Introduction

A detailed simulation of the geometry and of the involved physics processes is required from the very early stages of a new instrument development. Indeed both the development and the optimization of new-instrument concepts, performed with detailed comparisons between different configurations, as well as the detailed characterization of instrument responses and the verification of the scientific objectives of a new instrument, require the same detailed simulation.

Geant4 is an object-oriented toolkit for the simulation of high-energy physics detectors that is now widely used in nuclear physics, medical physics, astrophysics, space applications, radiation background studies, etc. Geant4 is supported by a large international collaboration with the participation of various institutes around the world. It is also an experiment in the application of rigorous software engineering methodologies.

In particular the physics is open to the user, who has the possibility to select among different models of the same physics process or to extend existing models to cover new requirements.

Polarized photon interactions in Geant4

The Compton and Rayleigh processes are affected by the polarization of the incoming radiation, and even an unpolarized beam acquires a certain degree of polarization after a Compton or Rayleigh event. So a description of these processes in which the polarization is present is particularly relevant even though the incoming radiation is not polarized.

Introduction

The GEM is one of the recently developed micro-pattern gas detectors. A dense pattern of through-holes is drilled in an insulator substrate, which is typically polyimide, sandwiched by thin copper foils. The surface and cross-section micrographs of a GEM are shown in Figure 8.1. When high voltage is applied to the copper electrodes in an appropriate gas, the GEM works as an electron multiplier. GEMs are used in many fields such as high energy and nuclear physics, X-ray imaging, etc. In astrophysics, photoelectric X-ray polarimeters, in which the GEM is a key device to multiply an electron cloud whilst retaining its shape, are the most interesting application.

We have produced GEMs since 2002 for X-ray polarimeters. The standard method to produce GEMs is a wet etching technique, while our method is laser etching, which has many advantages. Cylindrical holes are easily formed with the laser etching. The capability to drill cylindrical holes helps in forming finer-pitch holes on a thicker substrate.

Introduction

Owing to quadrupole anisotropy in the radiation field, the scattered emission in certain resonance X-ray lines should be polarized. Anisotropy can be due to A) the centrally concentrated gas distribution and B) the gas bulk motions.

Introduction

A photon carries information about its direction, time of arrival, energy and polarization. Satellite missions such as Uhuru, Ariel V, HEAO-1 and ROSAT located hundreds of sources in their all-sky surveys, and missions such as HEAO-1, ASCA, Suzaku, RXTE, Chandra, and XMM/Newton characterized source spectra and time variability. The X-ray community now needs polarization information to reveal the intrinsic small-scale geometry of astrophysical systems and to evaluate physical processes in regions near compact objects which cannot be resolved via imaging, spectroscopy or timing.

X-ray radiation is polarized when the production mechanism has an implicit directionality, such as when electrons interact with a magnetic field to produce cyclotron and synchrotron emission. In radio quasars, for example, the amplitude of the polarization is a diagnostic for the X-ray emission mechanism and the polarization direction associates the X-ray emission with particular regions identified in milli-arc-second radio images. Polarization also occurs when X-rays are scattered by electrons, a common process in the highly ionized environments of compact X-ray sources.

Introduction

The light emitted in the vicinity of the compact object has different properties when absorbed by a detector at infinity. First of all the photons are usually emitted by fast-moving matter – orbiting, falling into or being ejected from the central body with very high speeds. Thus the effects of Einstein's special relativity, the Doppler shift and aberration, change the photon energy and the direction of its polarization. The beaming effect in the direction of emitting matter motion may be quite significant as well. It is worth mentioning that although these effects are those of special relativity, the high velocities causing them are due to large gravity of the central object. Therefore we need to use general relativity in order to evaluate them properly.

However, general relativity has even more direct impact on the properties of these photons. They move from a strong gravity region, often dragged along by the rotating space-time if the compact body has large angular momentum.

Introduction

X-ray astronomy deals with the most violent and compact spots in the Universe, such as the surfaces of pulsars, the close orbits around giant black holes, and the blast waves of supernova explosions. By making efficient use of the few photons emitted by disks around black holes and other objects, astronomers have successfully applied photometry, imaging and spectroscopy to these energetic and often variable sources. But polarimetry has been largely ignored at X-ray wavelengths because of the inefficiency of the existing instruments.

Introduction

One of the most important advances in neutron star (NS) astrophysics in the last decade has been the detection and detailed studies of surface (or near-surface) X-ray emission from a variety of isolated NSs. This has been made possible by X-ray telescopes such as Chandra and XMM-Newton. Such studies can potentially provide invaluable information on the physical properties and evolution of NSs (e.g. equation of state at super-nuclear densities, cooling history, surface magnetic fields and compositions, different NS populations). The inventory of isolated NSs with detected surface emission includes: (i) radio pulsars: e.g. the phase-resolved spectroscopic observations of the ‘three musketeers’ revealed the geometry of the NS polar caps; (ii) magnetars (AXPs and SGRs): e.g. the quiescent emission of magnetars consists of a black body at T ∼ 0.5 keV with a power-law component (index 2.7–3.5), plus significant emission up to ∼ 100 keV; (iii) central compact objects (CCOs) in SNRs: these now include six to eight sources, several have P, measurements and two have absorption lines; (iv) thermally-emitting isolated NSs: these are a group of seven nearby (≲1 kpc) NSs with low (∼1032 erg s−1) X-ray luminosities and long (3–10 s) spin periods, and recent observations have revealed absorption features in many of the sources.

Introduction

Sensitive X-ray polarimetry promises to reveal unique and crucial information about physical processes in and structures of neutron stars, black holes, and ultimately all classes of X-ray sources. We do not review the astrophysical problems for which X-ray polarization measurements will provide new insights, as these will be discussed in some detail in many of the presentations at this conference.

Despite major progress in X-ray imaging, spectroscopy, and timing, there have been only modest attempts at X-ray polarimetry. The last such dedicated experiment, conducted by Bob Novick (Columbia University) over three decades ago, had such limited observing time (and sensitivity) that even ∼10% degree of polarization would not have been detected from some of the brightest X-ray sources in the sky. Statistically significant X-ray polarization was detected in only one X-ray source, the Crab Nebula.

History

The first positive detection of X-ray polarization was performed in a sounding-rocket experiment that viewed the Crab Nebula in 1971. Using the X-ray polarimeter on the Orbiting Solar Observatory (OSO)-8, this result was confirmed with a 19-σ detection (P = 19.2%±1.0%), conclusively proving the synchrotron origin of the X-ray emission.