High-energy Radiation Sources Research Articles

Particle acceleration in axisymmetric pulsar current sheets

The equatorial current sheet in pulsar magnetospheres is often regarded as an ideal site for particle acceleration via relativistic reconnection. Using 2D spherical particle-in-cell simulations, we investigate particle acceleration in the axisymmetric pulsar magnetosphere as a function of the injected plasma multiplicity and magnetization. We observe a clear transition from a highly charge-separated magnetosphere for low plasma injection with little current and spin-down power, to a nearly force-free solution for high plasma multiplicity characterized by a prominent equatorial current sheet and high spin-down power. We find significant magnetic dissipation in the current sheet, up to 30% within 5 light-cylinder radii in the high-multiplicity regime. The simulations unambiguously demonstrate that the dissipated Poynting flux is efficiently channeled to the particles in the sheet, close to the Y-point within about 1-2 light cylinder radii from the star. The mean particle energy in the sheet is given by the upstream plasma magnetization at the light cylinder. The study of particle orbits shows that all energetic particles originate from the boundary layer between the open and the closed field lines. Energetic positrons always stream outward, while high-energy electrons precipitate back towards the star through the sheet and along the separatrices, which may result in auroral-like emission. Our results suggest that the current sheet and the separatrices may be the main source of high-energy radiation in young pulsars.

Discovery of X-ray pulsations in the Be/X-ray binary IGR J21343+4738

We report on the discovery of X-ray pulsations in the Be/X-ray binary IGR J21343+4738 during an XMM-Newton observation. We obtained a barycentric corrected pulse period of 320.35+-0.06 seconds. The pulse profile displays a peak at low energy that flattens at high energy. The pulse fraction is 45+-3$% and independent of energy within the statistical uncertainties. The 0.2-12 keV spectrum is well fit by a two component model consisting of a blackbody with kT=0.11+-0.01 keV and a power law with photon index Gamma=1.02+-0.07. Both components are affected by photoelectric absorption with a equivalent hydrogen column density NH=(1.08+-0.15)x 10^{22} cm^{-2} The observed unabsorbed flux is 1.4x10^{-11} erg cm^{-2} s^{-1} in the 0.2-12 keV energy band. Despite the fact that the Be star's circumstellar disc has almost vanished, accretion continues to be the main source of high energy radiation. We argue that the observed X-ray luminosity (LX~10^{35} erg s^{-1}) may result from accretion via a low-velocity equatorial wind from the optical companion.

Editorial: Welcome to Structural Dynamics-A new open-access journal co-published by AIP Publishing and the American Crystallographic Association.

Welcome to the first issue of Structural Dynamics, a new, open-access journal devoted to the science of time-evolving structures. Structural Dynamics is being launched at a key moment. Almost twenty five years ago the birth of femtosecond spectroscopy opened a new era in science by its ability to probe in “real-time” the motion of atomic nuclei in molecules of all sizes, solids, and liquids. In recent years, huge efforts have enabled experimental tools that combine the atomic resolution of time (femtoseconds) with that of length (sub-Angstrom). First, the advent of a new generation of light sources, the so-called X-ray free electron lasers (XFELs), which deliver highly coherent hard X-ray pulses of typically a few tens of femtosecond duration and with fluxes several orders of magnitude higher than the picosecond pulses of synchrotrons. Almost simultaneously, high current sources of ultrashort pulses of electrons boosted the fields of ultrafast electron diffraction and microscopy. Thus, electron and X-ray diffraction and electron microscopy are now routinely implemented with time resolutions down to tens of femtoseconds. XFELs are also opening the field for different schemes of static or time-resolved imaging thanks to their high coherence, while single shot diffraction experiments are becoming a reality. Finally, they are ideal for photon-greedy X-ray spectroscopies such as X-ray emission spectroscopy (XES) or resonant inelastic X-ray scattering (RIXS). Lab-based sources of pulsed high energy radiation are also pushing the area of structural studies to new levels, not only by methods such as X-ray diffraction, but also by studies of the electronic structure of molecular and condensed matter systems since in the end, it is the electronic structure that determines the chemical bond. Thus vacuum ultraviolet/soft X-ray lab sources using the process of high harmonic generation (HHG) are playing an increasingly important role in this respect. In addition to these new sources, there has been an impressive development of methodologies which allow new insights into the mapping of time-evolving structures of molecular and biological systems, liquids and solid materials. Photoelectron spectroscopies have recently been implemented for liquids, several new variants of X-ray spectroscopy (XES, RIXS) have now entered the time domain. A major development of the past 10–15 years has been the implementation of multidimensional spectroscopies, which are analogues of NMR in the optical domain, enabling a higher sensitivity and a higher time resolution to be achieved. These multidimensional spectroscopies in the infrared, the visible and the ultraviolet have become important structural tools due to their ability to probe correlations among different excited centers in a given system, be it a chemical, biological or condensed matter one. Efforts are even being deployed to push these multidimensional spectroscopies into the X-ray domain by means of XFELs, which are currently the only instruments that make such schemes possible. We are now on the cusp of a new “Age” in the field of structural studies. Structural Dynamics is the first journal entirely devoted to this field, with a wide range of methodologies that interrogate in “real-time” not only the evolving geometric structure, but also the underlying electronic structure of assemblies of atoms, as well as correlations between them. Because length and time scale together, the journal covers structural phenomena ranging from femtoseconds to microseconds. It is the ideal time for us to proudly launch what we are sure will become the top open access journal entirely dedicated to time-dependent structure determination. As part of promoting Structural Dynamics the first 50 papers to be published will be free – now is the time to submit your best and most exciting work before the queue is filled.

Dark matter in the Galactic center

AbstractIn addition to boasting the highest density of baryonic matter in our galaxy, the center of the Milky Way is also believed to contain an extremely high density of dark matter particles. While dark matter is expected to be gravitationally subdominant to baryons near the Galactic center, many models allow for the annihilation of dark matter into standard model particles, a phenomenon which could be a significant source of high energy radiation in the Galactic center region. In fact, standard models of the dark matter density distribution and annihilation spectrum predict that dark matter at the Galactic center would (1) produce the brightest flux from dark matter annihilation of any region in the sky, and (2) contribute a significant portion of the total γ-ray luminosity observed within several degrees of the Galactic center by the Fermi/LAT. This makes the effort to understand and differentiate the morphologies and spectral features of dark matter and astrophysical γ-ray emission at the Galactic center potentially rewarding. Here, I will summarize the recent developments in indirect searches for dark matter annihilation at the Galactic center, and discuss several of the difficulties in producing accurate models of the high energy astrophysical emission. Finally, I will comment on current efforts to produce multi-wavelength models which better constrain or indicate a dark matter signal at the Galactic center.

Optical control of hard X-ray polarization by electron injection in a laser wakefield accelerator

Laser-plasma particle accelerators could provide more compact sources of high-energy radiation than conventional accelerators. Moreover, because they deliver radiation in femtosecond pulses, they could improve the time resolution of X-ray absorption techniques. Here we show that we can measure and control the polarization of ultra-short, broad-band keV photon pulses emitted from a laser-plasma-based betatron source. The electron trajectories and hence the polarization of the emitted X-rays are experimentally controlled by the pulse-front tilt of the driving laser pulses. Particle-in-cell simulations show that an asymmetric plasma wave can be driven by a tilted pulse front and a non-symmetric intensity distribution of the focal spot. Both lead to a notable off-axis electron injection followed by collective electron–betatron oscillations. We expect that our method for an all-optical steering is not only useful for plasma-based X-ray sources but also has significance for future laser-based particle accelerators.

Angular distribution of photoelectrons emitted by the laboratory soft and hard X-ray radiation sources

XPS spectrometers are typically equipped with the Mg Kα radiation and/or Al Kα radiation sources. However, for some analyses, it is convenient to use the laboratory sources emitting high energy X-rays, e.g. to avoid coincidence of peaks due to photoelectrons and Auger electrons, or to increase the information depth. On the other hand, the mathematical formalism of quantitative XPS analysis is based on the theoretical photoemission cross section that is valid for photoelectrons emitted with a relatively low kinetic energy (the so-called dipole approximation – DA). For high kinetic energy photoelectrons, the photoemission cross section needs to be modified with nondipolar parameters. An important issue for an analyst is the correction of the formalism of quantitative analysis for the photoelectron elastic scattering effects. The Monte Carlo program has been developed for simulation of transport of photoelectrons emitted from polycrystalline or amorphous solids by four X-ray radiation sources: Mg Kα, Al Kα, Zr Lα and Ti Kα. Calculations were performed for photoelectrons emitted in elemental solids with a wide range of atomic numbers: Al, Cu, Ag and Au. The photoelectron emission event was described within the DA and the model modified with the multipole correction (non-dipole approximation – NDA). It has been found that, in a typical XPS configuration, the dipole approximation is of sufficient accuracy for the Mg Kα and Al Kα radiation sources. The difference between photoelectron signal intensities calculated within the DA and NDA did not exceed 8% when elastic photoelectron elastic scattering is neglected. For high-energy radiation sources, Zr Lα and Ti Kα, the difference may reach 25% in this geometry, and thus the NDA model is recommended for calculations. The photoelectron elastic collisions are found to decrease the difference between the DA and NDA models. An important result of the present analysis is the observation that the NDA formalism can be corrected for elastic scattering effects in the same way as the DA formalism, i.e. with two parameters, Qx and βx. Furthermore, the expressions for calculating these parameters derived for the DA model and for the Mg Kα and Al Kα sources are found to be also applicable to photoelectron emitted by the Zr Lα and Ti Kα sources. However, new expressions valid for all four radiation sources have been tentatively derived in the present work.

Hardness Assurance Testing for Proton Direct Ionization Effects

The potential for using the degraded beam of high-energy proton radiation sources for proton hardness assurance testing for ICs that are sensitive to proton direct ionization effects are explored. SRAMs were irradiated using high energy proton radiation sources (~67-70 MeV). The proton energy was degraded using plastic or Al degraders. Peaks in the SEU cross section due to direct ionization were observed. To best observe proton direct ionization effects, one needs to maximize the number of protons in the energy spectrum below the proton energy SEU threshold. SRIM simulations show that there is a tradeoff between increasing the fraction of protons in the energy spectrum with low energies by decreasing the peak energy and the reduction in the total number of protons as protons are stopped in the device as the proton energy is decreased. Two possible methods for increasing the number of low energy protons is to decrease the primary proton energy to reduce the amount of energy straggle and to place the degrader close to the DUT to minimize angular dispersion. These results suggest that high-energy proton radiation sources may be useful for identifying devices sensitive to proton direct ionization.

Multiwavelength Astronomy and CTA: X-rays

We discuss how future X-ray instruments which are under development can contribute to our understanding of the non-thermal Universe. Much progress has been made in the field of X-ray Astronomy recently, thanks to the operation of modern X-ray telescopes such as Chandra, XMM-Newton, Suzaku, and SWIFT, but more in-depth investigation awaits future missions. These future missions include ASTROSAT, NuSTAR, e-ROSITA, ASTRO-H and GEMS, which will be realized in the next decade, and also much larger projects such as Athena and LOFT, which have been proposed for the 2020’s. All of those are expected to bring a variety of novel observational results regarding astrophysical sources of high-energy particles and radiation, i.e. supernova remnants, neutron stars, stellar-mass black holes, active galaxies, and clusters of galaxies among others. The operation of the future X-ray instruments will proceed in parallel with the operation of Fermi-LAT and the Cherenkov Telescope Array. We emphasize that the synergy between the X-ray and γ-ray observations is particularly important, and that the planned X-ray missions, when in conjunction with the modern γ-ray observatories, will indeed provide a qualitatively better insight into the high-energy Universe.

Unbeamed γ-rays from low luminosity AGNs

Proton-proton collisions in two-temperature accretion flows may lead to substantial γ-ray emission through neutral pion production and decay. We present preliminary results of our study of such a hadronic γ-ray emission. We find that radiatively inefficient flows around rapidly rotating black holes can produce the MeV/GeV luminosities up to ∼ 1044 erg s−1. The hot-flow model predictions for the hadronic and the Comptonization components can put interesting constraints on the properties of the source of high-energy radiation in AGNs. As an example, we compare our model with X/γ-ray observations of Centaurus A and we find that the model with a maximally rotating black hole strongly overpredicts the γ-ray flux.

Migrating source of energetic radiation generated by thunderstorm activity

[1] We identify a migrating source of high energy radiation, lasting for several minutes, attributed to thunderstorm activities through the observations of radiation, atmospheric electric field, and meteorological radar echoes at several points. Our findings indicate that the energetic radiation is emitted continuously from a downward hemispherical surface without lightning, the bottom of which is about 300 m above sea level, and this source of radiation moves from north to south above the observation site at the speed of about 7 m/s. The radiation source probably moves along with the negatively charged region of the cloud at the height of around 1 km, because the estimated migration of the radiation source is consistent with the observed movement of atmospheric electric field variation between ground-based observation sites and with the wind speed and direction at about 1 km altitude. This movement implies that the intensive electric field produced by the charged region in the thundercloud generates a radiation source. In addition, our results suggest that the low altitude of radiation source is related to no lightning activity during the energetic radiation emission.

Experimental results on hard x-ray energy emitted by a low-energy plasma focus device: a radiographic image analysis

The PF-400J plasma focus device (400 J of energy) was studied in a similar way to a pulsed source of high-energy x-ray radiation. High-sensitivity fast-response commercial radiographic film and different metal filter array combinations were used to study the energy of the proper emitted x-ray. The effective energy of the hard x-ray radiation is studied and discussed in terms of its effective mean attenuation with the penetration distance on different metallic samples when the x-ray pulsed beam is supposed monoenergetic. These results were then used to assess the energetic properties of the pulsed radiation when different x-ray film–filter array combinations were irradiated between tens to almost 160 effective pulses of x-ray coming from the PF-400J device.

Detailed characterization of stellar high energy (FUV/EUV/X-ray) radiation fields during protoplanetary system formation

AbstractYoung stars undergoing the conversion of pre-main-sequence circumstellar disks into protoplanetary systems are strong sources of high energy (FUV/EUV/X-ray) radiation that controls the physical and chemical processes in their circumstellar environment out to hundreds of AU from the star. The high energy radiation resulting from magnetic activity and accretion onto the central star controls the thermal structure of disks, the formation process of planetesimals, and the photoexcitation and photoionization of protoplanets and young planetary atmospheres. Modeling of the dust and gas evolution requires an accurate understanding of the local radiation field throughout the ultraviolet (UV) and X-ray spectral regions, even those parts of the spectrum that are impossible to observe from Earth.Our current research efforts are directed towards developing a better understanding of UV (using HST and FUSE) and X-ray (using Chandra, XMM-Newton, and Swift) stellar activity and the resulting radiation fields during pre-main-sequence evolution from ages of a few to several hundred million years. These studies include extensive UV and X-ray spectral sampling of individual stars in nearby star formation regions and the various moving groups of the Local Association, including our HST Cycle 17 Large Project (GO-11616), which is using 111 HST orbits to observe 32 T Tauri stars with the COS UV spectrograph. Most young stars are well over 100 pc from the Sun and are consequently hard to observe in the UV and X-ray regions at even moderate spectral resolution. However, members of the Local Association, whose ages range from 7 Myr to a few hundred Myr, surround the Sun at distances of 50 pc or less and permit the detailed study of the later stages of the early evolution of stellar activity when gas giant and terrestrial protoplanets are forming. We illustrate our methodology using the 12 Myr old early-M dwarf AU Mic, which possesses a striking dust debris disk, as an example.

Engines of Discovery: A Century of Particle Accelerators

Part A. The Accelerators: Electrostatic Accelerators Cyclotrons Linacs Betatrons Synchrotrons Colliders Muon Accelerators Detectors Part B. The Applications: High-Energy and Nuclear Physics Synchrotron Radiation Sources Isotope Production and Cancer Therapy Accelerators Spallation Neutron Sources Accelerators in Industry National Security Energy and Environment Part C. The Future: A Final Word - Mainly to the Young.

Proton and neutrino extragalactic astronomy

The study of extragalactic sources of high energy radiation via the direct measurement of the proton and neutrino fluxes that they are likely to emit is one of the main goals for the future observations of the recently developed air showers detectors and neutrino telescopes. In this work we discuss the relation between the inclusive proton and neutrino signals from the ensemble of all sources in the universe and the resolved signals from the closest and brightest objects. We also compare the sensitivities of proton and neutrino telescopes and comment on the relation between these two new astronomies.

Using X-Ray Microtomography for Characterisation of Catalyst Particle Pore Structure

The use of a high-energy synchrotron radiation source has permitted the introduction of high-resolution X-ray microtomography imaging as a method of non-intrusive and non-destructive three-dimensional (3D)visualized characterization of porous media. The use of a combination of X-ray microtomography, along with mercury porosimetry and Low Melting Point Alloy (LMPA) intrusion potentially provides a more complete approach to more accurate catalyst pore structure characterisation and modelling. This combined approach has been applied to the study of an industrial alumina catalysts support. The data collected from the study has been used to develop and generate an improved pore network model basis, in which both heterogeneity and hierarchy are considered in obtaining an improved pore architecture and pore size distribution.

Turning Starlight into Gamma Rays

An overlooked gamma-ray production mechanism explains a mysterious source of high-energy radiation from space.

Estimated efficiency of slit collimation of a high-energy radiation source for radiometric testing of large objects

An approach to estimating the contribution of scattering to the integral flux of detected radiation is developed. The proposed method may be used for radiometric testing of large objects. A method for selecting parameters of slit collimators for sources of high-energy bremsstrahlung is proposed.

A gas pixel detector for x-ray polarimetry

Even though lacking of solid experimental verifications, X-ray polarimetry is strongly established as a deep diagnostic tool for probing the emission mechanisms in astronomical sources of high energy radiation. The recent development of new, more efficient instrumentation, as well as the renewed interest of the theoreticians, has drawn a significant attention to the field. Particularly, the exploitation of the photoelectric effect for deriving polarization information seems to promise a great advance in sensitivity with respect to the conventional techniques. To this aim we have designed, produced and tested a CMOS VLSI array of 2101 pixels (with 80 μm pitch), to be directly used as the charge collecting anode of a Gas Electron Multiplier (GEM). Each pixel is fully covered by a hexagonal metal electrode and each of these electrodes is individually connected to a full electronics chain, built immediately below it; in this sense detector and read-out electronics become virtually the same thing. Even though we focus our attention on the polarimetric applications, our achievements are highly significant for the whole field of development of gas detectors, which for the first time reach the level of integration and resolution typical of solid state detectors.

Collapsing Stars as Sources of High Energy Particles and Radiation

The particle acceleration and the non-thermal radiation in the magnetospheres of collapsing stars are considered. The collapsing stars can be powerful sources of the high-energy charged particles and the non-thermal radiation bursts. These bursts can be observed by means of modern astronomical instruments.

Radiosurgery for pituitary adenomas

Radiosurgery for pituitary adenomas