Showing papers on "Synchrotron radiation published in 2008"

A compact synchrotron radiation source driven by a laser-plasma wakefield accelerator

Abstract: Ultrashort light pulses are powerful tools for time-resolved studies of molecular and atomic dynamics1. They arise in the visible and infrared range from femtosecond lasers2, and at shorter wavelengths, in the ultraviolet and X-ray range, from synchrotron sources3 and free-electron lasers4. Recent progress in laser wakefield accelerators has resulted in electron beams with energies from tens of mega-electron volts (refs 5,6,7) to more than 1 GeV within a few centimetres8, with pulse durations predicted to be several femtoseconds9. The enormous progress in improving beam quality and stability5,6,7,8,10 makes them serious candidates for driving the next generation of ultracompact light sources11. Here, we demonstrate the first successful combination of a laser-plasma wakefield accelerator, producing 55–75 MeV electron bunches, with an undulator to generate visible synchrotron radiation. By demonstrating the wavelength scaling with energy, and narrow-bandwidth spectra, we show the potential for ultracompact and versatile laser-based radiation sources from the infrared to X-ray energies.

SYNCHROTRON SELF-COMPTON ANALYSIS OF TeV X-RAY-SELECTED BL LACERTAE OBJECTS

Abstract: We introduce a methodology for analysis of multiwavelength data from X-ray-selected BL Lac (XBL) objects detected in the TeVregime. By assuming that the radioYthroughYX-ray flux from XBLs is nonthermal synchrotron radiation emitted by isotropically distributed electrons in the randomly oriented magnetic field of a relativistic blazar jet, we obtain the electron spectrum. This spectrum is then used to deduce the synchrotron self-Compton (SSC) spectrum as a function of the Doppler factor, magnetic field, and variability timescale. The variability timescale is used to infer the comoving blob radius from light-travel time arguments, leaving only two parameters. With this approach, we accurately simulate the synchrotron and SSC spectra of flaring XBLs in the Thomson through Klein-Nishina regimes. Photoabsorption by interactions with internal jet radiation and the intergalactic background light (IBL) is included.Dopplerfactors,magneticfields,andabsolutejetpowersareobtainedbyfittingtheH.E.S.S.andSwiftdataof the recent giant TeV flare observed from PKS 2155� 304. For the H.E.S.S. and Swift data from 2006 July 28 and 30, respectively, Doppler factors k60 and absolute jet powers k10 46 ergs s � 1 are required for a synchrotron/SSC model to give a good fit to the data, for a low intensity of the IBL and a ratio of 10 times more energy in hadrons than nonthermal electrons. Fits are also madetoa TeVflare observedin 2001fromMrk 421 which require Doppler factorsk30 and jet powers k10 45 ergs s � 1 . Subject headingg BLLacertaeobjects:general — BLLacertaeobjects:individual(PKS2155� 304,Mrk421) — galaxies: active — radiation mechanisms: nonthermal

Synchrotron Self-Compton Analysis of TeV X-ray Selected BL Lacertae Objects

Abstract: We introduce a methodology for analysis of multiwavelength data from X-ray selected BL Lac (XBL) objects detected in the TeV regime. By assuming that the radio--through--X-ray flux from XBLs is nonthermal synchrotron radiation emitted by isotropically-distributed electrons in the randomly oriented magnetic field of a relativistic blazar jet, we obtain the electron spectrum. This spectrum is then used to deduce the synchrotron self-Compton (SSC) spectrum as a function of the Doppler factor, magnetic field, and variability timescale. The variability timescale is used to infer the comoving blob radius from light travel-time arguments, leaving only two parameters. With this approach, we accurately simulate the synchrotron and SSC spectrum of flaring XBLs in the Thomson through Klein-Nishina regimes. Photoabsorption by interactions with internal jet radiation and the intergalactic background light (IBL) is included. Doppler factors, magnetic fields, and absolute jet powers are obtained by fitting the {\em HESS} and {\em Swift} data of the recent giant TeV flare observed from \object{PKS 2155--304}. For the contemporaneous {\em Swift} and {\em HESS} data from 28 and 30 July 2006, respectively, Doppler factors $\gtrsim 60$ and absolute jet powers $\gtrsim 10^{46}$ ergs s$^{-1}$ are required for a synchrotron/SSC model to give a good fit to the data, for a low intensity of the IBL and a ratio of 10 times more energy in hadrons than nonthermal electrons. Fits are also made to a TeV flare observed in 2001 from Mkn 421 which require Doppler factors $\gtrsim 30$ and jet powers $\gtrsim 10^{45}$ erg s$^{-1}$.

Characteristics of synchrotron radiation

Abstract: In this paper, we review the principle of various synchrotron radiation sources and their characteristics. Spectral characteristics, the angular distribution, the polarization, and the frequency integrated power of the bending‐magnet radiation is discussed. Undulator radiation topics include the undulator harmonics, the spectrum at a given angle, the angular distribution at a given frequency, the effect due to the electron beam distribution, the angle‐integrated power, and the polarization. In addition to the mathematical analysis, we emphasize physical understanding based on the properties of the apparent trajectories. The wiggler as a limiting case of an undulator for a large orbit excursion. We establish the conditions under which a wiggler may be regarded as a sequence of bending magnets. A general discussion of the properties of synchrotron at finite, how it propagates through optical medium, how it forms interference patterns, etc., is given. (AIP)

High-Altitude Emission from Pulsar Slot Gaps: The Crab Pulsar

Abstract: We present results of a 3D model of optical to gamma-ray emission from the slot gap accelerator of a rotation-powered pulsar. Primary electrons accelerating to high-altitudes in the unscreened electric field of the slot gap reach radiation-reaction limited Lorentz factors of approx. 2 x 10(exp 7), while electron-positron pairs from lower-altitude cascades flow along field lines interior to the slot gap. The curvature, synchrotron and inverse Compton radiation of both primary electrons and pairs produce a broad spectrum of emission from infra-red to GeV energies. Both primaries and pairs undergo cyclotron resonant absorption of radio photons, allowing them to maintain significant pitch angles. Synchrotron radiation from pairs with a power-law energy spectrum from gamma = 10(exp 2) - 10(exp 5), dominate the spectrum up to approx. 10 MeV. Synchrotron and curvature radiation of primaries dominates from 10 MeV up to a few GeV. We examine the energy-dependent pulse profiles and phase-resolved spectra for parameters of the Crab pulsar as a function of magnetic inclination alpha and viewing angle zeta, comparing to broad-band data. In most cases, the pulse profiles are dominated by caustics on trailing field lines. We also explore the relation of the high-energy and the radio profiles, as well as the possibility of caustic formation in the radio cone emission. We find that the Crab pulsar profiles and spectrum can be reasonably well reproduced by a model with alpha = 45deg and zeta approx. 100deg or 80deg. This model predicts that the slot gap emission below 200 MeV will exhibit correlations in time and phase with the radio emission.

Gamma-Ray Studies of Blazars: Synchro-Compton Analysis of Flat Spectrum Radio Quasars

Abstract: We extend a method for modeling synchrotron and synchrotron self-Compton radiations in blazar jets to include external Compton processes. The basic model assumption is that the blazar radio through soft X-ray flux is nonthermal synchrotron radiation emitted by isotropically-distributed electrons in the randomly directed magnetic field of outflowing relativistic blazar jet plasma. Thus the electron distribution is given by the synchrotron spectrum, depending only on the Doppler factor $\delta_{\rm D}$ and mean magnetic field $B$, given that the comoving emission region size scale $R_b^\prime \lesssim c \dD t_v/(1+z)$, where $t_v$ is variability time and $z$ is source redshift. Generalizing the approach of Georganopoulos, Kirk, and Mastichiadis (2001) to arbitrary anisotropic target radiation fields, we use the electron spectrum implied by the synchrotron component to derive accurate Compton-scattered $\gamma$-ray spectra throughout the Thomson and Klein-Nishina regimes for external Compton scattering processes. We derive and calculate accurate $\gamma$-ray spectra produced by relativistic electrons that Compton-scatter (i) a point source of radiation located radially behind the jet, (ii) photons from a thermal Shakura-Sunyaev accretion disk and (iii) target photons from the central source scattered by a spherically-symmetric shell of broad line region (BLR) gas. Calculations of broadband spectral energy distributions from the radio through $\gamma$-ray regimes are presented, which include self-consistent $\gamma\gamma$ absorption on the same radiation fields that provide target photons for Compton scattering. Application of this baseline flat spectrum radio/$\gamma$-ray quasar model is considered in view of data from $\gamma$-ray telescopes and contemporaneous multi-wavelength campaigns.

Observation of synchrotron radiation from electrons accelerated in a petawatt-laser-generated plasma cavity.

Abstract: The dynamics of plasma electrons in the focus of a petawatt laser beam are studied via measurements of their x-ray synchrotron radiation. With increasing laser intensity, a forward directed beam of x rays extending to 50 keV is observed. The measured x rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. The critical energy of the measured synchrotron spectrum is found to scale as the Maxwellian temperature of the simultaneously measured electron spectra. At low laser intensity transverse oscillations are negligible as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser and enter a highly radiative regime with up to 5% of their energy converted into x rays.

Separation of anomalous and synchrotron emissions using WMAP polarization data

Abstract: The main goals of this study is to use the information from both WMAP intensity and polarization data to do a separation of the Galactic components, with a focus on the synchrotron and anomalous emissions. Our analysis is made at 23 GHz where the signal-to-noise ratio is the highest and the estimate of the CMB map is less critical. Our estimate of the synchrotron intensity is based on an extrapolation of the Haslam 408 MHz data with a spatially varying spectral index constrained by the WMAP 23 GHz polarization data and a bi-symmetrical spiral model of the galactic magnetic field with a turbulent part following a -5/3 power law spectrum. The 23 GHz polarization data are found to be compatible with a magnetic field with a pitch angle p=-8.5 degrees and an amplitude of the turbulent part of the magnetic field 0.57 times the local value of the field, in agreement with what is found using rotation measures of pulsars and polarized extinction by dust. The synchrotron spectral index between 408 MHz and 23 GHz obtained from polarization data and our model of the magnetic field has a mean value of Beta=-3.00 with a limited spatial variation with a standard deviation of 0.06. When thermal dust, free-free and synchrotron are removed from the WMAP intensity data, the residual anomalous emission is highly correlated with thermal dust emission with a spectrum in agreement with spinning dust models. Considering a classical model of the large scale Galactic magnetic field, we show that the polarization data of WMAP are in favor of a soft synchrotron intensity highly correlated with the 408 MHz data. Furthermore the combination of the WMAP polarization and intensity data brings strong evidence for the presence of unpolarized spinning dust emission in the 20-60 GHz range.

The multiwavelength variability of 3C 273

Abstract: Aims. We present an update of the 3C 273’s database hosted by the ISDC, completed with data from radio to gamma-ray observations over the last 10 years. We use this large data set to study the multiwavelength properties of this quasar, especially focussing on its variability behaviour. Methods. We study the amplitude of the variations and the maximum variability time scales across the broad-band spectrum and correlate the light curves in different bands, specifically with the X-rays, to search for possible connections between the emission at different energies. Results. 3C 273 shows variability at all frequencies, with amplitudes and time scales strongly depending on the energy and being the signatures of the different emission mechanisms. The variability properties of the X-ray band imply the presence of either two separate components (possibly a Seyfert-like and a blazar-like) or at least two parameters with distinct timing properties to account for the X-ray emission below and above ∼20 keV. The dominant hard X-ray emission is most probably not due to electrons accelerated by the shock waves in the jet as their variability does not correlate with the flaring millimeter emission, but seems to be associated to long-timescale variations in the optical. This optical component is consistent with being optically thin synchrotron radiation from the base of the jet and the hard X-rays would be produced through inverse Compton processes (SSC and/or EC) by the same electron population. We show evidence that this synchrotron component extends from the optical to the near-infrared domain, where it is blended by emission of heated dust that we find to be located within about 1 light-year from the ultraviolet source.

Reference-free X-ray spectrometry based on metrology using synchrotron radiation

Abstract: X-ray spectrometry is a wide spread technique for revealing reliable information concerning the elemental composition and binding state in various materials. Reference-free quantitation in X-ray spectrometry is based on the knowledge of both the instrumental and fundamental atomic parameters. Instrumental or experimental parameters involve the radiant power and spectral purity of the excitation radiation, the beam geometry, the solid angle of detection, and the response behavior and efficiency of the detector. The reliability of the quantitation equally depends on the relative uncertainty of the atomic fundamental parameters involved. Both the values and estimated uncertainty of atomic fundamental parameters given in the literature can be improved by dedicated experiments: By means of transmission and fluorescence experiments with tunable synchrotron radiation, the mass absorption coefficient and fluorescence yield of Al was determined. Furthermore, the transition probabilities of the fluorescence lines belonging to the Cu-Liii and Lii subshells were determined using a superconducting tunnel junction detector offering an energy resolution of about 10 eV in the soft X-ray range. Selected techniques and applications of reference-free X-ray spectrometry are presented. A particular advantage of reference-free quantitation modes is their capability to directly probe new materials without the need to wait for appropriate standard reference materials.

Towards a table-top free-electron laser

Abstract: Synchrotron radiation generated using an electron beam from a laser-driven accelerator opens the possibility of building an X-ray free-electron laser hundreds of times smaller than conventional facilities currently under construction.

Non-thermal emission from relativistic MHD simulations of pulsar wind nebulae: from synchrotron to inverse Compton

Abstract: Aims. The goal of this paper is twofold. In the first place we complet e the set of diagnostic tools for synchrotron emitting sourc es presented by Del Zanna et al. (Astron. Astrophys. 453, 621, 2006 ) with the computation of inverse Compton radiation from the same relativistic particles. Moreover we investigate, for the fi rst time, the gamma-ray emission properties of Pulsar Wind Nebulae in the light of the axisymmetric jet-torus scenario. Methods. The proposed method consists in evolving the relativistic MHD equations and the maximum energy of the emitting particles including adiabatic and synchrotron losses along streamlines. The particle energy distribution function is split in t wo components: one corresponding to the radio emitting electrons interpreted as a relic population born at the outburst of the supernova and the other associated with the wind population continuously accelerated at the termination shock and emitting up to the gamma-ray band. The inverse Compton emissivity is calculated using the general Klein-Nishina differential cross-section and three different photon targets for the relativistic particles are considered: the nebular synchrotron photons, photons associated with the far-infrared thermal excess and the cosmic microwave background. Results. When the method is applied to the simulations that better reproduce the optical and X-ray morphology of the Crab Nebula, the overall synchrotron spectrum can only be fitted assuming an excess of injected particles and a steeper power law (E −2.7 ) with respect to previous models. The resulting TeV emission has then the correct shape but is in excess of the data. This is rela ted to the magnetic field structure in the nebula as obtained by the simu lations, in particular the field is strongly compressed near the termination shock but with a lower than expected volume average. The jet-torus structure is found to be clearly visible in high-resolution gammaray synthetic maps too. We also present a preliminary exploration of time variability in the X and gamma-ray bands. We find variations with time-scales of about 2 years in both bands. The variability observed originates from the strongly time-dependent MHD motions inside the nebula.

X-ray-scattering information obtained from near-field speckle

Abstract: Whenever coherent radiation impinges on a scattering object, a speckled intensity pattern is produced. In the far field the speckle size and shape do not mirror any properties of the object. Here we show that, in spite of the limited spatial coherence of synchrotron radiation, speckles with remarkable properties can be observed when the sensor is placed in the near field. The statistical analysis of these speckles generates static and dynamic X-ray-scattering data, and the results from two typical scattering samples are given. When compared with conventional far-field techniques, the method enables a substantial increase of around four orders of magnitude in the beam size and power and opens the way to a previously inaccessible region of scattering angles. It also offers the possibility of tracking the spatio-temporal evolution of complex fluids and other inhomogeneous systems.

Neutrons and synchrotron radiation in engineering materials science : from fundamentals to material and component characterization

Abstract: PART I: GENERAL MICROSTRUCTURE AND PROPERTIES OF ENGINEERING MATERIALS INTERNAL STRESSES IN ENGINEERING MATERIALS TEXTURE AND TEXTURE ANALYSIS IN ENGINEERING MATERIALS PHYSICAL PROPERTIES OF PHOTONS AND NEUTRONS RADIATION SOURCES GENERATION AND PROPERTIES OF NEUTRONS PRODUCTION AND PROPERTIES OF SYNCHROTRON RADIATION PART II: METHODS INTRODUCTION TO DIFFRACTION METHODS FOR INTERNAL STRESS ANALYSES STRESS ANALYSIS BY ANGLE-DISPENSIVE NEUTRON DIFFRACTION STRESS ANALYSIS BY ENERGY-DISPERSIVE NEUTRON DIFFRACTION RESIDUAL STRESS ANALYSIS BY MONOCHROMATIC HIGH-ENERGY X-RAYS RESIDUAL STRESS ANALYSIS BY WHITE HIGH ENERGY X-RAYS REFLECTION MODE TRANSMISSION MODE DIFFRACTION IMAGING FOR MICROSTRUCTURE ANALYSIS BASICS OF SMALL-ANGLE SCATTERING METHODS SMALL-ANGLE NEUTRON SCATTERING DECOMPOSITION KINETICS IN COPPER-COBALT ALLOY SYSTEMS: APPLICATIONS OF SMALL-ANGLE X-RAY SCATTERING New Developments in Neutron Tomography NEUTRON AND SYNCHROTRON -RADIATION-BASED IMAGING FOR APPLICATIONS IN MATERIALS SCIENCE - FROM MACRO- TO NANOTOMOGRAPHY mu-TOMOGRAPHY OF ENGINEERING MATERIALS DIFFRACTION ENHANCED IMAGING PART III: NEW AND EMERGING METHODS 3D X-RAY DIFFRACTION MICROSCOPE 3D MICRON-RESOLUTION LAUE DIFFRACTION QUANTITATIVE ANALYSIS OF THREE-DIMENSIONAL PLASTICS STRAIN FIELD USING MARKERS AND X-RAY ABSORPTION TOMOGRAPHY COMBINED DIFFRACTION AND TOMOGRAPHY PART IV: INDUSTRIAL APPLICATIONS DIFFRACTION-BASED RESIDUAL STRESS ANALYSIS APPLIED TO PROBLEMS IN THE AIRCRAFT INDUSTRY OPTIMIZATION OF RESIDUAL STRESSES IN CRANKHAFTS

Delta undulator for Cornell energy recovery linac

Abstract: In anticipation of a new era of synchrotron radiation sources based on energy recovery linac techniques, we designed, built, and tested a short undulator magnet prototype whose features make optimum use of the unique conditions expected in these facilities. The prototype has pure permanent magnet (PPM) structure with 24 mm period, 5 mm diameter round gap, and is 30 cm long. In comparison with conventional undulator magnets it has the following: (i) full x-ray polarization control.---It may generate varying linear polarized as well as left and right circular polarized x rays with photon flux much higher than existing Apple-II--type devices. (ii) 40% stronger magnetic field in linear and approximately 2 times stronger in circular polarization modes. This advantage translates into higher x-ray flux. (iii) Compactness.---The prototype can be enclosed in a $\ensuremath{\sim}20\text{ }\text{ }\mathrm{cm}$ diameter cylindrical vacuum vessel. These advantages were achieved through a number of unconventional approaches. Among them is control of the magnetic field strength via longitudinal motion of the magnet arrays. The moving mechanism is also used for x-ray polarization control. The compactness is achieved using a recently developed permanent magnet soldering technique for fastening PM blocks. We call this device a ``Delta'' undulator after the shape of its PM blocks. The presented article describes the design study, various aspects of the construction, and presents some test results.

Dots, Clumps, and Filaments: The Intermittent Images of Synchrotron Emission in Random Magnetic Fields of Young Supernova Remnants

Abstract: Nonthermal X-ray emission in some supernova remnants originates from synchrotron radiation of ultrarelativistic particles in turbulent magnetic fields. We address the effect of a random magnetic field on synchrotron emission images and spectra. A random magnetic field is simulated to construct synchrotron emission maps of a source with a steady distribution of ultrarelativistic electrons. Nonsteady localized structures (dots, clumps, and filaments), in which the magnetic field reaches exceptionally high values, typically arise in the random field sample. These magnetic field concentrations dominate the synchrotron emission (integrated along the line of sight) from the highest energy electrons in the cutoff regime of the distribution, resulting in an evolving, intermittent, clumpy appearance. The simulated structures resemble those observed in X-ray images of some young supernova remnants. The lifetime of X-ray clumps can be short enough to be consistent with that observed even in the case of a steady particle distribution. The efficiency of synchrotron radiation from the cutoff regime in the electron spectrum is strongly enhanced in a turbulent field compared to emission from a uniform field of the same magnitude.

Protein crystallography with a micrometre-sized synchrotron-radiation beam.

Abstract: For the first time, protein microcrystallography has been performed with a focused synchrotron-radiation beam of 1 µm using a goniometer with a sub-micrometre sphere of confusion. The crystal structure of xylanase II has been determined with a flux density of about 3 × 1010 photons s−1 µm−2 at the sample. Two sets of diffraction images collected from different sized crystals were shown to comprise data of good quality, which allowed a 1.5 A resolution xylanase II structure to be obtained. The main conclusion of this experiment is that a high-resolution diffraction pattern can be obtained from 20 µm3 crystal volume, corresponding to about 2 × 108 unit cells. Despite the high irradiation dose in this case, it was possible to obtain an excellent high-resolution map and it could be concluded from the individual atomic B-factor patterns that there was no evidence of significant radiation damage. The photoelectron escape from a narrow diffraction channel is a possible reason for reduced radiation damage as indicated by Monte Carlo simulations. These results open many new opportunities in scanning protein microcrystallography and make random data collection from microcrystals a real possibility, therefore enabling structures to be solved from much smaller crystals than previously anticipated as long as the crystallites are well ordered.

Nonthermal Radiation from Pulsar Wind Nebulae

Abstract: We study the nonthermal emission of pulsar wind nebulae (PWNe) from radio to TeV gamma-ray energies with a simplified time-dependent injection model. In this model, a relativistic wind of particles driven by a central pulsar with a spin-down power L(t) is blown into the ambient medium and a shock wave is formed, which accelerates the particles to very high energies through the Fermi acceleration mechanism in the PWN. The relativistic particles in the PWN therefore consist of two components, one coming directly from the pulsar magnetosphere and the other from shock acceleration in the PWN. The model PWN spectra follow a broken power law with different indices and a break energy E-b. The accelerated particles produce nonthermal photons through synchrotron radiation and inverse Compton scattering off soft photon fields. We apply this model to the Crab Nebula, the PWN in MSH 15 - 52, and HESS J1825 - 137, which have all been found to emit very high energy (VHE) photons. Our results indicate that ( 1) the observed data, ranging from radio to VHE gamma-rays, for the Crab Nebula can be reproduced by this model, where the emission from radio to medium-energy gamma-rays is from the synchrotron emission due to the injected electrons, whereas the high-energy photons primarily come from inverse Compton scattering of the high-energy electrons on synchrotron photons; and ( 2) the TeV emission from the PWN in MSH 15 - 52 and HESS J1825 - 137 mainly comes from the inverse Compton scattering of the high-energy electrons on infrared photons.

Tunable narrowband terahertz emission from mastered laser–electron beam interaction

Abstract: A tunable source of coherent narrowband terahertz radiation is realized by using a laser to modulate the emission characteristics of a relativistic electron beam. In the quest for sources of optical radiation in the terahertz domain, promising candidates are nonlinear optical processes occurring when an intense laser beam interacts with a material medium1,2,3,4,5. Besides conventional media (such as crystals), relativistic electrons also show striking nonlinear collective behaviours, which can lead to powerful laser-induced coherent emission6,7, revealing huge potentials of these devices as terahertz sources8. However, up to now only broadband emissions were reported, and experimental control of their radiation properties, such as their spectra9,10, remained an important challenge. Here, we demonstrate the possibility of mastering the coherent emission experimentally by producing tunable narrowband terahertz radiation. The interaction is made to occur between an electron beam and laser pulses possessing a longitudinal quasi-sinusoidal modulation, and the narrowband emission occurs in a region of quasi-uniform magnetic field. The process therefore strongly differs from classical synchrotron radiation experiments, where narrowband emission occurs inside a periodic magnetic field.

From diffraction to imaging: New avenues in studying hierarchical biological tissues with x-ray microbeams (Review)

Abstract: Load bearing biological materials such as bone or arthropod cuticle have optimized mechanical properties which are due to their hierarchical structure ranging from the atomic/molecular level up to macroscopic length scales. Structural investigations of such materials require new experimental techniques with position resolution ideally covering several length scales. Beside light and electron microscopy, synchrotron radiation based x-ray imaging techniques offer excellent possibilities in this respect, ranging from full field imaging with absorption or phase contrast to x-ray microbeam scanning techniques. A particularly useful approach for the study of biological tissues is the combination x-ray microbeam scanning with nanostructural information obtained from x-ray scattering [small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS)]. This combination allows constructing quantitative images of nanostructural parameters with micrometer scanning resolution, and hence, covers two length scales at once. The present article reviews recent scanning microbeam SAXS/WAXS work on bone and some other biological tissues with particular emphasis on the imaging capability of the method. The current status of instrumentation and experimental possibilities is also discussed, and a short outlook about actual and desirable future developments in the field is given.

Differential phase contrast with a segmented detector in a scanning X‐ray microprobe

Abstract: Scanning X-ray microprobes are unique tools for the nanoscale investigation of specimens from the life, environmental, materials and other fields of sciences. Typically they utilize absorption and fluorescence as contrast mechanisms. Phase contrast is a complementary technique that can provide strong contrast with reduced radiation dose for weakly absorbing structures in the multi-keV range. In this paper the development of a segmented charge-integrating silicon detector which provides simultaneous absorption and differential phase contrast is reported. The detector can be used together with a fluorescence detector for the simultaneous acquisition of transmission and fluorescence data. It can be used over a wide range of photon energies, photon rates and exposure times at third-generation synchrotron radiation sources, and is currently operating at two beamlines at the Advanced Photon Source. Images obtained at around 2 keV and 10 keV demonstrate the superiority of phase contrast over absorption for specimens composed of light elements.

Simulating cosmic rays in clusters of galaxies – III. Non-thermal scaling relations and comparison to observations

Abstract: Complementary views of galaxy clusters in the radio synchrotron, hard X-ray inverse Compton and high-energy γ-ray regimes are critical in calibrating them as high-precision cosmological probes. We present predictions for scaling relations between cluster mass and these non-thermal observables. To this end, we use high-resolution simulations of a sample of galaxy clusters spanning a mass range of almost two orders of magnitudes, and follow self-consistent cosmic ray physics on top of the radiative hydrodynamics. We model relativistic electrons that are accelerated at cosmological structure formation shocks and those that are produced in hadronic interactions of cosmic rays with ambient gas protons. Calibrating the magnetic fields of our model with Faraday rotation measurements, the synchrotron emission of our relativistic electron populations matches the radio synchrotron luminosities and morphologies of observed giant radio haloes and minihaloes surprisingly well. Using the complete sample of the brightest X-ray clusters observed by ROSAT in combination with our γ-ray scaling relation, we predict GLAST to detect about ten clusters allowing for Eddington bias due to the scatter in the scaling relation. The expected brightest γ-ray clusters are Ophiuchus, Fornax, Coma, A3627, Perseus and Centaurus. The high-energy γ-ray emission above 100 MeV is dominated by pion decays resulting from hadronic cosmic ray interactions. We provide an absolute lower flux limit for the γ-ray emission of Coma in the hadronic model which can be made tighter for magnetic field values derived from rotation measurements to match the GLAST sensitivity, providing thus a unique test for the possible hadronic origin of radio haloes. Our predicted hard X-ray emission, due to inverse Compton emission of shock accelerated and hadronically produced relativistic electrons, falls short of the detections in Coma and Perseus by a factor of 50. This casts doubts on inverse Compton interpretation and reinforces the known discrepancy of magnetic field estimates from Faraday rotation measurements and those obtained by combining synchrotron and inverse Compton emission.

Subnanosecond time-resolved x-ray measurements using an organic-inorganic perovskite scintillator

Abstract: We have developed a fast x-ray detector using an organic-inorganic perovskite scintillator of phenethylamine lead bromide (PhE-PbBr4) The scintillator had a dominant light emission with a fast decay time of 99 ns An x-ray detector equipped with a 09-mm-thick PhE-PbBr4 crystal was used to detect nuclear resonant scattering in N61i (the first excited level: 6741 keV; lifetime: 76 ns) by using synchrotron radiation With this detector, we could successfully record the decaying gamma rays emitted from N61i with a time resolution of 07 ns (full width at half maximum) and a relatively high detection efficiency of 24%

A Revisit of the Phase-resolved X-Ray and Gamma-Ray Spectra of the Crab Pulsar

Abstract: We use a modified outer-gap model to study the multifrequency phase-resolved spectra of the Crab pulsar. The emissions from both poles contribute to the light curve and the phase-resolved spectra. Using the synchrotron self-Compton mechanism and by considering the incomplete conversion of curvature photons into secondary pairs, the observed phase-averaged spectrum from 100 eV to 10 GeV can be explained very well. The predicted phase-resolved spectra can match the observed data reasonably well, too. We find that the emission from the north pole mainly contributes to leading wing 1. The emissions in the remaining phases are mainly dominated by the south pole. The widening of the azimuthal extension of the outer gap explains trailing wing 2. The complicated phase-resolved spectra for the phases between the two peaks, namely, trailing wing 1, the bridge, and leading wing 2, strongly suggest that there are at least two well-separated emission regions with multiple emission mechanisms—synchrotron radiation, inverse Compton scattering, and curvature radiation. Our best-fit results indicate that there may exist some asymmetry between the south and north poles. Our model predictions can be examined with GLAST.

Evidence of a Curved Synchrotron Spectrum in the Supernova Remnant SN 1006

Abstract: A joint spectral analysis of some Chandra ACIS X-ray data and Molonglo Observatory Synthesis Telescope radio data was performed for 13 small regions along the bright northeastern rim of the supernova remnant SN 1006. These data were fitted with a synchrotron radiation model. The nonthermal electron spectrum used to compute the photon emission spectra is the traditional exponentially cut off power law, with one notable difference: The power-law index is not a constant. It is a linear function of the logarithm of the momentum. This functional form enables us to show, for the first time, that the synchrotron spectrum of SN 1006 seems to flatten with increasing energy. The effective power-law index of the electron spectrum is 2.2 at 1 GeV (i.e., radio synchrotron-emitting momenta) and 2.0 at about 10 TeV (i.e., X-ray synchrotron-emitting momenta). This amount of change in the index is qualitatively consistent with theoretical models of the amount of curvature in the proton spectrum of the remnant. The evidence of spectral curvature implies that cosmic rays are dynamically important instead of being "test" particles. The spectral analysis also provides a means of determining the critical frequency of the synchrotron spectrum associated with the highest-energy electrons. The critical frequency seems to vary along the northeastern rim, with a maximum value of 1.1+1.0−0.5 × 1017 Hz. This value implies that the electron diffusion coefficient can be no larger than a factor of ~4.5-21 times the Bohm diffusion coefficient if the velocity of the forward shock is in the range 2300-5000 km s−1. Since the coefficient is close to the Bohm limit, electrons are accelerated nearly as fast as possible in the regions where the critical frequency is about 1017 Hz.

Particle Acceleration and Non-Thermal Emission in Pulsar Outer Magnetospheric Gap

Abstract: A two-dimensional electrodynamic model is used to study particle acceleration and non-thermal emission mechanisms in the pulsar magnetospheres. We solve distribution of the accelerating electric field with the emission process and the pair-creation process in meridional plane, which includes the rotational axis and the magnetic axis. By solving the evolutions of the Lorentz factor, and of the pitch angle, we calculate spectrum in optical through $\gamma$-ray bands with the curvature radiation, synchrotron radiation, and inverse-Compton process not only for outgoing particles, but also for ingoing particles, which were ignored in previous studies. We apply the theory to the Vela pulsar. We find that the curvature radiation from the outgoing particles is the major emission process above 10 MeV bands. In soft $\gamma$-ray to hard X-ray bands, the synchrotron radiation from the ingoing primary particles in the gap dominates in the spectrum. Below hard X-ray bands, the synchrotron emissions from both outgoing and ingoing particles contribute to the calculated spectrum. The calculated spectrum is consistent with the observed phase-averaged spectrum of the Vela pulsar. We show that the observed five-peak pulse profile in the X-ray bands of the Vela pulsar is reproduced by the inward and outward emissions, and the observed double-peak pulse profile in $\gamma$-ray bands is explained by the outward emissions.

On the X-ray feature associated with the Guitar nebula

Abstract: Context. A mysterious X-ray nebula, showing a remarkably linear geometry, was recently discovered close to the Guitar Nebula, the bow-shock nebula associated with B2224+65, which is the fastest pulsar known. The nature of this X-ray feature is unknown, and even its association with pulsar B2224+65 is unclear. Aims. We attempt to develop a self-consistent scenario to explain the complex phenomenology of this object. Methods. We assume that the highest energy electrons accelerated at the termination shock escape from the bow shock and diffuse into the ambient medium, where they emit synchrotron X-rays. The linear geometry should reflect the plane-parallel geometry of its ambient field. Results. We estimate the Lorentz factor of the X-ray emitting electrons and the strength of the magnetic field. The former (� 10 8 )i s close to its maximum possible value, while the latter, at 45 μG, is higher than typical interstellar values and must have been amplified in some way. The magnetic field must also be turbulent to some degree to trap the electrons sufficiently for synchrotron X-ray emission to occur effectively. We propose a self-consistent scenario in which, by some streaming instability, the electrons themselves generate a turbulent field in which they then diffuse. Some numerical coincidences are explained, and tests are proposed to verify our scenario. Conclusions. Electron leaking may be common in the majority of pulsar bow-shock nebulae, even though the X-ray nebulosity in general is too diffuse to be detectable.