The Gamma-RAy Polarimeter Experiment (GRAPE) is a concept for an astronomical, hard X-ray, Compton polarimeter operating in the 50–500 keV energy band. The instrument has been optimized for wide-field polarization measurements of transient outbursts from energetic astrophysical objects such as gamma-ray bursts and solar flares. The GRAPE instrument is composed of identical modules, each of which consists of an array of scintillator elements read out by a multi-anode photomultiplier tube (MAPMT). Incident photons Compton-scatter in plastic scintillator elements and are subsequently absorbed in inorganic scintillator elements; a net polarization signal is revealed by a characteristic asymmetry in the azimuthal scattering angles. We have constructed a prototype GRAPE module, containing a single CsI(Na) calorimeter element, which has been calibrated using a polarized hard X-ray beam and flown on an engineering balloon test flight. A full-scale scientific balloon payload, consisting of up to 36 modules, is currently under development. The first flight, a one-day flight scheduled for 2011, will verify the expected scientific performance with a pointed observation of the Crab Nebula. We will then propose long-duration balloon flights to observe gamma-ray bursts and solar flares.

We investigate the polarization of Compton scattered X-rays from relativistic jets in active galactic nuclei (AGN) using Monte-Carlo simulations. We consider three scenarios: scattering of photons from an accretion disk, scattering of cosmic microwave background (CMB) photons, and synchrotron self-Comptonization (SSC) within the jet. For Comptonization of thermal disk photons or CMB photons the maximum linear polarization attained is slightly over 20% at viewing angles close to 90°. The value decreases with the viewing inclination. For SSC, the maximum value may exceed 80%. The angle dependence is complicated, and it varies with the photon injection sites. Our study demonstrates that X-ray polarization, in addition to multi-wavelength spectra, can distinguish certain models for emission and particle acceleration in relativistic jets.

Radiation of X-ray bursts and of accretion shocks in weakly magnetized neutron stars in low-mass X-ray binaries is produced in plane-parallel atmospheres dominated by electron scattering. We first discuss polarization produced by single (non-magnetic) Compton scattering, in particular the depolarizing effect of high electron temperature, and then the polarization due to multiple electron scattering in a slab. We further predict the X-ray pulse profiles and polarization properties of nuclear-and accretion-powered millisecond pulsars. We introduce a relativistic rotation vector model, which includes the effect of rotation of polarization plane due to the rapid motion of the hot spot as well as the light bending. Future observations of the X-ray polarization will provide a valuable tool to test the geometry of the emission region in pulsars and its physical characteristics.

EXIST (Energetic X-ray Imaging Survey Telescope) is a proposed spaceborne observatory that combines a wide-field-of-view X-ray telescope (5–600 keV) with a pointed optical/infrared telescope, and possibly with a soft-X-ray telescope contributed by Italian collaborators. The primary science drivers of EXIST are the study of the high-redshift Universe and the epoch of re-ionization through the detection and follow-up observations of high-redshift gamma-ray bursts (GRBs) at z ∼ 10, the study of supermassive black holes in galaxies (including heavily obscured and dormant black holes), and the study of stellar-mass and intermediatemass black hole populations in the Milky Way galaxy and in the Local Group. In this contribution, we discuss the polarimetric capabilities of the EXIST hard X-ray telescope. Based on a pointed five-day observation (or based on four-months all-sky survey observations), EXIST can detect the hard X-ray polarization of 100 mCrab sources for polarization degrees down to 6%. The wide field of view of EXIST will make it possible to measure the polarization of transient events like GRBs and flaring galactic and extragalactic sources. We discuss the scientific potential of the hard-X-ray polarimetric measurements. The EXIST observations would allow us to (i) obtain qualitatively new constraints on the locale of particle acceleration in the vicinity of compact objects, (ii) gain key insights into the structure of jets from GRBs and active galactic nuclei, (iii) test high-order QED predictions in the extreme magnetic fields of neutron stars, and (iv) search for quantum gravity signatures (the helicity-dependence of the speed of light) with unprecedented sensitivity.

Multicolour black-body emission from the accretion disc around the black hole can be polarized on its way through the atmosphere above the accretion disc. We model this effect by assuming Kerr metric for the black hole, a standard thin disc for the accretion flow and Thomson scattering in the atmosphere. We compute the expected polarization degree and the angle as they can be measured for different inclinations of the observer, optical thickness of the atmosphere and different values of the black hole spin. All relativistic effects near a compact centre are taken into account. We also assess the perspectives for the next generation of X-ray polarimeters.

Pulsar wind nebulae (PWNe) were the first objects where non-thermal polarized synchrotron emission was detected. They are one of the best astrophysical labs available for the study of high-energy processes like particle acceleration, properties of relativistic outflows, and non-thermal emission. Their broad-band spectrum makes them a suitable target for many instruments, and to date they are the only objects for which there is clear and undisputed evidence for high-energy X-ray polarized emission. In recent years a canonical model has been established which has proved incredibly successful in explaining many of the observed features. All of this makes PWNe a prime candidate for any future X-ray polarimetry study. I will review here the current MHD model, what we know from polarization in the optical and radio band, and what we might learn from next-generation polarimetry.

PolariS (Polarimetry Satellite) is a Japanese small-satellite mission dedicated to polarimetry of X-ray and γ-ray sources. We aim to perform wide-band X-ray (2–80 keV) polarimetry of sources brighter than 10 mCrab, employing three hard X-ray mirrors and two types of polarimeters. X-ray and γ-ray polarimetry of transient sources with wide-field polarimeters is the second purpose. Most of the components have their prototype used or planned to be used in balloon and other satellite missions. Conceptual design is in progress for target launch in the mid 2010s.

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.