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.