R.A. Lewis

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.

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.

A diffusive light collection system for hard X-ray astronomical scintillation detectors

Abstract A novel design of a detector system for hard X-ray astronomy is described which permits large sensitive areas to be constructed on a modular basis. The system also incorporates high quality active shielding without the use of the phoswich system. The development and performance of the diffusive light collection optics are discussed.

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

Abstract 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 large area multitechnique experiment for hard X-ray astronomy

A balloon-borne multitechnique large-area experiment comprising two co-aligned detectors (3200 sq cm NaI and 1800 sq cm multiwire proportional counters) is described. The NaI array is an actively shielded scintillation device designed to operate over photon energies of 20 to 300 keV. The telescope comprises eight modules, each consisting of a 6-mm-thick NaI crystal that is actively shielded over the lower 2 pi steradians by a 5-cm-thick NaI(Tl) crystal. The two crystals are optically isolated and the scintillation light from the primary crystal is collected by means of a diffusive light collection system that is approximately 10 cm high and is set below the collimators. It is noted that each diffusion box is viewed by two photomultipliers set in electronic coincidence. This minimizes the effects of tube noise and raises the uniformity of response and the energy resolution of the unit. In addition, the shape of the diffusion box is designed to maximize the energy resolution of the telescope so that a typical value at 60 keV is 25 percent FWHM.

A high energy observation of the Coma Cluster region

Abstract High energy observation of the Coma Cluster of Galaxies was obtained during a balloon flight on July 15, 1987 with the MIFRASO telescope over the photon energy range 18 + 130 keV. The data have been obtained from the scintillation detector array and two proportional counters. The source excesses detected correspond to a statistical significance of 5.0 σ and 5.1 σ respectively. The present data are compared with previous observations of Coma Cluster and interpreted in terms of the Inverse Compton (IC) emission model. The existence of active galaxies associated with the Cluster as possible origin of the detected hard X-ray emission is also discussed.

The Hard X-Ray Telescope "MIFRASO": In Flight Performance

A large area, high sensitivity hard X-Ray experiment, operating in the energy range 15 = 300 keV, has been succesfully flown twice (1986 and 1987) from the balloon launching facility of Trapani-Milo (Sicily). This balloon borne multitechnique telescope was designed and built within the framework of an international collaboration between the Istituto di Fisica Cosmica and the Istituto di Astrofisica Spaziale, C.N.R., Italy and the Department of Physics of the Southampton University, U.K. The telescope consists of two different arrays; scintillation detectors(NaI 5.5 mm thick actively shielded) with a total area of 2,700 cm 2, and Multiwire Proportional Counters (MWPC) with a total sensitive area of 900 cm2. The scintillation detectors provide a large sensitive area over the energy range 20 300 keV, with moderate energy resolution, while the proportional counters cover the range 15 4. 130 keV with good spectral resolution (8% at 120 keV). During the two flights several cosmic sources have been observed and detected. Preliminary results will be presented on the Crab, used as an in-flight calibration source, and on the extragalactic objects 3C273, NGC 4151 and MCG 8-11-11.

EXPERIMENTAL GAMMA-RAY IMAGES GENERATED BY USE OF A ROTATING MODULATOR

Abstract The rotation modulation collimator is an accepted instrument for use in gamma-ray astronomy. Recently an adaption of this technique has been suggested which should improve the angular resolution and sensitivity by introducing some position sensitivity into the detection plane. A prototype of this instrument, the rotation modulation collimator, has been constructed and laboratory images generated in order to assess the potentiation for astronomical applications.