Search

Imaging arrays of thick CdTe detectors, with millimeter spatial resolution and better than 10 keV energy resolution at 1 MeV, are very promising for the calorimeter section of an Advanced Compton Telescope. Measurements with small CdTe detectors with Schottky contacts, and advances in fabrication technology, have led us to the design of an 8×8 pixel array, with 2×2 mm2 pixel size, as a first prototype module for large arrays. The detectors are arranged with the electric field direction perpendicular to the incoming gamma-ray direction, allowing several centimeters deep arrays, with the minimum amount of passive material. With the development of the 8×8, 10 mm deep array, we aim to demonstrate the feasibility of the concept for gamma-ray imaging.

The feasibility of a large-volume liquid xenon Compton telescope based on full event imaging in a time projection chamber has been demonstrated with the development of the balloon-borne instrument LXeGRIT. With its 400 cm2 sensitive area and 7 cm drift gap, the liquid xenon detector images γ-rays in the energy range from 200 keV to 25 MeV. The precise 3-dimensional localization of γ-ray interactions within the sensitive volume provides excellent background reduction capabilities. Together with the large efficiency of a homogeneous detector volume, LXeGRIT addresses the primary instrumental limitations encountered in this energy band. Following engineering tests at balloon altitude in 1997, LXeGRIT has been upgraded with a new trigger and data acquisition system, integrated with the existing readout electronics. Enhanced data transfer capability and onboard data storage were also implemented. LXeGRIT was successfully flown from Ft. Sumner, NM on May 7, 1999. The instrument worked as expected at balloon altitude. The large amount of data gathered during the 9.5 h flight are currently being analyzed. We describe the instrument and present some results from calibration data and preliminary results from the flight.

The instrumental-line background of the Compton telescope COMPTEL onboard the Compton Gamma-Ray Observatory is due to the activation and/or decay of a number of different isotopes. The major components of the COMPTEL instrumental-line background can be attributed to eight individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl. In addition, evidence for the presence of 27Mg has been obtained in the search for gamma-ray lines from supernovae. The identification of the instrumental lines with specific isotopes is based on the line energies as well as on the variation of the activity with time, cosmic-ray intensity and deposited radiation dose during passages through the South-Atlantic Anomaly. The characteristic variation of the activity due to an individual isotope depends on its life-time, orbital parameters, and the solar cycle.

The COMPTEL map of the 1.809 MeV line, which is attributed to the radioactive decay of 26Al, shows significant excess emission in the Cygnus region. We investigate counterparts of this emission, based on COMPTEL data accumulated over 8 years. Previous studies suggested that this flux is due to the integrated, but unresolved, nucleosynthesis from young, massive objects like Wolf Rayet (WR) stars and core collapse supernovae. Beside a static scenario of known point sources-WR stars and SNRs-we consider OB associations in a dynamic scenario to model the observed flux in the Cygnus region. We carry out population synthesis studies to determine the time-dependent production of 26Al and also kinetic as well as radiative energy. Furthermore we consider the impact of enhanced 26Al yields probably occurring in a fraction of massive close binary systems. We compare the nucleosynthesis results of our OB association model with recent measurements in the Cygnus region.

44Ti is one of the few radioactive nuclei produced abundantly in supernovae (SNe). Because its life time is comparable to the typical time between supernovae in the Galaxy, γ-ray lines from 44Ti provide a powerful tool for the search for optically obscured supernovae that occurred during the last few 100 years. We present our analysis of COMPTEL data (Cycle 1–6), comparing γ-ray line images and flux distributions with Monte Carlo simulations of SNe in the Galaxy. Assuming standard SNe distribution models and theoretical estimates of the 44Ti yields, we constrain the Galactic supernova rate. We compare these constraints with those obtained from the historical record covering the last millennium. Relating the current supernova rate to the observed 44Ti solar abundance through chemical evolution models, agreement can be obtained only if there is a special type of supernova (e.g., helium detonations) that produces large amounts of 44Ti, but with a very low event rate.

We present an updated version of COMPTEL’s 1.809 MeV sky survey. Based on eight years of observations we compare results from different imaging techniques using background from adjacent energy bands. We confirm the previously reported characteristics of the galactic 1.809 MeV emission, specifically an extended galactic ridge emission, mainly concentrated towards the inner galaxy, a peculiar emission feature in the Cygnus region, and a low-intensity ridge extending towards Carina and Vela. Because this gamma ray line is due to the decay of radioactive 26Al, predominantly synthesized in massive stars, one anticipates flux enhancements aligned with regions of recent star formation. This is born out by the observations. In particular the Cygnus feature, first presented in 1996 based on three years of COMPTEL data, is confirmed. Based on the stellar population we distinguish three prominent areas in this region, for which we separately derive fluxes, and discuss interpretations.

26Al emission at 1.809 MeV has been measured by several different instruments. The constraints set by each are very different, due to differing sky exposures and background estimation methods. We report on a first combined analysis of data from the COMPTEL, OSSE, and SMM instruments. The COMPTEL 1.8 MeV image as deconvolved with the Maximum-Entropy algorithm was tested for its flux normalization and its fine structure in the inner Galaxy, using data from SMM and OSSE. We find consistency of the OSSE data with COMPTEL’s image structure, in particular the choice of the ‘stopping point’ of the iterative deconvolution method. The SMM data suggest substantially more total flux than that given by the COMPTEL map. We discuss possible explanations, in particular large-scale emission outside the plane of the Galaxy, and outline future work on combined data from the three instruments.

Nucleosynthesis models predict the production of 60 Fe by the same massive stars which are responsible for 26 Al synthesis. With a radioactive decay time similar to 26 Al , the gamma-ray line emission at 1.173 and 1.332 MeV is predicted to be ∼16% of the 1.809 MeV 26 Al line intensity, from the same source regions. We investigate with COMPTEL all-sky data from CGRO Phases 1–5 whether this source of 60 Fe can be detected, using the known spectral signature plus the spatial distribution as imaged with COMPTEL 26 Al measurements. Uncertainties in spatial signature of the instrumental and continuum background limit the sensitivity, such that only an upper limit of ∼44% (2σ) is quoted at this time.

The all-sky data from CGRO Phases 1–5 provide the most detailed constraints to date on the 26Al distribution in the Galaxy. With improved modelling of the instrumental background, different source distribution models can be distinguished. We derive large-scale parameters from first-order models, i.e., a characteristic scale height of 130 pc, and a Galactocentric scale radius of 5 kpc. Spiral structure of the 26Al source distribution improves the fits, as molecular gas provides a better description than geometrical models in the inner Galaxy. We note that infrared emission from dust also matches the 26Al emission quite well. Hints for contributions from more local and localized 26Al components such as Loop I and the Gould Belt are compatible with a massive star origin of 26Al.