Medium Energy Gamma-ray Astronomy Research Articles

A compact instrument for gamma-ray burst detection on a CubeSat platform I

The Educational Irish Research Satellite 1 (EIRSAT-1) is a 2U CubeSat being developed under ESA’s Fly Your Satellite! programme. The project has many aspects, which are primarily educational, but also include space qualification of new detector technologies for gamma-ray astronomy and the detection of gamma-ray bursts (GRBs). The Gamma-ray Module (GMOD), the main mission payload, is a small gamma-ray spectrometer comprising a 25 mm × 25 mm × 40 mm cerium bromide scintillator coupled to an array of 16 silicon photomultipliers. The readout is provided by IDE3380 (SIPHRA), a low-power and radiation tolerant readout ASIC. GMOD will detect gamma-rays and measure their energies in a range from tens of keV to a few MeV. Monte Carlo simulations were performed using the Medium Energy Gamma-ray Astronomy Library to evaluate GMOD’s capability for the detection of GRBs in low Earth orbit. The simulations used a detailed mass model of the full spacecraft derived from a very high-fidelity 3D CAD model. The sky-average effective area of GMOD on board EIRSAT-1 was found to be 10 cm2 at 120 keV. The instrument is expected to detect between 11 and 14 GRBs, at a significance greater than 10σ (and up to 32 at 5σ), during a nominal one-year mission. The shape of the scintillator in GMOD results in omni-directional sensitivity which allows for a nearly all-sky field of view.

The MEGA Project for Medium Energy Gamma-ray Astronomy

The Medium Energy Gamma-ray Astronomy (MEGA) telescope concept will soon be proposed as a MIDEX mission. This mission would enable a sensitive all-sky survey of the medium-energy gamma-ray sky (0.4–50 MeV) and bridge the huge sensitivity gap between the COMPTEL and OSSE experiments on the Compton Gamma Ray Observatory and the visionary Advanced Compton Telescope (ACT) mission. The scientific goals include compiling a much larger catalog of sources in this energy range, performing far deeper searches for supernovae, better measuring the galactic continuum and line emissions, and identifying the components of the cosmic diffuse gamma-ray emission. MEGA records and images gamma rays by completely tracking Compton and pair creation events in a stack of double-sided Si strip detectors surrounded by a pixellated CsI calorimeter. A prototype instrument has been developed and calibrated in the laboratory and at a gamma-ray beam facility. We present calibration results from the prototype and describe the proposed satellite mission.

The Compton and pair creation telescope MEGA

MEGA, short for Medium Energy Gamma-ray Astronomy, is the development of a new technology telescope in the energy band 0.4–50 MeV. The wide energy range of MEGA, which spans nuclear γ-ray lines and energetic continuum spectra, the large field of view, and the capacity for polarimetry enables unique investigations into cosmic nucleosynthesis, particle accelerators around compact objects, and explosive high-energy events. We describe the development and tests of a prototype detector. Results from laboratory tests using radioactive sources and from a beam test calibration are presented and an outlook of a potential space mission is sketched.

Development and calibration of the tracking Compton/Pair telescope MEGA

We describe the development and tests of the prototype for a new telescope for Medium Energy Gamma-ray Astronomy (MEGA) in the energy band 0.4–50 MeV. As a successor to COMPTEL and EGRET (at low energies), MEGA aims to improve the sensitivity for astronomical sources by at least an order of magnitude. It could thus fill the severe sensitivity gap between scheduled or operating hard-X-ray and high-energy gamma-ray missions and open the way for a future Advanced Compton Telescope. MEGA records and images γ-rays by completely tracking Compton and Pair creation events in a stack of double-sided Si-strip track detectors surrounded by a pixelated CsI calorimeter. A scaled down prototype has been built and we describe technical details of its design and properties. Results from calibrations using radioactive sources and from measurements with an accelerator generated, fully polarized, γ-ray beam are presented and an outlook to future plans with MEGA is given.

The MEGA project

We describe the development of a new telescope for Medium Energy Gamma-Ray Astronomy (MEGA) for the energy band 0.4–50 MeV. As a successor to COMPTEL and EGRET (low energies), MEGA aims to improve the sensitivity for astronomical sources by at least an order of magnitude. It could thus fill the severe sensitivity gap between scheduled or operating hard-X-ray and high-energy γ-ray missions and open the way for a future Advanced Compton Telescope. MEGA records and images γ-rays by completely tracking Compton and Pair creation events in a stack of double sided Si-strip track detectors surrounded by a pixelated CsI calorimeter. A scaled down prototype has been built and calibrations using radioactive sources and exposures to an accelerator generated γ-ray beam were performed in 2003. A balloon flight is planned for 2004.

Data analysis for the MEGA prototype

The data analysis for combined Compton and Pair telescopes like the Medium Energy Gamma-Ray Astronomy telescope (MEGA) separates into three basic steps: the starting point is the calibration and low-level data-analysis. It is followed by the most critical part, the event reconstruction, which has to identify all event types (pair-creation, Compton events, charged particles, etc.) while effectively suppressing different kinds of background (photons from below, chance coincidences, activation, etc.). The last stage, the high level data analysis, is dominated by image reconstruction, which in our case is performed by a technique called list-mode maximum-likelihood expectation-maximization. The current performance of the data analysis algorithms is demonstrated by calibration measurements of the MEGA prototype.

Development of silicon strip detectors for a medium energy gamma-ray telescope

We report on the design, production, and testing of advanced double-sided silicon strip detectors under development at the Max-Planck-Institute as part of the Medium Energy Gamma-ray Astronomy (MEGA) project. The detectors are designed to form a stack, the “tracker,” with the goal of recording the paths of energetic electrons produced by Compton-scatter and pair-production interactions. Each layer of the tracker is composed of a 3×3 array of 500 μm thick silicon wafers, each 6 cm×6 cm and fitted with 128 orthogonal p and n strips on opposite sides (470 μm pitch). The strips are biased using the punch-through principle and AC-coupled via metal strips separated from the strip implant by an insulating oxide/nitride layer. The strips from adjacent wafers in the 3×3 array are wire-bonded in series and read out by 128-channel TA1.1 ASICs, creating a total 19 cm×19 cm position-sensitive area. At 20°C a typical energy resolution of 15–20 keV FWHM, a position resolution of 290 μm, and a time resolution of ∼1 μs is observed.

Present and future instrumentation for high-energy astrophysics in Italy

The high-energy astrophysics community in Italy is involved in several major international projects, which concern the study, design, development, and operation of telescopes in the field of high-energy astronomy and cosmic ray physics. A number of balloon- or space-borne missions, with important Italian participation, are presently either operative or recently ended (BeppoSAX, XMM-Newton, HETE-2), just launched (INTErnational Gamma-Ray Astrophysics Laboratory, in October 2002), under development (Claire, Swift, AGILE, Gamma-Ray Large Area Telescope), or under study (XEUS, Constellation-X, Lobster, Medium Energy Gamma-ray Astronomy, MAX, Gamma Air Watch, Extreme Universe Space Observatory, Interstellar/Intergalactic Medium and gamma ray Burst Observatory Spectroscopy Survey, Hard X-ray Modulation Telescope). The techniques, detection systems, and design concepts chosen for these telescopes are different and depend upon their operative energy range and scientific objectives. This work will focus on the current status of these instruments and on the possible future developments.

The MEGA advanced Compton telescope project

The goal of the Medium Energy Gamma-ray Astronomy (MEGA) telescope is to improve sensitivity at medium gamma-ray energies (0.4–50 MeV) by at least an order of magnitude over that of COMPTEL. This will be achieved with a new compact design that allows for a very wide field of view, permitting a sensitive all-sky survey and the monitoring of transient and variable sources. The key science objectives for MEGA include the investigation of cosmic high-energy particle accelerators, studies of nucleosynthesis sites using gamma-ray lines, and determination of the large-scale structure of galactic and cosmic diffuse background emission. MEGA records and images gamma-ray events by completely tracking both Compton and pair creation interactions in a tracker of double-sided silicon strip detectors and a calorimeter of CsI crystals able to resolve in three dimensions. We present initial laboratory calibration results from a small prototype MEGA telescope.

Low and medium energy gamma-ray astronomy — present status and future aspects

During the last few years quite some progress has been achieved in the field of low and medium energy gamma-ray astronomy below about 30 MeV. Gamma rays from the galactic center and anti-center region have been detected, which require a high interstellar electron flux in the 100 MeV range, if they are predominantly diffuse in nature. Though the Crab pulsar and its nebula are still the only galactic gamma-ray sources which definitely have been detected, some recently determined upper limits to the gamma-ray fluxes of other radio pulsars are close to the theoretically expected values. Active galaxies seem to have a maximum of luminosity in the range between several 100 keV and a few MeV and, therefore, are of special interest. First observational results have been reported on the Seyfert galaxies NGC 4151 and MCG 8-11-11, and the radio galaxy CenA. The nature of the diffuse cosmic gamma-ray component at low gamma-ray energies is not yet solved. Unresolved active galaxies are good candidates for its origin. Considering the present status of gamma ray astronomy the study of galactic sources like radio pulsars and the unidentified high energy gamma-ray sources, the Milky Way as a whole, active galaxies and the diffuse cosmic sky seem to be the prime targets for broad band observations below 30 MeV in the GRO area. An unexplored field like that of low energy gamma-ray astronomy, however, is always open for surprises.

Gamma Ray Detection with Long NaI(Tl) Scintillator Bars

Extensive position, energy and timing resolution measurements have been made for a l00×5×5 cm3 NaI(Tl) scintillator bar. The arrival times and the pulse heights of signals from two photomultiplier tubes, one at each end of the bar, are used to provide the energy loss, linear position and time-of-flight for gamma ray interactions in the bar over the energy range from 0.5 to 20 MeV. The scintillator bar has an approximately exponential roll-off with a mean attenuation coefficient of 0.015 cm-1. At 0.662 MeV, resolutions (FWHM) of 5 cm, 9.4% and 10 ns were obtained for position, energy and timing, respectively. The timing resolution is 6 ns at 1.25 MeV. At 6.13 MeV, a spatial resolution of 2.2 cm was obtained. This bar is a prototype element of a Compton scatter telescope that uses both plastic and NaI(TI) scintillator bars. The technique is being used for medium energy gamma ray astronomy from a balloon platform. The telescope will have 3.5° (FWHM) angle and 6% energy resolutions at 6 MeV.

Medium energy gamma ray astronomy with transpacific balloon flights

Transpacific balloon flights with the University of California, Riverside (UCR) double scatter telescope are discussed. With flight durations from 5 days up to perhaps 15 days the long observation times necessary for medium energy (1–30 MeV) gamma ray astronomy can be obtained. These flights would be made under the auspices of the Joint U.S.-Japan Balloon Flight Program at NASA. We propose that flights can provide at least 30 hours of observation time per flight for many discrete source candidates and 120 hours for detecting low intensity cosmic gamma ray bursts.

Double Scatter Telescope for Medium Energy Gamma Ray Astronomy from a Satellite

A large area (1 m2) medium energy (1-30 MeV) telescope for gamma ray astronomy is discussed. This telescope utilizes the double scattering of gamma rays between two scintillator arrays with directionial discrimination by means of time-of-flight. The first and second arrays consist of a series of plastic and NaI(Tl) scintillators, respectively, in the shape of long (1 m) linear elements viewed by photomultiplier tubes at both ends. The lateral position of the interaction in the plastic is determined by timing and in the NaI(Tl) by pulse height. Neutron-induced background is eliminated by using a pulse shape discrimination plastic scintillator. At 6 MeV, the telescope has an area-efficiency factor of 300 cm2, an energy resolution of 8% (FWHM), an angular resolution of 3° (HWHM) and a sensitivity of 5 × 10-6 γ/cm2sec for line emission. Discrete sources can be located to 0.5°.

Significance of medium-energy gamma-ray astronomy in the study of cosmic rays

Medium energy (about 10 to 30 MeV) gamma ray astronomy provides information on the product of the galactic electron cosmic ray intensity and the galactic matter to which the electrons are dynamically coupled by the magnetic field. Because high energy (greater than 100 MeV) gamma ray astronomy provides analogous information for the nucleonic cosmic rays and the relevant matter, a comparison between high energy and medium energy gamma ray intensities provides a direct ratio of the cosmic ray electrons and nucleons throughout the galaxy. A calculation of gamma ray production by electron bremsstrahlung shows that: bremsstrahlung energy loss is probably not negligible over the lifetime of the electrons in the galaxy; and the approximate bremsstrahlung calculation often used previously overestimates the gamma ray intensity by about a factor of two. As a specific example, expected medium energy gamma ray intensities are calculated for the speral arm model.