Starting from 1960s, a great number of missions and experiments have been performed for the study of the high-energy sky. This review gives a wide vision of the most important space missions and balloon experiments that have operated in the 10–600 keV band, a crucial window for the study of the most energetic and violent phenomena in the Universe. Thus it is important to take the stock of the achievements to better establish what we have still to do with future missions in order to progress in this field, to establish which are the technologies required to solve the still open issues and to extend our knowledge of the Universe.

Hard X-Ray/Soft Gamma-Ray Experiments and Missions: Overview and Prospects

Starting from 1960s, a great number of missions and experiments have been performed for the study of the high--energy sky. This review gives a wide vision of the most important space missions and balloon experiments that have operated in the 10--600 keV band, a crucial window for the study of the most energetic and violent phenomena in the Universe. Thus it is important to take the stock of the achievements to better establish what we have still to do with future missions in order to progress in this field, to establish which are the technologies required to solve the still open issues and to extend our knowledge of the Universe.

Hard X-ray/Soft gamma-ray experiments and missions: overview and prospects

The planned International X-ray Observatory (IXO) was the designated successor to the extremely suc- cessful XMM Newton and Chandra X-ray observatories. Aiming to provide high sensitivity, high spatial, spectral and timing resolution observations in the 0.1 keV – 40 keV energy range, the mission would have greatly extended our knowledge of the universe. The international project was canceled due to the pullout of NASA in 2010. Subsequently, ESA pursued an European-only mission: A Telescope for High ENergy Astrophysics (ATHENA).
Both missions foresaw the use of a DePFET-based Wide Field Imager (WFI) for spectroscopic imag- ing observations in the energy range between 0.1 keV – 15 keV. The WFI would offer a high quan- tum efficiency, high spatial (≤ 10arcsec), moderate spectral (∆E ≈ 70eV@1keV) and high timing resolution (< 20μs) for imaging observations. The planned high sensitivity of ≈ 10−17 erg cm−2 s−1 for a 100ks observation translates into a maximum cosmic particle-induced background rate of ≈ 10−4 cts keV−1 cm−2 s−1 – 10−3 cts keV−1 cm−2 s−1. A rate which at the second Lagrangian point of the Earth-Sun system is only feasible if an optimized shielding concept and efficient background detection and reduction algorithms are employed. The study and optimization of these background reduction con- cepts, using a Geant4 Monte-Carlo simulation and the accompanying software development, is the core topic of this work.
After an introduction to X-ray astronomy in which the requirements for a next-generation X-ray ob- servatory are discussed and an overview of the IXO and ATHENA missions is given the simulation envi- ronment and the Geant4 Monte-Carlo toolkit are introduced. In the course of doing so the requirements for the simulation are defined and it is asserted that the radioactive decay simulation of Geant4 does not provide adequate functionality for an X-ray astronomy application. In particular, it is found that the code is not well verified and validated and that an appropriate means of simulating the cosmic-ray induced delayed background is not provided. As a response to this problem an extensive verification and self-consistent validation effort on radioactive decays in Geant4 was undertaken which has resulted in a new radioactive decay code for Geant4. This software is more accurate and significantly faster than the existing code. It includes a novel statistical sampling approach which appreciates the fact that for most experiments radioactive decays and the resulting radiation are a statistical observable. Furthermore, a self-consistent long-term activation simulation which requires minimal user input is included. The new code was extensively verified with Evaluated Nuclear Structure Data File (ENSDF) data and validated with High Purity Germanium (HPGe) detector measurements.
As a concrete application example this code was included into the IXO/ATHENA simulation environ- ment. Using this environment which was validated with XMM Newton EPIC-pn background measure- ments and cosmic ray activation measurements from the Space Shuttle STS-53 mission an extensive characterization of the prompt and delayed on-orbit background was performed. These studies resulted in an optimized graded-Z shielding design which is needed for a fluorescence emission free background; flexible pattern detection and rejection algorithms with > 99% rejection efficiency; as well as a novel approach utilizing an electric field to accelerate secondary electrons to energies above the detection limit thereby additionally reducing the background by ≈ 50%.
In conclusion, a background estimate of (6.42 ± 2.03) × 10−4 cts keV−1 cm−2 s−1 has been obtained for the ATHENA WFI. If a 200 kV cm−1 accelerating field is used a lower rate of (2.70 ± 2.67) × 10−4 cts keV−1 cm−2 s−1 can be achieved. The contribution of the delayed background component was estimated at (0.21 ± 0.05) × 10−4 cts keV−1 cm−2 s−1 after a 10 year mission time. All rates are within the WFI background requirements.
The background studies reported upon in the work are deemed applicable and beneficial for a wide range of silicon pixel detector applications. The code development work which has resulted in a new radioactive decay simulation for Geant is considered useful for an even broader range of experiments including applications in low background detectors, material sciences, radiation safety, nuclear non- proliferation studies, medical physics and homeland security.

Studies on the Background of the Wide Field Imager aboard the IXO and ATHENA X-Ray Telescopes

The Oriented Scintillation Spectrometer Experiment (OSSE) consists of four large volume NaI detectors that are optimized to observe gamma-rays in the 0.05 - 10.0 MeV energy band. Each detector can be oriented independently and has a wide variety of observation and telemetry modes, leading to a complex data analysis environment. The OSSE data analysis task is rendered even more complicated by the on-orbit environment, which exhibits a rich set of variations as a function of energy, point direction, orbital phase and total time in orbit. Since the typical OSSE signal to noise ratio is very low, understanding and accounting for background variations is a critical and complex task. In order to deal with the OSSE data analysis challenge, a system of databases and software have been developed. These include a series of well defined data products that are handled first by a production data analysis system performing routine processing and then by a scientific analysis system which allows individual scientists to examine and manipulate OSSE data.

OSSE data flow and analysis

Clusters of black holes as point-like gamma-ray sources

: We report on ~120 days of observations of Cygnus X-1 with OSSE onboard the Compton Observatory. Emission is detected in the range 50 keV to 1 MeV and we find evidence for a continuum of hard X-ray spectra over a broad range of fluxes rather than the existence of distinct flux states. Comparisons of the source spectra with various theoretical models show that an exponentially truncated power law best describes the average spectrum in the OSSE energy band. Although we have measured a new minimum in the hard X-ray flux from the source, no evidence was found for either a broad 1 MeV feature or a narrow 511 keV line previously reported in association with a low flux state. Upper limits on such emission features are an order of magnitude lower than earlier reported detections. The 5.6-day periodicity of the source measured at optical wavelengths was not detected with a sensitivity to the rms modulation fraction of 5% in the 60-140 keV energy band.

Gamma Ray Observations of Cygnus X-1 With OSSE

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Arthur Holly Compton Gamma Ray Observatory is described. An overview of the operation and control of the instrument is given, together with a discussion of typical observing strategies used with OSSE and basic data types produced by the instrument. Some performance measures for the instrument are presented that were obtained from pre-launch and in-flight data. These include observing statistics, continuum and line sensitivity, and detector effective area and gain stability.

/pdf/operation-and-performance-of-the-osse-instrument-4uz6d4a3lx.pdf

Operation and Performance of the OSSE Instrument

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Gamma Ray Observations of Cygnus X-1 With OSSE

Operation and Performance of the OSSE Instrument