Hard X-ray imaging with microchannel plate optics

We investigate, by ray-trace simulation, two hard X-ray telescopes based on microchannel plate (MCP) optics. The first is the, by now well known, lobster-eye geometry, while the second is a novel design. The second design uses a pair of MCPs with square channels packed in a radial fashion and represents a conical approximation to the Wolter type I configuration. We show that such telescopes can provide X-ray imaging at energies up to 100 keV. Effective area may be scaled arbitrarily by co-aligning many MCP optics. As an example, we calculate that 50 parallel Wolter I units each of 60 mm diameter and 5 m focal length yield a sensitivity of 1 milli Crab at 45 keV in a 105 second integration.     

Point source detection performance of Hard X-ray Modulation Telescope imaging observation

The Hard X-ray Modulation Telescope(HXMT) will perform an all-sky survey in the hard X-ray band as well as deep imaging of a series of small sky regions.We expect various compact objects to be detected in these imaging observations. Point source detection performance of HXMT imaging observation depends not only on the instrument but also on the data analysis method that is applied since images are reconstructed from HXMT observed data with numerical methods. The denoising technique used plays an important part in the HXMT imaging data analysis pipeline along with demodulation and source detection. In this paper we have implemented several methods for denoising HXMT data and evaluated the point source detection performances in terms of sensitivities and location accuracies. The results show that direct demodulation with 1-fold cross-correlation should be the default reconstruction and regularization method, although both sensitivity and location accuracy could be further improved by selecting and tuning numerical methods in data analysis used for HXMT imaging observations.     

Hard X-ray all-sky imaging with BATSE/CGRO

The large Area Detectors (LADs) of the BATSE experiment aboard the Compton Gamma-ray Observatory have been used recently as the first hard X-ray all-sky imager at energies between 20 keV and 300 keV. The Earth occultation process is formulated in terms of a curved Radon transform convoluted by a step transform in a selected field of view (FOV) ranging from 5°×5° to 40°×40°. The Maximum Entropy Method is then used to reconstruct an image in the hard X-ray sky. Multiple images of different regions of the sky can be produced simultaneously. A source location accuracy of 0.1° for strong sources and a sensitivity limit of 100 mCrab have been achieved in an one-day integration period.Invited Paper on Imaging in High Energy Astronomy, Sept., 1994, Capri, Italyalso Universities Space Research Association     

Hard X-ray imaging of a solar gradual hard X-ray burst on April 1, 1981

An intense solar X-ray burst occurred on April 1, 1981. X-ray images of this gradual hard X-ray burst were observed with the hard X-ray telescope aboard the Hinotori satellite for the initial ten minutes of rise and maximum phases of the burst. The hard X-ray images (13–29 keV) look like a large loop without considerable time variation of an elongated main source during the whole observation period. The main X-ray source seems to lie along a ridge of a long coronal arcade 2 × 10⁴ km above a neutral line, while a tangue-like sub-source may be another large coronal loop although the whole structure of the X-ray source looks like a large semi-circular loop. Both nonthermal and hot thermal (3–4 × 10⁷ K) electrons are contributing to the source image. The ratio of these components changed in a wide range from 2.3 to 0.4 during the observation, while the image was rather steady. It suggests that both heating and accelerations of electrons are occurring simultaneously in a common source. Energetic electrons of 15–30 keV would be collisionally trapped in the coronal magnetic loops with density of the order of 10¹¹ cm^–3.     

Hard X-ray imaging via focusing optics with mosaic crystals

In spite of the tremendous potential of hard X-ray astronomy (>10 keV) for studying high energy phenomena in celestial objects, the current generation of direct-viewing telescopes is heavily noise limited. It can accurately study only the strongest sources. Thus focusing of hard X-rays is mandatory in order to overcome these sensitivity limitations. Several focusing techniques of hard X-rays (>10 keV) are under study. We will discuss the Bragg diffraction technique and the imaging performance of a concentrator configuration based on this technique. Apart from its unprecedented flux sensitivity, the Bragg concentrators show intrinsic capabilities as polarimeters.     

Prospects of hard X-ray polarimetry with Astrosat-CZTI

Astrosat is the first Indian satellite mission dedicated for astronomical studies. It is planned for launch during 2014 and will have five instruments for multi-wavelength observations from optical to hard X-rays. Cadmium Zing Telluride Imager (CZTI) is one of the five instruments aiming for simultaneous X-ray spectroscopy and imaging in the energy range of 10 keV to 100 keV (along with all sky photometric capability unto 250 keV). It is based on pixilated CZT detector array with total geometric area of 1024 cm². It will have two-dimensional coded mask for medium resolution X-ray imaging. The CZT detector plane will be realized using CZT detector modules having integrated readout electronics. Each CZT detector module consists of 4 cm × 4 cm CZT with thickness of 5 mm which is further pixilated into 16 × 16 array of pixels. Thus each pixel has size of 2.5 mm × 2.5 mm and thickness of 5 mm. Such pixilated detector plane can in principle be used for hard X-ray polarization measurements based on the principle of Compton scattering by measuring azimuthal distribution of simultaneous events in two adjacent pixels. We have carried out detailed Geant4 simulations for estimating polarimetric capabilities of CZTI detector plane. The results indicate that events in the energy range of 100 keV to 250 keV, where the 5 mm thick CZT detector has significant detection efficiency, can be used for polarimetric studies. Our simulation results indicate the minimum detectable polarization (MDP) at the level of ～ 10% can be achieved for bright Crab like X-ray sources with exposure time of ～500 ks. We also carried out preliminary experiments to verify the results from our simulations. Here we present detailed method and results of our simulations as well as preliminary results from the experimental verification of polarimetric capabilities of CZT detector modules used in Astrosat CZTI.     

POLARIX: a pathfinder mission of X-ray polarimetry

Since the birth of X-ray astronomy, spectral, spatial and timing observation improved dramatically, procuring a wealth of information on the majority of the classes of the celestial sources. Polarimetry, instead, remained basically unprobed. X-ray polarimetry promises to provide additional information procuring two new observable quantities, the degree and the angle of polarization. Polarization from celestial X-ray sources may derive from emission mechanisms themselves such as cyclotron, synchrotron and non-thermal bremsstrahlung, from scattering in aspheric accreting plasmas, such as disks, blobs and columns and from the presence of extreme magnetic field by means of vacuum polarization and birefringence. Matter in strong gravity fields and Quantum Gravity effects can be studied by X-ray polarimetry, too. POLARIX is a mission dedicated to X-ray polarimetry. It exploits the polarimetric response of a Gas Pixel Detector, combined with position sensitivity, that, at the focus of a telescope, results in a huge increase of sensitivity. The heart of the detector is an Application-Specific Integrated Circuit (ASIC) chip with 105,600 pixels each one containing a full complete electronic chain to image the track produced by the photoelectron. Three Gas Pixel Detectors are coupled with three X-ray optics which are the heritage of JET-X mission. A filter wheel hosting calibration sources unpolarized and polarized is dedicated to each detector for periodic on-ground and in-flight calibration. POLARIX will measure time resolved X-ray polarization with an angular resolution of about 20 arcsec in a field of view of 15 × 15 arcmin and with an energy resolution of 20% at 6 keV. The Minimum Detectable Polarization is 12% for a source having a flux of 1 mCrab and 10⁵ s of observing time. The satellite will be placed in an equatorial orbit of 505 km of altitude by a Vega launcher. The telemetry down-link station will be Malindi. The pointing of POLARIX satellite will be gyroless and it will perform a double pointing during the earth occultation of one source, so maximizing the scientific return. POLARIX data are for 75% open to the community while 25% + SVP (Science Verification Phase, 1 month of operation) is dedicated to a core program activity open to the contribution of associated scientists. The planned duration of the mission is one year plus three months of commissioning and SVP, suitable to perform most of the basic science within the reach of this instrument. A nice to have idea is to use the same existing mandrels to build two additional telescopes of iridium with carbon coating plus two more detectors. The effective area in this case would be almost doubled.     

Hard X-ray imaging evidence of nonthermal and thermal burst components

We analyze hard X-ray imaging observations of three flares, showing widely different characteristics, in order to try and discriminate the relative efficiency of heating and acceleration in the primary energy release. Using a simplified approach, we compute the hard X-ray distribution and energy deposition due to accelerated electrons, with beam and ambient plasma parameters appropriate for each of the observed events. The results are convolved with the Hard X-Ray Imaging Spectrometer (HXIS) instrumental response and compared with observations. We find that: (a) Many observations are compatible with thick target processes, and with the possibility that flares may have high (>20%) acceleration efficiency. (b) Single hard X-ray sources should be very common in the data available at present (HXIS and HINOTORI), as it is the case, as well as a transition from chromospheric footpoints to single source structures. The latter cannot then be directly interpreted as thermal sources. (c) In the particular case of a limb flare, associated with a rather weak high energy burst, we show that the spatial and spectral behavior of the hard X-ray emission is incompatible with pure nonthermal processes. We thus propose that the observed emission was principally due to the strong heating intrinsic to a reconnection process within the region of interaction between two magnetic structures which are seen in the soft X-ray data. (d) We also study the heating effect of a beam, due to Coulomb losses, during its passage through the flare loops. In some cases, rather large and localized temperature increases can be expected to appear within short timescales ( 1 s), leading to a combination of nonthermal plus thermal output in the hard X-ray spectrum, which renders virtually impossible the determination of the underlying beam parameters. We finally discuss the extent to which our conclusions are valid, considering the instrumental limitations as well as the simple physical treatment that we apply.     

Polarisation measurements with a CdTe pixel array detector for Laue hard X-ray focusing telescopes

Polarimetry is an area of high energy astrophysics which is still relatively unexplored, even though it is recognized that this type of measurement could drastically increase our knowledge of the physics and geometry of high energy sources. For this reason, in the context of the design of a Gamma-Ray Imager based on new hard-X and soft gamma ray focusing optics for the next ESA Cosmic Vision call for proposals (Cosmic Vision 2015-2025), it is important that this capability should be implemented in the principal on-board instrumentation. For the particular case of wide band-pass Laue optics we propose a focal plane based on a thick pixelated CdTe detector operating with high efficiency between 60–600keV. The high segmentation of this type of detector (1–2mm pixel size) and the good energy resolution (a few keV FWHM at 500keV) will allow high sensitivity polarisation measurements (a few % for a 10mCrab source in 10⁶s) to be performed. We have evaluated the modulation Q factors and minimum detectable polarisation through the use of Monte Carlo simulations (based on the GEANT 4 toolkit) for on and off-axis sources with power law emission spectra using the point spread function of a Laue lens in a feasible configuration.     

A Hard X-ray Two-Ribbon Flare Observed with Yohkoh/HXT

The Yohkoh hard X-ray telescope (HXT) observed hard X-rays from the impulsive phase of a long-duration event (LDE) occurring on 14 July 2000. The Yohkoh soft X-ray telescope (SXT) and other instruments observed a large arcade, with width and length ∼30 000 km and ∼120 000 km, respectively. In hard X-rays, for the first time, a two-ribbon structure was clearly observed in the energy range above 30 keV. This result suggests that electrons are in fact accelerated in the whole system of this arcade, not merely in a particular dominant loop. We analyzed the motions of bright kernels in the two hard X-ray ribbons in detail. Assuming these bright kernels to be footpoints of newly reconnected loops, we infer from their motions that the loops reconnecting early are highly sheared, while the loops reconnecting later are less sheared. We have also analyzed the hard X-ray spectra of the two ribbons independently. At the outer edge of a ribbon, the spectrum tends to be harder than that in the inner edge. This suggests that higher-energy electrons precipitate at the footpoints of outer loops and lower ones do at those of inner loops. We discuss what kind of model can support this tendency.     

Development of the super high angular resolution principle for X-ray imaging

Development of the Super High Angular Resolution Principle (SHARP) for coded-mask X-ray imaging is presented. We prove that SHARP can be considered as a generalized coded mask imaging method with a coding pattern comprised of diffraction-interference fringes in the mask pattern. The angular resolution of SHARP can be improved by detecting the fringes more precisely than the mask’s element size, i.e. by using a detector with a pixel size smaller than the mask’s element size. The proposed mission SHARP-X for ...     

Observations of high velocities inHii regions with a two-etalon Fabry-Pérot spectrometer

High internal motions of the ionized material in theHii regions M 8, M16, M 17 and the Orion Nebula were searched for with a two-etalon Fabry-Perot monochromotor. The profiles of the [Oiii], 5007 Å and in one case the 4959 Å line were obtained at many positions from these nebulae. Non-gaussian wings of up to –60 km/s were found on the profiles from M 17 and M 16 over regions several minutes across. Line doubling of up to 20 km/s was definitely found in M17 and M 8. Small components with radial velocities of up to –55 km/s with respect to the means were suspected in M 8 and the Orion Nebula.     

Hard x-ray and gamma-ray imaging with solid state detectors

Solid state detectors are used in x-ray and gamma-ray astronomy primarily for their fine spectroscopy. For some cases (e.g., gamma-ray observations with Ge detectors), the spectroscopy and sensitivity requirements drive the design of the aperture systems and only moderate-quality imaging is possible. In other cases (e.g., hard x-ray observations), the detectors can be finely segmented for highquality imaging. The new room-temperature solid-state detectors like CdZnTe and HgI₂ are naturally well-suited for imaging. Because of their high atomic numbers, photoelectric absorption dominates over Compton scattering to >200 keV. This, combined with their high densities, allows thin detectors to be used with segmented contacts. Position resolution in the detector plane can be on 100 m scales giving sub-arcmin angular resolutions.     

The performance of a large echelle grating in a solar spectrometer

A number of tests of a large diffraction grating are described. It is shown that useful information about the performance of the grating in a spectrometer can be obtained from an analysis of the diffracted wavefront by the method of Stroke. Comparison of the instrumental profile at three wavelengths strongly suggests that the influence of changes in the angle of diffraction on the profile cannot be neglected. A resolving power of 6.9 × 10⁵ is attained at 6328 Å. At the same wavelength the first order Rowland ghosts in the ninth order of diffraction have an intensity of only 8 × 10^-4 of the parent line. The influence of stray light on the central intensities of deep Fraunhofer lines is shown to be small but by no means negligible.     

Hard X-ray Source Distributions on EUV Bright Kernels in a Solar Flare

We explore the hard X-ray source distributions of an C1.1 flare occurred on 14 December 2007. Both Hinode/EIS and RHESSI observations are used. One of EIS rasters perfectly covers the double hard X-ray footpoints, where the EUV emission appears strong from the cool line of He ii (log T=4.7) to the hot line of Fe xvi (log T=6.4). We analyze RHESSI X-ray images at different energies and different times before the hard X-ray maximum. The results show a similar topology for the time-dependent source distribution (i.e. at 14:14:35 UT) as that for energy-dependent source distribution (i.e. at a given energy band of 6 – 9 keV) overlapped on EUV bright kernels, which seems to be consistent with the evaporation model.     

Quasi-periodic Variations in the Hard X-ray Emission of a Large Arcade Flare

We investigated the quasi-periodic oscillations of the hard X-ray (HXR) emission of the large flare of 2 November 1991 using HXR light curves and soft X-ray and HXR images recorded with the Yohkoh X-ray telescopes. We analysed these observations and report five main results: i) The observations confirm that electrons are accelerated in oscillating magnetic traps that are contained within the cusp magnetic structure. ii) The chromospheric upflow increases the density within the magnetic traps, which in turn together with the higher amplitude of the trap oscillations increases the amplitude of the HXR pulses. iii) This increase stops when the density inside the traps increases progressively and inhibits the acceleration of electrons. iv) The model of oscillating magnetic traps is able to explain the time variation of the electron precipitation, the strong asymmetry in the precipitation of the accelerated electrons, and the systematic differences in the precipitation of 15 and 25 keV electrons. v) We have obtained direct observational evidence that strong HXR pulses are the result of the inflow into the accelerated volume of dense plasma from chromospheric evaporation.     

Lucifers,a photoelectric radial-velocity spectrometer

A spectrometer dedicated to the measurement of stellar radial velocities has been developed at the University of Canterbury and the Mt John University Observatory. The spectrometer scans a spectrum from the Observatory's 1-metre McLellan reflecting telescope and fibre-fed échelle with an oscillating mask having 2447 rectangular slots representing absorption lines in the spectrum of the star Centauri A covered by the wavelength range 397 to 570 nm in orders 40 to 58 of the spectrograph and measures the light passing through the mask as a function of mask position. A dedicated computer constructs a cross-correlation function to which a Gaussian distribution function is fitted. The difference between the radial velocities of a star and a zero-velocity reference spectrum provided by a hollow-cathode iron emission lamp is calculated from the Gaussian parameters. The sources of random error in the system are discussed and its magnitude for stars of spectral types F0 to M3.5 is estimated. Systematic errors in the system are also discussed.     

Hard X-ray bremsstrahlung produced by electrons escaping a high-temperature thermal source in a solar flare

The problem of production of flare hard X-rays by bremsstrahlung from hot thermal escaping electrons (Skrynnikov and Somov, 1982) in a chromospheric plasma is studied.The Landau kinetic equation is solved near the thermal source of energized electrons in a homogeneous magnetic tube to compute the anisotropic inhomogeneous distribution of the thermal escaping electrons.The intensity and polarization of hard X-rays is also computed and a comparison of theoretical results with observational data is made.On leave from: Istituto di Astronomia, Largo E, Fermi 5, I-50125 Firenze, Italy.     

FREGATE,a gamma-ray spectrometer for HETE

The FREnch GAmma-ray TElescope (FREGATE) on board HETE, is aimed to provide accurate spectral information on-ray bursts at low energies. Four cleaved NaI crystals provide a good sensitivity between 5 keV and 400 keV. In this paper we describe the main characteristics of this instrument: its geometry, energy resolution, and sensitivity, as well as the modes of operation and the data recorded.