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IXPE scientific payload comprises of three telescopes, each composed of a mirror and a photoelectric polarimeter based on the Gas Pixel Detector design. The three focal plane detectors, together with the unit which interfaces them to the spacecraft, are named IXPE Instrument and they will be built and calibrated in Italy; in this proceeding, we will present how IXPE Instrument will be calibrated, both on-ground and in-flight. The Instrument Calibration Equipment is being finalized at INAF-IAPS in Rome (Italy) to produce both polarized and unpolarized radiation, with a precise knowledge of direction, position, energy and polarization state of the incident beam. In flight, a set of four calibration sources based on radioactive material and mounted on a filter and calibration wheel will allow for the periodic calibration of all of the three IXPE focal plane detectors independently. A highly polarized source and an unpolarized one will be used to monitor the response to polarization; the remaining two will be used to calibrate the gain through the entire lifetime of the mission., 11 pages, 8 figures

Scheduled to launch in late 2021 the Imaging X-ray Polarimetry Explorer (IXPE) is a Small Explorer Mission designed to open up a new window of investigation -- X-ray polarimetry. The IXPE observatory features 3 identical telescope each consisting of a mirror module assembly with a polarization-sensitive imaging x-ray detector at its focus. An extending beam, deployed on orbit provides the necessary 4 m focal length. The payload sits atop a 3-axis stabilized spacecraft which among other things provides power, attitude determination and control, commanding, and telemetry to the ground. During its 2-year baseline mission, IXPE will conduct precise polarimetry for samples of multiple categories of x-ray sources, with follow-on observations of selected targets. IXPE is a partnership between NASA and the Italian Space Agency (ASI).

While X-ray Spectroscopy, Timing and Imaging have improved verymuch since 1962, when the first astronomical non-solar source was discovered, especially with the launch of Newton/X-ray Multi-Mirror Mission, Rossi/X-ray Timing Explorer and Chandra/Advanced X-ray Astrophysics Facility, the progress of X-ray polarimetry has been meager. This is in part due to the lack of sensitive polarization detectors, in part due to the fate of approved missions and in part because the celestial X-ray sources appeared less polarized than expected. Only one positive measurement has been available until now. Indeed the eight Orbiting Solar Observatory measured the polarization of the Crab nebula in the 70s. The advent of techniques of microelectronics allowed for designing a detector based on the photoelectric effect in gas in an energy range where the optics are efficient in focusing X-rays. Herewe describe the Instrument, which is the major contribution of the Italian collaboration to the SmallExplorer mission called IXPE, the Imaging X-ray Polarimetry Explorer, which will be flown in late 2021. The instrument, is composed of three Detector Units, based on this technique, and a Detector Service Unit. Three Mirror Modules provided by Marshall Space Flight Center focus X-rays onto the detectors. In the following we will show the technological choices, their scientific motivation and the results from the calibration of the Instrument. IXPE will perform imaging, timing and energy resolved polarimetry in the 2-8 keV energy band opening this window of X-ray astronomy to tens of celestial sources of almost all classes., 25 pages, 12 figures, 5 tables (accepted for publication in Astronomical Journal)

IXPE (Imaging X-ray Polarimetry Explorer) is a NASA SMEX in a partnership with ASI. The focal plane Detector Units (DUs) and the Detector Service Unit (DSU) were developed by the Italian research Institutes INAF-IAPS and INFN and were manufactured by OHB-I. IXPE will investigate X-ray polarimetry in the 2-8 keV energy band. The payload comprises three identical telescopes, each composed of a mirror and a detector unit with an X-ray polarimeter based on the Gas Pixel Detector (GPD). A stray-light collimator (SLC) is mounted on the top of the DU to shield the GPD from background X-rays not coming from the optics. At the bottom of the SLC, an ions-UV filter is mounted to reduce the thermal load and to prevent ions and UV from entering the DU. The ions-UV filters consist mainly of 1 um LUXFilm (based on polyimide). During on-ground calibration activities of the IXPE DUs, X-ray transparency of DU-FM ions-UV filters was measured with monochromatic X-ray at 2.7 keV and 6.4 keV at INAF-IAPS.

The NASA/ASI Imaging X-ray Polarimetry Explorer, which will be launched in 2021, will be the first instrument to perform spatially resolved X-ray polarimetry on several astronomical sources in the 2-8 keV energy band. These measurements are made possible owing to the use of a gas pixel detector (GPD) at the focus of three X-ray telescopes. The GPD allows simultaneous measurements of the interaction point, energy, arrival time, and polarization angle of detected X-ray photons. The increase in sensitivity, achieved 40 years ago, for imaging and spectroscopy with the Einstein satellite will thus be extended to X-ray polarimetry for the first time. The characteristics of gas multiplication detectors are subject to changes over time. Because the GPD is a novel instrument, it is particularly important to verify its performance and stability during its mission lifetime. For this purpose, the spacecraft hosts a filter and calibration set (FCS), which includes both polarized and unpolarized calibration sources for performing in-flight calibration of the instruments. In this study, we present the design of the flight models of the FCS and the first measurements obtained using silicon drift detectors and CCD cameras, as well as those obtained in thermal vacuum with the flight units of the GPD. We show that the calibration sources successfully assess and verify the functionality of the GPD and validate its scientific results in orbit; this improves our knowledge of the behavior of these detectors in X-ray polarimetry., Comment: 22 pages, 16 figures, published on Journal of Astronomical Telescopes, Instruments, and Systems

SPIE grants to authors (and their employers) of papers, posters, and presentation recordings published in SPIE Proceedings or SPIE Journals on the SPIE Digital Library the right to post an author-prepared version or an official version (preferred version) of the published paper, poster, or presentation recording on an internal or external repository controlled exclusively by the author/employer, or the entity funding the research, provided that (a) such posting is noncommercial in nature and the paper, poster, or presentation recording is made available to users without charge; (b) an appropriate copyright notice and citation appear with the paper, poster, or presentation recording; and (c) a link to SPIE’s official online version of the paper, poster, or presentation recording is provided using the DOI (Document Object Identifier) link. This authorization does not extend to third-party web sites not owned and maintained by the author/employer such as ResearchGate, Academia.edu, YouTube, etc. SPIE content published under a Creative Commons CC-BY license is exempt from the above requirements. The Gas Pixel Detector (GPD) is an X-ray polarimeter that exploits the photoelectric effect to measure the polarization and to obtain the image of astrophysical sources. This detector is on board the IXPE (Imaging X-ray Polarimetry Explorer) mission selected by NASA in the framework of the Explorer program scheduled for the launch in 2021. We report on tests carried out with a laboratory prototype of the GPD to verify the performance as a function of the temperature in a large temperature range between 15°C and 40°C.

The Gas Pixel Detector, recently developed and continuously improved by Pisa-INFN in collaboration with IASF-Roma (INAF), can visualize the tracks produced within a low Z gas by photoelectrons of few keV. By reconstructing the impact point and the original direction of the photoelectrons, the GPD can measure the polarization plane of X-Ray photons, while preserving the information on the absorption point, the energy and the time of arrival of individual photons. Applied to X-ray Astrophysics, in the focus of grazing incidence telescopes, it can perform angular and energy resolved polarimetry with a large improvement of sensitivity, when compared with the conventional techniques of Bragg diffraction at 45° and Compton scattering around 90°. This configuration has been the basis of POLARIX and HXMT, two pathfinder missions, and was included in the baseline design of IXO, the very large X-ray telescope under study by NASA, ESA and JAXA. We have recently improved the design of this low energy polarimeter (2–10 keV) by modifying the geometry of the absorption cell to minimize any systematic effect that could leave a residual polarization signal for non-polarized source. We will report on the testing of this new concept.

In this work we present an active Compton scattering polarimeter as a focal plane instrument able to extend the X-ray polarimetry towards hard X-rays. Other authors have already studied various instrument design by means of Monte Carlo simulations, in this work we will show for the first time the experimental measurements of "tagging efficiency" aimed to evaluate the polarimeter sensitivity as a function of energy. We performed a characterization of different scattering materials by measuring the tagging efficiency that was used as an input to the Monte Carlo simulation. Then we calculated the sensitivity to polarization of a design based on the laboratory set-up. Despite the geometry tested is not optimized for a realistic focal plane instrument, we demonstrated the feasibility of polarimetry with a low energy threshold of 20 keV. Moreover we evaluated a Minimum Detectable Polarization of 10% for a 10 mCrab source in 100 ks between 20 and 80 keV in the focal plane of one multilayer optics module of NuSTAR. The configuration used consisted of a doped p-terphenyl scatterer 3 cm long and 0.7 cm of diameter coupled with a 0.2 cm thick LaBr3 absorber., Comment: Accepted in press on Astroparticle Physics, 2013

COMpton Polarimeter with Avalanche Silicon readout (COMPASS) is a research and development project that aims to measure the polarization of X-ray photons through Compton Scattering. The measurement is obtained by using a set of small rods of fast scintillation materials with both low-Z (as active scatterer) and high-Z (as absorber), all read-out with Silicon Photomultipliers. By this method we can operate scattering and absorbing elements in coincidence, in order to reduce the background. In the laboratory we are characterising the SiPMs using different types of scintillators and we are optimising the performances in terms of energy resolution, energy threshold and photon tagging efficiency. We aim to study the design of two types of satellite-borne instruments: a focal plane polarimeter to be coupled with multilayer optics for hard X-rays and a large area and wide field of view polarimeter for transients and Gamma Ray Bursts. In this paper we describe the status of the COMPASS project, we report about the laboratory measurements and we describe our future perspectives.

X-ray polarimetry is a hot topic and, as a matter of fact, a number of missions dedicated to the measurement of the polarization in the ∼2-8 keV energy range with photoelectric devices are under advanced study by space agencies. The Gas Pixel Detector (GPD), developed and continuously improved in Italy by Pisa INFN in collaboration with INAF-IAPS, is the only instrument able to perform imaging polarimetry; moreover, it can measure photon energy and time of arrival. In this paper, we report on the performance of a GPD prototype assembled with flight-like materials and procedures. The remarkably uniform operation over a long period of time assures a straightforward operation in orbit and support the high readiness level claimed for this instrument.

At the core of the AGILE scientific instrument, designed to operate on a satellite, there is the Gamma Ray Imaging Detector (GRID) consisting of a Silicon Tracker (ST), a Cesium Iodide Mini-Calorimeter and an Anti-Coincidence system of plastic scintillator bars. The ST needs an on-ground calibration with a g-ray beam to validate the simulation used to calculate the energy response function and the effective area versus the energy and the direction of the g rays. A tagged g-ray beam line was designed at the Beam Test Facility (BTF) of the INFN Laboratori Nazionali of Frascati (LNF), based on an electron beam generating g-rays through bremsstrahlung in a position-sensitive target. The g-ray energy is deduced by difference with the post-bremsstrahlung electron energy [1,2]. The electron energy is measured by a spectrometer consisting of a dipole magnet and an array of position sensitive silicon strip detectors, the Photon Tagging System (PTS). The use of the combined BTF-PTS system as tagged photon beam requires understanding the efficiency of g-ray tagging, the probability of fake tagging, the energy resolution and Contents lists available at SciVerse ScienceDirect

Silicon drift detectors (SDD) of small dimensions (up to 1 cm 2 ) have been successfully employed in X-ray spectroscopy due to their small anode geometry, which allows to minimize the electronic noise due to the readout device. Many applications, however, require large sensitive areas to be covered (e.g. X-ray astronomy), so that these detectors are effectively impractical. We present the spectroscopic performance of a 53 cm 2 sensitive area, multi-anode SDD, measured at room temperature using an eight-channel readout setup. The measurements, taken using 55 Fe and 241 Am sources, and X-ray tubes generating energies down to 2 keV, show energy resolutions in the range 290–570 eV FWHM, at 20 °C, depending on the number of anodes collecting the signal. Further developments we are carrying out could improve the detector characteristics and allow to approach the performance of small area SDDs.

During its first 2 years of operation, the gamma-ray AGILE satellite almost completed a full study of the gamma-ray sky. This paper presents the preliminary results of the systematic study performed on the AGILE data to search for GeV counterparts and to derive flux upper limits of the TeV sources detected by various instruments (MAGIC, HESS, VERITAS, Cangaroo, MILAGRO, ARGO, … .).

The Gas Pixel Detector belongs to the very limited class of gas detectors optimized for the measurement of X-ray polarization in the emission of astrophysical sources. The choice of the mixture in which X-ray photons are absorbed and photoelectrons propagate, deeply affects both the energy range of the instrument and its performance in terms of gain, track dimension and ultimately, polarimetric sensitivity. Here we present the characterization of the Gas Pixel Detector with a 1 cm thick cell filled with dimethyl ether (DME) at 0.79 atm, selected among other mixtures for the very low diffusion coefficient. Almost completely polarized and monochromatic photons were produced at the calibration facility built at INAF/IASF-Rome exploiting Bragg diffraction at nearly 45 degrees. For the first time ever, we measured the modulation factor and the spectral capabilities of the instrument at energies as low as 2.0 keV, but also at 2.6 keV, 3.7 keV, 4.0 keV, 5.2 keV and 7.8 keV. These measurements cover almost completely the energy range of the instrument and allows to compare the sensitivity achieved with that of the standard mixture, composed of helium and DME., 20 pages, 11 figures, 5 tables. Accepted for publication by NIMA

AGILE is an Italian Space Agency mission dedicated to the exploration of the gamma-ray Universe. The AGILE, very innovative instrument, combines for the first time a gamma-ray imager (sensitive in the range 30 MeV–50 GeV) and a hard X-ray imager (sensitive in the range 18–60 keV). An optimal angular resolution and very large fields of view are obtained by the use of state-of-the-art Silicon detectors integrated in a very compact instrument. AGILE was successfully launched on April 23, 2007 from the Indian base of Sriharikota and was inserted in an optimal low-particle background equatorial orbit. AGILE will provide crucial data for the study of Active Galactic Nuclei, Gamma-Ray Bursts, unidentified gamma-ray sources, galactic compact objects, supernova remnants, TeV sources, and fundamental physics by microsecond timing. The AGILE Cycle-1 pointing program started on 2007 December 1, and is open to the international community through a Guest Observer Program.

SuperAGILE is a coded mask experiment based on silicon microstrip detectors. It operates in the 15-45 keV nominal energy range, providing crossed one-dimensional images of the X-ray sky with an on-axis angular resolution of 6 arcmin, over a field of view in excess of 1 steradian. It was designed as the hard X-ray monitor of the AGILE space mission, a small satellite of the Italian Space Agency devoted to image the gamma-ray sky in the 30 MeV - 50 GeV energy band. The AGILE mission was launched in a low-earth orbit on 23^{rd} April 2007. In this paper we describe the SuperAGILE experiment, its construction and test processes, and its performance before flight, based on the on-ground test and calibrations., Comment: Nucl. Instr. and Meth. A, in press. Full version with figures included in the text, now available at: ftp://vesta.rm.iasf.cnr.it/pub/mferoci/ It will also be linked at the official AGILE web pages

In this paper we describe the electronics of the SuperAGILE X-ray imager on-board AGILE satellite and the instrumentation developed to test and improve the Front-End and digital electronics of the flight model of the imager. Although the working principle of the instrument is very well established, and the conceptual scheme simple, the budget and mechanical constraints of the AGILE small mission made necessary the introduction of new elements in SuperAGILE, regarding both the mechanics and the electronics. In fact the instrument is contained in a View the MathML source?44×44×16cm3 volume, but the required performance is quite ambitious, leading us to equip a sensitive area of View the MathML source?1350cm2 with 6144 Silicon ??strips detectors with a pitch of View the MathML source121?m and a total length of View the MathML source?18.2cm. The result is a very light and power-cheap imager with a good sensitivity (View the MathML source?15mCrab in 1 day in View the MathML source15-45keV), high angular resolution View the MathML source(6arcmin) and gross spectral resolution. The test-equipment is versatile, and can be easily modified to test FEE based on self-triggered, data-driven and sparse-readout ASICs such as XA family chips.

The Gas Pixel Detector (GPD) is an imaging X-ray polarimeter with a moderate spectral resolution and a very good position resolution. 1, 2 The GPD derives this information from the true 2-d charge image of the photoelectron track produced in gas and collected by an ASIC CMOS chip after its drift and its multiplication. In this paper we report on the experimental results of the study of the effect of a strong magnetic field in reducing the diffusion and increasing the sensitivity for a GPD filled with one bar of He-DME 20-80. We generated a magnetic field of about 1600 Gauss by means of commercial magnets made of an alloy of Neodymium-Iron-Boron configured as one ring and one cylinder. We compared the pixel size distributions and the modulation curves with and without magnets at two different drift fields, corresponding to different nominal diffusion properties, with both polarized and unpolarized sources. The results obtained show that a not sensitive improvement is present at this fields implying that a much larger magnetic field is necessary with this mixture, albeit a shift on the position angle of the modulation curve, derived from a polarized source, is observed.

X-rays are particularly suited to probe the physics of extreme objects. However, despite the enormous improvements of X-ray Astronomy in imaging, spectroscopy and timing, polarimetry remains largely unexplored. We propose the photoelectric polarimeter Gas Pixel Detector (GPD) as an instrument candidate to fill the gap of more than thirty years of lack of measurements. The GPD, in the focus of a telescope, will increase the sensitivity of orders of magnitude. Moreover, since it can measure the energy, the position, the arrival time and the polarization angle of every single photon, allows to perform polarimetry of subsets of data singled out from the spectrum, the light curve or the image of source. The GPD has an intrinsic very fine imaging capability and in this work we report on the calibration campaign carried out in 2012 at the PANTER X-ray test facility of the Max-Planck-Institut f\"ur extraterrestrische Physik of Garching (Germany) in which, for the first time, we coupled it to a JET-X optics module with a focal length of 3.5 m and an angular resolution of 18 arcsec at 4.5 keV. This configuration was proposed in 2012 aboard the X-ray Imaging Polarimetry Explorer (XIPE) in response to the ESA call for a small mission. We derived the imaging and polarimetric performance for extended sources like Pulsar Wind Nebulae and Supernova Remnants as case studies for the XIPE configuration, discussing also possible improvements by coupling the detector with advanced optics, having finer angular resolution and larger effective area, to study with more details extended objects., Comment: Accepted for publication in The Astrophysical Journal Supplement

The Joint European X-ray Telescope (JET-X) was the core instrument of the Russian Spectrum-X-gamma space observatory. It consisted of two identical soft X-ray (0.3 - 10 keV) telescopes with focusing optical modules having a measured angular resolution of nearly 15 arcsec. Soon after the payload completion, the mission was cancelled and the two optical flight modules (FM) were brought to the Brera Astronomical Observatory where they had been manufactured. After 16 years of storage, we have utilized the JET-X FM2 to test at the PANTER X-ray facility a prototype of a novel X-ray polarimetric telescope, using a Gas Pixel Detector (GPD) with polarimetric capabilities in the focal plane of the FM2. The GPD was developed by a collaboration between INFN-Pisa and INAF-IAPS. In the first phase of the test campaign, we have re-tested the FM2 at PANTER to have an up-to-date characterization in terms of angular resolution and effective area, while in the second part of the test the GPD has been placed in the focal plane of the FM2. In this paper we report the results of the tests of the sole FM2, using an unpolarized X-ray source, comparing the results with the calibration done in 1996., Author's accepted manuscript posted to arXiv.org as permitted by Springer's Self-Archiving Policy. The final publication is available at http://rd.springer.com/article/10.1007%2Fs10686-013-9365-y

SuperAGILE is the X-ray monitor of AGILE, a satellite mission for gamma-ray astronomy, and it is the first X-ray imaging instrument based on the technology of the silicon microstrip detectors combined with a coded aperture imaging technique. The SuperAGILE detection plane is composed of four 1-D silicon microstrip detector modules, mechanically coupled to tungsten coded mask units. The detector strips are separately and individually connected to the input analogue channels of the front-end electronics, composed of low-noise and low-power consumption VLSI ASIC chips. SuperAGILE can produce 1-D images with 6 arcmin angular resolution and ∼2–3 arcmin localisation capability, for intense sources, in a field of view composed of two orthogonal areas of 107°×68°. The time resolution is 2 μs, the overall dead time is ∼5 μs and the electronic noise is ∼7.5 keV full-width at half-maximum. The resulting instrument is very compact (40×40×14 cm 3 ), light (10 kg) and has low power consumption (12 W). AGILE is a mission of the Agenzia Spaziale Italiana and its launch is planned in 2007 in a low equatorial Earth orbit. In this contribution we present SuperAGILE and discuss its performance and scientific objectives.

Context: AGILE is a gamma-ray astrophysics mission which has been in orbit since 23 April 2007 and continues to operate reliably. The gamma-ray detector, AGILE-GRID, has observed Galactic and extragalactic sources, many of which were collected in the first AGILE Catalog. Aims: We present the calibration of the AGILE-GRID using in-flight data and Monte Carlo simulations, producing Instrument Response Functions (IRFs) for the effective area A_eff), Energy Dispersion Probability (EDP), and Point Spread Function (PSF), each as a function of incident direction in instrument coordinates and energy. Methods: We performed Monte Carlo simulations at different gamma-ray energies and incident angles, including background rejection filters and Kalman filter-based gamma-ray reconstruction. Long integrations of in-flight observations of the Vela, Crab and Geminga sources in broad and narrow energy bands were used to validate and improve the accuracy of the instrument response functions. Results: The weighted average PSFs as a function of spectra correspond well to the data for all sources and energy bands. Conclusions: Changes in the interpolation of the PSF from Monte Carlo data and in the procedure for construction of the energy-weighted effective areas have improved the correspondence between predicted and observed fluxes and spectra of celestial calibration sources, reducing false positives and obviating the need for post-hoc energy-dependent scaling factors. The new IRFs have been publicly available from the Agile Science Data Centre since November 25, 2011, while the changes in the analysis software will be distributed in an upcoming release.

We show that meaningful, highly sensitive x-ray polarimetry with imaging capability is possible with a small mission tailored to the NASA Explorer program. Such a mission—derived from the Imaging X-ray Polarimetry Explorer (IXPE) proposed to a previous NASA call—takes advantage of progress in light-weight x-ray optics and in gas pixel detectors to achieve sensitive time-resolved, spectrometric, imaging polarimetry. We outline the main characteristics and requirements of this mission and provide a realistic assessment of its scientific utility for modeling point-like and extended x-ray sources and for studying physical processes (including questions of fundamental physics).

The background of the Gas Pixel Detector is expected to be negligible for polarimetry of point sources due to the intrinsic low atomic number and density of the He-DME mixtures and to its imaging properties. Also the background for extended sources is expected to be negligible at least down to the smallest flux for sensitive polarimetry in a reasonable observing time. However in the spatial distribution of the background in a laboratory environment we observed an accumulation on the edges of the sensitive plane due to the presence of the nearby cell walls. We recently developed gas pixel detectors with a new design of the gas cell having a larger distance of the walls from the sensitive plane. In this paper we compare the spatial distribution of the measured background for the two design and their residual systematics. Also the impact of the background in the case of SgrB2 a faint extended source in the galactic center region is evaluated.

The Gas Pixel Detector, developed and continuously improved by Pisa INFN in collaboration with INAF-IAPS, can visualize the tracks produced within a low Z gas by photoelectrons of few keV. By reconstructing the impact point and the original direction of the photoelectrons, the GPD can measure the linear polarization of X-rays, while preserving the information on the absorption point, the energy and the time of arrival of individual photons. The Gas Pixel Detector filled with He-DME mixture at 1 bar is sensitive in the 2-10 keV energy range and this configuration has been the basis of a number of mission proposals, such as POLARIX or XPOL on-board XEUS/IXO, or the X-ray Imaging Polarimetry Explorer (XIPE) submitted in response to ESA small mission call in 2012. We have recently improved the design by modifying the geometry of the absorption cell to minimize any systematic effect which could leave a residual polarization signal for non polarized source. We report on the testing of this new concept with preliminary results on the new design performance.

The possibility to perform polarimetry in the soft X-ray energy band (2-10 keV) with the Gas Pixel Detector, filled with low Z mixtures, has been widely explored so far. The possibility to extend the technique to higher energies, in combination with multilayer optics, has been also hypothesized in the past, on the basis of simulations. Here we present a recent development to perform imaging polarimetry between 6 and 35 keV, employing a new design for the GPD, filled with a Ar-DME gas mixture at high pressure. In order to improve the efficiency by increasing the absorption gap, while preserving a good parallel electric field, we developed a new configuration characterized by a wider gas cell and a wider GEM. The uniform electric field allows to maintain high polarimetric capabilities without any decrease of spectroscopic and imaging properties. We present the first measurements of this prototype showing that it is now possible to perform imaging and spectro-polarimetry of hard X-ray sources.

At the core of the AGILE scientific instrument, designed to operate on a satellite, there is the Gamma Ray Imaging Detector (GRID) consisting of a Silicon Tracker (ST), a Cesium Iodide Mini-Calorimeter and an Anti-Coincidence system of plastic scintillator bars. The ST needs an on-ground calibration with a γ-ray beam to validate the simulation used to calculate the energy response function and the effective area versus the energy and the direction of the γ rays. A tagged γ-ray beam line was designed at the Beam Test Facility (BTF) of the INFN Laboratori Nazionali of Frascati (LNF), based on an electron beam generating γ rays through bremsstrahlung in a position-sensitive target. The γ-ray energy is deduced by the difference with the post-bremsstrahlung electron energy 1-.2 The electron energy is measured by a spectrometer consisting of a dipole magnet and an array of position sensitive silicon strip detectors, the Photon Tagging System (PTS). The use of the combined BTF-PTS system as tagged photon beam requires understanding the efficiency of γ-ray tagging, the probability of fake tagging, the energy resolution and the relation of the PTS hit position versus the γ-ray energy. This paper describes this study comparing data taken during the AGILE calibration occurred in 2005 with simulation.

Large-area multi-anode Silicon Drift Detectors (SDDs) have X-ray imaging and spectroscopic characteristics that make them extremely attractive in the perspective of their applications to the field of space astrophysics. We describe here the imaging performance of such a detector, originally developed for particle tracking in the ALICE experiment at LHC, as derived by laboratory measurements and Monte Carlo simulations. Despite an anode pitch of View the MathML source, we measured a position resolution as high as View the MathML source by charge weighting in the anode direction, for photon energies in the range 2–10 keV. These results are comparable to those obtained with the same detectors in particle tracking. Notwithstanding the 1-D nature of the devices, as far as their read-out is concerned, we envisaged an algorithm that exploits the charge diffusion to reconstruct the position of the photon absorption point also along the drift direction (that is, the one formally not position-sensitive). With the current set-up, the position resolution was measured as View the MathML source in the same energy range as above. Such 2-D imaging capability in a 1-D detector, although asymmetric, is highly useful in space applications, where the power and the complexity requested by a 2-D read-out system is sometimes unaffordable. Keywords: High Energy Astrophysics; Silicon Drift Detectors; X-rays

The SuperAGILE experiment is the hard X-ray monitor of the AGILE mission. It is a 2 x one-dimensional imager, with 6-arcmin angular resolution in the energy range 18 - 60 keV and a field of view in excess of 1 steradian. SuperAGILE is successfully operating in orbit since Summer 2007, providing long-term monitoring of bright sources and prompt detection and localization of gamma-ray bursts. Starting on October 2009 the AGILE mission lost its reaction wheel and the satellite attitude is no longer stabilized. The current mode of operation of the AGILE satellite is a Spinning Mode, around the Sun-pointing direction, with an angular velocity of about 0.8 degree/s (corresponding to 8 times the SuperAGILE point spread function every second). In these new conditions, SuperAGILE continuously scans a much larger fraction of the sky, with much smaller exposure to each region. In this paper we review some of the results of the first 2.5 years of "standard" operation of SuperAGILE, and show how new implementations in the data analysis software allows to continue the hard X-ray sky monitoring by SuperAGILE also in the new attitude conditions.

The New Hard X-Ray Imaging and Polarimetric Mission makes a synergic use of Hard X-Ray Imaging, Spectroscopy and Polarimetry, as independent diagnostic of the same physical systems. It exploits the technology of multi-layer optics that, with a focal length of 10 m, allow for spectroscopic and imaging, with a resolution from 15 to 20 arcseconds, on the band 0.2 - 80 keV. One of the four telescopes is devoted to polarimetry. Since the band of a photoelectric polarimeter is not that wide, we foresee two of them, one tuned on the lower energy band (2-10 keV) and another one tuned on higher energies (6 - 35 keV). The blurring due to the inclined penetration of photons in the gas , thanks to the long focal length is practically negligible. In practice the polarimeters fully exploit the resolution the telescope and NHXM can perform angular resolved simultaneous spectroscopy and polarimetry on the band 2 - 35 keV. We are also studying the possibility to extend the band up to 80 keV by means of a focal plane scattering polarimeter.

SuperAGILE is the hard X-ray monitor of the AGILE gamma ray mission, in orbit since 23$^{rd}$ April 2007. It is an imaging experiment based on a set of four independent silicon strip detectors, equipped with one-dimensional coded masks, operating in the nominal energy range 18-60 keV. The main goal of SuperAGILE is the observation of cosmic sources simultaneously with the main gamma-ray AGILE experiment, the Gamma Ray Imaging Detector (GRID). Given its $\sim$steradian-wide field of view and its $\sim$15 mCrab day-sensitivity, SuperAGILE is also well suited for the long-term monitoring of Galactic compact objects and the detection of bright transients. The SuperAGILE detector properties and design allow for a 6 arcmin angular resolution in each of the two independent orthogonal projections of the celestial coordinates. Photon by photon data are continuously available by the experiment telemetry, and are used to derive images and fluxes of individual sources, with integration times depending on the source intensity and position in the field of view. In this paper we report on the main scientific results achieved by SuperAGILE over its first two years in orbit, until April 2009., Comment: Accepted for publication on Astronomy & Astrophysics

The XEUS mission incorporates two satellites: the Mirror Spacecraft with 5 m2 of collecting area at 1 keV and 2 m2 at 7 keV, and an imaging resolution of 5" HEW and the Payload Spacecraft which carries the focal plane instrumentation. XEUS was submitted to ESA Cosmic Vision and was selected for an advanced study as a large mission. The baseline design includes XPOL, a polarimeter based on the photoelectric effect, that takes advantage of the large effective area which permits the study of the faint sources and of the long focal length, resulting in a very good spatial resolution, which allows the study of spatial features in extended sources. We show how, with XEUS, Polarimetry becomes an efficient tool at disposition of the Astronomical community., 10 pages, 6 figures

The SuperAGILE experiment is the hard X-ray monitor of the Italian AGILE gamma-ray mission. SuperAGILE is the first experiment for X-ray astronomy employing single-sided silicon microstrip detectors in space. The fine position sensitivity of these detectors allowed to design a coded mask imaging experiment very compact and resource-effective, operating in the 18–60 keV energy range. In this paper we present the design of the experiment, its development as well as its first operations in orbit, and a brief summary of the main scientific results achieved after 12 months of nominal observations.

AGILE, an Italian Space Agency (ASI) mission dedicated to the exploration of the gamma-ray Universe, was successfully launched on April 23, 2007 from the Indian base of Sriharikota in an low-particle background equatorial orbit at 550 km height. AGILE gamma ray range cover the 30 MeV – 50 GeV band with its detectors combined together in the Gamma-Ray Imaging Detector (GRID). An X-ray imager (SuperAGILE) operates in the 18 – 60 keV band. Some of the payload instruments are also operated to detect transient X and gamma events with the main target being the Gamma Ray Burst in the SuperAGILE band and above 300 keV. The GRID instrument consists of a Silicon-Tungsten Tracker, a Caesium Iodide Mini-Calorimeter (MCAL), and a plastic-scintillator Anticoincidence system (ACS). The GRID achieves an angular resolution of about 15’ for bright sources with an unprecedented large field-of-view about 2.5 sr with a time resolution of few microsec and a very reduced dead-time (less than 200 microsec for a gamma detection). The hard X-ray imager (SuperAGILE) combines a tungsten coded mask with Si detector with a technology identical to the Si-Tracker. It is on top of the gamma-ray detector and has an optimal angular resolution (about 6 arcmin) with good sensitivity over about 1 sr field of view (10–15 mCrab on axis for a 1-day integration). The burst monitoring combines the data from SuperAGILE and from MCAL which operates independently with a specific operative mode from 300 keV up to several tens of MeV. All the detector operations are mastered by the Payload Data Handling Unit (PDHU) that includes several operation for data selection and background rejection. AGILE has very innovative scientific payload for what concern both its detectors and its operative mode. After more than one year of in orbit operation the in-flight performances of the instruments as well as the operation of the background reducing algorithm are presented and the main scientific results achieved are discussed.

SuperAGILE (SA) is the hard X-ray monitor of the AGILE small satellite mission, launched on 23rd April 2007. The monitor is based on four one-dimensional coded-mask detectors. In spite of the compactness (45×45×15 cm3) and lightness (5 kg), the experiment has high angular resolution (6 arcmin) and point source location accuracy (

The Gas Pixel Detector (GPD) is a new generation device which, thanks to its 50 um pixels, is capable of imaging the photoelectrons tracks produced by photoelectric absorption in a gas. Since the direction of emission of the photoelectrons is strongly correlated with the direction of polarization of the absorbed photons, this device has been proposed as a polarimeter for the study of astrophysical sources, with a sensitivity far higher than the instruments flown to date. The GPD has been always regarded as a focal plane instrument and then it has been proposed to be included on the next generation space-borne missions together with a grazing incidence optics. Instead in this paper we explore the feasibility of a new kind of application of the GPD and of the photoelectric polarimeters in general, i.e. an instrument with a large field of view. By means of an analytical treatment and measurements, we verify if it is possible to preserve the sensitivity to the polarization for inclined beams, opening the way for the measurement of X-ray polarization for transient astrophysical sources. While severe systematic effects arise for inclination greater than about 20 degrees, methods and algorithms to control them are discussed., Comment: 11 pages, 8 figures

We devised and built a versatile facility for the calibration of the next generation X-ray polarimeters with unpolarized and polarized radiation. The former is produced at 5.9 keV by means of a Fe55 radioactive source or by X-ray tubes, while the latter is obtained by Bragg diffraction at nearly 45 degrees. Crystals tuned with the emission lines of X-ray tubes with molybdenum, rhodium, calcium and titanium anodes are employed for the efficient production of highly polarized photons at 2.29, 2.69, 3.69 and 4.51 keV respectively. Moreover the continuum emission is exploited for the production of polarized photons at 1.65 keV and 2.04 keV and at energies corresponding to the higher orders of diffraction. The photons are collimated by means of interchangeable capillary plates and diaphragms, allowing a trade-off between collimation and high fluxes. The direction of the beam is accurately arranged by means of high precision motorized stages, controlled via computer so that long and automatic measurements can be done. Selecting the direction of polarization and the incidence point we can map the response of imaging devices to both polarized and unpolarized radiation. Changing the inclination of the beam we can study the systematic effects due to the focusing of grazing incidence optics and the feasibility of instruments with large field of view., Comment: 12 pages, 11 figures

An X-ray photoelectric polarimeter based on the Gas Pixel Detector has been proposed to be included in many upcoming space missions to fill the gap of about 30 years from the first (and to date only) positive measurement of polarized X-ray emission from an astrophysical source. The estimated sensitivity of the current prototype peaks at an energy of about 3 keV, but the lack of readily available polarized sources in this energy range has prevented the measurement of detector polarimetric performances. In this paper we present the measurement of the Gas Pixel Detector polarimetric sensitivity at energies of a few keV and the new, light, compact and transportable polarized source that was devised and built to this aim. Polarized photons are produced, from unpolarized radiation generated with an X-ray tube, by means of Bragg diffraction at nearly 45 degrees. The employment of mosaic graphite and flat aluminum crystals allow the production of nearly completely polarized photons at 2.6, 3.7 and 5.2 keV from the diffraction of unpolarized continuum or line emission. The measured modulation factor of the Gas Pixel Detector at these energies is in good agreement with the estimates derived from a Monte Carlo software, which was up to now employed for driving the development of the instrument and for estimating its low energy sensitivity. In this paper we present the excellent polarimetric performance of the Gas Pixel Detector at energies where the peak sensitivity is expected. These measurements not only support our previous claims of high sensitivity but confirm the feasibility of astrophysical X-ray photoelectric polarimetry., 15 pages, 12 figures. Accepted for publication in NIMA

We devised and built a light, compact and transportable X-ray polarized source based on the Bragg diffraction at nearly 45 degrees. The source is composed by a crystal coupled to a small power X-ray tube. The angles of incidence are selected by means of two orthogonal capillary plates which, due to the small diameter holes (10 um) allow good collimation with limited sizes. All the orders of diffraction defined by the crystal lattice spacing are polarized up to the maximum order limited by the X-ray tube voltage. Selecting suitably the crystal and the X-ray tube, either the line or the continuum emission can be diffracted, producing polarized photons at different energies. A very high degree of polarization and reasonable fluxes can be reached with a suitable choice of the capillary plates collimation. We present the source and test its performances with the production of nearly completely polarized radiation at 2.6, 5.2, 3.7 and 7.4 keV thanks to the employment of graphite and aluminum crystals, with copper and calcium X-ray tubes respectively. Triggered by the very compact design of the source, we also present a feasibility study for an on-board polarized source, coupled to a radioactive Fe55 nuclide and a PVC thin film, for the calibration of the next generation space-borne X-ray polarimeters at 2.6 and 5.9 keV., Comment: 12 pages, 22 figures

The XAA1.2 is a custom ASIC chip for silicon microstrip detectors adapted by Ideas for the SuperAGILE instrument on board the AGILE space mission. The chip is equipped with 128 input channels, each one containing a charge preamplifier, shaper, peak detector and stretcher. The most important features of the ASIC are the extended linearity, low noise and low power consumption. The XAA1.2 underwent extensive laboratory testing in order to study its commandability and functionality and evaluate its scientific performances. In this paper we describe the XAA1.2 features, report the laboratory measurements and discuss the results emphasizing the scientific performances in the context of the SuperAGILE front-end electronics., Comment: 30 pages, 19 figures, accepted for publication by Nuclear Inst. and Methods in Physics Research A

The AGILE Mission will explore the gamma-ray Universe with a very innovative instrument combining for the first time a gamma-ray imager (sensitive in the range 30 MeV - 50 GeV) and a hard X-ray imager (sensitive in the range 15-45 keV). An optimal angular resolution and a large field of view are obtained by the use of state-of-the-art Silicon detectors integrated in a very compact instrument. AGILE will be operational at the beginning of 2007 and it will provide crucial data for the study of Active Galactic Nuclei, Gamma-Ray Bursts, unidentified gamma-ray sources, Galactic compact objects, supernova remnants, TeV sources, and fundamental physics by microsecond timing.

The Flight Model of the SuperAGILE experiment was calibrated on-ground using an X-ray generator and individual radioactive sources at IASF Rome on August 2005. Here we describe the set-up, the measurements and the preliminary results of the calibration session carried out with the X-ray generator. The calibration with omnidirectional radioactive sources are reported elsewhere. The beam was collimated using a two slits system in order to reach a rectangular spot at the detector approximately 1800 μm × 100 μm in size. The long dimension was aligned with the detector strip, so that the short dimension could fall within one single detector strip (121 μm wide). The detector was then slowly moved continuously such that the beam effectively scanned along the coding direction. This measurement was done both at detection plane level (i.e., without collimator and mask) to characterize the detector response, and at experiment level (i.e., with collimator, mask and digital electronics), to study the imaging response. Aim of this calibration is the measurement of the imaging response at 0, 10 and 20 degrees off-axis, with a parallel beam, although spatially limited to a ~2 mm long section of the coded mask.

SuperAGILE is the hard X-ray (15-45 keV) imager for the gamma-ray mission AGILE, currently scheduled for launch in early 2007. It is based on 4 Si-microstrip detectors, with a total geometric area of 1444 cm2 (max effective area 230 cm2), equipped with 4 one-dimensional coded masks. The 4 detectors are perpendicularly oriented, in order to provide pairs of orthogonal one-dimensional images of the X-ray sky. The field of view of each 1-D detector is 107° x 68°, at zero response, with an overlap in the central 68° x 68° area. The angular resolution on axis is 6 arcmin. We present here the current status of the hardware development and the scientific perspective.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The Flight model of SuperAGILE experiment was calibrated on-ground on August 2005 at IASF-Rome laboratories using standard radioactive X-rays sources. These omnidirectional sources were positioned at approximately 2 meters distance from the experiment. A method to correct for the beam divergence has been developed in order to use these measurements to derive information about the point spread function of the experiment for infinite distance sources. In this paper we describe the set-up of the measurements, the method to correct for the beam divergence and show preliminary results of the data analysis.

The XAA1.2 is a custom ASIC chip for space applications built using a 0.8 μ m complementary metal oxide semiconductor technology on epitaxial layer. It has been selected as the front-end electronics chip of the SuperAGILE experiment on board the AGILE space mission, although it is not specifically designed as a radiation hard device. To study the XAA1.2 sensitivity to Single Event Effects and Total Dose Effects we irradiate this chip at the SIRAD facility of the Laboratori Nazionali INFN of Legnaro. In this paper we describe the experimental set-up and the measurements. We then discuss how the results can be scaled to the cosmic rays environment in a low-Earth orbit.

The XAA1.2 chip is a low noise, self-triggered, data-driven and sparse readout ASIC chip with 128 input channels designed as a front-end electronic circuit for silicon-microstrip detectors and manufactured by Ideas ASA (Norway). The XAA1.2 has been selected as the front-end electronic circuit of the SuperAGILE experiment on-board the AGILE satellite mission. This chip underwent to extensive laboratory tests to evaluate its scientific performances. Particularly we have measured the electronic noise and threshold voltage in both configurations stand alone and bonded to a silicon microstrip detector and we have tested the chip thermal stability and radiation damage. In this paper we describe the measurements and we discuss the results.

In this paper we describe the instrumentation and the software tools we developed to test the SuperAGILE Front-End Electronics (SAFEE) and Interface Electronics (SAIE). The SAFEE is based on twelve XAA1.2 ASICs (produced by IDE-AS). The Test Equipment hardware is composed of commercial VME modules and laboratory developed boards. Commercial VME boards were used for data acquisition and SAFEE handling. Laboratory developed boards provide signal conditioning, pulse generation, trigger system and timing. The VME based architecture assured a stable system for a period of years and a very high acquisition rate. The choice of 'laboratory-developed' boards allowed an easy and cost effective continuous improvement of the system. Two Linux running PC were used, one for the "System Control" and data acquisition, the other one for data reduction and archiving. The s/w for DAQ, data-reduction, and analysis also was laboratory-developed and based on well-known tools.

The AGILE gamma-ray mission is in its Phase C-D. The Engineering model of the Payload has been built and tested, and the construction of the flight model has started. We present here the status of the SuperAGILE experiment, the 15-40 keV imaging monitor, based on Silicon microstrip technology and equipped with one dimensional coded masks. We show the design of the experiment and the results of testing campaigns carried out on the engineering model of the experiment.