A. J. Court

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

Abstract 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 cm2 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.

Hard X-ray variability of NGC 4151

During a balloon flight on July 15, 1987 of the MIFRASO telescope, a region of the sky containing the Seyfert galaxy NGC 4151 was studied over the photon energy range 15-335 keV. A 13-sigma excess, interpreted as originating from the direction of NGC 4151, was detected in this energy band and the corresponding emission spectrum is represented by a single power law with a photon spectral index of alpha = 1.5 + or {minus} 0.1. With respect to the July 1986 observation of this source with the same telescope, the alpha slope is statistically unchanged, while the X-ray flux in the energy range 27-80 keV shows, at the 95-percent confidence level, an increase of about 40 percent. An observation of the Crab Nebula, that proves the stability of the telescope response through the two measurements of NGC 4151, is also described. 48 refs.

Hard X-ray observation of NGC 4151 with the Mifraso telescope

During a balloon flight on July 29, 1986 of the Mifraso telescope, a region of the sky containing the Seyfert galaxy NGC 4151 was studied over the photon energy range 15-300 keV. A 5.5 sigma excess, interpreted as originating from the direction of NGC 4151, was detected in the energy band 27-80 keV. The corresponding emission spectrum may be represented by a single power law with a photon spectral index of alpha = 1.5 + or - 0.6. The result is compared to previous hard X-ray observations of this source. Apart from this measurement, an observation of the Crab Nebula as an in-flight calibration source of the telescope is described. 33 refs.

An imaging telescope for soft gamma-ray astronomy: The preliminary in-flight tests

A large area (6000 cm2) actively shielded low energy gamma-ray telescope is going to be built by an Anglo-Italian collaboration. The telescope, named ZEBRA, will be capable of producing images of the X and gamma ray sky in the energy range 0.015–20 MeV with an intrinsic angular resolution of a few tenths of a degree. A prototype detector has been built in order to experimentally study the main characteristics of the detection plane. The preliminary results obtained during a balloon flight from Trapani, Sicily in July 1981 are presented.

A position sensitive detector using a NaI(Tl)/photomultiplier tube combination for the energy range 200 keV to 10 MeV

Abstract The performance of the position sensitive detector for the ZEBRA low energy gamma-ray imaging telescope is described. The detector consists of 9 position sensitive Nal(TI) elements each 5.8 × 5.0 × 56.0 cm viewed at either end of the long axis by 2 in. photomultiplier tubes. The total active area is 2470 cm 2 with an average positional resolution of 2.1 cm and energy resolution of 15% FWHM at 661.6 keV. The method of flight calibration is described together with the provision within the on-board electronics to correct for sources of error in the calculation of event energy loss and position. The results presented are obtained from the calibration phase of the ZEBRA telescope project.

Laboratory Gamma-Ray Images Using the ZEBRA Telescope

A series of approximately 100 laboratory images have been made with the ZEBRA astronomical telescope using a range of radioactive sources. These images were aimed at assessing the performance of the telescope and included combinations of point sources, extended sources, ring sources as well as vignetted sources. Besides linear reconstruction techniques a range of non linear image restoration processes are discussed.

Nonthermal X-ray emission from 3C 273: the core of a knotty problem?

The quasar 3C 273 was observed in the hard X-ray region of the spectrum (15-300 keV) during a stratospheric balloon flight on July 15, 1987. The emitted photon spectrum is well described by a single power law of photon index -1.61 which corresponds to a hard (20-200 keV) X-ray luminosity of 0.8 x 10 to the 47th ergs/s. This flux is consistent with the 1978/1979 HEAO 1 and AIT/MPI measurements and about a factor of three below the high state measured in 1981 (AIT/MPI). The implications of these results with respect to the location of the production site and mechanism are discussed. 26 refs.

Performance of a day time star sensor for a stabilized balloon platform

A modified version of a CCD (charge-coupled device) star tracker originally designed for use on the ROSAT X-ray astronomy satellite has been built for use on a three-axis-stabilized balloon platform. The first flight of this star sensor was planned for May 1988 from the NASA balloon base at Palestine, Texas. Descriptions are given of the star sensor head, flight electronics, and steering system. The expected performance of this instrument is described along with the preflight results, which confirm its performance. >

The Structure of the ZEBRA Telescope, the Integration Tests and the First Calibration Results

The ZEBRA telescope is a balloon borne low energy gamma ray instrument, sensitive in the photon energy range 0.2-10MeV. The main characteristics and components of the telescope are described, and its performance during laboratory calibration is discussed, after its integration with the flight electronics.

The in-flight performance of the ZEBRA day-time star sensor

Abstract The low energy balloon-borne gamma-ray telescope ZEBRA is capable of producing images of the sky with a resolution of 1° and a point source location accuracy of a few arcminutes. In order to fully exploit the sensitivity of this instrument, a platform stabilised in three axes to this precision has been developed, which however has an absolute pointing accuracy of 30′-1°. Therefore there is a need to have on board a system which will allow post facto reconstruction of the absolute pointing direction of the telescope as a function of time, for observations in both day- and night-time conditions. An independently steerable starsensor has been constructed, based on a CCD imaging system, for this purpose. The results obtained with this device during the first flight of the ZEBRA telescope in May 1989 from the NASA balloon base in Palestine, Texas are presented.