THE SCIENTIFIC INSTRUMENT

1. Measurement principle and payload overview

Interactions of photons with matter in the e-ASTROGAM energy range is dominated by Compton scattering from 0.3 MeV up to about 15 MeV in silicon, and by electron-positron pair production in the field of a target nucleus at higher energies. e-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 the 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 angles depend 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). Photon tracks are shown in black, and electron and/or positron tracks in red.

Pair events produce two main tracks from the created electron and positron. Tracking of the initial opening angle and of the plane spanned by the electron and positron tracks enables direct back-projection of the source position. Multiple scattering of the pair in the tracker material (or any intervening passive materials) leads to broadening of the tracks and limits the angular resolution. The nuclear recoil taking up an unmeasured momentum results in an additional small uncertainty. The energy of the gamma-ray is measured using the Calorimeter and information on the electron and positron multiple scattering in the Tracker. Polarization information in the pair domain is given by the azimuthal orientation of the electron-positron plane.
The e-ASTROGAM payload is shown in Figure 2. It consists of three main detectors:

  • A Silicon Tracker in which the cosmic gamma-rays undergo a rst Compton scattering or a pair conversion; it is 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 particle; it is made of an array of small scintillation crystals (33,856 CsI (Tl) bars of 5x5x80 mm3) read out by silicon drift photodetectors to achieve the required energy resolution (4.5% at 662 keV);
  • An Anticoincidence system(AC) composed of a standard AC shielding and a Time of Flight, located below the instrument, to veto the particle background; it is designed with plastic scintillator tiles with a detection eciency exceeding 99.99%.

The payload is completed by a Payload Data Handling Unit (PDHU) and a Power Supply Unit (PSU) located below the Calorimeter inside the platform together with the back-end electronics (BEE). 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 (including maturity margins) are 999 kg and 1340 W, respectively.
Especially for the Compton mode at low energies, but also more broadly over the entire energy range covered by e-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.

Figure 2

Figure 2 Overview of the e-ASTROGAM payload showing the silicon Tracker, the Calorimeter and the Anticoincidence system.

1.1 Silicon Tracker

The Si Tracker is the heart of the e-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 with the AGILE and Fermi missions. The e-ASTROGAM Tracker needs double sided strip detectors (DSSDs) to work also as a Compton telescope.
The essential characteristics of the e-ASTROGAM Tracker are:

  • its light mechanical structure minimizing the amount of passive material within the detection volume to enable the tracking of low-energy Compton electrons and electron-positron pairs, and improve the point spread function in both the Compton and pair domains by reducing the effect of multiple Coulomb scattering;
  • its fine spatial resolution of less than 40 µm (< 1=6 of the microstrip pitch) obtained by analog readout of the signals (as in the AGILE Tracker);
  • its charge readout with an excellent spectral resolution of ∼ 6 keV FWHM (noise level in the baseline configuration) obtained with an ultra low-noise FEE, in order to accurately measure low-energy deposits produced by Compton events; the energy threshold is 15 keV.
  • The Si Tracker comprises 5600 DSSDs arranged in 56 layers (100 DSSDs per layer). It is divided in four towers of 5 · 5 DSSDs. The total detection area amounts to 9025 cm 2 and the total Si thickness to 2.8 cm, which corresponds to 0.3 radiation length on axis, and a probability of a Compton interaction at 1 MeV of 40%. Such a stacking of relatively thin detectors enables an ecient tracking of the electrons and positrons produced by pair conversion, and of the recoil electrons produced by Compton scattering. The DSSD signals are readout by 860,160 independent, low-power electronics channels with self-triggering capability.

    1.2 Calorimeter

    The e-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 33,856 parallelepiped bars of CsI(Tl) of 8 cm length and 5 · 5 mm2 cross section, read out by silicon drift detectors (SDDs) at both ends, arranged in an array of 529 (= 23 · 23) elementary modules comprising each 64 crystals (see Figure 3). The Calorimeter thickness - 8 cm of CsI(Tl) - makes it a 4.3 radiation-length detector having an absorption probability of a 1-MeV photon on axis of 88%.
    The Calorimeter detection principle and architecture are based on the heritage of the space instruments INTEGRAL/PICsIT, AGILE/MCAL and Fermi/LAT, as well as on the particle physics experiment LHC/ALICE at CERN. However, the e-ASTROGAM calorimeter features two major improvements with respect to the previous instruments:

  • the energy resolution is optimized to a FWHM of 4.5% at 662 keV (scaling with the inverse of the square root of the energy) by the use of low-noise SDDs for the readout of the scintillation signals, combined with an appropriate ultra low-noise FEE;
  • the spatial resolution is improved by measuring the depth of interaction in the detector from a suitable weighting function of the recorded scintillation signals at both ends; the position resolution along the CsI(Tl) bars is ∼ 5 mm FWHM, i.e. comparable to the resolution in the X-Y plane given by the crystal cross section (5 · 5 mm2). Accurately measuring the 3D position and deposited energy of each interaction is essential for a proper reconstruction of the Compton events.
  • The simultaneous data set provided by the Silicon Tracker, the Calorimeter and the Anticoincidence system constitutes the basis for the gamma-ray detection. However, the Calorimeter will also have the capability to trigger the gamma-ray event processing independently of the Tracker, in order to search for fast transient events such as GRBs and terrestrial gamma-ray flashes.

    Figure 3

    Figure 3 Overview of the Calorimeter and of one of its 529 (= 23 · 23) basic modules comprising 64 CsI(Tl) crystals.

    1.3 Anticoincidence system

    The third main detector of the e-ASTROGAM payload consists of an Anticoincidence system composed of two main parts: (1) a standard Anticoincidence, named Upper-AC, made of segmented panels of plastic scintillators covering the top and four lateral sides of the instrument, requiring a total active area of about 5.2 m2, and (2) a Time of Flight (ToF), aimed at rejecting the particle background produced by the platform. The Upper-AC detector is segmented in 33 plastic tiles (6 tiles per lateral side and 9 tiles for the top). All scintillator tiles are coupled to silicon photomultipliers (SiPM) by optical fibers. The architecture of the Upper-AC detector is fully derived from the successful design of the AGILE and Fermi/LAT AC systems. In particular, their segmentation has proven successful at limiting the "backsplash" self-veto, therefore the dead time of the instrument. The Upper-AC particle background rejection is designed to achieve a relativistic charged particle detection ineciency lower than 10−4, a standard value already realized in current space experiments. In addition to the panel segmentation, providing coarse information on the part of the detector that has been hit, we are also considering the possibility of even ner position resolution based on the analysis of the relative light output of multiple fibers.
    In the baseline design, the Upper-AC system covers the entire instrument from five sides, leaving open the bottom for design considerations of cabling to the S/C bus, the layout of heat pipes, etc. The bottom side of the instrument is protected by the ToF to discriminate the particles coming out from the instruments wrt the particles entering in the instrument. The ToF is composed by two scintillator layers separated by 50 cm. The required timing resolution is of 300 ps. The readout will be performed by SiPM connected with Time Digital Converter (TDC). The ToF will be based on technologies well proven in space (AMS/PAMELA satellites).

    1.4 Data Handling and Power Supply

    The e-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) scienti c 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 are: (i) BEE 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 to meet the telemetry requirements; (iii) scientific data compression; (iv) formatting of the compressed data into telemetry packets. The hearth of the PDHU architecture is based on a powerful Digital Signal Processor (DSP) running the payload on-board software.
    The PSU is in charge of generating the required payload voltages with high DC/DC conversion efficiency and distributing them to the other sub-systems.

    1.5 Trigger logic and data flow architecture

    The e-ASTROGAM on-board scientific data processing is composed of two main trigger pipelines, the gamma-ray acquisition mode and the Calorimeter burst search. Both are based on the experience of the AGILE and Fermi missions. The simultaneous data sets provided by the silicon Tracker, the Calorimeter and the AC constitute the basis for the gamma-ray detection and processing. The gamma-rays trigger logic is structured on two main levels: Level-1 (fast: 5-10 μs logic, hardware); and Level-2 (asynchronous, 50 μs processing, software). Figure 4 shows the expected data rates at the input of the Level-1 and at the output of the Level-1 and Level-2 trigger stages.
    Level-1 is a hardware trigger logic with fast response implemented in the silicon Tracker BEE providing a preliminary discrimination between Compton and pair-producing photon events and a rst cut of background events. Discrimination criteria based on the hit multiplicity in the Tracker and in the Calorimeter can provide optimal algorithms to identify Compton events. The Level-1 trigger configuration is de ned to save the largest possible number of potential Compton events.
    Level-2 is a software trigger stage carried out by the PDHU and aimed, at further reducing the residual particle and photon background of the pair data set and at finalizing the selection of the Compton events. The hearth of the Level-2 trigger stage consists of the track reconstruction of the candidate pair events implemented with Kalman Filter techniques. The Level-2 trigger is a full asynchronous processing stage and does not increase the dead time of the instrument. At the end, the Compton events and the pair events surviving the Level-2 trigger are collected in dedicated telemetry packets and sent to the ground.
    The Calorimeter burst search is a software algorithm implemented by the PDHU. The burst search is based on the integration and processing of a proper set of rate meters measuring the trend of the background and foreground counting rates. Since the expected impulsive signals (gamma-ray bursts and terrestrial gamma-ray flashes) are strongly energy and timescale dependent, the rate meters are integrated on different timescales (in the range 100 μs - 10 s) and energy ranges (in the overall range 30 keV - 200 MeV).

    Figure 4

    Figure 4 Expected data flow of the on-board e-ASTROGAM gamma-ray data acquisition system.

    2. Performance assessment

    The scienti c performances of the e-ASTROGAM instrument were evaluated by detailed numerical simulations with the 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 Xray and gamma-ray telescopes on ground and in space, such as COMPTEL, NCT, and NuSTAR. BoGEMMS (Bologna Geant4 Multi-Mission Simulator) is a software for simulation of payload of Xand gamma-ray missions, which has been developed at the INAF/IASF Bologna. It has already been applied to several hard X-ray/gamma-ray instruments and mission projects, including Simbol-X, NHXM, Gamma-Light, AGILE, and GAMMA-400. Both software packages exploit the Geant4 toolkit to model the geometrical and physical parameters of the detectors and simulate the interactions of photons and particles in the instrument. The numerical mass model of e-ASTROGAM used to simulate the performance of the instrument is shown in Figure 5. 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.

    Figure 5

    Figure 5 Geant4/MEGAlib mass model of the e-ASTROGAM telescope, with a simulated pair event produced by a 30-MeV photon.

    2.1 Background model

    For best environmental conditions, e-ASTROGAM should be launched into a quasi-equatorial (inclination i < 2:5°) low-Earth orbit (LEO) at a typical altitude of 550 km. The background environment in such an orbit is now well-known (Figure 6), thanks to the Beppo-SAX mission, which measured the radiation environment on a low-inclination (i ∼ 4°), 500 - 600 km altitude orbit almost uninterruptedly during 1996 - 2002 and the on-going AGILE mission, which scans the gamma-ray sky since 2007 from a quasi-equatorial (i ∼ 2:5°) LEO at an average altitude of 535 km. The dominant sources of background for the e-ASTROGAM telescope in the MeV domain are the cosmic di use gamma-ray background, the atmospheric gamma-ray emission, the reactions induced by albedo neutrons, and the background produced by the radioactivity of the satellite materials activated by fast protons and alpha particles. 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 spacecraft, as well as by the cosmic diffuse radiation and the atmospheric gamma-ray emission.

    Figure 6

    Figure 6 Background environment of e-ASTROGAM on its equatorial LEO (550 km). The satellite will be exposed to Galactic cosmic rays (mainly protons and electrons) modulated by the geomagnetic field, semi-trapped secondary protons and leptons, as well as to albedo neutrons and atmospheric gamma rays. The cosmic diffuse X- and gamma-ray radiation (in green) is the dominant background component below a few hundred keV, but it is also a fundamental science topic for e-ASTROGAM above a few MeV

    2.2 Angular and spectral resolutions

    e-ASTROGAM will achieve an unprecedented angular resolution both in the MeV domain and above a few hundreds of MeV, i.e. improving the angular resolution of the CGRO/COMPTEL telescope and that of the Fermi/LAT instrument by a factor of ∼ 4 at 1 MeV and 1 GeV, respectively.
    In the pair production domain, the PSF improvement over Fermi/LAT is due to (i) the absence of heavy converters in the Tracker, (ii) the light mechanical structure of this detector minimizing the amount of passive material within the detection volume and thus enabling a better tracking of the secondary electrons and positrons, and (iii) the analog readout of the DSSD signals allowing a fine spatial resolution of about 40 μm (∼ 1/6 of the microstrip pitch). In the Compton domain, thanks to the fine spatial and spectral resolutions of both the Tracker and the Calorimeter, the e-ASTROGAM angular resolution will be close to the physical limit induced by the Doppler broadening due to the velocity of the target atomic electrons.
    e-ASTROGAM will also significantly improve the energy resolution with respect to COMPTEL, e.g. by a factor of ∼ 3.2 at 1 MeV, where it will reach a 1 σ resolution of ΔΕ/Ε = 1.3%(Figure 7). In the pair production domain above 30 MeV, the simulated spectral resolution is within 20-30%.

    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 40° to 50° off-axis angle and then degrades for larger incident angles. For example, the field of view at 1 MeV amounts to 46° half width at half maximum (HWHM), with a fraction-of-sky coverage in zenith pointing mode of 23%, corresponding to Ω = 2:9 sr.

    Figure 7

    Figure 7 Left panel { e-ASTROGAM on-axis angular resolution compared to that of COMPTEL and Fermi/LAT. In the Compton domain, the presented performance of e-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 for a 30° point source. The Fermi/LAT PSF is from the Pass 8 analysis (release 2 version 6) and corresponds to the FRONT and PSF event type. Right panel { 1 δ energy resolution of COMPTEL and e-ASTROGAM in the Compton domain after event reconstruction and selection on the ARM.

    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. With the e-ASTROGAM characteristics (size, Si plane spacing, overall geometry), the field of view is found to be > 2:5 sr above 10 MeV.

    2.3 Effective area and continuum sensitivity

    Improving the sensitivity in the medium-energy gamma-ray domain (1-100 MeV) by one to two orders of magnitude compared to previous missions is the main requirement for the proposed e-ASTROGAM mission. Such a performance will open an entirely new window for discoveries in the high-energy Universe. Tables 2 and 3 present the simulated effective area and continuum sensitivity in the Compton and pair-production domains. The sensitivity below 10 MeV is largely independent of the source location (inner galaxy vs. high latitude), because the diffuse gamma-ray background is not a major background component in the Compton domain.
    Figure 1 shows the e-ASTROGAM continuum sensitivity for a 1-year e ective exposure of a high Galactic latitude source. Such an e ective exposure will be reached for broad regions of the sky after 3 years of operation, given the very large eld of view of the instrument. We see that e-ASTROGAM would provide an important leap in sensitivity over a wide energy band, from about 200 keV to 100 MeV. At higher energies, e-ASTROGAM would also provide a new vision of the gamma-ray sky thanks to its unprecedented angular resolution, which would reduce the source confusion that plagues the current Fermi-LAT and AGILE images near the Galactic plane (see, e.g., the 3FGL catalog).

    2.4 Line sensitivity

    Table 4 shows the e-ASTROGAM 3δ 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 (OTE) assuming 5 × 5 dithering observations. The reported line widths are from SPI observations of 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. We see that e-ASTROGAM will achieve a major gain in sensitivity compared to SPI for all gammaray lines, the most signi cant improvement being for the 847 keV line from Type Ia SNe. With the predicted line sensitivity, e-ASTROGAM will also (i) provide a much better map of the 511 keV radiation from positron annihilation in the inner Galaxy, (ii) uncover ∼10 young, 44Ti-rich SN remnants in the Galaxy and thus provide new insight on the explosion mechanism of core-collapse SNe (iii) detect for the first time the expected line from 22Na decay in novae hosted by ONe white dwarfs, (iv) provide a new constraint on the nuclear equation of state of neutron stars by detectingthe predicted redshifted 2.2 MeV line from Scorpius X-1, and (iv) measure the energy density of low-energy cosmic rays in the inner Galaxy to better understand the role of these particles in the Galactic ecosystem.

    Table 2 e-ASTROGAM performance in the Compton domain simulated with MEGAlib v2.26.01. The 3δ continuum sensitivity is for the detection of a point source on axis after an observation time T obs = 106 s.

    Table 2

    (a) Source spectrum is an E-2 power-law in the range ΔΕ.
    (b) ARM radius. Note that the best sensitivity results are obtained for a selection on the ARM radius slightly larger than the optimal ARM.
    (c) Effective area after event selection optimized for sensitivity.
    (d) Total background including the atmospheric γ-ray background, the cosmic γ-ray background, the activation induced by primary and semi-trapped particles (mainly protons), and the prompt reactions from primary (i.e. cosmic-ray) protons, as well as from secondary protons and leptons (electrons and positrons).

    Table 3 e-ASTROGAM performance in the pair-production domain simulated with BoGEMMS v2.0.1, together with Kalman v1.5.0 and Trigger v1.0.0. All results are for a 30°off-axis source and for Tobs = 106s.

    Table 3

    (a) Source spectrum is an E-2 power-law in the range ΔΕ.
    (b) Point Spread Function (68% containment radius) derived from a single King function fit of the angular distribution.
    (c) Effective area after event selection.
    (d) The background for the Galactic Center is assumed to be 3 times larger than that of the Inner Galaxy.

    Table 4 e-ASTROGAM line sensitivity (3δ in 106s) compared to that of INTEGRAL/SPI.

    Table 4

    2.5 Polarization response

    Both Compton scattering and pair creation partially preserve the linear polarization information of incident photons. In a Compton telescope, the polarization signature is reflected in the probability distribution of the azimuthal scatter angle. In the pair domain, the polarization information is given by the distribution of azimuthal orientation of the electron-positron plane. e-ASTROGAM will be able to perform unprecedented polarization measurements thanks to the fine 3D position resolution of both the Si Tracker and the Calorimeter, as well as the light mechanical structure of the Tracker, which is devoid of any heavy absorber in the detection volume.
    The left panel of Figure 8 shows an example of a polarigramme in the 0.2 - 2 MeV range (i.e. in the Compton domain), simulated with MEGAlib. The calculations assume a 100% polarized emission from a 10 mCrab-like source observed on axis. From the obtained modulation (μ100 = 0:36), we find that at low energies (0.2 - 2 MeV), e-ASTROGAM will be able to achieve a Minimum Detectable Polarization (MDP) at the 99% confidence level as low as 0.7% for a Crab-like source in 1 Ms (statistical uncertainties only). After one year of effective exposure of the Galactic center region, the achievable MDP99 for a 10 mCrab source will be 10%. With such a performance, e-ASTROGAM will be able to study the polarimetric properties of many pulsars, magnetars, and black hole systems in the Galaxy.
    The right panel of Figure 8 shows the number of GRBs detectable by e-ASTROGAM as a functionof MDP99 in the 150-300 keV band. The total number of GRBs detected by e-ASTROGAM will be ∼600 in 3 years of nominal mission lifetime. Here, the GRB emission spectrum has been approximated by a typical Band function with α= -1:1, β= -2:3, and Epeak = 0:3 MeV, and the response of e-ASTROGAM to linearly polarized GRBs has been simulated at several off-axis angles in the range [0°; 90°]. The number of GRBs with polarization measurable with e-ASTROGAM has then been estimated using the Fourth BATSE GRB Catalog. We see in Figure 8 that e-ASTROGAM should be able to detect a polarization fraction of 20% in about 42 GRBs per year, and a polarization fraction of 10% in ∼16 GRBs per year. This polarization information, combined with spectroscopy over a wide energy band, will provide unambiguous answers to fundamental questions on the sources of the GRB highly relativistic jets and the mechanisms of energy dissipation and high-energy photon emission in these extreme astrophysical phenomena.

    Figure 8

    Figure 8 Left panel - e-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 = 0.36. Right panel - Cumulative number of GRBs to be detected by e-ASTROGAM as a function of the minimum detectable polarization at the 99% confidence level.