1. Measurement principle and payload overview

Interactions of photons with matter in the ASTROGAM energy range is dominated by Compton scattering from 0.1 MeV up to about 15 MeV in silicon, and by electron-positron pair production in the field of a target nucleus at higher energies. ASTROGAM maximizes its efficiency for imaging and spectroscopy of energetic gamma rays by using both processes. Figure 1 shows representative topologies for Compton and pair events.

For Compton events, point interactions of the gamma ray in tracker and calorimeter produce spatially resolved energy deposits, which have to be reconstructed in sequence using the redundant kinematic information from multiple interactions. Once the sequence is established, two sets of information are used for imaging: the total energy and the energy deposit in the first interaction measure the first Compton scatter angle. The combination with the direction of the scattered photon from the vertices of the first and second interactions generates a ring on the sky containing the source direction. Multiple photons from the same source enable a full deconvolution of the image, using probabilistic techniques. For energetic Compton scatters (above ~1 MeV), measurement of the track of the scattered electron becomes possible, resulting in a reduction of the event ring to an arc, hence further improving event reconstruction. Compton scattering depends on polarization of the incoming photon, hence careful statistical analysis of the photons for a strong (e.g., transient) source yields a measurement of the degree of polarization of its high-energy emission.

Figure 1

Figure 1 Representative event topologies for Compton events without (left) and with electron tracking (center) and for a pair event (right panel).

Pair events produce two main tracks from the electron and positron at small opening angle. Tracking of the initial opening angle and the plane spanned by electron and positron enables direct back-projection of the source. Multiple scattering in the tracker material (or any intervening passive materials) leads to broadening of the tracks and limits the angular resolution at low energies. The nuclear recoil taking up an unmeasured momentum results in a small uncertainty, usually negligible compared to instrumental effects. The energy of the gamma ray is measured using the calorimeter. Polarization information in the pair domain is given by the azimuthal orientation of the electron-positron plane.

The ASTROGAM payload is shown in Figure 2. It consists of three main detectors:

  • A Silicon Tracker in which the cosmic gamma rays undergo a first Compton scattering or a pair conversion, based on the technology of double sided Si strip detectors to measure the energy and the 3D position of each interaction with an excellent energy and spatial resolution;
  • A 3D-imaging Calorimeter to absorb and measure the energy of the secondary particles, which is made of an assembly of small scintillation crystals (12,544 CsI (Tl) bars of 5x5x50 mm3) readout by silicon drift photodetectors to achieve the required energy resolution (4.5% at 662 keV);
  • An Anticoincidence (AC) system to veto the prompt-reaction background induced by trapped, solar, or cosmic-ray charged particles, design with plastic scintillators covering the instrument to detect single charged relativistic particles with an efficiency exceeding 99.99%.

The payload is completed by a Data Handling and Power Supply Units located below the Calorimeter inside the platform together with the back-end electronics. The PDHU is in charge of the payload internal control, the scientific data processing, the operative mode management, the on-board time management, and the telemetry and telecommand management. The total payload mass and power budget are 300 kg and 524 W, respectively.

Figure 2

Figure 2 Overview of the ASTROGAM payload showing the Silicon Tracker, the Calorimeter and the Anticoincidence system.

Especially for the Compton mode at low energies, but also more broadly over the entire energy range covered by ASTROGAM, it is important to keep the amount of passive materials on the top and at the sides of the detector to a minimum, to reduce background production in the field of view and to optimize angular and energy resolutions. In addition, the passive materials between the Tracker layers, and between the Tracker and the Calorimeter must be minimized for best performance. This enters the considerations for both the mechanical design, weighing minimal mass vs. mechanical stiffness, and the electronics layout, where a balance must be found between signal amplification and analog/digital conversion in the front-end electronics (FEE) close to the detector, and trigger and event selection decisions further away, ideally below the telescope in back-end electronics and on-board computers. The instrument harness is also optimized in this regard.

1.1 Silicon Tracker

The gamma ray Tracker is the heart of the ASTROGAM payload. It is based on the silicon strip detector technology widely employed in medical imaging and particle physics experiments (e.g., ATLAS and CMS at LHC), and already applied to the detection of gamma rays in space by the AGILE and Fermi missions. The ASTROGAM Tracker will employ double sided strip detectors (DSSDs) to work also as a Compton telescope. The Tracker configuration is based on 70 planes of Silicon DSSDs; each plane is 60 x 60 cm2 large, and each detector is made of 9.5 x 9.5 cm2 Silicon tiles.

1.2 Calorimeter

The ASTROGAM Calorimeter is a pixelated detector made of a high-Z scintillation material Thallium activated Cesium Iodide for an efficient absorption of Compton scattered gamma rays and electron-positron pairs. It consists of an array of 12,544 parallelepiped bars of CsI(Tl) of 5x5x50 mm3 dimension, read by silicon drift detectors at both ends. The Calorimeter thickness 5 cm of CsI(Tl) makes it a 2.7 radiation-length detector having an absorption probability of a 1-MeV photon of 72%.

1.3 Anticoincidence

The ASTROGAM Anticoincidence (AC) system is made of segmented panels of plastic scintillators covering the top and four lateral sides of the instrument, with three plastic tiles per side. The top plastic scintillators are 6 mm thick, whereas the lateral panels are 5 mm thick. All scintillator tiles are coupled to silicon photomultipliers (SiPM) by optical fibers. The architecture of the AC detector is fully derived by the successful design of the AGILE and Fermi/LAT AC systems. The AC particle background rejection is designed to achieve a relativistic charged particle detection inefficiency lower than 10-4 (a standard value already realized in current space experiments).

1.4 Data Handling

The ASTROGAM payload is completed by a Payload Data Handling Unit (PDHU) and a Power Supply Unit (PSU). The PDHU is in charge of carrying out the following principal tasks: (i) payload internal control; (ii) scientific data processing; (iii) operative modes management; (iv) on board time management; (v) Telemetry and Telecommand management. The main functions related to the scientific data processing will be: (i) the BEEs interfacing through dedicated links to acquire the scientific data; (ii) the real-time software processing of the collected Silicon Tracker, Anticoincidence and Calorimeter scientific data aimed at rejecting background events; (iii) the formatting of the selected good events into telemetry packets.

1.5 Trigger logic and data flow architecture

The ASTROGAM on-board scientific data processing will be composed of two main trigger pipelines, the gamma ray acquisition mode and the Calorimeter Burst search. The simultaneous data set provided by the Silicon Tracker, the Calorimeter and the AC constitutes the basis for the gamma-ray detection and processing. The gamma-rays trigger logic will be structured on two main levels: Level-1 (fast: 5-10 μs logic, hardware); and Level-2 (asynchronous, 50 μs processing, software).

2. Performance assessment

The scientific performances of the ASTROGAM instrument were evaluated by detailed numerical simulations, using two different sets of software tools: MEGAlib and Bogemms. The MEGAlib package was originally developed for analysis of simulation and calibration data related to the Compton scattering and pair creation telescope MEGA. It has then been successfully applied to a wide variety of hard X-ray/gamma-telescopes on ground and in space, such as COMPTEL, NCT and NuSTAR. MEGAlib contains a geometry and detector description tool for the detailed modeling of different detector types and characteristics, and provides a Monte Carlo simulation program based on Geant4, together with specialized Compton event reconstruction algorithms.

Bogemms is a Geant4-based simulation tool specialized on the reconstruction of pair creation events. It was initially developed to reproduce AGILE data accumulated during the pre-launch testing and post-launch commissioning phases and was then validated from in-flight data from the AGILE gamma-ray imager detector. The results of Bogemms and MEGAlib were crosschecked in the gamma-ray energy range between 10 and 100 MeV.

Figure 3 shows the mass model of ASTROGAM used for these simulations. An accurate mass model that includes passive material in the detector and its surroundings, true energy thresholds and energy and position measurement accuracy, as well as a roughly accurate S/C bus mass and position are crucial to the modeling. In particular, care was taken to include all passive materials close to the Si and CsI(Tl) detectors.

All these components were carefully modeled using the MEGAlib environment tools. In the pair domain above 10 MeV, the background is mainly induced by fast particles (mainly leptons) impinging the S/C, as well as by the cosmic diffuse radiation and the atmospheric gamma-ray emission.

Figure 3

Figure 3 Geant4/MEGAlib mass model of the ASTROGAM telescope on the platform, with a simulated pair event produced by a 30-MeV photon.

2.1 Angular and energy resolutions

ASTROGAM will achieve an unprecedented point spread function (PSF) above 50 MeV and a very good angular resolution in the Compton scattering domain as well, e.g. better than that of COMPTEL by a factor of 4 at 5 MeV (Figure 4). In the latter regime, the δθ values reported in Figure 4 are the FWHM of the optimal ARM obtained after selection on the reconstructed energy (2σ on the full-energy peak) and the Compton scatter angle (e.g. Φ between 0 and 40 at 500 keV). In the pair production domain, the ASTROGAM Tracker properties are ideal for a very accurate determination of the event topology, which include the vertex determination and the particle tracking. The absence of heavy converters and a ultra-light mechanical structure makes very probable the gamma-ray conversion in the Silicon detectors. At this point, the PSF above 30 MeV is dictated by the combination of multiple scattering in Silicon detectors and the tracking reconstruction algorithm. The ASTROGAM PSF above 30 MeV is therefore expected and simulated to be substantially better than those of Fermi and AGILE, as represented in Figure 4.

The right panel of Figure 4 shows the ASTROGAM spectral resolution in the Compton domain. Above 30 MeV the spectral energy resolution is expected to be within 20-30%.

Figure 4

Figure 4 Left panel ASTROGAM on-axis angular resolution compared to that of COMPTEL and Fermi/LAT. In the Compton domain, the presented performance of ASTROGAM and COMPTEL is the FWHM of the angular resolution measure (ARM). In the pair domain, the point spread function (PSF) is the 68% containment radius. The Fermi/LAT PSF is from the Pass 7 data. Right panel 1σ energy resolution of COMPTEL and ASTROGAM in the Compton domain after event reconstruction and selection on the ARM.

2.2 Field of View

The ASTROGAM field of view (FoV) was evaluated from detailed simulations of the incident angle dependence of the sensitivity. It amounts to 41 HWHM at 1 MeV, with a fraction of sky coverage in zenith pointing mode of 21%, corresponding to Ω = 2.6 sr. In the pair production domain, the field-of-view assessment is also based on in-flight data from the AGILE and Fermi-LAT gamma-ray imager detectors. The consolidated FoV is 2.5 sr, but ASTROGAM characteristics (size, Si plane spacing, overall geometry) make possible an even larger FoV.

Figure 5

Figure 5 shows the FoV of a 1-month AGILE pointing of the Galactic Center region. The ASTROGAM FoV will be similar or larger than that of AGILE and Fermi-LAT, 2.5 sr.

2.3 Effective area and continuum sensitivity

Table 1 compares the effective area of ASTROGAM with that of INTEGRAL/SPI, CGRO/COMPTEL, Fermi/LAT and AGILE. Below 10 MeV, the reported values correspond to the detection of full-energy gamma rays from a monochromatic source on axis. We see that the effective area of ASTROGAM at low energies will be about two times larger than that of SPI and 7.5 times larger than that of COMPTEL (at 1 MeV). However, the gain in sensitivity of ASTROGAM will be much higher (see below), thanks to its unprecedented background rejection capability.

Table 1 Effective area (in cm2) for the detection of a point source on axis.

Table 1

Figure 6

Figure 6 ASTROGAM source 5-σ sensitivity in pointing mode (dark blue curves) for a 1-month observation of a source at 30 degrees off-axis in the inner Galaxy (left panel) and for a high-latitude source (right panel). Also shown are the Fermi-front-LAT Pass7V_6 sensitivity (green curves) and CTA sensitivity for 15 hr integration (cyan curves). Sensitivies in the MeV up to nearly the GeV energies are background dominated. At higher energies, sensitivities are photon limited: here we show the limit sensitivities assuming at least N=5 high-energy photons detected within the 99% confidence radius. ASTROGAM is assumed to be pointing in a LEO orbit with an overall exposure efficiency of 0.6 similar to AGILE's (checked with real data). Fermi-LAT is assumed to be in sky-scanning mode with an overall exposure efficiency per single source of 0.16 (as checked with real data).

In Figure 6 we compare the ASTROGAM source sensitivity in pointing mode for a 1-month integration in the inner Galaxy and in an extragalactic field. Also shown are the sensitivities of Fermi-front-LAT in sky scanning mode (the standard mode for Fermi) and for CTA (15 hr integration). ASTROGAM will have a better sensitivity than INTEGRAL and COMPTEL in the range 0.3-20 MeV by a factor of 10-30. A better sensitivity compared to Fermi up to 1 GeV is obtainable by a combination of pointing strategy and much improved angular resolution.

Figure 7

Figure 7 Point source continuum sensitivity of different X and γ-ray instruments (adapted from Figure 1 in Takahashi et al. 2013). The curves for Chandra/ACIS-S, Suzaku/HXD (PIN, GSO), INTEGRAL/IBIS and ASTRO-H (HXI, SGD) are given for an observing time Tobs = 100 ks. The COMPTEL and EGRET sensitivities are given for the observing time accumulated during the whole duration of the CGRO mission (Tobs ~ 9 years). The Fermi/LAT sensitivity is for a high Galactic latitude source and Tobs = 1 year (Atwood et al. 2009). For MAGIC (Aleksic et al. 2012), H.E.S.S. (Aharonian et al. 2006) and CTA (Actis et al. 2011) the sensitivities are given for Tobs = 50 hours. The ASTROGAM sensitivity is for an effective exposure of 1 year of a high Galactic latitude source (see text). Sensitivities above 30 MeV are given at the 5-sigma confidence level, whereas those below 10 MeV (30 MeV for COMPTEL) are at 3-sigma.

Figure 7 shows the ASTROGAM continuum sensitivity (at 3σ below 10 MeV and 5σ above, assuming a spectral bin width ΔΕ = Ε) for a 1-year effective exposure of a high Galactic latitude source, on a plot from Takahashi et al. (2013) compiling the sensitivity of various X and gamma-ray instruments. Such an effective exposure will be reached for broad regions of the sky after ~3 years of operation, given the very large field of view of the instrument (see Fig.7). We see that ASTROGAM will provide an important leap in sensitivity in a wide energy band, from about 300 keV to 100 MeV. At higher energies, ASTROGAM will also provide a new vision of the gamma-ray sky thanks to its unprecedented angular resolution.

2.4 Line sensitivity

Table 2 shows the ASTROGAM 3σ sensitivity for the detection of key gamma-ray lines from pointing observations, together with the sensitivity of the INTEGRAL Spectrometer (SPI). The latter was obtained from the INTEGRAL Observation Time Estimator assuming 5x5 dithering observations. The reported line widths are from SPI observations for the 511 and 847 keV lines (SN 2014J), and from theoretical predictions for the other lines. Noteworthy, the neutron capture line from accreting neutron stars can be significantly redshifted and broadened (FWHM between 10 and 100 keV) depending on the geometry of the mass accretion.

Table 2 ASTROGAM line sensitivity (3σ in 106 s) compared to that of INTEGRAL/SPI.

Table 2

ASTROGAM will achieve a major gain in line sensitivity compared to SPI, e.g., by factors of 26 and 11 for the 847 and 1157 keV lines, respectively, which will enable the detection of several SNe Ia during the 3.5 years of mission duration, as well as a significant number of young, 44Ti-rich SN remnants.

2.5 Polarization response

Both Compton scattering and pair creation partially preserve the polarization information of incident, linearly polarized photons. In a Compton telescope, the polarization signature (polarization angle and fraction) is reflected in the probability distribution of the azimuthal scatter angle. In the pair domain, the polarization information is given by the azimuthal orientation of the electron-positron plane. ASTROGAM will be able to perform unprecedented polarization measurements throughout its bandwidth, thanks to the light mechanical structure of the Si Tracker, which is devoid of any heavy absorber in the detection volume, and to the fine 3D position resolution of the Tracker and the Calorimeter.

Polarized radiation is generally expected from sources in which the phase space distribution of the radiating particle population is restricted by external forces. In ordered magnetic fields this leads to polarized synchrotron or curvature radiation; in beams and jets it can lead to polarized bremsstrahlung or inverse Compton emission; non-isotropic scattering of photons will also polarize the emissions. Polarization therefore allows unique insight into the geometry of high-energy sources. ASTROGAM has a great capability of detecting polarization in the MeV range as well as near 100 MeV employing the characteristic response of Compton scattering and pair creation to polarized gamma photons. Figure 8 shows the polarization detection capability of ASTROGAM.

Figure 8

Figure 8 Left panel ASTROGAM polarization response (polarigramme) in the 0.2 2 MeV range for a 100% polarized, 10 mCrab-like source observed on axis for 106 s. The corresponding modulation is μ100 = 0.34. Right panel Polarigramme measured with INTEGRAL/IBIS (1σ error bars) for the Crab emission between 200 and 800 keV in the off-pulse and bridge phase intervals (Forot et al. 2008). The measured polarization fraction is >88%.

The left panel of Figure 8 shows an example of a simulated polarigramme for a 100% polarized emission from a 10 mCrab-like source. The comparison with the Crab polarization data recorded by the INTEGRAL/IBIS telescope (right panel) clearly illustrates the quantum leap in polarization capability that ASTROGAM will perform. Thus, ASTROGAM will be able to achieve a 3σ Minimum Detectable Polarization (MDP) as low as 1.1% for a Crab-like source in 1 Ms (statistical uncertainties only). After one year of effective exposure of the Galactic center region, the achievable MDP for a 10 mCrab source will be 17%. ASTROGAM will thus enable the study of the polarimetric properties of many black hole systems in the Galaxy. In addition, ASTROGAM will detect the polarization of several dozen extragalactic GRBs and AGNs.