E. Quadrini

IBIS: The Imager on-board INTEGRAL

The IBIS telescope is the high angular resolution gamma-ray imager on-board the INTEGRAL Observatory, suc- cessfully launched from Baikonur (Kazakhstan) the 17th of October 2002. This medium size ESA project, planned for a 2 year mission with possible extension to 5, is devoted to the observation of the gamma-ray sky in the energy range from 3 keV to 10 MeV (Winkler 2001). The IBIS imaging system is based on two independent solid state detector arrays optimised for low (15 1000 keV) and high (0:175 10:0 MeV) energies surrounded by an active VETO System. This high eciency shield is essential to minimise the background induced by high energy particles in the highly excentric out of van Allen belt orbit. A Tungsten Coded Aperture Mask, 16 mm thick and1 squared meter in dimension is the imaging device. The IBIS telescope will serve the scientific community at large providing a unique combination of unprecedented high energy wide field imaging capability coupled with broad band spectroscopy and high resolution timing over the energy range from X to gamma rays. To date the IBIS telescope is working nominally in orbit since more than 9 month.

Hard X-rays from Coma Cluster region

A high-energy nonthermal component in the 18-130 keV energy band was detected by balloon-borne observations of the region containing the Coma Cluster of galaxies. The emitted photon spectrum is well-described by a single power law of photon index -1.77 and an intensity of 8.9 x 10 to the -5th photons/sq cm/s/KeV at 50 keV. The emission supports the presence of a hard tail, and could be due to the inverse Compton mechanism of relativistic electrons permeating the cluster off the 2.7 K background radiation or the presence in the field of an active galactic nucleus. 30 refs.

CdZnTe detector for hard x-ray and gamma-ray focusing telescope

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.

Hard X-ray variability of NGC 4151

During a balloon flight on July 15, 1987 of the MIFRASO telescope, a region of the sky containing the Seyfert galaxy NGC 4151 was studied over the photon energy range 15-335 keV. A 13-sigma excess, interpreted as originating from the direction of NGC 4151, was detected in this energy band and the corresponding emission spectrum is represented by a single power law with a photon spectral index of alpha = 1.5 + or {minus} 0.1. With respect to the July 1986 observation of this source with the same telescope, the alpha slope is statistically unchanged, while the X-ray flux in the energy range 27-80 keV shows, at the 95-percent confidence level, an increase of about 40 percent. An observation of the Crab Nebula, that proves the stability of the telescope response through the two measurements of NGC 4151, is also described. 48 refs.

Hard X-ray observation of NGC 4151 with the Mifraso telescope

During a balloon flight on July 29, 1986 of the Mifraso telescope, a region of the sky containing the Seyfert galaxy NGC 4151 was studied over the photon energy range 15-300 keV. A 5.5 sigma excess, interpreted as originating from the direction of NGC 4151, was detected in the energy band 27-80 keV. The corresponding emission spectrum may be represented by a single power law with a photon spectral index of alpha = 1.5 + or - 0.6. The result is compared to previous hard X-ray observations of this source. Apart from this measurement, an observation of the Crab Nebula as an in-flight calibration source of the telescope is described. 33 refs.

A three-dimensional CZT detector as a focal plane prototype for a Laue Lens telescope

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 mm2 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.

Superlow-state hard X-ray observation of Cygnus X-1

Hard-X-ray observations of Cyg X-1, obtained using the multiwire proportional counters and scintillator arrays of the Milan-Frascati-Southampton balloon-borne instrument package during July 1987, are reported. The results are combined with previous observational data and presented in tables and graphs. A very low flux similar to that measured by the HEAO-3 gamma-ray spectrometer in 1979 and in agreement with low-energy observations by the Ginga satellite during August 1987 is detected, confirming that the object is in a superlow state. The photon spectrum is found to be well fit by a Comptonized model with electron temperature 56 keV and optical depth 2.3, or by a simple power law with photon index alpha = 1.92 and intensity 1.10 crab. 32 refs.

IBIS ground calibration

We present an overview of results obtained from IBIS ground calibrations. The spectral and spatial characteristics of the detector planes and surrounding passive materials have been determined through a series of calibration campaigns. Measurements of pixel gain, energy resolution, detection uniformity, efficiency and imaging capability are presented.

Development of a 3D CZT detector prototype for Laue Lens telescope

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 mm2 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.

A dedicated digital unit for event recognition and centroiding in photon counting intensified CCDs

A dedicated digital electronic system for real time acquisition and processing of data arrays with input rate as high as 20 MHz is presented. The system identifies and determines the center coordinates, to sub-pixel accuracy of events produced by a photon counting intensified CCD. Each photon event is represented, on the CCD matrix, by a charge distribution of approximately Gaussian profile. Identification is performed by a morphological analysis of the event profile. A number of characteristics of the expected profile is checked by dedicated units, operating in parallel, with an internal pipeline architecture that renders it possible to produce a result every 50 ns. System operation is managed by a microprogrammable control unit. The system prototype has been implemented by means of field programmable gate arrays (FPGA).