Introduction

We are discussing the state of the art of X-ray polarization detection techniques in this conference, so that it is important to remind ourselves of the expected X-ray polarization properties of various astrophysical objects. In this paper, we give a brief overview of the expected X-ray polarization signatures of magnetic neutron stars found in diverse situations, e.g. in accretion-powered pulsars, low-mass X-ray binaries (LMXBs), recycled pulsars, isolated neutron stars and finally the fascinating magnetars. We concentrate here only on some aspects of the basic physics of radiation propagation around magnetized neutron stars which lead to some basic, expected polarization features in the X-rays which we consider relatively robust. Accordingly, our discussion here is qualitative. Quantitative aspects of a few of these features have been described by other participants of the conference, and detailed calculations on some other aspects will be reported elsewhere.

The X-ray emission we are concerned with here is basically thermal emission from the surface of the neutron star, powered by accretion or otherwise. This radiation propagates through the neutron-star atmosphere, then through the accretion columns over the magnetic poles of the neutron star if it is an accreting one, and finally through the neutron-star magnetosphere.

Advances in X-ray astronomy over the almost five decades since the first rockets were launched have been impressive in the domains of imaging, spectroscopy and timing. On the contrary, polarimetry has not progressed much since the historic results of the Columbia team headed by Bob Novick with rockets and with the OSO-8 satellite. The introduction, since Einstein, of X-ray telescopes and imaging detectors produced a dramatic jump in the sensitivity of X-ray missions. Polarimetry based on the conventional techniques of Bragg diffraction and Compton scattering has suffered from the increased mismatching in terms of sensitivity which resulted in the preclusion of the whole extragalactic sky. Moreover the shift from satellites stabilized on one axis to those stabilized on three axes made cumbersome the hosting of polarimeters, which needed the rotation of the whole instrument, in the focal plane of telescopes. As a consequence no polarimeter was included in the final design of Einstein, Chandra and XMM-Newton.

The advent of a new generation of detectors, to be combined with large area X-ray telescopes, has renewed interest in X-ray polarimetry, as demonstrated by the several polarimetric missions recently proposed. One of them, GEMS, has been recently selected by NASA within the SMEX program, with a launch due in 2014. There are discussions in Italy about the possibility of a national X-ray mission including polarimetry, to be launched in the same time frame.

Introduction

Observations in the γ-ray regime allow us to scrutinize some of the most energetic emission processes associated with cosmic sources. Unlike other wave bands that mainly show emission due to thermal reprocessing within hot gases, the majority of γ-ray emission is decidedly non-thermal in nature and is generally produced directly by electrons and other elementary particles in magnetic fields. The electrons which produce the γ-rays are extremely energetic and often at the upper limit of the capability of the accelerator that produces them.

Introduction

Magnetic cataclysmic variables (mCVs) and Ultra-compact double degenerate binaries (UCDs) are potential X-ray polarization sources. The mCVs contain a magnetic white dwarf accreting material from a low-mass, Roche-lobe filling companion star. There are two major types: (i) the AM Herculis binaries (AM Hers, also known as polars) and (ii) the intermediate polars (IPs) (see). In AM Hers, the white-dwarf magnetic field (B ∼ 107 −108 G) is strong enough to lock the whole system into synchronous rotation. It also prohibits the formation of an accretion disk, and the accretion flow is channelled by the magnetic field into the magnetic polar regions of the white dwarf.

Introduction

Polarimetry is a powerful diagnostic tool to study spatially unresolved sources at cosmological distances, such as gamma-ray burst (GRB) afterglows. Radiation mechanisms that produce similar spectra can be disentangled by means of their polarization signatures. Also, polarization provides unique insights into the geometry of the source, which remains hidden in the integrated light.

Historically, essentially all interpretative studies about GRB afterglow polarimetry have been based on the cosmological fireball model, which we will also use as a reference for our discussion. Afterglow polarization studies have indeed the advantage that different models are often almost indistinguishable in terms of radiation output in the optical, but produce markedly distinct predictions about polarization.

In this proceeding, we will briefly review in Section 32.2 what we have derived by optical afterglow polarimetric observations and discuss the most recent development in the field in Section 32.3.

Introduction

X-ray astronomy has been much advanced by three observations: spectroscopy, timing, and imaging. Also in the hard X-ray region, these three observations will be realized by ASTRO-H and XEUS. However, the observation of the polarization is at the moment left out in spite of its potential usefulness. This is because of the difficulty of developing polarimeters with high sensitivity. Since the origin of the polarization is often due to nonthermal radiation processes such as synchrotron radiation, observations in the hard X-ray region are possibly more important than those in the soft X-ray region: it is expected that the degree of polarization in the hard X-ray region would be higher than that at lower energies.

Introduction

The study of the polarization of X-ray sources may justifiably be called the next frontier in X-ray astronomy. Such measurements can reveal the physics and astrophysics under extreme conditions that are obtained in the presence of intense magnetic fields and strong gravity surrounding compact objects like pulsars and black holes and those encountered in high Mach number shocks. X-ray polarimetry of astronomical sources is a deep and sensitive probe of the astrophysical conditions that are obtained in these sources. Polarized X-rays are generated through a wide range of physical processes: synchrotron radiation, inverse-Compton scattering of low-energy polarized photons, curvature and thermal radiation in the intense magnetic fields in the polar caps of pulsars, cyclotron emission in neutron star and magnetar magnetic fields, and Thomson scattering from anisotropic systems like jets, disks and plane parallel atmospheres.

Introduction

Polarization of the prompt signal is a key ingredient to understand the gammaray-burst phenomenon. In fact very different scenarios (for example Poyntingflux-driven or baryon-driven models) differ widely on the predicted polarization level and, at the same time, agree on most of the other measurable parameters.

The bulk of the energy in the prompt signal of GRB is emitted around 100 keV so this is the part of the spectrum that contains the most valuable information. At this energy, the Compton effect dominates. Fortunately the Compton cross section is dependent on the incoming photon polarization, being maximal for scattering angles perpendicular to the polarization direction.

The desirable features for a GRB polarimeter are:

• It should be a space instrument because gamma-rays do not penetrate the atmosphere.

• It should feature a large angular dependence to collect as many GRB as possible.

• […]

Introduction

Gamma-ray bursts (GRBs) are amongst the most energetic events in the universe, and have stimulated intense observational and theoretical research. Theoretical models indicate that a refined understanding of the inner structure of GRBs, including the geometry and physical processes close to the central engine, requires the exploitation of high-energy X-ray and γ-ray polarimetry. To date, observations have been of limited sensitivity and subject to poorly understood systematics. POlarimeters for Energetic Transients (POET) is a SMEX mission concept that is capable of measuring the high-energy polarization of GRBs and other sources of astronomical interest.

Introduction

It has now been a little more than 100 years since the first reported laboratory measurements of γ-ray polarization based on the use of Compton scattering. Although the first efforts to apply this technique in high-energy (X-ray and γ-ray) astronomy took place almost 40 years ago, this area of research is still in its infancy. This is a notoriously difficult area of research, compounded by the combination of low flux levels, high background rates and instrumental artifacts that can often mimic a polarization signature. Nonetheless, all of the recent polarization measurements have relied on this approach.

Experimental considerations

Scattering polarimetry relies on experimental methods that are based on the scattering of photons off electrons. The scattering of photons off single electrons is variably referred to as Compton scattering or, at lower energies, as Thomson scattering. Thomson scattering is the classical limit of Compton scattering in which there is no loss of energy to the electron. At lower energies, coherent scattering off the atomic electron cloud can also be important.

Introduction

The Gamma-RAy Polarimeter Experiment (GRAPE) is a scintillator-based Compton polarimeter designed to observe polarized astrophysical phenomena in the hard X-ray energy band (50–500 keV). Although intended primarily for observations of bright, transient events such as gamma-ray bursts (GRBs) and solar flares, GRAPE may also be operated in a collimated, pointed mode.

Introduction

Observations of extended jets in AGN by Chandra have revealed that the origins of their X-ray emission is less trivial than previous thought (see for X-ray jet surveys). The X-rays may arise from various processes. The polarization in the radio and optical bands suggests that the emission is generated by the synchrotron process. Thus, synchrotron and synchrotron self-Compton (SSC) emission are candidates for the X-ray continuum emission. However, the X-rays can also be generated from external Comptonization (EC) of disk black-body radiation or of the CMB. It has been suggested that X-ray polarization measurements are able to discriminate these competing emission mechanisms.

Introduction

Polarization has proved to be a valuable tool in determining the geometry of the emission region in radio pulsars. For X-ray pulsars, the data are not yet available, but their interpretation in any case is not going to be easy, because of the strong magnetic field effects on the radiation transport. Discovery of millisecond coherent pulsations during X-ray bursts in nearly 20 low-mass X-ray binaries (so-called nuclear-powered millisecond pulsars (NMSP) see and in the persistent emission of at least eight sources (accretion-powered millisecond pulsars (AMSP) see opens a completely new range of possibilities. The emission in these cases is produced at the surface of a rapidly spinning, weakly magnetized neutron star.

Introduction

We consider polarization originating from a Keplerian, geometrically thin and optically thick accretion disc near a black hole. At each radius the accretion disc emits black body radiation, the temperature of which is given by the Novikov-Thorne expression for the outer part of the standard disc. The thermal photons are scattered in the atmosphere of the disc and thus the observed radiation becomes polarized. We assume multiple Thomson scattering with different optical depths of the disc atmosphere. The effect of hardening of the energy of photons due to scattering is taken into account via the hardening factor that increases the effective temperature.

Once the photons leave the atmosphere the polarization vector can be rotated due to strong gravity of the black hole. The energy of photons is shifted by the gravitational and Doppler effects.

Introduction

Pulsar wind nebulae (PWNe) are bubbles of relativistic particles and magnetic field created when the ultra-relativistic wind from a pulsar interacts with the ambient medium, either SNR or ISM. The prototype, and the best studied of this entire class of objects, is the Crab Nebula. The canonical model of PWNe was first presented by Rees & Gunn, developed by Kennel & Coroniti, and is based on a relativistic MHD description. The pulsar wind is confined inside the SNR, and slowed down to non-relativistic speeds in a strong termination shock (TS).

PolariS concept

PolariS (Polarimetry Satellite) is a Japanese small satellite mission dedicated for polarimetry of X-ray and γ-ray sources. Design of the PolariS is now being discussed by the PolariS working group, which consists of 34 members from 11 institutes mainly in Japan, though informal international collaboration is under way.

The main purpose of the PolariS mission is wide-band X-ray (2–80 keV) polarimetry of sources brighter than 10 mCrab. We expect X-ray polarimetry with small satellites to be realized within several years. When measuring X-ray polarization of various type of sources, we consider the energy dependence of the polarization (degree and direction) to be essential to study the geometry and emission mechanisms of different objects, as has been pointed out by theoretical calculation. Furthermore, the hard X-ray band above 10 keV is of particular importance, since the physics we are exploring with polarimetry is in most cases nonthermal.