Hard X-ray Astronomy Research Articles

Modified form of the Kompaneets equation

Comptonization is a very important phenomenon in astrophysics. The Kompaneets equation describes the Comptonization process in up-Comptonization (hν̄ ≪ kTe), while it fails in describing the down-Comptonization (hν̄ ≫ kTe), which is the most important radiative transfer process in hard X-ray and γ-ray astronomy. In this study we extend the Kompaneets equation to a new modified equation which works in a more general Comptonization process, including up- and down-Comptonization, suitable for the cases hν̄ ≪ kTe, hν̄ ≫ kTe, and hν̄ ∼ kTe. Numerical solutions for the evolution behavior of Gaussian emission lines show excellent agreement between the classical equations and our new equation for up-Comptonization, while the big differences are displayed for down-Comptonization. Based on this extended equation, the modified Sunyaev-Zel’dovich effect is investigated. Instead, some typical calculated results for up- and down-Comptonization in X-ray astronomy are compared between the new equation and the Kompaneets equation. The potential applications of the extended equation in astrophysics are also highly emphasized for further study.

Carbon-based Fresnel optics for hard x-ray astronomy.

We investigate the potential of large-scale diffractive-refractive normal-incidence transmission lenses for the development of space-based hard x-ray telescopes with an angular resolution in the range of (10-6-10-3) arcsec over a field of view that is restricted by the available detector size. Coherently stepped achromatic lenses with diameters up to 5m for compact apertures and 13m in the case of segmentation provide an access to spectrally resolved imaging within keV-wide bands around the design energy between 10 and 30keV. Within an integration time of 106 s, a photon-limited 5σ sensitivity down to (10-9-10-7) s-1 cm-2 keV-1 can be achieved depending on the specific design. An appropriate fabrication strategy, feasible nowadays with micro-optical technologies, is considered and relies on the availability of high-purity carbon or polymer membranes. X-ray fluorescence measurements of various commercially available carbon-based materials prove for most of them the existence of a virtually negligible contamination by critical trace elements such as transition metals on the ppm level.

Curved focusing crystals for hard X-ray astronomy

A lens made by a properly arranged array of crystals can be used to focus x-rays of energy ranging from 30 to 500 keV for x-ray astronomy. Mosaic or curved crystals can be employed as x-ray optical elements. In this work self standing curved focusing Si and GaAs crystals in which the lattice bending is induced by a controlled damaging process on one side of planar crystals are characterized. Diffraction profiles in Laue geometry have been measured in crystals at x-ray energies E = 17, 59 and 120 keV. An enhancement of diffraction efficiency is found in asymmetric geometries.

Simulation of the charge collection and signal response of a HPGe double sided strip detector using MGS

This paper aims to present Multi geometry Simulation (MGS), a software intended for the characterization of the signal response of solid state detectors. Its main feature is the calculation of the pulse shapes induced at the electrodes of the detector by a photon–semiconductor interaction occurring at a specific position inside the detector volume. The program uses numerical methods to simulate the drift of the charge carriers generated by the interaction, as the movement of these particles induces the useful signal for detection to the electrodes. After the description of the tool fundamentals, an example of application is presented where MGS was used for simulating a High Purity Germanium (HPGe) double sided strip detector conceived for hard X-ray astronomy. Simulated and measured pulse shapes are compared for interactions occurring at different depths in the detector volume. The comparison focuses on the difference in time of arrival between the anode and cathode pulses, as this measure allows, together with the X/Y information retrieved from the strips, a 3D determination of the photon interaction point, which is an important feature of the detector. A good matching between simulations and measurements is obtained, with a discrepancy less than 0.5mm between the measured and the simulated depth of the interaction, for an 11mm thick detector.

Swiftdiscovery of the orbital period of the high mass X-ray binary IGR J015712−7259 in the Small Magellanic Cloud

In the last years the hard X-ray astronomy has made a significant step forward, thanks to the monitoring of the IBIS/ISGRI telescope on board the INTEGRAL satellite and of the Burst Alert Telescope (BAT) on board of the Swift observatory. This has provided a huge amount of novel information on many classes of sources. We have been exploiting the BAT survey data to study the variability and the spectral properties of the new high mass X-ray binary sources detected by INTEGRAL. In this letter we investigate the properties of IGR J015712-7259. We perform timing analysis on the 88-month BAT survey data and on the XRT pointed observations of this source. We also report on the broad-band 0.2-150 keV spectral analysis. We find evidence for a modulation of the hard-X-ray emission with period P_o=35.6 days. The significance of this modulation is 6.1 standard deviations. The broad band spectrum is modeled with an absorbed power law with photon index Gamma 0.4 and a steepening in the BAT energy range modeled with a cutoff at an energy of ~13 keV.}

Preparation of bent crystals as high-efficiency optical elements for hard x-ray astronomy

Curved crystals, instead of flat mosaic crystals, can be used as optical elements of a Laue lens for hard x- and gamma-ray astronomy to increase the diffraction efficiency. We propose to achieve the bending of the crystals by a controlled surface damaging, which introduces defects in a layer of few tens nanometers in thickness undergoing a highly compressive strain. Several oriented silicon and gallium arsenide wafer crystals have been treated. The local and mean curvature radii of each sample have been determined by means of high resolution x-ray diffraction measurements in Bragg condition at low energy (8 keV). Silicon samples showed spherical curvatures, whereas GaAs-treated samples evidenced elliptical curvatures with major axes corresponding to the 〈110〉 crystallographic directions. Curvature radii between 3 and 70 m were easily obtained in wafers of different thicknesses. The characterization of GaAs samples performed in Laue geometry at gamma-ray energy of 120 keV confirmed the increase of the diffraction efficiency in the bent crystals.

High-resolution x-ray characterization of mosaic crystals for hard x-ray astronomy

GaAs, Cu, CdTe, and CdZnTe crystals have been studied as optical elements for lenses for hard x-ray astronomy. High-resolution x-ray diffraction at 8 keV in Bragg geometry and at synchrotron at energies up to 500 keV in Laue geometry has been used. A good agreement was found between the mosaicity evaluated in Bragg geometry at 8 keV with x-ray penetration of the order of few tens of micrometers and that derived at synchrotron in transmission Laue geometry at higher x-ray energies. Mosaicity values in a range between a few to 150 arcsec were found in all the samples but, due to the presence of crystal grains in the cm range, CdTe and CdZnTe crystals were found not suitable. Cu crystals exhibit a mosaicity of the order of several arcmin; they indeed were found to be severely affected by cutting damage which could only be removed with a very deep etching. The full width at half maximum of the diffraction peaks decreased at higher x-ray energies showing that the peak broadening is affected by crystallite size. GaAs crystals grown by Czochralski method showed a mosaic spread up to 30 arcsec and good diffraction efficiency up to energies of 500 keV. The use of thermal treatments as a possible method to increase the mosaic spread was also evaluated.

A large area LaBr3/NaI phoswich for hard X-ray astronomy

In terms of energy resolution, temporal response to burst events, and thermal stability, lanthanum bromide doped with Ce is a much better choice than the traditional NaI(Tl) scintillator for hard X-ray astronomy. We present the test results of a phoswich detector with a diameter of 101.6mm consisting of 6mm thick LaBr3:Ce and 40mm thick NaI(Tl), which is the largest one of this type reported so far. The measured energy resolution is 10.6% at 60keV, varying inversely proportional to the square root of the energy, and the energy nonlinearity is found to be less than 1%, as good as those of smaller phoswiches. The coupled scintillators and phototube also show excellent uniformity across the detecting surface, with a deviation of 0.7% on the pulse amplitude produced by 60keV gamma-rays. Thanks to the large ratio of light decay times of NaI(Tl) and LaBr3:Ce, 250ns versus 16ns, pulse shape discrimination is much easier for this combination than for NaI(Tl)/CsI(Na). As the light decay time of LaBr3:Ce is about 15 times faster than that of NaI(Tl), this phoswich is more suitable for detection of bright, transient sources such as gamma-ray bursts and soft gamma-ray repeaters. The internal activity of lanthanum produces a count rate of about 6countss−1 at 37.5keV in the detector. This peak could be used for in-flight spectral calibration and gain correction.

High diffraction efficiency at hard X-ray energy in a silicon crystal bent by indentation

The diffraction properties of a crystalline silicon plate of which one face was mechanically indented have been studied. This treatment induced a permanent curvature in the sample, which allowed a diffraction efficiency of 88% for 150 keV photons,i.e.a reflectivity of 64% including the absorption. This efficiency is constant over 14 arcseconds and is very close to the theoretical expectation, meaning that the curvature is highly homogeneous. The technique enables the fabrication, in a very reproducible fashion, of crystals for the realization of an astronomical hard X-ray concentrator (Laue lens).

Characteristics and performance of thin LaBr 3(Ce) crystal for hard X-ray astronomy

We have developed a new detector using thin lanthanum bromide crystal (32 × 3 mm) for use in X-ray astronomy. The instrument was launched in high altitude balloon flight on two different occasions, December 21, 2007, which reached a ceiling altitude of 4.3 mbs and April 25, 2008 reaching a ceiling altitude 2.8 mbs. The observed background counting rate at the ceiling altitude of 4 mbs was ∼4 × 10 −3 ct cm −2 s −1 keV −1 sr −1. This paper describes the details of the experiment, the detector characteristics, and the background behaviour at the ceiling altitude.

Calibration of the Gamma-RAy Polarimeter Experiment (GRAPE) at a polarized hard X-ray beam

The Gamma-RAy Polarimeter Experiment (GRAPE) is a concept for an astronomical hard X-ray Compton polarimeter operating in the 50–500 keV energy band. The instrument has been optimized for wide-field polarization measurements of transient outbursts from energetic astrophysical objects such as gamma-ray bursts and solar flares. The GRAPE instrument is composed of identical modules, each of which consists of an array of scintillator elements read out by a multi-anode photomultiplier tube (MAPMT). Incident photons Compton scatter in plastic scintillator elements and are subsequently absorbed in inorganic scintillator elements; a net polarization signal is revealed by a characteristic asymmetry in the azimuthal scattering angles. We have constructed a prototype GRAPE module containing a single CsI(Na) calorimeter element, at the center of the MAPMT, surrounded by 60 plastic elements. The prototype has been combined with custom readout electronics and software to create a complete “engineering model” of the GRAPE instrument. This engineering model has been calibrated using a nearly 100% polarized hard X-ray beam at the Advanced Photon Source at Argonne National Laboratory. We find modulation factors of 0.46±0.06 and 0.48±0.03 at 69.5 and 129.5 keV, respectively, in good agreement with Monte Carlo simulations. In this paper we present details of the beam test, data analysis, and simulations, and discuss the implications of our results for the further development of the GRAPE concept.

Hard X-ray imaging mission NeXT

One of the major fields, which should be explored following Chandra and XMM-Newton is the hard X-ray sky up to several tens of keV with certain imaging capabilities. A new X-ray telescope mission or Non-thermal Energy eXploration Telescope (NeXT) in Japan is the first satellite mission proposed for a hard X-ray telescope using multilayer supermirror. The hybrid X-ray imager will be placed on the focal plane to observe hard X-ray images with CdTe pixel detector underneath a thin CCD for soft X-ray imaging. For high-resolution spectroscopy, TES type calorimeters are considered without cryogen. Non-imaging hard X-ray and soft gamma-ray detector is studied to achieve unprecedented sensitivity. NeXT will be a sort of pathfinder of hard X-ray astronomy not only technically but also scientifically.

Development of ASICs for CdTe pixel and line sensors

We are developing low noise analog ASICs for hard X-ray and gamma-ray astronomy. They are based on a sub-micron CMOS process. Our design includes a finely-adjustable high-resistance circuit, which, for example, enables us to realize a pole-zero-cancellation in the analog chain. A two-dimensional ASIC with a fast read-out scheme and a one-dimensional ASIC with an improved analog design are developed and verified. With the latter ASIC connected with a CdTe detector, we have obtained a radioactive source spectrum with an energy resolution of 4.6 keV (FWHM) at 59.5 keV.

Evolution of X-ray spectra in down-Comptonization. A comparison of the extended Kompaneets equation with Monte Carlo simulation and the Ross-McCray equation

The original Kompaneets equation fails to describe down-Comptonization, which is the most important radiative transfer process in hard X-ray and γ-ray astronomy, compared with up-Comptonization. In this paper, we improve our previous derivation of the extended Kompaneets equation and present it more clearly. The new equation can be used to describe a more general Comptonization process, including up- and down-Comptonization, suitable for any case, hν � kTe, hν � kTe and hν ∼ kTe. The condition of the original Kompaneets equation hν � kTe is no longer necessary. Using the extended equation, we give some typical solutions in X-ray astronomy, and compare them with those of the prevailing Monte Carlo simulations and the Ross-McCray equation. The excellent consistency between the extended Kompaneets equation and Monte Carlo simulation or Ross-McCray equation confirms the correctness of our extended Kompaneets equation. The numerical solution of the extended Kompaneets equation is less expensive in terms of computational time than the Monte Carlo simulation. Another advantage of the equation method is the simplicity and the clarity in physics. The potential applications in X-ray and γ-ray astronomy are also emphasized.

Measurement of the background spectrum of a CdTe detector at balloon altitudes

The CdTe and the CdZnTe detectors are considered to be promising detectors for the future of astronomical hard X-ray observations due to their high stopping power and high energy resolution. For the preparation of future experiments, we have performed a simple balloon experiment using a small CdTe detector (2 mm cubic) as a piggy-back mission in 2000. Its energy range was 20–140 keV band with an energy resolution of 3.0 keV at 60 keV. The balloon reached an altitude of 36.9 km, and then gradually descended. During the 10-h flight, we successfully observed the background spectrum. The flux of the background was ∼1.5 × 10−2 s−1 cm−2 keV−1 at 50 keV, being comparable to those of the past observations measured by CdZnTe detectors. We also found that the count-rate increases as the atmospheric pressure increases. No apparent line emission was observed.

Measurement of the background spectrum of a CdTe detector at balloon altitudes

The CdTe and the CdZnTe detectors are considered to be promising detectors for the future of astronomical hard X-ray observations due to their high stopping power and high energy resolution. For the preparation of future experiments, we have performed a simple balloon experiment using a small CdTe detector (2 mm cubic) as a piggy-back mission in 2000. Its energy range was 20–140 keV band with an energy resolution of 3.0 keV at 60 keV. The balloon reached an altitude of 36.9 km, and then gradually descended. During the 10-h flight, we successfully observed the background spectrum. The flux of the background was ∼1.5 × 10 −2 s −1 cm −2 keV −1 at 50 keV, being comparable to those of the past observations measured by CdZnTe detectors. We also found that the count-rate increases as the atmospheric pressure increases. No apparent line emission was observed.

Large-area CdTe diode detector for space application

The current status of Schottky CdTe diode detectors, especially in view of their space application for hard X-ray and gamma-ray astronomy, are reported. For practical use in space science, a large-area CdTe diode with a size of 21.5×21.5 mm 2 and a thickness of 0.5 mm was developed. A good energy resolution, 2.8 keV (FWHM) at −20°C, and high homogeneity to within 0.2% over the detector were achieved for the spectral performance. This device has successfully passed a series of tests required for its use in space, in view of utilizing Japanese M-V rockets. The tests include the mechanical environment test, vacuum test, long run for weeks and proton-beam radiation. Initial results from a 2×2 segmented electrode large-area device with a guard-ring are also presented.

Design and performance of a ruggedized large-area CZT detector module for hard X-ray astronomy

We discuss the design and performance of a large area (40cm2) CZT module for space-borne X-ray astronomy applications. This module is based upon the 32mm×32mm CZT crossed strip detectors that have been under development at UCSD. This compact design includes readout electronics and allows for efficient tiling of modules to form large detector planes for wide field of view coded mask imagers, or for efficient packaging within an anticoincidence shield at the focus of a hard X-ray telescope.

Imaging in Hard X-ray Astronomy

The energy range of hard X-rays is a key waveband to the study of high energy processes in celestial objects, but still remains poorly explored. In contrast to direct imaging methods used in the low energy X-ray and high energy gamma-ray bands, currently imaging in the hard X-ray band is mainly achieved through various modulation techniques. A new inversion technique, the direct demodulation method, has been developed since early 90s. with this technique, wide field and high resolution images can be derived from scanning data of a simple collimated detector. The feasibility of this technique has been confirmed by experiment, balloon-borne observation and analyzing simulated and real astronomical data. Based the development of methodology and instrumentation, a high energy astrophysics mission – Hard X-ray Modulation Telescope (HXMT) has been proposed and selected in China for a four-year Phase-A study. The main scientific objectives are a full-sky hard X-ray (20–200 keV) imaging survey and high signal-to-noise ratio timing studies of high energy sources.

Improvement on the light yield of a high-Z inorganic scintillator GSO(Ce)

Cerium-doped gadolinium silicic dioxide crystal, GSO(Ce), is a high-Z non-hydroscopic scintillator that gives higher light yield than BGO, and can potentially replace NaI(Tl), CsI(Tl) and BGO in many applications. Its production cost, however, has been substantially higher than any of them, while its energy resolution has been worse than that of NaI(Tl) or CsI(Tl). The merit did not overcome these deficiencies except in limited applications. We developed a low background phoswich counter (the well-type phoswich counter) for the Hard X-ray Detector of the Astro-E project based on GSO scintillator. In the developmental work, we have succeeded in improving the light yield of GSO(Ce) by 40–50%. For energies above 500 keV , a large GSO(Ce) crystal (4.5 cm×4.5φ cm) now gives energy resolution comparable to or better than the best NaI(Tl) when read out with a phototube. With a small GSO(Ce) crystal (5×5×5 mm 3) and a photodiode, an energy resolution comparable to or better than the best CsI(Tl) has been obtained. With this improved performance, we find that the much higher photopeak efficiency and the shorter scintillation decay time of GSO(Ce) offsets its higher cost for many applications. We summarize our past developmental work to decrease radioactive contamination and to increase light yield of GSO(Ce) for astronomical hard X-ray detection. Included also are measurements done after the unsuccessful launch of the Astro-E mission. The work is still continuing for the remake version of Astro-E Hard X-ray Detector.