Showing papers by "G. Di Cocco published in 2006"

INTEGRAL observations of the blazar 3C 454.3 in outburst

Abstract: In Spring 2005, the blazar 3C 454.3 underwent a dramatic outburst at all wavelengths from mm to X-rays. This prompted INTEGRAL observations, accomplished in 15-18 May 2005. The source was detected by the INTEGRAL instruments from 3 to 200 keV in a bright state (${\sim} 5 \times 10^{-10}$ erg s -1 cm -2 ), at least a factor of 2-3 higher than previously observed. This is one of the brightest blazar detections achieved by INTEGRAL . During the 2.5 days of INTEGRAL monitoring, we detected a ~20% decrease in the hard X-rays (20-40 keV), indicating that we have sampled the decaying part of the flare. The decrease is less apparent in the soft X-rays (5-15 keV). The simultaneous optical variations are weakly correlated with those at soft X-rays, and not clearly correlated with those at hard X-rays. The spectral energy distribution exhibits two components, as typically seen in blazars, which can be modeled with synchrotron radiation and inverse Compton scattering occurring in a region external to the broad line region.

XMM-Newton observations of a sample of gamma-ray loud active galactic nuclei

Abstract: (abridged) We performed a homogeneous and systematic analysis of simultaneous X-ray and optical/UV properties of a group of 15 gamma-ray loud AGN, using observations performed with XMM-Newton. The sample is composed of 13 blazars (6 BL Lac and 7 Flat-Spectrum Radio Quasar) and 2 radio galaxies, that are associated with detections at energies >100 MeV. The data for 7 of them are analyzed here for the first time, including the first X-ray observation of PKS 1406-706. Then, the spectral characteristics of the sources in the present sample have been compared with those in previous catalogs of blazars and other AGN, in order to search for any difference or long term change. All the selected sources appear to follow the classic "blazar sequence" and the spectral energy distributions (SED) built with the present X-ray and optical/UV data and completed with historical data, confirm the findings of previous studies on this type of sources. Some sources display interesting features worth noting: four of them, namely AO 0235+164, PKS 1127-145, S5 0836+710 and PKS 1830-211 show the presence of an intervening absorption system along the line of sight, but only the latter is known to be surely gravitationally lensed. AO 0235+164 was detected during an outburst and its SED shows a clear shift of the synchrotron peak. 3C 273 shows a change in the state with respect to the previous BeppoSAX observations, that can be interpreted as an increase of the Seyfert-like component and a corresponding decline of the jet emission. This is consistent with the monitoring at radio wavelengths performed during the same years. PKS 1406-706 is detected with a flux higher than in the past, but with a corresponding low optical flux. Moreover, although it is classified as FSRQ, the SED can be modelled with a simple synchrotron self-Compton model.

Simultaneous X-ray and optical observations of S5 0716+714 after the outburst of March 2004

Abstract: At the end of March 2004, the blazar S5 0716 + 714 underwent an optical outburst that prompted for quasi-simultaneous target-of-opportunity observations with the INTEGRAL and XMM-Newton satellites. In this paper, we report the results of the XMM-Newton and INTEGRAL OMC data analysis. The X-ray spectrum is well-represented by a concave broken power-law model, with the break at about 2 keV. In the framework of the synchrotron self-Compton model, the softer part of the spectrum, which is described by a power law of index a ≃ 1.8 (f v oc v -α ), is probably due to synchrotron emission, while the harder part of the spectrum, which has a ≃ 1, is due to inverse Compton emission. The blazar shows the long and short-term variability typical of low-frequency peaked BL Lac (LBL): the former is manifested by a gradual decrease in the optical flux from the peak as observed by ground telescopes at the end of March 2004, while the latter is characterized by soft X-ray and optical flares on time scales from a few thousand seconds to few hours. We can follow spectral variations on sub-hour time scales and study their correlation with the flux variability. We find evidence that the peak energy of the time-resolved spectra is increasing with flux. The modeling of the spectral energy distribution compared with archival observations suggests that the long-term variability (from outburst to quiescence or viceversa) could be due to a change in the injected power, while the short-term variability (flares) could be explained with changes in the slope of the distribution of the electrons.

XMM-Newton observations of a sample of γ-ray loud active galactic nuclei

Abstract: Aims. To understand the nature of γ-ray loud active galactic nuclei (AGN) and the mechanisms for the generation of high-energy γ-rays. Methods. We performed a homogeneous and systematic analysis of simultaneous X-ray and optical/UV properties of a group of 15 γ-ray loud AGN, using observations performed with XMM-Newton. The sample is composed of 13 blazars (6 BL Lac and 7 Flat-Spectrum Radio Quasar) and 2 radio galaxies that are associated with detections at energies >100 MeV. The data for 7 of them are analyzed here for the first time, including the first X-ray observation of PKS 1406 - 706. The spectral characteristics of the sources in the present sample were compared with those in previous catalogs of blazars and other AGN, to search for difference or long term changes. Results. All the selected sources appear to follow the classic "blazar sequence" and the spectral energy distributions (SED) built with the present X-ray and optical/UV data and completed with historical data, confirm the findings of previous studies on this type of source. Some sources display interesting features: four of them, namely AO 0235 + 164, PKS 1127 - 145, S5 0836 + 710 and PKS 1830 - 211 show the presence of an intervening absorption system along the line of sight, but only the last is known to be gravitationally lensed. AO 0235 + 164 was detected during an outburst and its SED shows a clear shift of the synchrotron peak. 3C 273 shows a change in state with respect to the previous BeppoSAX observations that can be interpreted as an increase of the Seyfert-like component and a corresponding decline of the jet emission. This is consistent with the monitoring at radio wavelengths performed during the same period. PKS 1406-706 is detected with a flux higher than in the past, but with a corresponding low optical flux. Although it is classified as FSRQ, the SED can be modelled with a simple synchrotron self-Compton model.

INTEGRAL and XMM–Newton observations of the X-ray pulsar IGR J16320−4751/AX J1631.9−4752

Abstract: We report on observations of the X-ray pulsar IGR J16320−4751 (a.k.a. AX J1631.9−4752) performed simultaneously with INTEGRAL and XMM-Newton. We refine the source position and identify the most likely infrared counterpart. Our simultaneous coverage allows us to confirm the presence of X-ray pulsations at ∼ 1300 s, that we detect above 20 keV with INTEGRAL for the first time. The pulse fraction is consistent with bein g constant with energy, which is compatible with a model of polar accretion by a pulsar. We study the spectral properties of IGR J16320−4751 during two major periods occurring during the simultaneous coverage with both satellites, namely a flare and a non-flare p eriod. We detect the presence of a narrow 6.4 keV iron line in both periods. The presence of such a feature is typical of supergiant wind accretors such as Vela X-1 or GX 301−2. We inspect the spectral variations with respect to the pulse phase during the non-flare period, a nd show that the pulse is solely due to variations of the X-ray flux emitted by the source and no t to variations of the spectral parameters. Our results are therefore compatible with the source being a pulsar in a High Mass X-ray Binary. We detect a soft excess appearing in the spectra as a blackbody with a temperature of∼0.07 keV. We discuss the origin of the X-ray emission in IGR J16320−4751: while the hard X-rays are likely the result of Compton emission produced in the close vicinity of the pulsar, based on energy argument we suggest that the soft excess is likely the emission by a collisionally energised cloud in which the compact object is embedded.

INTEGRAL observations of the Crab pulsar

Abstract: Aims. The paper presents the timing and spectral analysis of several observations of the Crab pulsar performed with INTEGRAL in the energy range 3-500 keV. Methods. All these observations, when summed together provide a high statistics data set which can be used for accurate phase resolved spectroscopy. A detailed study of the pulsed emission at different phase intervals is performed. Results. The spectral distribution changes with phase showing a characteristic reverse S shape of the photon index. Moreover the spectrum softens with energy, in each phase interval, and this behavior is adequately modeled over the whole energy range 3-500 keV with a single curved law with a slope variable with Log(E), confirming the BeppoSAX results on the curvature of the pulsed emission. The bending parameter of the log-parabolic model is compatible with a single value of 0.14±0.02 over all phase intervals. Conclusions. Results are discussed within the three-dimensional outer gap model.

The AGILE mission and its scientific instrument

Abstract: 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.

A short hard X-ray flare from the blazar NRAO 530 observed by INTEGRAL

Abstract: We report about a short flare from the blazar NRAO 530 occurred on 17 February 2004 and detected serendipitously by the IBIS/ISGRI detector on board INTEGRAL . In the $20{-}40$ keV energy range, the source, that is otherwise below the detection limit, is detected at a level of ≈ 2 $\times$ 10 -10 erg cm -2 s -1 during a time interval of less than 2000 s, which is about a factor 2 above the detection threshold. At other wavelengths, only nearly-simultaneous radio data are available (1 observation at 2 cm on 11 February 2004), indicating a moderate increase of the polarization. This appears to be the shortest time variability episode ever detected in a high luminosity blazar at hard X-rays, unless the blazar is contaminated by the presence of an unknown unresolved rapidly varying source.

The Mini-Calorimeter detector for the AGILE mission

Abstract: AGILE is an ASI space mission for high energy astrophysics in the gamma ray energy range 30MeV-50GeV, and in the X-ray band (10keV-40keV). AGILE is composed of three detecting systems: a Tungsten-Silicon Tracker, a Csl Mini-Calorimeter and a Silicon based X-ray detector (Super-Agile), plus an anticoincidence system for background rejection. The satellite will have good imaging performances (with angular resolution of a few arc-minutes in the gamma ray band), good timing resolution and a large field of view (about 1/5 of the sky). AGILE high energy detector is composed of a Silicon Tracker, a Mini-Calorimeter, the Anticoincidence and the Data Handling system The Mini-Calorimeter can also work as a stand-alone gamma ray detector in the energy range 250keV-250MeV, with no imaging capabilities, for the detection of transients and gamma ray burst events and for the evaluation of gamma ray background fluctuations. The Mini-Calorimeter is made of 30 independent elements of 37.5 × 2.3 × 1.5 cm stacked in two layers. Each detector scintillator signal is read out via two photodiodes glued on the small surfaces of the bar. The characteristics of the engineering model detectors has been deeply investigated and will be reported.

Characteristics and Performance of the GAW Experiment for a Large Field of View Cerenkov Gamma-ray Telescope

Abstract: One of the intents of the ground-based gamma-ray astronomy is to obtain a sky survey in the TeV energy region, and nowadays this target can be reached with giant arrays of telescopes, which however need many pointings due to their small field of view. A different approach is on the basis of GAW, acronym for Gamma Air Watch, an array of three relatively small Cerenkov telescopes which differentiate from the existing and presently planned telescopes for two main features: the adoption of a refractive optics system as light collector with a large field of view capability, and the use of single photoelectron counting as detector working mode. During a first phase, the focal plane detector of the GAW telescopes will be implemented in a reduced configuration to test the sensitivity and to prove the feasibility of the method; then the focal plane will be enlarged to cover a field of view of 24°×24°; pointing along different North-South directions, GAW would reach a survey of 360°×60° region of the sky. In this paper, the GAW expected performance are reported as evaluated in the case of the Calar Alto site, Spain, 2150 m a.s.l., where GAW is planned to be located within 2007. GAW is a collaboration effort of Research Institutes in Italy, Portugal and Spain.

INTEGRAL observations of the Crab pulsar

Abstract: The paper presents the timing and spectral analysis of several observations of the Crab pulsar performed with INTEGRAL in the energy range 3-500 keV. All these observations, when summed together provide a high statistics data set which can be used for accurate phase resolved spectroscopy. A detailed study of the pulsed emission at different phase intervals is performed. The spectral distribution changes with phase showing a characteristic reverse S shape of the photon index. Moreover the spectrum softens with energy, in each phase interval, and this behavior is adequately modeled over the whole energy range 3-500 keV with a single curved law with a slope variable with Log(E), confirming the BeppoSAX results on the curvature of the pulsed emission. The bending parameter of the log-parabolic model is compatible with a single value of 0.14+/-0.02 over all phase intervals. Results are discussed within the three-dimensional outer gap model.

The MEGA Project for Medium Energy Gamma-ray Astronomy

Abstract: The Medium Energy Gamma-ray Astronomy (MEGA) telescope concept will soon be proposed as a MIDEX mission This mission would enable a sensitive all-sky survey of the medium-energy gamma-ray sky (04–50 MeV) and bridge the huge sensitivity gap between the COMPTEL and OSSE experiments on the Compton Gamma Ray Observatory and the visionary Advanced Compton Telescope (ACT) mission The scientific goals include compiling a much larger catalog of sources in this energy range, performing far deeper searches for supernovae, better measuring the galactic continuum and line emissions, and identifying the components of the cosmic diffuse gamma-ray emission MEGA records and images gamma rays by completely tracking Compton and pair creation events in a stack of double-sided Si strip detectors surrounded by a pixellated CsI calorimeter A prototype instrument has been developed and calibrated in the laboratory and at a gamma-ray beam facility We present calibration results from the prototype and describe the proposed satellite mission

Simultaneous X-ray and optical observations of S5~0716+714 after the outburst of March 2004

Abstract: At the end of March 2004, the blazar S5 0716+714 underwent an optical outburst that prompted for quasi-simultaneous target-of-opportunity observations with the INTEGRAL and XMM-Newton satellites. In this paper, we report the results of the XMM-Newton and INTEGRAL OMC data analysis. The X-ray spectrum is well-represented by a concave broken power-law model, with the break at about 2 keV. In the framework of the synchrotron self-Compton model, the softer part of the spectrum, which is described by a power law of index $\alpha \simeq 1.8$ ($f_ u \propto u^{-\alpha}$), is probably due to synchrotron emission, while the harder part of the spectrum, which has $\alpha \simeq 1$, is due to inverse Compton emission. The blazar shows the long and short-term variability typical of low-frequency peaked BL Lac (LBL): the former is manifested by a gradual decrease in the optical flux from the peak as observed by ground telescopes at the end of March 2004, while the latter is characterized by soft X-ray and optical flares on time scales from a few thousand seconds to few hours. We can follow spectral variations on sub-hour time scales and study their correlation with the flux variability. We find evidence that the peak energy of the time-resolved spectra is increasing with flux. The modeling of the spectral energy distribution compared with archival observations suggests that the long-term variability (from outburst to quiescence or viceversa) could be due to a change in the injected power, while the short-term variability (flares) could be explained with changes in the slope of the distribution of the electrons.

Design of a high update-rate star sensor for arcsec-level attitude determination from balloon-borne X/γ astronomy platforms

Abstract: This paper describes the design of a star sensor based upon a high dynamic range CCD in order to reach an arcsec-level attitude determination in balloon-borne missions. A custom star identification software was developed and laboratory-tested on a prototype assembled using commercial components. A set of numerical simulations have been carried out to study the dependence on the pointing precision of the centroid position accuracy, the number of detected stars and the effect of the image focusing. Moreover, the role of the electronic noise and the discrete pixel structure on the light signals is identified by the analysis of numerical simulations. Laboratory tests confirm that the arcsec pointing accuracy with a 1 Hz update rate can be achieved with our combination of custom-developed software and selected hardware components.

Gamma-Ray Astrophysics with AGILE

Abstract: AGILE will explore the gamma‐ray Universe with a very innovative instrument combining for the first time a gamma‐ray imager and a hard X‐ray imager. AGILE will be operational in spring 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 AGILE instrument is designed to simultaneously detect and image photons in the 30 MeV – 50 GeV and 15 – 45 keV energy bands with excellent imaging and timing capabilities, and a large field of view covering ∼ 1/5 of the entire sky at energies above 30 MeV. A CsI calorimeter is capable of GRB triggering in the energy band 0.3–50 MeV AGILE is now (March 2007) undergoing launcher integration and testing. The PLSV launch is planned in spring 2007. AGILE is then foreseen to be fully operational during the summer of 2007.