A. Mauri

Extreme particle acceleration in the microquasar Cygnus X-3

Super-massive black holes in active galaxies can accelerate particles to relativistic energies, producing jets with associated γ-ray emission. Galactic ‘microquasars’, which are binary systems consisting of a neutron star or stellar-mass black hole accreting gas from a companion star, also produce relativistic jets, generally together with radio flares. Apart from an isolated event detected in Cygnus X-1, there has hitherto been no systematic evidence for the acceleration of particles to gigaelectronvolt or higher energies in a microquasar, with the consequence that we are as yet unsure about the mechanism of jet energization. Here we report four γ-ray flares with energies above 100 MeV from the microquasar Cygnus X-3 (an exceptional X-ray binary that sporadically produces radio jets). There is a clear pattern of temporal correlations between the γ-ray flares and transitional spectral states of the radio-frequency and X-ray emission. Particle acceleration occurred a few days before radio-jet ejections for two of the four flares, meaning that the process of jet formation implies the production of very energetic particles. In Cygnus X-3, particle energies during the flares can be thousands of times higher than during quiescent states.

Discovery of extreme particle acceleration in the microquasar Cygnus X-3

Super-massive black holes in active galaxies can accelerate particles to relativistic energies, producing jets with associated γ-ray emission. Galactic ‘microquasars’, which are binary systems consisting of a neutron star or stellar-mass black hole accreting gas from a companion star, also produce relativistic jets, generally together with radio flares. Apart from an isolated event detected in Cygnus X-1, there has hitherto been no systematic evidence for the acceleration of particles to gigaelectronvolt or higher energies in a microquasar, with the consequence that we are as yet unsure about the mechanism of jet energization. Here we report four γ-ray flares with energies above 100 MeV from the microquasar Cygnus X-3 (an exceptional X-ray binary that sporadically produces radio jets). There is a clear pattern of temporal correlations between the γ-ray flares and transitional spectral states of the radio-frequency and X-ray emission. Particle acceleration occurred a few days before radio-jet ejections for two of the four flares, meaning that the process of jet formation implies the production of very energetic particles. In Cygnus X-3, particle energies during the flares can be thousands of times higher than during quiescent states.

The Ibis-Picsit detector onboard integral

PICsIT is the high-energy detector layer of the IBIS Imager, composed of 4096 CsI(Tl) scintillator detectors 8:4 8:4 300 mm in size with PhotoDiode readout. The detector operates in the 175 keV 20.4 MeV range and its data generation modes make it possible to collect information from single events and multiple coincident events. PICsIT is surrounded by the active BGO VETO and is located about 3 metres below the coded mask. The entire PICsIT plane is physically divided into 8 modules and logically divided into smaller units. The overall performance of the plane is directly related to the behaviour of each individual pixel, including its electronics, the system interconnection logic, and interaction with the other sub-systems. Pixels and electronic parameters were monitored constantly during instrument assembly. The following report describes PICsIT design and contains a summary of on-ground test results.

IBIS/PICsIT in-flight performances

PICsIT (Pixellated Imaging CaeSium Iodide Telescope) is the high energy detector of the IBIS telescope on-board the INTEGRAL satellite. PICsIT operates in the gamma-ray energy range between 175 keV and 10 MeV, with a typical energy resolution of 10% at 1 MeV, and an angular resolution of 12 arcmin within a100 square degree field of view, with the possibility to locate intense point sources in the MeV region at the few arcmin level. PICsIT is based upon a modular array of 4096 independent CsI(Tl) pixels,0.70 cm 2 in cross-section and 3 cm thick. In this work, the PICsIT on-board data handling and science operative modes are described. This work presents the in-flight performances in terms of background count spectra, sensitivity limit, and imaging capabilities.

The AGILE Instrument

AGILE is an ASI gamma-ray astrophysics space Mission which will operate in the 30 MeV - 50 GeV range with imaging capabilities also in the 10 - 40 keV range. Primary scientific goals include the study of AGNs, gamma-ray bursts, Galactic sources, unidentified gamma-ray sources, diffuse Galactic and extragalactic gamma-ray emission, high-precision timing studies, and Quantum Gravity testing. The AGILE scientific instrument is based on an innovative design of three detecting systems: (1) a Silicon Tracker, (2) a Mini-Calorimeter, and (3) an ultralight coded mask system with Si-detectors (Super-AGILE). AGILE is designed to provide: (1) excellent imaging in the energy bands 30 MeV-50 GeV (5-10 arcmin for intense sources) and 10-40 keV (1-3 arcmin); (2) optimal timing capabilities, with independent readout systems and minimal deadtimes for the Silicon Tracker, Super-AGILE and Mini-Calorimeter; (3) large field of view for the gamma-ray imaging detector (~3 sr) and Super-AGILE (~1 sr). AGILE will be the only Mission entirely dedicated to source detection above 30 MeV during the period 2004-2006.

ICARUS ASIC: a 16 channel photodiode read out system

In the framework of the INTEGRAL satellite, the second medium size mission of the ESA Horizon 2000 plus plan, it is required that the gamma ray detector of IBIS, one of the main on board instruments, exhibits position sensitive capabilities. This detector will be realised in two different planes covering different energy ranges. Although constructed with different technologies (Solid State and Scintillator detectors), both layers will be assembled using arrays of independent detectors (pixels) whose size will determine the spatial resolution of the instrument. In this approach each pixel needs its individual electronic chain able to process the charge delivered via a low noise charge amp and a shaping stage. Each chain will be completed with a trigger discriminator and peak and hold stage that will maintain the signal amplitude information locally for successive read out. To fulfil the constraints of space and power available for the project, these electronic chains can only be realised using ASIC technology. In this work the ICARUS ASIC used to read-out the signals delivered by PhotoDiodes (PD) employed in the high energy plane of the IBIS instrument is described and the first performance evaluation reported.

PICsIT: the high-energy detection plane of the IBIS instrument onboard INTEGRAL

IBIS is one of the two main instruments onboard the INTEGRAL gamma-ray satellite. IBIS will produce images of the gamma- ray sky in the region between 15 keV and 10 MeV by means of a coded mask coupled to a double-layer position sensitive detector. PICsIT is the detection layer optimized for high energy. It has a total area of 3065 cm2 and is composed by 4096 individual pixels made of CsI(Tl) crystal, each one with its proper electronic chain. The single units are 0.75 cm2 in area, and 3 cm thick. The front end electronics are designed so that analogue circuits, with their low noise figure, will allow the exploitation of the spectroscopic characteristics of the detector. The digital circuits will allow PICsIT to operate in anticoincidence with an active shield, and to deliver the interaction time of occurrence of the events.

AGILE and gamma-ray astrophysics

Abstract The study of γ rays is fundamental for our understanding of the universe: γ rays probe the most energetic phenomena occurring in nature, and several signatures of new physics are associated with the emission of γ rays. The main science objectives and the status of the new generation high-energy gamma-ray astrophysics experiment AGILE are presented.

ICARUS-SDD: a 16 channel ASIC for silicon drift detectors read-out

A prototype Application Specific Integrated Circuit (ASIC) to read out signals from Silicon Drift Detector has been realised and characterised. The device is based on the ICARUS ASIC that is currently employed into the PICsIT detector on board the INTEGRAL satellite http://astro.estec.esa.nl/Integral/integral.html. The first stage of the charge preamplifiers collecting the signals from the detectors is mounted externally to the ASIC. The ASIC functional behaviour is described and its performances are summarised and discussed.

The mini-calorimeter for the AGILE satellite

AGILE is a small space mission of the Italian Space Agency (ASI) devoted to observations for astrophysics in the gamma-ray energy range 30 MeV-50 GeV with a simultaneous window in the X-ray band 10 keV-40 keV. AGILE payload is made of a tungsten-silicon tracker, a CsI mini-calorimeter for the high energy band and a silicon based X-ray detector (SuperAgile); an anticoincidence system carries out background rejection. In the gamma-rays band the satellite will have a field of view of about 1/5 of the sky, with angular resolution of a few arc-minutes and good timing resolution. For the high energy AGILE detection principle is based on the pair production process that arises from the interaction between photons and the tungsten layers above the silicon tracker. The silicon tracker is designed to determine the direction of the incoming radiation, while the mini-calorimeter evaluates the energy of the interacting photons and particles. For the detection of transients and gamma-ray burst events, the mini-calorimeter will also work as a stand-alone gamma-ray detector, with no imaging capabilities, covering the energy range 250 keV-250 MeV. The mini-calorimeter is made of an array of position sensitive CsI(Tl) scintillator bars with photodiode read-out. The characteristics of its detector elements and the front end electronics signal processing allow a moderate position reconstruction of the detected event. In this paper a description of the mini-calorimeter is reported as well as a summary of performances reached with the pre-flight instrument models already built and tested.