The basic ideas of accretion onto magnetized neutron stars are outlined. These are applied to a simple model of the structure of the plasma mound sitting at the magnetic poles of such a star, in which upward diffusion of photons is balanced by their downward advection. This steady flow model of the plasma's dynamical state is used to compute the emission of polarized X-raysfrom the optically thick, birefringent medium. The linear polarization of the continuum radiation emerging from the quasi-static mound is found to be as much as 40 percent at some rotation phases, but is insensitive to the geometry of the accretion flow. The role of the accretion shock, whose detailed polarimetric and spectral characteristics have yet to be calculated, is emphasized as the final determinant of the properties of the emerging X-rays. Some results describing the fully time dependent dynamics of the flow are also presented. In particular, steady flow onto a neutron star is shown to exhibit formation of 'photon bubbles', regions of greatly reduced plasma density filled with radiation which form and rise on millisecond time scale. The possible role of these complex structures in the flow for the formation of the emergent spectrum is briefly outlined.

Degenerate stellar objects are expected to be strong sources of polarized X-ray emission. This is particularly true for strongly magnetized neutron stars, e.g. accretion or rotation powered pulsars, and gamma ray bursters. In these, linear polarization degrees well in excess of 30 percent are expected. Weaker magnetic field stellar sources, such as old neutron stars in low mass binary systems, white dwarfs and black holes are expected to have polarization degrees in the range 1-3 percent. A great interest attaches to the detection of polarization in these objects, since this would provide invaluable information concerning the geometry, radiation mechanism and magnetic field strength, necessary for testing and proving models of the structure and evolution of stars in their late stages. In this paper we review the theoretical models of the production of polarized radiation in compact stellar X-ray sources, and discuss the possibility of detecting these properties using currently planned detectors to be flown in space.

Broad emission lines from many accretion powered sources have been observed. These include the UV and the optical lines from active galactic nuclei and broad X-ray lines from galactic black hole candidates. It has been demonstrated that the broad Balmer lines of two radio galaxies, Arp 102B and 3C 332, can be fitted very well by the line profile from a relativistic Keplerian thin disk. The broad iron line of Cyg X-1 can also be fitted reasonably well by the relativistic disk model although the X-ray resolution at present is not high enough to reveal the details of the line profile yet. Since the lines which originate from the relativistic accretion disk could be polarized due to electron scattering in the accretion disk corona, the spectropolarimetric observation could provide a useful diagnostic of the relativistic disk model for these broad emission lines.

This paper presents and analyzes some new features of the scattering of x rays from the condensed matter systems. In particular the polarization and incident frequency degrees of freedom now available at synchrotron sources allow us to focus on some interesting relatively unexplored magnetic properties of solids. After an introduction presenting the general features of this scattering we will look in some detail at elastic scattering experiments in Ho (a spiral anti-ferromagnetic) and UBe13 (a so called heavy fermion anti-ferromagnet). We will also consider spin glasses such as dilute Fe in Cu. This system is an example of a random magnetic system with competing interactions. It undergoes an interesting and rather subtle phase transition. The order parameter for this transition has never been observed. We will present the ideas behind a rather unique magnetic elastic x-ray experiment designed to make a direct measurement of the elusive spin glass order parameter. In all cases we will cover without going into great detail, the basic phenomenon and try to focus on what new physics can be learned from such elastic x-ray scattering experiments. The second part of the talk will present ideas on so called magnetic compton scattering. These experiments require circularly polarized x-rays. They allow us to measure, with rather high accuracy, the ground state wave function of Ferromagnetic systems. The best characterized but still not well understood examples of such systems are the elements Ni and Fe. Preliminary experiments on these ideal elements will be used to demonstrate the kind of information we can obtain. We will also show how this information can be compared with microscopic calculations. In the future, greater resolution should allow us to answer some of the still unanswered basic questions in the theory of itinerant magnetism.

Theoretical results on x-ray resonant magnetic scattering at the M4,5-edges of Uranium compounds and circular magnetic x-ray dichroism at the L3-edge of heavy rare earth systems are reported, with the aim of providing an interpretation of recent experiments. Near an inner-shell absorption threshold, the effect of spin-orbit and exchange interactions is reflected in the presence of magnetization-sensitive components in the anomalous scattering amplitude; as a consequence, x-ray magnetic scattering can exhibit a dramatic enhancement. This has been demonstrated experimentally and theoretically. In the x-ray region, strong magnetic effects can be observed in absorption (linear and circular dichroism), the Kerr effect, and Faraday rotation. Dispersive and absorptive processes are determined by the forward scattering amplitude; hence, the above-mentioned magneto-optical phenomena can be brought together into a general formulation. Recently, magnetization-sensitive effects, associated with x-ray photoemission, have also been observed and discussed. In this paper we report ab initio atomic calculation of resonant magnetic scattering (XRES) at the M4,5-edges of the Uranium compounds. Also, we present calculations of circular magnetic x-ray dichroism at the L3-edge of the GdyieldsTm series.

Bragg reflecting perfect crystals such as silicon are doubly birefringent and dichroic. Phase coherence is preserved during Bragg reflection and therefore all the polarizing optics familiar in the visible band can be realized for x rays. Design principles are discussed for three key devices; tuneable high intensity linear polarizers with extinction ratios greater than 106; Bragg/Fresnel Rhombs that are the only high efficiency tuneable quarter wave plates so far described; and x-ray phase compensators. The linear x-ray polarizers and the quarter wave rhomb both rely on multiple Bragg reflections in channel cut crystals. Less efficient linear polarizers, which rely on x-ray linear dichroism to separate orthogonal polarization states, have been used in some experiments as have dichroic wave plates for the conversion of linearly polarized x-rays into elliptically polarized beams. These are likely to be less useful than bending magnets (for linear polarization) or helical insertion devices (for circular polarization) in practice as their efficiency and polarization ratio is rather poor.

A polarimeter for x-ray and vacuum ultraviolet (XUV) radiation was built to measure the spatial and spectral dependence of the polarization of the light produced by the new undulator at the U5 beamline at NSLS. The fourth-harmonic radiation was measured, and it does not agree with predictions based on ideal simulation codes in the far-field approximation.

Two short-wavelength optical components are described. The first, the whispering-gallery mirror, uses many glancing-incidence reflections to deflect a beam through a large net angle. Because the Fresnel coefficient for each reflection depends upon the state of polarization, the whispering-gallery mirror can act both as a polarizer and as a birefringent element. The second, the multilayer polarizing beamsplitter, is a Brewster-angle reflector thin enough to allow partial transmission of the incident beam. Its behavior can be surprising; in some cases the same polarization mode is preferred on both reflection and transmission.

Synchrotron radiation is inherently highly polarized, and this fact makes synchrotron radiation useful for probing the directional and/or helical nature of the matter via polarization sensitive experiment. Here we summarize briefly the nature of the polarization experiments and the polarization characteristics of synchrotron radiation.

A three-dimensional position sensitive imaging detector has been proposed for the prospective ESA/NASA gamma-ray satellite, INTEGRAL. The instrument is based on two layers of bar shaped CsI(T1) crystals viewed by photodiodes. The GEANT3 Monte Carlo simulation package has been used to assess the spectroscopic and enhanced imaging performance of this detector and an original routine has been written to assess its capabilities as a Compton polarimeter. A description of the algorithm of this routine is given with the results of both GEANT3 and the polarization simulations.

The Stellar X-Ray Polarimeter (SXRP) uses the polarization sensitivity of a graphite Bragg crystal and a lithium Thomsom scattering target to measure the polarization of X-rays from astrophysical sources. The SXRP is a focal plane detector for the Soviet-Danish SODART telescopes which will be launched on the Soviet Spectrum-X-Gamma mission. The SXRP will be the third orbiting stellar X-ray polarimeter, and should provide an order of magnitude increase in polarization sensitivity over its predecessors.

Recent experiments conducted to study the vectorial photoelectric effect with CsI, Al2O3 and Si photocathodes at 2.69 keV indicate null results. Detailed analysis shows that previously measured modulation can be well explained by geometrical misalignment and a combination of the asymmetric shape of the incident X-ray beam and a small detection area of the photoelectron detector. After the elimination of the sources of spurious modulation, we observed a modulation factor of less than 3 percent for a grazing incidence angle as small as 5 deg. There is no observable difference in the pulse height distribution between s and p states.

In this paper, we briefly describe measurements of the 60 - 200 eV photoemission from Caesium Iodide, Gold, and Aluminum photocathodes made on beamline 6.1 of the SERC Daresbury synchrotron radiation source (SRS). Measurements were made near grazing incidence using both s- (electric vector perpendicular to the plane of incidence) and p- polarized (electric vector parallel to the plane of incidence) radiation. Our data supports the existence of an 'x-ray vectorial effect'--a pronounced linear polarization dependence of the photoyield--observed in earlier experiments at Leicester and Columbia Universities and elsewhere. For CsI, novel measurements are also reported of: radiation damage and annealing, and the temperature dependence of the photoyield.

A brief review of using soft x-ray resonant magnetic scattering in the study of magnetic thin films and multilayers is given. Results from recent studies of thin Fe films and Fe/Gd multilayers are used as examples to demonstrate the information that can be obtained and the unique features of this technique. Comparison is made with related techniques: magneto- optical Kerr effect, Faraday effect, and magnetic circular dichroism.

Soft-x-ray magnetic circular dichroism (SXMCD) is a powerful tool to study the element and site specific magnetic moments in multi-component systems. This is demonstrated for a thin metal film overlayer system and a rare-earth/transition-metal compound. The magnetic properties of ultrathin Ni films on a Cu(100) substrate are investigated at the Ni L2,3 absorption edges. Temperature dependent SXMCD data of various film thickness yield Curie temperatures and the critical exponents used in scaling theories which describe two dimensional magnetic phase transitions. The magnetic moments of Gd and Fe in gadolinium iron garnet (Gd3Fe5O12) were probed at 77 K and 300 K at the Gd M4,5 and at the Fe L2,3 absorption edges. The SXMCD signal at each edge allows the magnetic ordering for each specific ion to be independently determined, and the temperature dependence confirms that the reversal of the macroscopic magnetization is due to the reversal of each local magnetic moment of the Fe and the Gd atoms when the compensation point (Tcomp equals 288 K) is crossed.

Synchrotron radiation is a good source of polarized radiation in the x-ray regime. The radiation obtained from a bending magnet source is linearly polarized in the bending plane and has a varying degree of circular polarization away from the bending plane. This feature of synchrotron radiation can be taken advantage of with proper optics to selectively use the type of polarized radiation required for the experiment in question. Linear polarized radiation is used to study the anisotropic nature of electronic and atomic structure by x-ray absorption techniques from single crystal and oriented powder samples. We will give a specific example of the use of linearly polarized x-ray absorption spectroscopy measurements for the study of the magnetically oriented layered copper oxide superconductors. While such linear dichroism measurements help identify the symmetry of the empty electronic states, circular dichroism measurements in magnetic systems help in determining the spin contribution to the absorption process. We will discuss magnetic circular dichroism measurements of the ordered-disordered invar alloy Fe3Pt.

We propose the use of recently developed technique of circular intensity differential scattering (CIDS), as extended to the soft x-ray region of the spectrum (16 eV to 500 eV), to study the higher order organization of the eukaryotic chromosome. CIDS is the difference in scattering power of an object when illuminated by right circularly polarized vs. left circularly polarized electromagnetic radiation of arbitrary wavelength. CIDS has been shown to be a very sensitive measure of the helical organization of the scattering object, e.g., the eukaryotic chromosome. Preliminary results of measurements of samples of bacteriophages and octopus sperm done at SRC, Wisconsin, show the technique to be very sensitive to the dimensional parameters of the particles interrogated by circularly polarized light.

The magnetic circular dichroism and absorption spectra of gaseous benzene were measured using the four meter normal incidence monochromator and the Aladdin storage ring at the Synchrotron Radiation Center, University of Wisconsin-Madison. This is an extension of benzene magnetic circular dichroism measurements to higher energies than was previously possible. The magnetic circular dichroism gives additional information about the electronic structure of benzene. An initial interpretation of the magnetic circular dichroism results are presented and compared to proposed assignments. Various methods for extending to higher energies circular dichroism and other measurements which need a source of intense variably polarized electromagnetic radiation are summarized. Plans for extension of measurements to higher energies are also described.

The degree of beam polarization at the end of beam line U3C of the National Synchrotron Light Source has been measured. The polarization was measured at nine photon energy points from 219 to 1136 eV. Scattering from helium gas at 90 degrees was used to separate the beam component polarized with its electric vector parallel to the synchrotron electron orbit from that polarized perpendicular to the orbit. The simultaneous count rates of two gas flow proportional counters, one placed in the orbital plane perpendicular to the beam, the other perpendicular to both the orbital plane and the beam, were taken to be proportional to the intensities of the two components. The measured values of the experiment were lower than originally expected when compared to the theory and similar measurements in the literature. However, if an .8 milliradian vertical offset of the central ray was assumed, the measured values fitted the theoretical curve to within the counting statistics of the measurement. The technique shows promise as a beam line alignment tool.