Within the AHEAD consortium a mission concept named ASTENA (Advanced Surveyor of Transient Events and Nuclear Astrophysics) is proposed to address the top-priority themes identified by the AHEAD Science Advisory Group: Gamma-Ray Bursts and Nuclear Astrophysics. GRBs are among the most intriguing phenomena of the Universe, which thanks to their vast luminosities can be used to probe the first billion years of cosmic history, i.e. the era of first stars and black-holes. In spite of great advancements in the GRB astronomy since the BeppoSAX discovery of afterglows, several issues concerning both the prompt emission and the afterglow are still open. Concerning the prompt emission, for example, the emission mechanism of the radiation and the energy dissipation site (internal shocks? external shocks? photosphere?) are far from being understood. What is required is an accurate determination of the photon spectrum from few keV up to tens of MeV, and importantly, a measurement of the polarization of the radiation. The emission of the afterglow has been deeply investigated with Swift in the energy band from 0.5 to 10 keV, showing that an understanding of the origin of the emission mechanism requires spectral information extending to much higher energies, as already suggested by a few studies at < 60 keV (e.g., Kouveliotou et al. 2013, ApJ 779, L1). Landmark progress on this issue therefore requires polarization capabilities and a passband extending well beyond 60 keV.
Concerning nuclear astrophysics, a fundamental issue concerns the origin of the 511 keV positron annihilation line discovered with INTEGRAL/SPI in the Galactic center. According to the INTEGRAL results the emission is diffuse, but the poor imaging capability of INTEGRAL (at the best with a resolution of 12 arcmin with ISGRI) does not allow one to establish whether what appears diffuse is indeed the superposition of the emission from point-like sources, such as micro-quasars. The important role played by micro-quasars as sources of positron annihilation line emission has also been established with INTEGRAL (Siegert et al. 2016, Nature 531, 341). Another open issue in nuclear astrophysics concerns the determination and understanding of the nuclear burning processes in Type-1a supernovae. This requires a study of the intensity and time behavior of the expected lines emitted by the heavy elements produced in supernova explosions. Instrument concept to address the IWG requirements.
With the above considerations in mind, we propose to perform a feasibility study of a configuration of two instruments:
a) a wide field monitor/spectrometer (WFM/S), with a passband from 1 keV to 20 MeV, made of a
suitable number of detection modules, each consisting of an array of long bars of scintillator with very small cross section, and readout from both sides with solid state thin detectors (e.g. Silicon Drift Detectors, SDD). One of the SDD is used as soft X-ray Position Sensitive Detector. A possible crystal material is CsI(Tl), but also other faster crystals such as LSO(Ce) or CeBr3 should be examined. The detector modules are coupled to a light coded mask, for obtaining a GRB localization accuracy of order of ~1 arcmin between 1 and 30/50 keV. The number of modules, equipped with collimators, should be sufficient to achieve the required sensitivity to GRBs. The order of magnitude of the total detection area is 18000 cm2. The modules are slightly misaligned with each other tin order o achieve a wide FOV (> 1 sr).
b) a narrow field telescope (NFT), made of a broad-band Laue lens (50 – 600/700 keV) of a 20 m focal length, based on the exploitation of bent crystals, like those under development in Ferrara (FOV= 3.5 arcmin, angular resolution ≈20”). The NFT is coupled to a high efficiency (>80% above 600 keV) focal plane position sensitive detector, with 3D spatial resolution of at least 300 µm in the (X,Y) plane, fine spectroscopic response (1% @511 keV) and with polarization sensitivity.
With the WFM/S, we expect to accurately determine the energy spectrum of GRB prompt emission in the broadest band ever achieved with a single instrument, to measure the gamma-ray polarization of, at least, the brightest GRBs and to search for electromagnetic counterparts of Gravitational Wave events. In addition, with adequate scintillator bars and fast electronics, the Lorentz invariance for the brightest events can be tested. With the NFT, which is >~100 times more sensitive at a few hundred keV than any other past or planned mission, we can carry out for the first time a long-sought study of the afterglow spectrum of GRBs up to high energies (600/700 keV), including its polarization level. We can also establish, thanks to its high angular resolution (about 20”), whether the 511 keV positron annihilation line is due to the superposition of emission from point-like sources. In addition, we can address many Legacy Science topics mentioned in the Call, such as the origin of the high energy emission from magnetars, the first determination of the spectrum of blazars out to z~8 in between the two Synchrotron and Compton bumps, the determination of the sources that give rise to the gamma-ray diffuse background. For example, one could determine the high-energy cutoff from spectra of relatively bright AGN and study how this depends on the physics of the accretion (e.g. BH mass, Eddington ratio). We emphasize that the unprecedented sensitivity of the NFT and the combination with the WFM/S implies a large discovery space of this configuration. Moreover, such an instrument concept, thanks to the lightweight of the Laue lens and compactness of the wide field instrument, is expected to be within the limits imposed by an ESA Medium Size Mission.
The ASTENA mission, conceived within the AHEAD framework, consists of two coaligned instruments, a broad band Wide Field Monitor/Spectrometer WFM/S and a broad band Narrow Field Telescope (NFT). In the NFT a large geometric area Laue lens (3 m maximum diameter with a 20 m focal length) allows to focus the radiation of the 50 - 700 keV energy pass-band. Differently from other proposed Laue lenses in the past, the NFT is made of optimized thickness bent crystal tiles, made with Silicon (for the lower energy part of the lens pass-band) and Germanium (dedicated to the upper energy threshold). With these assumption we have optimized the NFT Field of View (FoV) to 3.5 arcmin with the angular resolution of 20”. The Laue lens is coupled with a high efficiency (>80% above 600 keV) focal plane position sensitive detector, with 3D spatial resolution of at least 300 µm in the (X,Y) plane and fine spectroscopic response (1% @511 keV) and with polarization sensitivity. In this SPIE contribution we will discuss the NFI geometry simulated with the MEGAlib toolkit and we will discuss its performances by simulating broad band and narrow energy typical sources, giving finally the instrument performances.
e-ASTROGAM is a concept for a breakthrough observatory space mission carrying a γ-ray telescope dedicated to the study of the non-thermal Universe in the photon energy range from 0.15 MeV to 3 GeV. The lower energy limit can be pushed down to energies as low as 30 keV for gamma-ray burst detection with the calorimeter. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with remarkable polarimetric capability. Thanks to its performance in the MeV–GeV domain, substantially improving its predecessors, e-ASTROGAM will open a new window on the non-thermal Universe, making pioneering observations of the most powerful Galactic and extragalactic sources, elucidating the nature of their relativistic outflows and their effects on the surroundings. With a line sensitivity in the MeV energy range one to two orders of magnitude better than previous and current generation instruments, e-ASTROGAM will determine the origin of key isotopes fundamental for the understanding of supernova explosion and the chemical evolution of our Galaxy. The mission will be a major player of the multiwavelength, multimessenger time-domain astronomy of the 2030s, and provide unique data of significant interest to a broad astronomical community, complementary to powerful observatories such as LISA, LIGO, Virgo, KAGRA, the Einstein Telescope and the Cosmic Explorer, IceCube-Gen2 and KM3NeT, SKA, ALMA, JWST, E-ELT, LSST, Athena, and the Cherenkov Telescope Array.
The development of new focusing optics based on wide band Laue lenses operating from ~60 keV up to several hundred
keV is particularly challenging. This type of hard X-ray or gamma ray optics requires a high performance focal plane
detector in order to exploit to the best their intrinsic capabilities. We describe a three dimensional (3D) position sensitive
detector prototype suitable as the basic module for a high efficiency Laue lens focal plane detector. This detector
configuration is currently under study for use in a balloon payload dedicated to performing a high significance
measurement of the polarization status of the Crab between 100 and 500 keV. The prototype is made by packing 8 linear
modules, each composed of one basic sensitive unit bonded onto a thin supporting ceramic layer. Each unit is a drift strip
detector based on a CZT crystal, irradiated transversally to the electric field direction. The anode is segmented into 8
detection cells, each comprising one collecting strip and 8 surrounding drift strips. The drift strips are biased by a voltage
divider. The cathode is divided into 4 horizontal strips for the reconstruction of the Z interaction position. The detector
readout electronics is based on RENA-3 ASIC and the data handling system uses a custom electronics based on FPGA to
provide the ASIC setting, the event handling logic, and the data acquisition. This paper mainly describes the components
and the status of the undergoing activities for the construction of the proposed 3D CZT prototype and shows the results
of the electronics tests.
Today it is widely recognised that a measurement of the polarization status of cosmic sources high energy emission is a
key observational parameter to understand the active production mechanism and its geometry. Therefore new
instrumentation operating in the hard X/soft γ rays energy range should be optimized also for this type of measurement.
In this framework, we present the concept of a small high-performance spectrometer designed for polarimetry between
100 and 1000 keV suitable as a stratospheric balloon-borne payload dedicated to perform an accurate and reliable
measurement of the polarization status of the Crab pulsar, i.e. the polarization level and direction. The detector with 3D
spatial resolution is based on a CZT spectrometer in a highly segmented configuration designed to operate as a high
performance scattering polarimeter. We discuss different configurations based on recent development results and
possible improvements currently under study. Furthermore we describe a possible baseline design of the payload, which
can be also seen as a pathfinder for a high performance focal plane detector in new hard X and soft gamma ray focussing
telescopes and/or advanced Compton instruments. Finally we present preliminary data from Montecarlo undergoing
studies to determine the best trade-off between polarimetric performance and detector design complexity.
DUAL will study the origin and evolution of the elements and explores new frontiers of physics: extreme energies that
drive powerful stellar explosions and accelerate particles to macroscopic energies; extreme densities that modify the laws
of physics around the most compact objects known; and extreme fields that influence matter in a way that is unknown on
Earth. The variability of these extreme objects requires continuous all-sky coverage, while detailed study demands an
improvement in sensitivity over previous technologies by at least an order of magnitude.
The DUAL payload is composed of an All-Sky Compton Imager (ASCI), and two optical modules, the Laue-Lens Optic
(LLO) and the Coded-Mask Optic (CMO). The ASCI serves dual roles simultaneously, both as an optimal focal-plane
sensor for deep observations with the optical modules and as a sensitive true all-sky telescope in its own right for all-sky
surveys and monitoring. While the optical modules are located on the main satellite, the All-Sky Compton Imager is
situated on a deployable structure at a distance of 30 m from the satellite. This configuration not only permits to maintain
the less massive payload at the focal distance, it also greatly reduces the spacecraft-induced detector background, and,
above all it provides ASCI with a continuous all-sky exposure.
We report on the development of a 3D position sensitive prototype suitable as focal plane detector for Laue lens
telescope. The basic sensitive unit is a drift strip detector based on a CZT crystal, (~19×8 mm<sup>2</sup> area, 2.4 mm thick),
irradiated transversally to the electric field direction. The anode side is segmented in 64 strips, that divide the crystal in 8
independent sensor (pixel), each composed by one collecting strip and 7 (one in common) adjacent drift strips. The drift
strips are biased by a voltage divider, whereas the anode strips are held at ground. Furthermore, the cathode is divided in
4 horizontal strips for the reconstruction of the third interaction position coordinate. The 3D prototype will be made by
packing 8 linear modules, each composed by one basic sensitive unit, bonded on a ceramic layer. The linear modules
readout is provided by a custom front end electronics implementing a set of three RENA-3 for a total of 128 channels.
The front-end electronics and the operating logics (in particular coincidence logics for polarisation measurements) are
handled by a versatile and modular multi-parametric back end electronics developed using FPGA technology.
The importance of hard X-ray astronomy (>10 keV) is now widely recognized. Recently both ESA and NASA have
indicated in their guidelines for the progress of X- and γ-ray astronomy in the next decade the development of new
instrumentation working in the energy range from the keV to the MeV region, where important scientific issues are still
open, exploiting high sensitivity for spectroscopic imaging and polarimetry observations. The development of new
concentrating (e.g. multilayer mirror) telescopes for hard X-rays (10 -100 keV) and focusing instruments based on Laue
lenses operating from ~60 keV up to a few MeV is particularly challenging. We describe the design of a threedimensional
(3D) depth-sensing position sensitive device suitable for use as the basic unit of a high efficiency focal
plane detector for a Laue lens telescope. The sensitive unit is a drift strip detector based on a CZT crystal, (10×10 mm<sup>2</sup>
area, 2.5 mm thick), irradiated transversally to the electric field direction. The anode is segmented into 4 detection cells,
each comprising one collecting strip and 8 drift strips. The drift strips are biased by a voltage divider, whereas the anode
strips are held at 0 V. The cathode is divided in 4 horizontal strips for the reconstruction of the Z interaction position.
The 3D prototype will be made by packing 8 linear modules, each composed of 2 basic sensitive units, bonded onto a
ceramic layer together with the readout electronics.
The science drivers for a new generation soft gamma-ray mission are naturally focused on the detailed study of
the acceleration mechanisms in a variety of cosmic sources. Through the development of high energy optics in the
energy energy range 0.05-1 MeV it will be possible to achieve a sensitivity about two orders of magnitude better
than the currently operating gamma-ray telescopes. This will open a window for deep studies of many classes of
sources: from Galactic X-ray binaries to magnetars, from supernova remnants to Galaxy clusters, from AGNs
(Seyfert, blazars, QSO) to the determination of the origin of the hard X-/gamma-ray cosmic background, from
the study of antimatter to that of the dark matter. In order to achieve the needed performance, a detector with
mm spatial resolution and very high peak efficiency is needed. The instrumental characteristics of this device
could eventually allow to detect polarization in a number of objects including pulsars, GRBs and bright AGNs. In
this work we focus on the characteristics of the focal plane detector, based on CZT or CdTe semiconductor sensors
arranged in multiple planes and viewed by a side detector to enhance gamma-ray absorption in the Compton
regime. We report the preliminary results of an optimization study based on simulations and laboratory tests,
as prosecution of the former design studies of the GRI mission which constitute the heritage of this activity.
The energy range above 50 keV is important for the study of many open problems in high energy astrophysics such as,
non thermal mechanisms in SNR, the study of the high energy cut-offs in AGN spectra, and the detection of nuclear and
annihilation lines. In the framework of the definition of a new mission concept for hard X and soft gamma ray (GRI-
Gamma Ray Imager) for the next decade, the use of Laue lenses with broad energy band-passes from 100 to 1000 keV is
under study. This kind of instruments will be used for deep study the hard X-ray continuum of celestial sources. This
new telescope will require focal plane detectors with high detection efficiency over the entire operative range, an energy
resolution of few keV at 500 keV and a sensitivity to linear polarization. We describe a possible configuration for the
focal plane detector based on CdTe/CZT pixelated layers stacked together to achieve the required detection efficiency at
high energy. Each layer can either operate as a separate position sensitive detector and a polarimeter or together with
other layers in order to increase the overall full energy efficiency. We report on the current state of art in high Z
spectrometers development and on some activities undergoing. Furthermore we describe the proposed focal plane option
with the required resources and an analytical summary of the achievable performance in terms of efficiency and
To date polarimetry in astrophysics in the energy domain from hard X-rays up to soft gamma-rays has not been pursued due to the difficulties involved in obtaining sufficient sensitivity. Indeed for those few instruments which are capable of performing this type of measurement, polarimetry itself plays a secondary role in the mission schedule, as the efficiencies and polarimetric Q factors are relatively limited. In order to perform efficient polarimetric measurements for hard X and soft gamma-ray sources, with an instrument of relatively robust and simple design, a CdTe based telescope (CIPHER: Coded Imager and Polarimeter for High Energy Radiation) is under study. This instrument is based on a thick (10 mm) CdTe position sensitive spectrometer comprising four modules of 32×32 individual pixels, each with a surface area of 2×2 mm<sup>2</sup>. The polarimetric performance and design optimisation of the CIPHER detection surface have been studied by use of a Monte Carlo code based on GEANT4 modules. Simulations show that we can achieve a Q factor better than 0.5 and Compton double event efficiency better than 11% between 100 keV and 1 MeV. Herein we will present and discuss the general problems that affect polarimetric measurements in space such as the inclination of the source with respect to the telescope optical axis and space background radiation. Q factor calculations for several beam inclinations as well as for space background together with simulated astronomical sources will be presented and discussed.
The polarisation of astrophysical source emissions in the energy range from a few tens of keV up to the MeV region is an almost unexplored field of high energy astrophysics. In order to improve the capabilities of performing polarimetric studies of hard X and soft gamma ray sources through Compton polarimetry, a CdTe based telescope (CIPHER: Coded Imager and Polarimeter for High Energy Radiation) is under study. This instrument is based on a thick (10 mm) CdTe position sensitive spectrometer made of four modules of 32x32 individual pixels, each with a surface area of 2x2 mm<SUP>2</SUP>, corresponding to about 160 cm<SUP>2</SUP> active detection area. This detector, due to its intrinsic geometry, could allow efficient polarimetric measurements to be made between 100 keV and 1 MeV. In order to predict the polarimetric performance and to optimise the design and concept of the CIPHER detection plane, a Monte Carlo code based on GEANT4 library modules was developed to simulate the detector behaviour under a polarised photon flux. The Compton double event efficiency, as well bi-dimensional double event distribution maps and the corresponding polarimetric modulation factor will be presented and discussed. Modulation (Q) factors better than 0.30 and double event total efficiencies over 10 % for an energy range from 100 keV to 1 MeV have been obtained.
Even though it is recognized that the study of polarization from cosmic high-energy sources can give very important information about the nature of the emission mechanism, to date very few measurements have been attempted. For several years we have proposed the use of a thick CdTe array as a position sensitive spectrometer for hard X- and soft gamma-ray astronomy, a design which is also efficient for use as a polarimeter at energies above approximately 100 keV. Herein we describe the preliminary results of our study of a polarimeter based on 4096 CdTe microcrystals that we would like to develop for a high altitude balloon experiment. We present the telescope concept with a description of each subsystem together with some results on activities devoted to the optimization of the CdTe detector units' response. Furthermore we give an evaluation of the telescope performance in terms of achievable spectroscopic and polarimetric performance. In particular we will show the results of Monte Carlo simulations developed to evaluate the efficiency of our detector as a hard X ray polarimeter.