Papers by Gaetano Gerardi
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The Astrophysical Journal, 2006
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We present numerical simulation results for the accretion disk models where advection or radial v... more We present numerical simulation results for the accretion disk models where advection or radial velocity is important. We use Smoothed Particle Hydrodynamics. We reproduce the solution topologies predicted for these disks and show that shocks may form for a large region of the parameter space (Molteni, Lanzafame and Chakrabarti, 1994; Molteni, Ryu and Chakrabarti, 1996). When viscosity is steadily increased the shocks become weaker until they disappear altogether (Chakrabarti & Molteni, 1995). When shocks are not predicted, the flow can still form a shock but may oscillates back and forth. This is true even for viscous flows (Lanzafame, Molteni and Chakrabarti, 1997). In presence of cooling, such as bremsstrahlung, the shocks also oscillate when the cooling time scale is comparable with the infall time scale (Molteni, Sponholz & Chakrabarti, 1996). In a more recent simulation using TVD method, we show that these shocks are stable even for non-Axisymmetric instability (Molteni, Toth and Kuznetov ). Advective disks of constant viscosity parameter need not join with a Keplerian disk smoothly. We show this deviation of the angular momentum of these disks from a Keplerian one. When shocks do not form, the deviation from the Keplerian disk roughly becomes larger for smaller viscosities and vice-versa. This we show by SPH techniques. We believe that steady spectral states, such as hard and soft states (which may be due to larger and smaller inner edge of the Keplerian disk) as well as time-dependent X-ray variations as observed in, e.g., QPOs may be due to different manifestation of the same set of equations.
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Proceedings of the International Astronomical Union
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ABSTRACT
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Proceedings of the International Astronomical Union
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ABSTRACT
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The paper presents the low-level languages implemented up to date to program the PAPIA machine. T... more The paper presents the low-level languages implemented up to date to program the PAPIA machine. The parallel assembly-level P-MACRO package, the microcode level instruction set and a machine simulating environment are described.
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Astronomy and Astrophysics
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ABSTRACT
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Journal of the Korean Astronomical Society
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Proceedings of the International Astronomical Union
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Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2015
ABSTRACT Photon counting detectors with energy resolving capabilities are desired for high flux X... more ABSTRACT Photon counting detectors with energy resolving capabilities are desired for high flux X-ray imaging. In this work, we present the performance of a pixelated Schottky Al/p-CdTe/Pt detector (4×4) coupled to a custom-designed digital readout electronics for high flux measurements. The detector (4×4×2 mm3) has an anode layout based on an array of 16 pixels with a geometric pitch of 1 mm (pixel size of 0.6 mm). The 4-channel readout electronics is able to continuously digitize and process the signals from each pixel, performing multi-parameter analysis (event arrival time, pulse shape, pulse height, pulse time width, etc.) even at high fluxes and at different throughput and energy resolution conditions. The spectroscopic response of the system to monochromatic X-ray sources, at both low and high rates, is presented with particular attention to the mitigation of some typical spectral distortions (pile-up, baseline shifts and charge sharing). At a photon counting rate of 520 kcps/pixel, the system exhibits an energy resolution (FWHM at 59.5 keV) of 4.6%, 7.1% and 9% at throughputs of 0.9%, 16% and 82%, respectively. Measurements of Ag-target X-ray spectra also show the ability of the system to perform accurate estimation of the input counting rate up to 1.1 Mcps/pixel.
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Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2014
ABSTRACT Modern spectrometers are currently developed by using digital pulse processing (DPP) sys... more ABSTRACT Modern spectrometers are currently developed by using digital pulse processing (DPP) systems, showing several advantages over traditional analog electronics. The aim of this work is to present digital strategies, in a time domain, for the development of real time high-rate high-resolution spectrometers. We propose a digital method, based on the single delay line (SDL) shaping technique, able to perform multi-parameter analysis with high performance even at high photon counting rates. A robust pulse shape and height analysis (PSHA), applied on single isolated time windows of the detector output waveforms, is presented. The potentialities of the proposed strategy are highlighted through both theoretical and experimental approaches. To strengthen our approach, the implementation of the method on a real-time system together with some experimental results are presented. X-ray spectra measurements with a semiconductor detector are performed both at low and high photon counting rates (up to 1.1 Mcps).
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Journal of Instrumentation, 2013
ABSTRACT New generation spectroscopy systems have advanced towards digital pulse processing (DPP)... more ABSTRACT New generation spectroscopy systems have advanced towards digital pulse processing (DPP) approaches. DPP systems, based on direct digitizing and processing of detector signals, have recently been favoured over analog pulse processing electronics, ensuring higher flexibility, stability, lower dead time, higher throughput and better spectroscopic performance. In this work, we present the performance of a new real time DPP system for X-ray and gamma ray semiconductor detectors. The system is based on a commercial digitizer equipped with a custom DPP firmware, developed by our group, for on-line pulse shape and height analysis. X-ray and gamma ray spectra measurements with cadmium telluride (CdTe) and germanium (Ge) detectors, coupled to resistive-feedback preamplifiers, highlight the excellent performance of the system both at low and high rate environments (up to 800 kcps). A comparison with a conventional analog electronics showed the better high-rate capabilities of the digital approach, in terms of energy resolution and throughput. These results make the proposed DPP system a very attractive tool for both laboratory research and for the development of advanced detection systems for high-rate-resolution spectroscopic imaging, recently proposed in diagnostic medicine, industrial imaging and security screening.
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Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2013
ABSTRACT Digital pulse processing (DPP) systems, based on direct digitizing and processing of det... more ABSTRACT Digital pulse processing (DPP) systems, based on direct digitizing and processing of detector signals, have recently been favoured over analog electronics, ensuring higher flexibility, stability, lower dead time and better spectroscopic performance. In this work, we present the performance of a new real time DPP system for X-ray and gamma ray semiconductor detectors. The system is based on a commercial digitizer equipped with a custom DPP firmware, developed by our group, for on-line pulse height and shape analysis. X-ray and gamma ray spectra measurements with cadmium telluride (CdTe) and germanium (Ge) detectors highlight the excellent performance of the system both at low and high rate environments (up to 800 kcps). These results make the proposed DPP system a very attractive tool for both laboratory research and the development of advanced detection systems for high-rate-resolution spectroscopic imaging, recently proposed in diagnostic medicine, industrial imaging and security screening.
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Papers by Gaetano Gerardi