal monochromator/polarizer for the purpose of test and calibration of the polarimeter. Preliminary test results from the developmental activities are presented here.

There are two kinds of gamma-ray burst (GRB) afterglows. One is the fireball afterglow that is the radiation of the external shock(s) driven by the GRB remnant. The other is the emission from the late ejecta launched by the prolonged activity of the central engine, i.e. the central engine afterglow. The former seems to be only weakly polarized and thus is not suitable for the upcoming X-ray polarimetry. For the latter, the polarization property is less clear. Some central engine afterglows, such as energetic flares and the plateau followed by a sharp drop, might be highly polarized because the outflows powering these behaviors may be Poynting-flux dominated. Furthermore, the breakdown of the symmetry of the visible emitting region may be hiding in some X-ray data and will give rise to interesting polarization signatures. For example, in the high latitude emission model for the sharp X-ray decline strong polarization evolution is possible. An XRT-like detector but with polarization capability on board a Swift-like satellite would be suitable to detect these possible signals.

The degree and the temporal evolution of linear polarization in the prompt and afterglow emission of gamma-ray bursts is a very robust diagnostic of some key features of gamma-ray-burst jets and their micro and macro physics. In this contribution, I review the current status of the theory of polarized emission from GRB jets during the prompt, optical flash, and afterglow emission. I compare the theoretical predictions to the available observations and discuss the future prospects from both the theoretical and observational standpoints.

We investigated the capability of the hybrid photon counting pixel detector Timepix to measure the degree of linear X-ray polarization between 27 and 84 keV. Due to its ability to measure energy deposition or the detection time in each pixel, both photoelectric effect and Compton scattering in the sensor can be exploited. The analyzing power exploiting photoelectric effect was found to be small compared to X-ray-sensitive CCDs due to the larger pixel pitch (55 μm) of the Timepix. We were able to measure a polarization asymmetry of (0.96±0.02)% between vertical and horizontal double-hit events in neighbouring pixels. The polarization asymmetry was measured with dependence on the energy deposition in the sensor. Asymmetries range between 0.2% at 29 keV and 3.4% at 78 keV. In order to exploit the polarization signature of Compton scattering in the sensor, the time-to-shutter mode of the Timepix was used. We measured a large modulation factor of about 68.1% in good agreement with simulations.

ASTRO-H is a next-generation JAXA X-ray satellite to be launched in 2014. The Soft Gamma-ray Detector (SGD) onboard ASTRO-H is a semi-conductor Compton camera with a narrow field-of-view (FOV) to achieve very low background. Although the SGD is primarily a spectrometer in the 40–600 keV energy band, it is also sensitive to polarization in the 50–200 keV energy band. This paper describes instrument design, expected performance, and experimental validation of polarimetric performance of the SGD.