Balloon-borne Hard X-ray Telescope Research Articles

InFOCμS Hard X-ray Imaging Telescope

InFOCμS is a new generation balloon-borne hard X-ray telescope with focusing optics and spectroscopy. We had a successful 22.5-hour flight from Fort Sumner, NM on September 16,17, 2004. In this paper, we present the performance of the hard X-ray telescope, which consists of a depth-graded platinum/carbon multilayer mirror and a CdZnTe detector. The telescope has an effective area of 49 cm2 at 30 keV, an angular resolution of 2.4 arcmin (HPD), and a field of view of 11 arcmin (FWHM) depending on energies. The CdZnTe detector is configured with a 12 × 12 segmented array of detector pixels. The pixels are 2 mm square, and are placed on 2.1 mm centers. An averaged energy resolution is 4.4 keV at 60 keV and its standard deviation is 0.36 keV over 128 pixels. The detector is surrounded by a 3-cm thick CsI anti coincidence shield to reduce background from particles and photons not incident along the mirror focal direction. The inflight background is 2.9 × 10−4 cts cm−2 sec−1 keV−1 in the 20–50 keV band.

Production and performance of the inFOCmicroS 20-40-keV graded multilayer mirror.

The International Focusing Optics Collaboration for microCrab Sensitivity (InFOCmicroS) balloonborne hard x-ray telescope incorporates graded Pt/C multilayers replicated onto segmented Al foils to obtain the significant effective area at energies previously inaccessible to x-ray optics. Reflectivity measurements of individual foils demonstrate our capability to produce a mass quantity of multilayered foils with a rms roughness of 0.5 nm. The effective area of the completed mirror is 78 and 22 cm2 at 20 and 40 keV, respectively. The measured half-power diameter is 2.0 +/- 0.6 are min (90% confidence). The successful completion of this mirror demonstrates its applicability to future x-ray telescopes such as Constellation-X.

Hard X-ray imaging observations: Science and technology

High sensitivity hard X-ray data by means of focusing optics is crucially important to investigate active galaxies and cluster of galaxies. We have developed focusing telescopes with multilayer coatings. The energy band is broadened by multilayers with graded periodic length, so called ‘ Supermirrors’. We are successful to obtain hard X-ray images in the energy band from 25 to 40 keV with a demonstration model of telescope with 20 mirror shells of supermirrors. The application of supermirrors to the balloon-borne hard X-ray telescope is discussed.

Background reduction in a balloon-borne hard X-ray telescope

We have been carrying out a series of balloon flight experiments using a large area proportional counter based (xenon filled) hard X-ray telescope and have studied in detail the spectral and temporal characteristics of several cosmic hard X-ray sources. The detector system uses multi-cell multi-layer configuration and for background reduction passive shielding, active anti-coincidence on three sides of the detector and mutual anti-coincidence for the active cells are employed. Further, K-gating technique is used to improve the energy resolution and detection efficiency. We present the effectiveness of the various background rejection techniques using a Monte-Carlo simulation and critically evaluate the possible improvements in the background rejection.

Hard X-ray pulse profile of GX1+4

We have observed the Galactic center X-ray pulsar GX 1+4 using a balloon-borne hard X-ray telescope. Hard X-ray pulse profiles and spectral parameters were obtained on two occasions; during the high state of December 1993 when the hard X-ray pulse fraction was very low and in March 1995 when there was an indication of the pulse period variation sign reversal. We find significant differences in the pulse shape on these two occasions: the hard X-ray pulse profile changed from a smooth sinusoidal to a double peak structure. The details of the observations are presented and the implications of these results on the accretion process near the compact object are outlined.

Optical, X-ray and γ-ray observations of compact objects in globular clusters

In the past three years, a new era of study of globular clusters has begun with multiwavelength observations from the current generation of astronomical telescopes in space. We review the recent results obtained from our studies of compact binaries and x-ray sources in globulars with ROSAT and HST as well as our balloon-borne hard x-ray telescope EXITE and ground-based observations (CTIO). With ROSAT, we have obtained the most sensitive high resolution soft x-ray images of clusters which show multiple low luminosity sources in cluster cores that are likely indicative of the long-sought population of cataclysmic variables (CVs). We have obtained deep Hα images of two clusters with HST and found CV candidates for 3 of the ROSAT sources in the core of NGC 6397. New CTIO imaging and spectroscopy of two ‘dim source’ fields in ω-Cen are also described. With EXITE we carried out the first hard x-ray imaging observations of the cluster 47 Tuc; such studies can ultimately limit the populations of millisecond pulsars and pulsar emission mechanisms. A long ROSAT exposure on 47 Tuc also shows probable cluster diffuse emission, possibly due to hot gas from ablating millisecond pulsars. Multiwavelength studies of globular clusters may provide new constraints on problems as diverse as the origin of CVs and LMXBs and the origin of hot gas in globulars.

Fabrication and flight performance of a large area balloon borne hard X-ray telescope

We describe the fabrication and flight performance of a balloon-borne large area hard X-ray (20–100 keV) telescope for spectral studies of discrete cosmic X-ray sources. The telescope consists of two multi-wire Xenon filled proportional counters of effective area 1200 cm 2 each, mounted on an orientable platform. It can be pre-programmed to track any celestial source with a pointing accuracy of 0.5 degrees. For one hour of observation the telescope has a 5 σ detection sensitivity of 10 −5 ph cm −2 s −1. The laboratory test results and the performance in a series of balloon flights conducted in 1984–1986 period is discussed and the preliminary results obtained for some X-ray sources are presented.

The background counting rates in a balloon borne hard X-ray telescope

A detailed Monte Carlo model of a hard (20–300 keV) X-ray astronomical telescope has been developed in order to calculate the energy loss distribution of the unwanted background noise events in the prime detection elements. The spectral distributions of the background rates measured at balloon altitudes over Palestine, Texas are compared to the predictions of the theoretical model. Good agreement has been found in terms of both the overall intensity level as well as the spectral distribution.

A two-dimensional position-sensitive spectroscopic proportional counter for balloon-borne hard X-ray astronomy

A new design of position sensitive spectroscopic proportional counter is described, for use in a balloon borne hard x-ray telescope. Initial position and spectral resolution data from a one-dimensional laboratory prototype are reported. With this device, the final telescope will have an angular resolution of better than 10 minutes of arc.

A large area telescope for balloon-borne hard X-ray astronomy

A balloon-borne hard X-ray telescope is described that has been designed to make highly sensitive observations of cosmic sources in the energy range 15 to 300 keV. The payload is characterized by a combination of NaI(Tl) based detector of a novel design and spectroscopic proportional counters, providing a total sensitive area of 5000 cm 2 and a 1 standard deviation sensitivity at the level of about 4 × 10 −6 photons cm −2 s −1 keV −1. All of the telescope sub-systems are described including the micro-processor based orientation platform that provides a pointing stability of better than 10 arc min. The physical characteristics of the detectors are included along with a summary of the telescope performance during a balloon flight in 1982.