Yuri Evangelista

A set of x-ray polarimeters for the New Hard X-ray Imaging and Polarimetric Mission

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.

Scientific performances of the XAA1.2 front-end chip for silicon microstrip detectors

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.

The on-ground calibrations of SuperAGILE: II. Finite distance radioactive sources

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.

Performance of the Gas Pixel Detector: an x-ray imaging polarimeter for upcoming missions of astrophysics

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.

The on-ground calibrations of SuperAGILE: I. X-ray pencil beam

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.

The LOFT wide field monitor simulator

We present the simulator we developed for the Wide Field Monitor (WFM) aboard the Large Observatory For Xray Timing (LOFT) mission, one of the four ESA M3 candidate missions considered for launch in the 2022–2024 timeframe. The WFM is designed to cover a large FoV in the same bandpass as the Large Area Detector (LAD, almost 50% of its accessible sky in the energy range 2–50 keV), in order to trigger follow-up observations with the LAD for the most interesting sources. Moreover, its design would allow to detect transient events with fluxes down to a few mCrab in 1-day exposure, for which good spectral and timing resolution would be also available (about 300 eV FWHM and 10 μs, respectively). In order to investigate possible WFM configurations satisfying these scientific requirements and assess the instrument performance, an end-to-end WFM simulator has been developed. We can reproduce a typical astrophysical observation, taking into account both mask and detector physical properties. We will discuss the WFM simulator architecture and the derived instrumental response.

Imaging x-ray sources at a finite distance in coded-mask instruments.

We present a method for the correction of beam divergence in finite distance sources imaging through coded-mask instruments. We discuss the defocusing artifacts induced by the finite distance showing two different approaches to remove such spurious effects. We applied our method to one-dimensional (1D) coded-mask systems, although it is also applicable in two-dimensional systems. We provide a detailed mathematical description of the adopted method and of the systematics introduced in the reconstructed image (e.g., the fraction of source flux collected in the reconstructed peak counts). The accuracy of this method was tested by simulating pointlike and extended sources at a finite distance with the instrumental setup of the SuperAGILE experiment, the 1D coded-mask x-ray imager onboard the AGILE (Astro-rivelatore Gamma a Immagini Leggero) mission. We obtained reconstructed images of good quality and high source location accuracy. Finally we show the results obtained by applying this method to real data collected during the calibration campaign of SuperAGILE. Our method was demonstrated to be a powerful tool to investigate the imaging response of the experiment, particularly the absorption due to the materials intercepting the line of sight of the instrument and the conversion between detector pixel and sky direction.

SuperAGILE at launch

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.

The large area detector onboard the eXTP mission

The eXTP (enhanced X-ray Timing and Polarimetry) mission is a major project of the Chinese Academy of Sciences (CAS) and China National Space Administration (CNSA) currently performing an extended phase A study and proposed for a launch by 2025 in a low-earth orbit. The eXTP scientific payload envisages a suite of instruments (Spectroscopy Focusing Array, Polarimetry Focusing Array, Large Area Detector and Wide Field Monitor) offering unprecedented simultaneous wide-band X-ray spectral, timing and polarimetry sensitivity. A large European consortium is contributing to the eXTP study and it is expected to provide key hardware elements, including a Large Area Detector (LAD). The LAD instrument for eXTP is based on the design originally proposed for the LOFT mission within the ESA context. The eXTP/LAD envisages a deployed 3.4 m2 effective area in the 2-30 keV energy range, achieved through the technology of the large-area Silicon Drift Detectors - offering a spectral resolution of up to 200 eV FWHM at 6 keV - and of capillary plate collimators - limiting the field of view to about 1 degree. In this paper we provide an overview of the LAD instrument design, including new elements with respect to the earlier LOFT configuration.

X-ray imaging and spectroscopy performance of a large area silicon drift chamber for wide-field x-ray astronomy applications

In the context of the design of wide-field of view experiments for X-ray astronomy, we studied the response to X-rays in the range between 2 and 60 keV of a large area Silicon Drift Chamber originally designed for particle tracking in high energy physics. We demonstrated excellent imaging and spectroscopy performance of monolithic 53 cm2 detectors, with position resolution as good as 30 μm and energy resolution in the range 300-570 eV FWHM obtainable at room temperature (20 °C). In this paper we show the results of test campaigns at the X-ray facility at INAF/IASF Rome, aimed at characterizing the detector performance by scanning the detector area with highly collimated spots of monochromatic X-rays. In these tests we used a detector prototype equipped with discrete read-out front-end electronics.