GRB 030329: 3 years of radio afterglow monitoring

Radio observations of gamma-ray burst (GRB) afterglows are essential for our understanding of the physics of relativistic blast waves, as they enable us to follow the evolution of GRB explosions much longer than the afterglows in any other wave band. We have performed a 3-year monitoring campaign of GRB 030329 with the Westerbork Synthesis Radio Telescopes and the Giant Metrewave Radio Telescope. Our observations, combined with observations at other wavelengths, have allowed us to determine the GRB blast wave physical parameters, such as the total burst energy and the ambient medium density, as well as to investigate the jet nature of the relativistic outflow. Further, by modelling the late-time radio light curve of GRB 030329, we predict that the Low-Frequency Array (30-240 MHz) will be able to observe afterglows of similar GRBs, and constrain the physics of the blast wave during its non-relativistic phase.     

The late X-ray afterglow of gamma-ray bursts

We have developed a functional fit which can be used to represent the entire temporal decay of the X-ray afterglow of gamma-ray bursts (GRBs). The fit delineates and parameterizes well-defined phases for the decay: the prompt emission; an initial steep decay; a shallow plateau phase; and finally, a powerlaw afterglow. For 20% of GRBs, the plateau phase is weak, or not seen, and the initial powerlaw decay becomes the final afterglow.We compare the temporal decay parameters and X-ray spectral indices for 107 GRBs discovered by Swift with the expectations of the standard fireball model including a search for possible jet breaks. For approximately 50% of GRBs, the observed afterglow is in accord with the model, but for the rest the temporal and spectral properties are not as expected. We identify a few possible jet breaks, but there are many examples where such breaks are predicted but are absent. We also find that the start time of the final afterglow decay, Ta, is associated with the peak of the prompt gamma-ray emission spectrum, Epeak, just as optical jet-break times, tj, are associated with Epeak in the Ghirlanda relation.     

X-ray optical parameters of a microcapillary lens

An analysis is made of the possibility of focusing x-rays using a new element in x-ray optics, termed a microcapillary x-ray lens. Pis’ma Zh. Tekh. Fiz. 24, 19–23 (December 26, 1998)     

Shock break-out: how a GRB revealed the beginnings of a supernova

In February 2006, Swift caught a gamma-ray burst (GRB) in the act of turning into a supernova, and made the first ever direct observations of the break-out and early expansion of a supernova shock wave. GRB 060218 began with an exceptionally long burst of non-thermal gamma-rays, lasting over 2000s, as a jet erupted through the surface of the star. While this was in progress, an optically-thick thermal component from the shock wave of the supernova explosion grew to prominence, and we were able to track the mildly relativistic expansion of this shell as the blackbody peak moved from the X-rays into the UV and optical bands. The initial radius of the shock implied that it was a blue supergiant that had exploded, but the lack of hydrogen emission lines in the supernova spectrum indicated a more compact star. The most likely scenario is that the shock ploughed into the massive stellar wind of a Wolf-Rayet progenitor, with the shock breaking-out and becoming visible to us once it reached the radius where the wind became optically-thin. I present the Swift observations of this landmark event, and discuss the new questions and answers it leaves us with.     

Study of the X-ray optical anisotropy of materials via X-ray interferometry

A new method for studying the X-ray optical anisotropy of materials which is based on the X-ray interferometric technique of measurement is described. Unlike the existing methods, in which the sample under study is placed in the path of one of the interfering beams, the test samples in the proposed method are placed directly in the path of two interfering beams; this eliminates the effect of other factors on the displacement of the interference moirefringes. In this method, moirefringes simultaneously occur during the same exposure both in the absence and in the presence of samples with different orientations of the optical axes. A relative displacement of moirefringes is observed in three different columns of the same beam, which makes it possible not only to simultaneously observe and immediately identify the presence of X-ray optical anisotropy but also to measure the values of refractive indexes n _o and n _e for a given sample (n _o is the refractive index for an emission with the polarization perpendicular to the principal cross section, and n _e is the refractive index for a beam with the polarization in the plane of the principal section and parallel to the optical axis). The new method is used to record the X-ray optical anisotropy of a cellophane film and to measure the values of refractive indexes n _o and n _e for cellophane. It is found that the cellophane film is an X-ray optically positive anisotropic medium.     

DQE measurement in a scintillating glass optical fiber detector for X-ray imaging

An example of the measurement of detective quantum efficiency is illustrated for a scintillating glass optical fiber detector used in X-ray imaging. We have shown the necessity to filter the input Poisson noise of the incoming beam, by the MTF (Modulation Transfer Function) of the detector, in order to obtain the real DQE (Detective Quantum Efficiency).     

The flowing afterglow in argon and helium: Differences in two reactor designs in application to the analysis of moisture

Excitation by rare gas metastables in a glow discharge afterglow is effective for the sensitive chemiluminescent detection of trace moisture. In the development of a flowing afterglow (FLAG) system for the study of effluent gases from high explosives, two approaches were taken in the design of a modular reactor. A glass reactor system with a tantalum shell hollow cathode source isolated by a Woods horn was compared to a 3 in. dia. steel reactor in which the reactor entrance port acts as the cathode. The sensitivity of both systems increases linearly with currents up to 15 mA in operation at 1 Torr argon or helium. The steel reactor provides an order of magnitude greater sensitivity for OH than the glass reactor. A continuous moisture background at 60°C over vacuum dried nitrocellulose of 0.9 nmol/gm-s (0.02 ppm) is demonstrated. With the steel system a photon count sensitivity for OH at 308.9 nm of 30000 counts/nmol in argon and 5200 counts/nmol in helium is obtained using a 0.4 nm bandpass.     

X-ray astronomy in the new millennium: a summary

Recent X-ray observations have had a major impact on topics ranging from proto-stars to cosmology. They have also drawn attention to important and general physical processes that currently limit our understanding of thermal and non-thermal X-ray sources. These include unmeasured atomic astrophysics data (wavelengths, oscillator strengths, etc.), basic hydromagnetic processes (e.g. shock structure, reconnection), plasma processes (such as electron-ion equipartition and heat conduction) and radiative transfer (in discs and accretion columns). Progress on these problems will probably come from integrative studies that draw upon observations, throughout the electromagnetic spectrum, of different classes of source. X-ray observations are also giving a new perspective on astronomical subjects, like the nature of galactic nuclei and the evolution of stellar populations. In addition, they are helping us to address central cosmological questions, including the measurement of the matter content of the Universe, understanding its overall luminosity density, describing its chemical evolution and locating the first luminous objects. X-ray astronomy has a healthy future with several international space missions under construction and in development.     

The circumstellar environment of rotating Wolf-Rayet Stars and the implications for GRB afterglows

If Wolf-Rayet stars are the progenitors of gamma-ray bursts (GRBs), they must rotate rapidly to produce the GRB. This rotation may affect their stellar-wind bubbles and possibly explain why so many GRB afterglows occur in a constant density medium.     

X-ray telescope onboard Astro-E: optical design and fabrication of thin foil mirrors

X-ray telescopes (XRT's) of nested thin foil mirrors are developed for Astro-E, the fifth Japanese x-ray astronomy satellite. Although the launch was not successful, the design concept, fabrication, and alignment procedure are summarized. The main purpose of the Astro-E XRT is to collect hard x rays up to 10 keV with high efficiency and to provide medium spatial resolution in limited weight and volume. Compared with the previous mission, Advanced Satellite for Cosmology and Astrophysics (ASCA), a slightly longer focal length of 4.5-4.75 m and a larger diameter of 40 cm yields an effective area of 1750 cm(2) at 8 keV with five telescopes. The image quality is also improved to 2-arc min half-power diameter by introduction of a replication process. Platinum is used instead of gold for the reflectors of one of the five telescopes to enhance the high-energy response. The fabrication and alignment procedure is also summarized. Several methods for improvement are suggested for the reflight Astro-E II mission and for other future missions. Preflight calibration results will be described in a forthcoming second paper, and a detailed study of images will be presented in a third paper.     

Erosion of amorphous carbon layers in the afterglow of oxygen microwave plasma

We present a study of the interaction of amorphous hydrogenated carbon deposits (a-C:H) with neutral oxygen atoms. Samples of a-C:H were exposed to an atomic oxygen rich atmosphere at surface temperatures from 473 K to 623 K. The source of atomic oxygen was microwave plasma, created in oxygen at pressures between 50 Pa and 200 Pa by means of a surfatron. The density of neutral oxygen atoms reached up to 4 ∙ 10²¹/m³. The densities of neutral atoms were measured by means of a catalytic probe. The erosion process was monitored by means of an infrared pyrometer. At each sample surface temperature, the erosion rates were found to linearly increase with the atomic oxygen density. This allowed us to calculate the reaction rate coefficient at corresponding surface temperatures. The reaction rate coefficient was found to increase exponentially with the sample surface temperature.     

Wet but not slippery: Boundary friction in tree frog adhesive toe pads.

Tree frogs are remarkable for their capacity to cling to smooth surfaces using large toe pads. The adhesive skin of tree frog toe pads is characterized by peg-studded hexagonal cells separated by deep channels into which mucus glands open. The pads are completely wetted with watery mucus, which led previous authors to suggest that attachment is solely due to capillary and viscous forces generated by the fluid-filled joint between the pad and the substrate. Here, we present evidence from single-toe force measurements, laser tweezer microrheometry of pad mucus and interference reflection microscopy of the contact zone in Litoria caerulea, that tree frog attachment forces are significantly enhanced by close contacts and boundary friction between the pad epidermis and the substrate, facilitated by the highly regular pad microstructure.     

Singlet delta oxygen concentration and the main process of its decrease in the afterglow of a DC discharge in oxygen flow

The concentration of O₂(a ¹Δ) molecules have been measured in the direct current discharge in an oxygen flow were measured in various cross sections in the gas flow channel after the discharge. It is shown that the decrease in the concentration can be described by a quadratic dependence of rate on the O₂(a ¹Δ) concentration. The rate constant of such a process is estimated to be ∼1 × 10⁻³¹ cm⁶/sec. Potential processes are considered. A three-body process in which an O(P) atom participates as the second particle apparently plays the key role     

Early X-ray equipment: a view from the South

The history of X-ray equipment starts at about the same time as Marconi's work on wireless telegraphy and is interwoven with the development of the thermionic valve and filament lamps. This article starts with brief biographical notes on its discoverer, W. C. Rontgen, followed by accounts of its early use, particularly during the South African War (1899-1902). It traces the early evolution of X-ray tubes, including Coolidge's introduction of the heated cathode followed by further improvements before the mid 1930s.     

A review of recent work in sub-nanometre displacement measurement using optical and X-ray interferometry

This paper reviews recent work in the field of displacement measurement using optical and X-ray interferometry at the sub-nanometre level of accuracy. The major sources of uncertainty in optical interferometry are discussed and a selection of recent designs of ultra-precise, optical-interferometer-based, displacement measuring transducers presented. The use of X-ray interferometry and its combination with optical interferometry is discussed.     

The fractal properties of a geometric model of the high-resolution X-ray diffractor

Perfect and mosaic crystals are conventionally used in X-ray monochromators operating in the energy range from several hundred to tens of thousands of electronvolts. The focusing X-ray optics for nonparallel beams employs either cylindrical bent crystals (the methods of Johann [1], Johansson [2], and Cauchois [3]) or crystals with spherical or toroidal [4] bending of the crystallographic planes. Special variants of high-resolution stepped-crystal diffractors [5–8] were developed to study the possibility of high-precision focusing of a monochromatic X-ray radiation. The fractal properties of a geometric model of such a high-resolution steppedcrystal diffractor are considered.     

The spherical X-ray wave focusing by a bent crystal in the Johann scheme

The focusing of a spherical X-ray beam during the Bragg reflection from a bent crystal in the Johann scheme is considered. It is demonstrated that, with an allowance for the spherical aberration of the beam, the wave intensity in the focus is determined by the square modulus of Airy’s function.     

Kinetics of reactions in CH4\N2 afterglow plasma : a simplified model

Processes in CH₄\N₂ microwave plasma are studied by computer experiment based on experimental data derived from the nitrogen flowing afterglow discharge. By means of a macroscopic kinetic approach, the original model consisting of 72 chemical reactions between 35 active species in the discharge is analysed in detail, the main attention being devoted both to the initial stages of methane decomposition and to the yield of stable products together with the most important reactions for the production of individual species. From these reactions the new simplified model is formulated.