X-ray Imaging Telescope Research Articles

Development of the hard X-ray imaging experiment timax: Laboratory images and first balloon flight

We present calibration results and laboratory images produced by the balloon-borne hard X-ray imaging telescope TIMAX. The images were produced with an241Am radioactive source placed 45 m away from the detector plane, in the center of the field of view. It is shown that the mask 3-antimask imaging reconstruction process, when combined with flat-fielding techniques, is very effective at recovering signal-to-noise ratio lost due to systematic non-uniformity in the background measured by the 35 detectors. The experiment was launched in June 8th, 1993 from Birigui, SP, Brazil, onboard a 186,000 m3 stratospheric balloon, and remained at an atmospheric depth of ∼2 g cm−2su for ∼8 hours. Even though no scientific data were gathered in this first flight, we obtained valuable engineering data and could also calculate the sensitivity of the experiment based on the instrumental background spectrum at balloon altitudes. In the 60–70 keV energy band, the experiment can detect 3σ sources at a level of ∼1.2 x 10−4 photons cm−2 s−1 keV−1 for an integration of 6 hours at 2.1 g cm−2.

Initial performance of the EXITE2 imaging phoswich detector/telescope for hard X-ray astronomy

The authors have developed, tested, and flown a large-area imaging phoswich detector/telescope for hard X-ray astronomy. This second-generation Energetic X-ray Imaging Telescope Experiment (EXITE) is designed to detect and image cosmic X-ray sources in the 20-600 keV energy range from a high-altitude balloon. Imaging is accomplished via the coded-aperture technique. Laboratory tests have shown that the detector achieves 14% (FWHM) energy resolution and /spl sim/10 mm (FWHM) spatial resolution at 60 keV, and that the pulse-shape discrimination circuit effectively distinguishes between NaI(Tl) and CsI(Na) events from /spl sim/40-400 keV. The first flight of the instrument was from Palestine, Texas, on 13-14 June 1993. The authors present details of the laboratory performance of the system and some flight results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Hard X-ray imaging of the Galactic black hole candidate GX 339 - 4

Imaging and spectral observations in the energy range 25-250 keV of the black hole candidate GX 339 - 4 have been obtained with the Energetic X-ray Imaging Telescope Experiment. Observations were made during a balloon flight from Alice Springs, Australia on UT 1989 May 8-10. A single source of nearly 6-sigma significance is detected near the center of the 3.4-deg field of view with a position consistent with GX 339 - 4. This is the first imaging observation of GX 339 - 4 at hard X-ray energies. This result confirms previously reported results from nonimaging experiments showing significant hard X-ray flux up to greater than about 60 keV, with a power-law spectral fit similar to the other black hole candidates such as Cygnus X - 1. The source may have been in an outburst state similar to that recently detected with BATSE on GRO.

X-Ray Observations of Stars: First Results from Rosat

At the time of writing (i.e., September 1991), the ROSAT all-sky survey has been completed and almost the entire sky has been scanned with an imaging X-ray telescope down to a limiting flux of approximately 2 10−13erg/s/cm2 in the pass band 0.1 — 2.0 keV; in regions of deeper exposures near the poles of the ecliptic considerably fainter flux limits have been achieved. While the processing and analysis of this huge body of data is still in progress and hence final results on the number of detected sources and their distribution in flux are not yet available, the total number of detected X-ray sources will be around 60 000. Preliminary results from optical identifications of selected fields show that about one quarter of the X-ray sources discovered at high galactic latitudes come from by comparison nearby stellar sources (Fleming 1991), while at lower galactic latitudes up to one half of the detected X-ray sources are of stellar origin; in areas occupied by star forming regions (for example, Orion) or open clusters (for example, Hyades or Pleiades) a large number of the detected X-ray sources can be identified with young stars, yielding up to 80 percent of the total source count as galactic stars. For the whole of the ROSAT all-sky survey we may therefore expect about one third of the total sources to be of stellar origin. The vast majority of these stellar X-ray sources is of coronal origin (i.e., late-type low mass stars). Only a relatively small number of stellar X-ray sources will be associated with early-type massive stars where the X-ray emission is thought to arise from instabilities in their radiatively driven winds or metal-poor degenerate stars where the X-ray emission comes from portions of the atmosphere considerably hotter than the optically visible photosphere. From the preliminary analyses performed so far it is already clear now that supersoft sources such as white dwarfs do not constitute a major fraction of the X-ray source population found in the ROSAT all-sky survey and the number of newly X-ray discovered white dwarfs will certainly be considerably less than one thousand. The X-ray emitting late-type stars are commonly referred to as ”active” stars, and the ROSAT all-sky survey catalog will comprise the most extensive list of such objects.

ROSAT: Early Resultsa

The general scientific objectives of ROSAT [1] are to perform (a) the first all sky surveys using imaging X-ray and EUV telescopes and (b) detailed investigations of interesting sources in a guest investigator programme.

A silicon diode array for hard X-ray imaging applications

The design requirements for a new generation hard X-ray imaging telescope are examined and various detector options are considered. A prototype array of 6×8 diodes (each 5 mm×5 mm) has been fabricated by Micron Semiconductors Limited on a 300 μm thick high resistivity silicon wafer to provide a basic module for a position sensitive detector for such an application. The design calls for the silicon to be totally depleted so that rear illumination by light from a scintillation crystal is possible. Tests have been carried out to determine the performance of the detector in comparison with commercially produced discrete diodes. These tests were carried out using a hybrid preamplifier but a design based on a new RAL multichannel amplifier is considered.

Observations from the Hinotori mission

The satellite Hinotori was launched in 1981 by the Institute of Space and Astronautical Science of Japan. Two major experiments on board the Hinotori satellite were a hard X-ray imaging telescope with modulation collimators, and a high dispersion soft X-ray crystal spectrometer utilizing the Bragg diffraction of X-rays on quartz crystals. These two instruments have revealed for the first time that solar flares show varying characteristics depending on the environment of flaring regions, and that flares produce plasmas as hot as 3-4 x 10 7 K.

Techniques for removing nonuniform background in coded-aperture imaging on the energetic X-ray imaging telescope experiment

It is found that the subtractive flat field technique for nonuniform background illumination is generally effective at removing background systematics for stationary mask experiments such as the energetic X-ray imaging experiment. The time dependence of intensity and the two-dimensional shape of the background detector image during the flight are explored. A flat field image is constructed from observations where X-ray sources were absent from the field of view. It is shown that this technique can successfully reduce RMS fluctuations to within a few percent of ideal Poisson statistics. The quality of the flat field does not appear to be a strong function of radius and can be used effectively out to the edge of the detector to remove the strong background ring.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Sub-arcsecond observations of the solar X-ray corona

SOFT X-ray observations are recognized as the best way to study the solar corona, as they are largely free of contaminating emission from other temperature regimes. They provide the only available method for seeing the corona on the disk, thereby avoiding the line-of-sight integration effects which are troublesome in limb observations. Here we present results from a high-resolution multilayer-coated X-ray imaging telescope, part of the Normal Incidence X-ray Telescope sounding rocket payload. From a flight at 16:35 UT on 11 September 1989 we obtained 40 images of the solar X-ray corona, with spatial resolution up to 0.75 arcsec. Images of the peak of a two-ribbon flare showed detailed structure within each ribbon, as well as the expected bright arches of emission connecting the ribbons. Active regions showed structure at a scale of 0.75 arcsec that was not visible in our 2-arcsec images taken during the same flight. The number of X-ray bright points1 is small, consistent with predictions based on the previous solar cycle2. The topology of the magnetic structure is complex and highly tangled, implying that the magnetic complexity of the photosphere is paralleled in the corona.

The ROSAT Mission

AbstractThe scientific payload of ROSAT consists of a 83 cm X-ray telescope (6 – 100 Å) and a 60 cm XUV-telescope (60 – 300 Å) which are looking parallel. An important objective of the mission is to perform the first all-sky survey with imaging X-ray telescopes providing an improvement in sensitivity by several orders of magnitude compared with previous surveys. A large number of new X-ray sources (~ 105) is expected to be discovered and located with an accuracy of 1 arcmin or better, depending on source strength. The sources discovered will represent almost all types of astronomical objects, ranging from nearby normal stars to distant quasars.After completion of the sky survey which will take half a year, the instruments will be used for detailed investigations of selected sources with respect to spatial structures, spectra and time variability. In this pointing mode, which will be open for guest observers, ROSAT is expected to provide substantial improvements over the imaging instruments of the Einstein observatory.

Calibration and performance of the Energetic X-ray Imaging Telescope Experiment

The final integration and testing of the Energetic X-ray Imaging Telescope Experiment (EXITE) are described. The detector and telescope as finally assembled are described, and problems as well as solutions are detailed. The preflight calibration of the position-sensitive NaI detector revealed unanticipated effects in the energy spectra. In particular, an effect due to surface roughness (lack of firm optical polishing) on the crystal was found and calibrated. The radial variation of gain and resolution in the detector was also measured, and so-called flat fielding methods have been derived to analyze the data. The integration of the EXITE detector and gondola systems is described. Several novel features were developed for the EXITE gondola, most notably a shock absorber system. The performance of the detector and gondola systems was evaluated on the first balloon flight from Australia (May 1988) and a second flight from Ft. Sumner, NM (October 1988).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The high throughput X-ray spectroscopy mission: XMM

The high throughput X-ray astrophysics mission is the second cornerstone in ESA's long-term space science programme. The long-duration X-ray observatory consists of three heavily nested X-ray imaging telescopes coupled to X-ray CCD cameras and gratings which provide a high throughput facility for cosmic X-ray spectroscopy. The mission is due for launch in 1998 with an anticipated lifetime of over ten years. The basic mission including the model payload is described and the capability of the observatory to tackle some of the more important scientific priorities are highlighted. Examples of some of the type of results we can expect from the mission are also provided. This observatory should enable major advances in X-ray astrophysics to be made at the turn of the century.

Geometrical aberration of a generalized Wolter type I telescope

Using two hyperboloids, we can control spherical aberration and coma of an imaging x-ray telescope. The coma-free condition is satisfied only at a particular radius of the first mirror. The best performance is obtained when we allow the coma-free condition at the outer edge and cancel about half of the defocused image for the largest height of an object by introducing spherical aberration.

Development of the EXITE Detector: A New Imaging Detector for 20 - 300 keV Astronomy

We report on the development and testing of a detector to be used in the Energetic X-ray Imaging Telescope Experiment (EXITE). It consists of a 34 cm diameter NaI(Tl) cyrstal coupled directly to a single large image intensifier tube with associated silicon PIN diode readout. The measured spatial and energy resolutions at 122 keV are 6mm (FWHM) and 9% (FWHM), respectively. This energy resolution is ~50% better than that of any previously flown hard x-ray experiment. These resolutions decrease with the square root of the energy of the incident x-ray, indicating that they are determined by the number of photons emitted in the NaI(Tl) scintillator light flash.

A search for X-ray emitting coronal structures in algol

Algol was observed with the low energy imaging X-ray telescope and the medium energy detectors on the ESA EKOSAT spacecraft during the time of secondary optical eclipse when the B star passes in front of its K type companion. An examination of the X-ray light curves allows us to set preliminary lower limits to the size of an X-ray emitting corona associated with the K star. The medium energy detector indicates a continuum temperature of 24.3.10 6 K. The detection of Pe XVII and Pe XVIII emission lines in an objective grating spectrum of the source positively indicates the presence of hot coronal material.

Hard X-ray imaging observations of solar hot thermal flares with the HINOTORI spacecraft

Two solar hard X-ray bursts of a new type (hot thermal flare) were observed with hard X-ray imaging telescopes and other instruments on Japanese spacecraft Hinotori. The flares have no clear impulsive phase below 40 keV and emit intense hard X-rays (10--50 keV) with extremely steep spectra from a small region with size (FWHM) of 10''--20''. This source contains a hot thermal plasma of (3--3.5) x 10/sup 7/ K with an emission measure of the order of 10/sup 49/ cm/sup -3/. One of the flares occurred just on the limb, and the centroid of the hard X-ray (14-38 keV) source was located at (6 +- 3) x 10/sup 3/ km above the photosphere. It is concluded that the energy continuously released goes into heating rather than acceleration almost throughout the flare. Typical impulsive flares may usually have a similar nature in the later phase (gradual phase) of the flare evolution. The number density of the hard X-ray emitting region was higher than (6-10) x 10/sup 10/ cm/sup -3/ even in the initial phase of the flare, probably due to its low altitude. We infer that this high density made acceleration difficult, and therefore unusually intense heating occurred from the startmore » of the flare.« less

New Methods in Development of X-Ray Optics for Plasma Diagnostics of Laser - Produced Plasma

A new technology was developed for manufacturing of precise and low-cost X-ray grazing incidence microscopic optics.Imaging experiments represent one of the main directions in the Czechoslovak X-ray astronomy program. The Space Research Department of the Astronomical Institute of the Czechoslovak Academy of Sciences in Ondřejov has been active in the development of X-ray mirrors since 1970. We have participated in 7 space X-ray imaging experiments. 6 experiments were flown onboard the Vertical 8, 9 and 11 rockets in the years 1979, 1981 and 1983 (Hudec et al. 1984a), one experiment was flown onboard the Soviet orbital station Salyut 7 in the year 1982 (Valníček et al. 1983 and Hudec et al. 1984b). The experiments represent both small solar X-ray imaging telescopes and big stellar X-ray telescope.

ROSAT

The scientific payload of the ROSAT spacecraft consists of a large X-ray telescope (6-100 Å) and a smaller Wide Field XUV Camera (60-300 Å) which are looking parallel. A primary objective of the mission is to perform the first all-sky survey with an imaging X-ray telescope leading to an improvement in sensitivity by several orders of magnitude compared with previous surveys. The spectral band covered in the survey mode by both telescopes ranges from 6 to 190 Å and can be divided into 6 "colours". A large number of near X-ray sources (≳ 105) is expected to be discovered and located with an accuracy of 1 arcmin or better, depending on source strength. The sources discovered will represent almost all types of astronomical objects, from nearby normal stars to distant quasars.After completion of the sky survey which will take half a year, the instruments will be used for detailed investigations of selected sources with respect to spatial structures, spectra and time variability. In this pointing mode, which will be open for guest observers, ROSAT is expected to provide substantial improvements over the imaging instruments of the Einstein observatory.The main ROSAT telescope consists of a fourfold nested mirror system with 83 cm aperture having three focal plane instruments. Two of them will be imaging proportional counters (0.1-2 keV) providing a field of view of 2°, an angular resolution of ≈ 30" in the pointing mode and a spectral resolution ΔE/E ≈ 45% FWHM at 1 keV. The third focal instrument will be a high resolution imager (≈ 3"). The Wide Field XUV Camera has an aperture of 58 cm and microchannel plate detectors as focal instruments. The average angular resolution is 2 arcmin over the field of view of 5°. Three spectral bands are defined by filters.ROSAT will be launched by the Space Shuttle in mid-1987.

Design and construction of the ROSAT 5 arcsec mirror assembly

The ROSAT mission, which is currently being prepared in W.-Germany, will perform the first soft X-ray all-sky survey by means of a large imaging X-ray telescope. Detailed calculations under the cost, volume and mass constraint of the satellite being a Shuttle payload have led to a design of the imaging optics with optimized geometry. The mirror system is of the Wolter type I configuration and includes four nested shells with a maximum aperture of 835 mm and a focal length of 2400 mm. The on-axis angular resolution of the mirror assembly has been specified to 5 arcsec with a scattering level as low as 3% for single reflection at 1.5 keV photon energy. Construction and technology studies have been completed by now and manufacturing of the first mirror shell has begun.

The ROSAT mission

A primary scientific objective of the ROSAT mission is to perform the first all-sky survey with an imaging X-ray telescope leading to an improvement in sensitivity by several orders of magnitude compared with previous surveys. A large number of new sources (≳ 105) will be discovered and located with an accuracy of 1 arcmin or better. These will comprise almost all astronomical objects from nearby normal stars to distant quasistellar objects. After completion of the survey which will take half a year the instrument will be used for detailed observations of selected sources with respect to spatial structure, spectra and time variability. In this mode which will be open for guest observers ROSAT will provide substantial improvement over the imaging instruments of the Einstein observatory.The main ROSAT telescope consists of a fourfold nested mirror system with 83 cm aperture having three focal plane instruments. Two of them will be imaging proportional counters (0.1 – 2 keV) providing a field of view of 2°, an angular resolution of ≈ 30″ in the pointing mode and a spectral resolution ΔE/E ≈ 45% FWHM at 1 keV. The third focal instrument will be a high resolution imager (≈ 3″). The main ROSAT telescope will be complemented by a parallel looking Wide Field camera which extend the spectral coverage into the XUV band.