Search

We report the results of a 3-weeks multi-wavelength campaign on the flat spectrum radio quasar 3C 273 carried out with the AGILE gamma-ray mission, covering the 30 MeV -50 GeV and 18-60 keV, the REM observatory (covering the near-IR and optical), Swift (near-UV/Optical, 0.2-10 keV and 15-50 keV), INTEGRAL (3 - 200 keV) and Rossi XTE (2-12 keV). This is the first observational campaign including gamma-ray data, after the last EGRET observations, more than 8 years ago. Our study was carried out using simultaneous light curves of the source flux from all the involved instruments, in the different energy ranges, in search for correlated variability. Then a time-resolved SED was used for a detailed physical modelling of the emission mechanisms. The source was detected in gamma-rays only in the second week of our campaign. We found indication of a possible anti-correlation between the emission at gamma-rays and at soft and hard X-rays, supported by the complete set of instruments. Instead, optical data do not show short term variability. Only in two EGRET observations (in 1993 and 1997) 3C 273 showed intra-observation variability in gamma-rays. The SED with almost simultaneous data, partially covers the regions of the synchrotron emission, the big blue bump, and the inverse-Compton. The soft X-ray emission is consistent with combined SSC and EC mechanisms, while hard X and gamma-ray emissions are compatible with EC from thermal photons of the disk. The time evolution of the SED is well interpreted and modelled in terms of an acceleration episode of the electrons population, leading to a shift in the IC peak towards higher energies., Comment: Regular paper, Acceped by Astronomy & Astrophysics, 25 pages, 7 tables, 5 figures

Context. We report the detection by the AGILE satellite of an intense gamma-ray flare from the gamma-ray source 3EG J1255-0549, associated to the Flat Spectrum Radio Quasar 3C 279, during the AGILE pointings towards the Virgo Region on 2007 July 9-13. Aims. The simultaneous optical, X-ray and gamma-ray covering allows us to study the spectral energy distribution (SED) and the theoretical models relative to the flaring episode of mid-July. Methods. AGILE observed the source during its Science Performance Verification Phase with its two co-aligned imagers: the Gamma- Ray Imaging Detector (GRID) and the hard X-ray imager (Super-AGILE) sensitive in the 30 MeV - 50 GeV and 18 - 60 keV respectively. During the AGILE observation the source was monitored simultaneously in optical band by the REM telescope and in the X-ray band by the Swift satellite through 4 ToO observations. Results. During 2007 July 9-13 July 2007, AGILE-GRID detected gamma-ray emission from 3C 279, with the source at ~2 deg from the center of the Field of View, with an average flux of (210+-38) 10^-8 ph cm^-2 s^-1 for energy above 100 MeV. No emission was detected by Super-AGILE, with a 3-sigma upper limit of 10 mCrab. During the observation lasted about 4 days no significative gamma-ray flux variation was observed. Conclusions. The Spectral Energy Distribution is modelled with a homogeneous one-zone Synchrotron Self Compton emission plus the contributions by external Compton scattering of direct disk radiation and, to a lesser extent, by external Compton scattering of photons from the Broad Line Region., Comment: Accepted for publication in Astronomy and Astrophysics

AGILE is a small space mission of the Italian Space Agency (ASI) devoted to gamma-ray and hard-X astrophysics, successfully launched on April 23 2007. The AGILE Payload is composed of three instruments: a gamma-ray imager based on a tungsten-silicon tracker (ST), for observations in the gamma ray energy range 30MeV - 50GeV, a Silicon based X-ray detector, SuperAGILE (SA), for imaging in the range 18keV - 60keV and a CsI(Tl) Mini-Calorimeter (MCAL) that detects gamma rays or charged particles energy loss in the range 300keV - 100MeV. MCAL is composed of 30 CsI(Tl) scintillator bars with photodiode readout at both ends, arranged in two orthogonal layers. MCAL can work both as a slave of the ST and as an independent gamma-ray detector for transients and gamma-ray bursts detection. In this paper a detailed description of MCAL is presented together with its performance., Comment: 36 pages, 15 figures, 2 tables, accepted for publication in Nucl. Instr. and Meth. A

We report the detection by the AGILE (Astro-rivelatore Gamma a Immagini LEggero) satellite of an intense gamma-ray flare from the source AGL J1511-0909, associated with the powerful quasar PKS 1510-089, during ten days of observations from 23 August to 1 September 2007. During the observation period, the source was in optical decrease following a flaring event monitored by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT). The simultaneous gamma-ray, optical, and radio coverage allows us to study the spectral energy distribution and the theoretical models based on the synchrotron and inverse Compton (IC) emission mechanisms. AGILE observed the source with its two co-aligned imagers, the Gamma-Ray Imaging Detector and the hard X-ray imager Super-AGILE sensitive in the 30 MeV - 50 GeV and 18 - 60 keV bands, respectively. Between 23 and 27 August 2007, AGILE detected gamma-ray emission from PKS 1510-089 when this source was located about 50 degrees off-axis, with an average flux of (270 +/- 65) x 10^{-8} photons cm^{-2} s^{-1} for photon energy above 100 MeV. In the following period, 28 August - 1 September, after a satellite re-pointing, AGILE detected the source at about 35 degrees off-axis, with an average flux (E > 100 MeV) of (195 +/- 30) x 10^{-8} photons cm^{-2} s^{-1}. No emission was detected by Super-AGILE, with a 3-sigma upper limit of 45 mCrab in 200 ksec. The spectral energy distribution is modelled with a homogeneous one-zone synchrotron self Compton (SSC) emission plus contributions by external photons: the SSC emission contributes primarily to the X-ray band, whereas the contribution of the IC from the external disc and the broad line region match the hard gamma-ray spectrum observed., Comment: 5 pages, 5 figures, accepted for publication in A&A

AGILE is a small gamma-ray astronomy satellite mission of the Italian Space Agency dedicated to high-energy astrophysics launched in 2007 April. Its 1 microsecond absolute time tagging capability coupled with a good sensitivity in the 30 MeV-30 GeV range, with simultaneous X-ray monitoring in the 18-60 keV band, makes it perfectly suited for the study of gamma-ray pulsars following up on the CGRO/EGRET heritage. In this paper we present the first AGILE timing results on the known gamma-ray pulsars Vela, Crab, Geminga and B 1706-44. The data were collected from 2007 July to 2008 April, exploiting the mission Science Verification Phase, the Instrument Timing Calibration and the early Observing Pointing Program. Thanks to its large field of view, AGILE collected a large number of gamma-ray photons from these pulsars (about 10,000 pulsed counts for Vela) in only few months of observations. The coupling of AGILE timing capabilities, simultaneous radio/X-ray monitoring and new tools aimed at precise photon phasing, exploiting also timing noise correction, unveiled new interesting features at sub-millisecond level in the pulsars' high-energy light-curves., Comment: Accepted for publication in The Astrophysical Journal. 14 pages in emulate-apj style, 2 tables, 9 figures (1 color). Replacement corrected Figure 6

The Mini-Calorimeter (MCAL) instrument on-board the AGILE satellite is a non-imaging gamma-ray scintillation detector sensitive in the 300keV-100MeV energy range with a total on-axis geometrical area of 1400cm^2. Gamma-Ray Bursts (GRBs) are one of the main scientific targets of the AGILE mission and the MCAL design as an independent self-triggering detector makes it a valuable all-sky monitor for GRBs. Furthermore MCAL is one of the very few operative instruments with microsecond timing capabilities in the MeV range. In this paper the results of GRB detections with MCAL after one year of operation in space are presented and discussed. A flexible trigger logic implemented in the AGILE payload data-handling unit allows the on-board detection of GRBs. For triggered events, energy and timing information are sent to telemetry on a photon-by-photon basis, so that energy and time binning are limited by counting statistics only. When the trigger logic is not active, GRBs can be detected offline in ratemeter data, although with worse energy and time resolution. Between the end of June 2007 and June 2008 MCAL detected 51 GRBs, with a detection rate of about 1 GRB/week, plus several other events at a few milliseconds timescales. Since February 2008 the on-board trigger logic has been fully active. Comparison of MCAL detected events and data provided by other space instruments confirms the sensitivity and effective area estimations. MCAL also joined the 3rd Inter-Planetary Network, to contribute to GRB localization by means of triangulation., Comment: 6 pages, 7 figures, accepted for publication by A&A 13/08/2008

[Abridged] We report on a multiwavelength observation of the blazar 3C 454.3 (which we dubbed "crazy diamond") carried out on November 2007 by means of the astrophysical satellites AGILE, INTEGRAL, Swift, the WEBT Consortium, and the optical-NIR telescope REM. 3C 454.3 is detected at a $\sim 19-\sigma$ level during the 3-week observing period, with an average flux above 100 MeV of $F_{\rm E>100MeV} = (170 \pm 13) \times 10^{-8}$ \phcmsec. The gamma-ray spectrum can be fit with a single power-law with photon index $\Gamma_{\rm GRID} = 1.73 \pm 0.16$ between 100 MeV and 1 GeV. We detect significant day-by-day variability of the gamma-ray emission during our observations, and we can exclude that the fluxes are constant at the 99.6% ($\sim 2.9 \sigma$) level. The source was detected typically around 40 degrees off-axis, and it was substantially off--axis in the field of view of the AGILE hard X-ray imager. However, a 5-day long ToO observation by INTEGRAL detected 3C 454.3 at an average flux of about $F_{\rm 20-200 keV} = 1.49 \times 10^{-3}$ \phcmsec with an average photon index of $\Gamma_{\rm IBIS} = 1.75 \pm 0.24$ between 20--200 keV. Swift also detected 3C 454.3 with a flux in the 0.3--10 keV energy band in the range $(1.23-1.40) \times 10^{-2}$ \phcmsec{} and a photon index in the range $\Gamma_{\rm XRT} = 1.56-1.73$. In the optical band, both WEBT and REM show an extremely variable behavior in the $R$ band. A correlation analysis based on the entire data set is consistent with no time-lags between the gamma-ray and the optical flux variations. Our simultaneous multifrequency observations strongly indicate that the dominant emission mechanism between 30 MeV and 30 GeV is dominated by inverse Compton scattering of relativistic electrons in the jet on the external photons from the broad line region., Comment: Accepted for publication in ApJ. Abridged Abstract. 37 pages, 14 Figures, 3 Tables

GRB 080514B is the first gamma ray burst (GRB), since the time of EGRET, for which individual photons of energy above several tens of MeV have been detected with a pair-conversion tracker telescope. This burst was discovered with the Italian AGILE gamma-ray satellite. The GRB was localized with a cooperation by AGILE and the interplanetary network (IPN). The gamma-ray imager (GRID) estimate of the position, obtained before the SuperAGILE-IPN localization, is found to be consistent with the burst position. The hard X-ray emission observed by SuperAGILE lasted about 7 s, while there is evidence that the emission above 30 MeV extends for a longer duration (at least ~13 s). Similar behavior was seen in the past from a few other GRBs observed with EGRET. However, the latter measurements were affected, during the brightest phases, by instrumental dead time effects, resulting in only lower limits to the burst intensity. Thanks to the small dead time of the AGILE/GRID we could assess that in the case of GRB 080514B the gamma-ray to X-ray flux ratio changes significantly between the prompt and extended emission phase., Comment: A&A letters, in press

We report the gamma-ray activity from the intermediate BL Lac S5 0716+714 during observations acquired by the AGILE satellite in September and October 2007. These detections of activity were contemporaneous with a period of intense optical activity, which was monitored by GASP-WEBT. This simultaneous optical and gamma-ray coverage allows us to study in detail the light curves, time lags, gamma-ray photon spectrum, and Spectral Energy Distributions (SEDs) during different states of activity. AGILE observed the source with its two co-aligned imagers, the Gamma-Ray Imaging Detector (GRID) and the hard X-ray imager (Super-AGILE), which are sensitive to the 30 MeV-50 GeV and 18-60 keV energy ranges, respectively. Observations were completed in two different periods, the first between 2007 September 4-23, and the second between 2007 October 24-November 1. Over the period 2007 September 7-12, AGILE detected gamma-ray emission from the source at a significance level of 9.6-sigma with an average flux (E>100 MeV) of (97 +/- 15) x 10^{-8} photons cm^{-2} s^{-1}, which increased by a factor of at least four within three days. No emission was detected by Super-AGILE for the energy range 18-60 keV to a 3-sigma upper limit of 10 mCrab in 335 ksec. In October 2007, AGILE repointed toward S5 0716+714 following an intense optical flare, measuring an average flux of (47 +/- 11) x 10^{-8} photons cm^{-2} s^{-1} at a significance level of 6.0-sigma. The gamma-ray flux of S5 0716+714 detected by AGILE is the highest ever detected for this blazar and one of the most intense gamma-ray fluxes detected from a BL Lac object. The SED of mid-September appears to be consistent with the synchrotron self-Compton (SSC) emission model, but only by including two SSC components of different variabilities., Comment: 5 pages, 5 figures, accepted by A&A Letters 18/8/2008

We present the AGILE gamma-ray observations of the field containing the puzzling gamma-ray source 3EG J1835+5918. This source is one of the most remarkable unidentified EGRET sources. An unprecedentedly long AGILE monitoring of this source yields important information on the positional error box, flux evolution, and spectrum. 3EG J1835+5918 has been in the AGILE field of view several times in 2007 and 2008 for a total observing time of 138 days from 2007 Sept 04 to 2008 June 30 encompassing several weeks of continuous coverage. With an exposure time approximately twice that of EGRET, AGILE confirms the existence of a prominent gamma-ray source (AGL J1836+5926) at a position consistent with that of EGRET, although with a remarkably lower average flux value for photon energies greater than 100 MeV. A 5-day bin temporal analysis of the whole data set of AGL J1836+5926 shows some evidence for variability of the gamma-ray flux. The source spectrum between 100 MeV and 1 GeV can be fitted with a power law with photon index in the range 1.6-1.7, fully consistent with the EGRET value. The faint X-ray source RX J1836.2+5925 that has been proposed as a possible counterpart of 3EG J1835+5918 is well within the AGILE error box. Future continuous monitoring (both by AGILE and GLAST) is needed to confirm the gamma-ray flux variability and to unveil the source origin, a subject that is currently being pursued through a multiwavelength search for counterparts., Comment: accepted for publication by Astron. & Astrophys, 4 pages, 2 figures, 2 tables

AIMS: Our goal is to understand the nature of blazars and the mechanisms for the generation of high-energy gamma-rays, through the investigation of the prototypical blazar PKS 2155-304, which shows complex behaviour. METHODS: We analyze simultaneous infrared-to-X-ray observations obtained with XMM-Newton and REM on November 7, 2006, when the source was in a low X-ray state. We perform a comparative analysis of these results with those obtained from previous observations in different brightness states. RESULTS: We found that the peak of the synchrotron emission moved from ultraviolet to optical wavelengths and the X-ray spectrum is best fit with a broken power law model with Gamma_2 ~ 2.4 harder than Gamma_1 ~ 2.6 and a break at about 3.5 keV. This suggests that the soft X-rays (E < 3.5 keV) are related to the high-energy tail of the synchrotron emission, while the hard X-rays (E > 3.5 keV) are from the energy region between the synchrotron and inverse-Compton humps. The different variability at energies below and above the break strengthens this hypothesis. Our results also stress the importance of monitoring this source at both low and high energies to better characterize its variability behaviour., Comment: 5 pages, 3 figures. A&A Letters, Accepted for publication

We report the first blazar detection by the AGILE satellite. AGILE detected 3C 454.3 during a period of strongly enhanced optical emission in July 2007. AGILE observed the source with a dedicated repointing during the period 2007 July 24-30 with its two co-aligned imagers, the Gamma-Ray Imaging Detector and the hard X-ray imager Super-AGILE sensitive in the 30 MeV-50 GeV and 18-60 keV, respectively. Over the entire period, AGILE detected gamma-ray emission from 3C 454.3 at a significance level of 13.8-$\sigma$ with an average flux (E$>$100 MeV) of $(280 \pm 40) \times 10^{-8}$ photons cm$^{-2}$ s$^{-1}$. The gamma-ray flux appears to be variable towards the end of the observation. No emission was detected by Super-AGILE in the energy range 20-60 keV, with a 3-$\sigma$ upper limit of $2.3 \times 10^{-3}$ photons cm$^{-2}$ s$^{-1}$. The gamma-ray flux level of 3C 454.3 detected by AGILE is the highest ever detected for this quasar and among the most intense gamma-ray fluxes ever detected from Flat Spectrum Radio Quasars., Comment: Accepted by Astrophysical Journal Letters; 14 pages, 3 EPS Figures, 1 Table

This Conference is the fifth of a series of Workshops on High Energy Gamma- ray Experiments, following the Conferences held in Perugia 2003, Bari 2004, Cividale del Friuli 2005, Elba Island 2006. This year the focus was on the use of gamma-ray to study the Dark Matter component of the Universe, the origin and propagation of Cosmic Rays, Extra Large Spatial Dimensions and Tests of Lorentz Invariance., Comment: 328 pages, 7.8Mb, Proceedings of the 5th SCINEGHE Workshop, June 18-20, 2007 http://www.roma2.infn.it/SciNeGHE07/

GRB 070724B is the first Gamma Ray Burst localized by SuperAGILE, the hard X-ray monitor aboard the AGILE satellite. The coordinates of the event were published $\sim 19$ hours after the trigger. The Swift X-Ray Telescope pointed at the SuperAGILE location and detected the X-ray afterglow inside the SuperAGILE error circle. The AGILE gamma-ray Tracker and Minicalorimeter did not detect any significant gamma ray emission associated with GRB 070724B in the MeV and GeV range, neither prompt nor delayed. Searches of the optical afterglow were performed by the Swift UVOT and the Palomar automated 60-inch telescopes without any significant detection. Similarly the Very Large Array did not detect a radio afterglow. This is the first GRB event with a firm upper limit in the 100 MeV -- 30 GeV energy range, associated with an X-ray afterglow., Comment: 7 pages, 4 figures (of which 2 in color), contains online material. Accepted for publication by Astronomy & Astrophysics Letters

Blazars are known to display strong and erratic variability at almost all the wavelengths of electromagnetic spectrum. Presently, variability studies at high-energies (hard X-rays, gamma-rays) are hampered by low sensitivity of the instruments. Nevertheless, the latest generation of satellites (INTEGRAL, Swift) have given suggestions not yet fully confirmed of variability on intraday timescales. Some specific cases recently observed are presented and physical implications are discussed (e.g. NRAO 530). The contribution that SIMBOL-X could give in this research topic is also outlined., Comment: 4 pages, 2 figures. Proceedings of the conference "SIMBOL-X: the hard X-ray Universe in focus", May 14-16, 2007, Bologna (Italy)

GAW, acronym for Gamma Air Watch, is a Research and Development experiment in the TeV range, whose main goal is to explore the feasibility of large field of view Imaging Atmospheric Cherenkov Telescopes. GAW is an array of three relatively small telescopes (2.13 m diameter) which differs from the existing and presently planned projects in two main features: the adoption of a refractive optics system as light collector and the use of single photoelectron counting as detector working mode. The optics system allows to achieve a large field of view (24x24 squared degrees) suitable for surveys of large sky regions. The single photoelectron counting mode in comparison with the charge integration mode improves the sensitivity by permitting also the reconstruction of events with a small number of collected Cherenkov photons. GAW, which is a collaboration effort of Research Institutes in Italy, Portugal and Spain, will be erected in the Calar Alto Observatory (Sierra de Los Filabres - Andalucia, Spain), at 2150 m a.s.l.). The first telescope will be settled within Autumn 2007. This paper shows the main characteristics of the experiment and its expected performance., Comment: 4 pages, 2 figures, PDF format, Proceedings of 30th ICRC, International Cosmic Ray Conference 2007, Merida, Yucatan, Mexico, 3-11 July 2007

We report about the status of the PICsIT layer of the imager IBIS. The instrument has been tested in both Spectral Imaging and Spectral Timing Mode. PICsIT Single Events, Multiple and ISGRI data of the Crab are simultaneously fitted to a power law model and fit parameters are compared with the standard values. We present a joint fit of SPI, ISGRI, PICsIT data of the long GRB041219, that was seen by the PICsIT instrument in both Spectral Imaging and Spectral Timing acquisition modes. This allowed us to generate a preliminary version of the instrumental response matrices for the PICsIT Spectral Timing mode., Comment: 4 pages, 5 figures. Proceedings of the 6th INTEGRAL Workshop, to be published in ESA SP-622

The idea that substantial absorption in AGN could be a definite characteristic of the early phases of QSO evolution was invoked to explain the submillimeter observations of X-ray absorbed AGN at z~1 to z~3, that have shown strong emission at 850 mkm. This strong emission is a signature of the copious star formation. In AGN evolution model, the main obscured growth phase of the QSO coincides with formation of the host galaxy spheroid, the completion of which indicates the beginning of the luminous, unobscured phase of the QSO's evolution. The verification of this model at redshifts up to z~6 appears possible via application of the gamma-ray resonant absorption method., Comment: 3 pages, 1 figure, contribution to proceedings of International Astrophysics Conference "Relativistic Astrophysics and Cosmology-Einsteins Legacy", held during November 7-11, 2005, Munich, Germany. To be printed in the Springer-Verlag series "ESO Astrophysics Symposia", Eds. B. Aschenbach, V. Burwitz, G. Hasinger, and B. Leibundgut

At the end of July 2006, the blazar PKS 2155-304 (z=0.116) underwent a strong outburst observed at TeV energies by HESS (up to 17 Crab flux level at E > 200 GeV). The Swift satellite followed the evolution of the source for about one month. The data analysis - reported in another paper - has shown that, despite the violent activity at TeV energies, the synchrotron energy distribution increased in normalization, but only with a small shift in frequency. In the present work, we fit the broad-band spectrum with a log-parabolic model, to search for indications of intrinsic curvature, which in turn is usually interpreted as a signature of energy-dependent acceleration mechanisms of electrons., Comment: 2 pages, 1 figure, 1 Table. Proceedings of the First GLAST Symposium, to be published

We present all the publicly available data, from optical/UV wavelengths (UVOT) to X-rays (XRT, BAT), obtained from Swift observations of the blazar PKS 2155-304, performed in response to the rapid alert sent out after the strong TeV activity (up to 17 Crab flux level at E > 200 GeV) at the end of July 2006. The X-ray flux increased by a factor of 5 in the 0.3-10 keV energy band and by a factor of 1.5 at optical/UV wavelengths, with roughly one day of delay. The comparison of the spectral energy distribution built with data quasi-simultaneous to the TeV detections shows an increase of the overall normalization with respect to archival data, but only a small shift of the frequency of the synchrotron peak that remains consistent with the values reported in past observations when the TeV activity was much weaker., Comment: 13 pages, 3 figures. ApJ Letters, accepted for publication

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 $\alpha \simeq 1.8$ ($f_\nu \propto \nu^{-\alpha}$), 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., Comment: 8 pages, 6 figures. Accepted for publication on A&A Main Journal. Revision of the English grammar

(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., Comment: 13 pages + 12 pages of on-line material; 6 figures, 12 Tables. Accepted for publication on A&A Main Journal. Revision of English grammar

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., Comment: 10 pages, 7 figures, accepted by Astronomy and Astrophysics

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 x 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., Comment: 5 pages, 4 figures. Accepted for publication on A&A Main Journal. Note added in proof

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 being 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 period. 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, and show that the pulse is solely due to variations of the X-ray flux emitted by the source and not 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., Comment: Accepted for publication in MNRAS, 10 pages, 8 figures

Notwithstanding the big efforts devoted to the investigation of the mechanisms responsible for the high-energy (E>100 MeV) gamma-ray emission in active galactic nuclei (AGN), the definite answer is still missing. The X-ray energy band (0.4-10 keV) is crucial for this type of study, since both synchrotron and inverse Compton emission can contribute to the formation of the continuum. Within an ongoing project aimed at the investigation of the gamma-ray emission mechanism acting in the AGN detected by the EGRET telescope onboard CGRO, we firstly focused on the sources for which X-ray and optical/UV data are available in the XMM-Newton public archive. The preliminary results are outlined here., Comment: 2 pages, no figures. Proceedings of the Conference "The X-ray Universe", San Lorenzo de El Escorial (Madrid, Spain), 26-30 September 2005

GAW, acronym for Gamma Air Watch, is a path-finder experiment to test the feasibility of a new generation of imaging atmospheric Cherenkov telescopes that join high flux sensitivity with large field of view capability. GAW is conceived as an array of three identical imaging telescopes disposed at the vertexes of an equilateral triangle, about 80 m side. Two main features characterize GAW with respect to all the existing and presently planned ground-based Cherenkov telescopes. The first difference concerns the optics system: GAW uses a Fresnel refractive lens (2.13 m diameter) as light collector instead of classical reflective mirror. The second main difference is the detection working mode used: the detector at the focal surface operates in single photoelectron counting mode instead of the usual charge integration one. The GAW array is planned to be located at the Calar Alto Observatory site, Spain, 2150 m a.s.l. During its first phase, only 6x6 degrees of the focal plane detector will be implemented; moving it along the field of view, the sensitivity of the telescopes will be tested observing the Crab Nebula with on-axis and off-axis pointing up to 20 deg and with energy threshold of 300 GeV and energy peak of 700 GeV. As path-finder, GAW will also monitor the Very High Energy activity of some flaring Blazars as well as will follow-up GLAST detections at high energies. In a second phase, the focal plane will be enlarged to cover a field of view of 24 deg (full); pointing along different North-South directions, GAW would reach a survey of 360x60 degrees region of the sky. GAW is a collaboration effort of Research Institutes in Italy, Portugal and Spain., Comment: 4 pages, 2 figures, PDF format. Accepted for publication on Proc. 29th ICRC, International Cosmic Rays Conference, Pune, India, 3-10 August 2005

The ESA INTErnational Gamma-Ray Astrophyisics Laboratory (INTEGRAL) is composed of two main instruments (IBIS and SPI) for the gamma-ray astrophysics (20 keV - 10 MeV), plus two monitors (JEM-X and OMC) for X-ray and optical counterparts. It was launched on October 17th, 2002, from the Baikonur cosmodrome (Kazakhstan) and it will be active at least up to 2008. A selection of its main scientific contributions obtained to date are presented., Comment: 8 pages, no figures. Proceedings of the workshop "Science with the New Generation of High-Energy Gamma-Ray Experiments", Cividale del Friuli (Italy), 30 May - 1 June 2005. Invited talk. Some typos corrected

The 10-100 keV region of the electromagnetic spectrum contains the potential for a dramatic improvement in our understanding of a number of key problems in high energy astrophysics. A deep inspection of the universe in this band is on the other hand still lacking because of the demanding sensitivity (fraction of microCrab in the 20-40 keV for 1 Ms integration time) and imaging (~15" angular resolution) requirements. The mission ideas currently being proposed are based on long focal length, grazing incidence, multi-layer optics, coupled with focal plane detectors with few hundreds microns spatial resolution capability. The required large focal lengths, ranging between 8 and 50 m, can be realized by means of extendable optical benches (as foreseen e.g. for the HEXIT-SAT, NEXT and NuSTAR missions) or formation flight scenarios (e.g. Simbol-X and XEUS). While the final telescope design will require a detailed trade-off analysis between all the relevant parameters (focal length, plate scale value, angular resolution, field of view, detector size, and sensitivity degradation due to detector dead area and telescope vignetting), extreme attention must be dedicated to the background minimization. In this respect, key issues are represented by the passive baffling system, which in case of large focal lengths requires particular design assessments, and by the active/passive shielding geometries and materials. In this work, the result of a study of the expected background for a hard X-ray telescope is presented, and its implication on the required sensitivity, together with the possible implementation design concepts for active and passive shielding in the framework of future satellite missions, are discussed., Comment: 13 pages, 6 figures. Proceedings of SPIE conference "Optics for EUV, X-Ray, and Gamma-Ray Astronomy II", San Diego (CA, USA), July 31st - August 4th, 2005, Vol. 5900. Full color figures are available at http://www.bo.iasf.cnr.it/~malaguti/papers/SPIE2005_1.ps.gz

A comparative XMM-Newton archival data spectral study of the ultraluminous X-ray source (ULX) M33 X-8 has been performed by using both the standard disk model and the newly developed slim disk models. The results of this analysis tend to confirm the hypothesis that M33 X-8 is an X-ray binary with a stellar-mass black hole accreting at super-Eddington rate., Comment: 6 pages, 4 figures, accepted for publication on Advances in Space Research, Special Issue Proceedings of 35th COSPAR (Paris, France, 18-25 July 2004)

The analysis of an INTEGRAL AO2 observation of the error contours of the EGRET source 3EG J1621+8203 is presented. The only source found inside the error contours for energies between 20 and 30 keV at 5 sigma detection significance is the FR I radio galaxy NGC 6251. This supports the identification of NGC 6251 with 3EG J1621+8203. The observed flux is higher and softer than observed in the past, but consistent with a variable blazar-like spectral energy distribution., Comment: 5 pages, 2 figures. Accepted for publication on A&A Main Journal

The Early Data Release of the Sloan Digital Sky Survey (SDSS) contains 150 Narrow-Line Seyfert 1 (NLS1) galaxies, most of them previously unknown. We present here the study of the X-ray emission from two of these active galaxies (SDSS J030639.57+000343.2 and SDSS J141519.50-003021.6), based upon XMM-Newton observations. The spectral and timing characteristics of the two sources are presented and compared against the typical properties of known NLS1 galaxies. We found that these two NLS1 are within the dispersion range of the typical values of this class of AGN, although with some interesting features that deserve further studies., Comment: 5 pages, 8 figures. Accepted for publication on A&A Main Journal

The possible identification by INTEGRAL of the EGRET source 3EG J1736-2908 with the active galactic nucleus GRS 1734-292 is discussed. The latter was discovered in 1990 and later identified with a Seyfert 1 galaxy. At the time of the compilation of the 3rd EGRET Catalog, it was not considered as a possible counterpart of the source 3EG J1736-2908, which remained unidentified. A detailed multiwavelength study of the EGRET error circle is presented, by including archival radio, soft- and hard-X observations, suggesting that GRS 1734-292 could be a likely counterpart of 3EG J1736-2908, even though this poses very interesting questions about the production mechanisms of gamma-rays with energies greater than 100 MeV., Comment: 6 pages, 3 figures. Accepted for publication on A&A Main Journal

Here we report results of an INTEGRAL-AO1 observation of the X-ray burst and atoll source Ser X-1 performed in May 2003. The object was observed for a total on-source time of 400 ks but nearly 8 degrees off-axis due to its amalgamation with an observation of SS 433, the pointing target source. Ser X-1 has been clearly detected up to 30 keV with unprecedented positional accuracy for high-energy emission. The 20-30 keV light curve showed substantial variability during the observation. Comparison with previous observations indicates that the source was in its high (`banana') state and displayed a soft spectrum during the INTEGRAL pointing. A (non simultaneous) radio-to-gamma-rays broad-band spectral energy distribution is also presented for the first time and discussed., Comment: 6 pages, 3 figures, 1 table; accepted for publication on Astronomy & Astrophysics, main journal. Final version of the paper including the A&A Language Editor's comments

The primary objective of the study of unidentified EGRET sources with INTEGRAL is to locate with a few arcminute accuracy the hard X-ray / soft gamma-ray counterparts within the EGRET error circle or to determine upper limits on their emission in the INTEGRAL energy range. The INTEGRAL Galactic Plane Scan (GPS) and Galactic Centre Deep Exposure (GCDE) data have been analysed and cross-correlated with the EGRET 3^rd Catalogue to search for counterparts of EGRET sources. The IBIS detection of a source within the EGRET error circle of 3EG J1736-2908 is presented and its possible identification with the active galaxy GRS 1734-292 is discussed. Finally, preliminary results on the cross-correlation between EGRET unidentified sources and the first data from the IBIS survey of the Galactic Centre are presented., Comment: 4 pages, 5 figures. Proceedings of the V INTEGRAL Workshop, Munich 16-20 February 2004. ESA SP-552

We here report results of an INTEGRAL observation of the X-ray burst and atoll source Ser X-1 performed on May 2003. The object was observed for a total of 400 ks but nearly 8 degrees off-axis due to the amalgamation with an observation of SS 433, the pointing target source. Ser X-1 was detected up to 30 keV with unprecedented positional accuracy for a high-energy emission; a sharp spectral drop is evident beyond this energy. Significant variability is seen in the 20-30 keV light curve. Comparison with previous observations indicates that the source was in its high (banana) state and displayed a soft spectrum during the INTEGRAL pointing. A (non simultaneous) broadband radio-to-gamma-rays broad-band spectral energy distribution for Ser X-1 is also presented for the first time., Comment: 4 pages, 3 figures. Presented at 5th INTEGRAL Workshop "The INTEGRAL Universe", Munich, Germany, 16-20 Feb 2004. To be published in the Workshop proceedings by EDP. Corrected following referee's comments; references added

We present observations with XMM-Newton of M33 X-8, the ultraluminous X-ray source (L_{0.5-10 keV} ~ 2x10^39 erg/s) closest to the centre of the galaxy. The best-fit model is similar to the typical model of Galactic black holes in very high state. Comparison with previous observations indicates that the source is still in a very high state after about 20 years of observations. No state transition has been observed even during the present set of XMM-Newton observations. We estimate the lower limit of the mass of the black hole >6 M_sun, but with proper parameters taking into account different effects, the best estimate becomes 12 M_sun. Our analysis favours the hypothesis that M33 X-8 is a stellar mass black hole candidate, in agreement with the findings of other authors. In addition, we propose a different model where the high luminosity of the source is likely to be due to orientation effects of the accretion disc and anisotropies in the Comptonized emission., Comment: 9 pages, 5 figures. Accepted for publication on A&A Main Journal. A bug in the introduction has been corrected (a citation)

We report on the spectral properties of a sample of 90 hard X-ray selected serendipitous sources detected in 12 XMM observations with 1~1.55 over the whole sample. We also find that the mean slope of the QSOs in our sample turns out to remain nearly constant (~1.8-1.9) between 0~10^(22) cm^(-2)) is ~30%, with little evolution in the range 2

We report preliminary results of a broadband spectral and temporal study of the black-hole binary Cyg X-1 performed with the IBIS telescope. Cyg X-1 was the first pointed celestial target of IBIS during the INTEGRAL Performance and Verification Phase, 2002 Nov.-Dec., for a total observing time of 2 Ms in both staring and dithering mode. Here, we report on only the staring, on-axis, observation performed in a stable instrument configuration. During the observing period the source was in its characteristic low/hard state, in which a few flares and dips have been detected. The IBIS/ISGRI results demonstrate that the INTEGRAL observatory offers a unique capability for studying correlations between hardness and/or flux in different bands over a wide photon energy range. One of our new results is finding that the hardness-flux correlation changes the sign twice over the 20-220 keV; first from positive to negative at 50 keV, and then back to positive at 120 keV. The former change appears to be due to the spectral curvature introduced by variable Compton reflection. The latter may be due spectral pivoting., Comment: 7 pages, 8 figures (4 in color), accepted for publication in A&A, INTEGRAL special issue

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 a \~100 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., Comment: 8 pages, 4 figures. Accepted for publication on A&A, special issue on First Science with INTEGRAL

We report on the association of an X-ray source (WGA J2025.1+3342), serendipitously found with BeppoSAX in two separate observations, with the unidentified EGRET source 3EG J2027+3429. The source is detected from 1 keV up to about 100 keV, has a flat (0.6-1.5) spectrum and is highly variable both in intensity and shape. The data indicate marginal evidence for an iron line in the source rest frame. The overall X-ray luminosity is 4 10^{45} erg s^{-1} typical of a quasar. The X-ray source is coincident in radio with a bright object characterized by a flat spectrum over the band 0.3-10 GHz while in optical it is identified with a quasar at redshift 0.22. All available data indicate a SED compatible with a low frequency peaked or red blazar type object. This identification is interesting because this is the second blazar found behind the galactic plane in the direction of the Cygnus region., Comment: 6 pages, 5 figures, 3 tables, accepted for publication in A&A

PICsIT (Pixellated Imaging CaeSium Iodide Telescope) is the high energy detector of the IBIS telescope on-board the INTEGRAL satellite. It consists of 4096 independent detection units, ~0.7 cm^2 in cross-section, operating in the energy range between 175 keV and 10 MeV. The intrinsically low signal to noise ratio in the gamma-ray astronomy domain implies very long observations, lasting 10^5-10^6 s. Moreover, the image formation principle on which PICsIT works is that of coded imaging in which the entire detection plane contributes to each decoded sky pixel. For these two main reasons, the monitoring, and possible correction, of the spatial and temporal non-uniformity of pixel performances, expecially in terms of gain and energy resolution, is of paramount importance. The IBIS on-board 22Na calibration source allows the calibration of each pixel at an accuracy of <0.5% by integrating the data from a few revolutions at constant temperature. The two calibration lines, at 511 and 1275 keV, allow also the measurement and monitoring of the PICsIT energy resolution which proves to be very stable at ~19% and ~9% (FWHM) respectively, and consistent with the values expected analytical predictions checked against pre-launch tests., Comment: 5 pages, 7 figures. Accepted for publication on A&A Special Issue on First Science with INTEGRAL

In the late afternoon of November 25, 2002 a gamma-ray burst (GRB) was detected in the partially coded field of view (about 7.3 deg from the centre) of the imager IBIS on board the INTEGRAL satellite. The instruments on-board INTEGRAL allowed, for the first time, the observation of the prompt gamma-ray emission over a broad energy band from 15 to 500 keV. GRB021125 lasted ~24 s with a mean flux of ~5.0 photons/cm^2/s in the 20-500 keV energy band, and a fluence of 4.8x10^-5 erg/cm^2 in the same energy band. Here we report the analysis of the data from the imager IBIS and the spectrometer SPI., Comment: 4 pages, 5 figures. Accepted for publication on A&A: Special Issue on First Science with INTEGRAL

We report on the identification of the optical and infrared counterparts of the ultraluminous X-ray source (ULX) XMMU J121214.5+131248 (NGC4168-ULX1). The optical spectrum yields a redshift of z=0.217, which implies that the ULX is not associated with the nearby galaxy NGC 4168, but rather with a background object. Optical spectral line ratios and the spectral energy distribution constructed from the available data indicate that the source is likely a starburst nucleus., Comment: 5 pages, 3 figures, 1 table, accepted for publication on A&A Letters

We have studied the average 3-200 keV spectra of Seyfert galaxies of type 1 and 2, using data obtained with BeppoSAX. The average Seyfert 1 spectrum is well-fitted by a power law continuum with photon spectral index Gamma~1.9, a Compton reflection component R~0.6-1 (depending on the inclination angle between the line of sight and the reflecting material) and a high-energy cutoff at around 200 keV; there is also an iron line at 6.4 keV characterized by an equivalent width of 120 eV. Seyfert 2's on the other hand show stronger neutral absorption (NH=3-4 x 10^{22} atoms cm-2) as expected but are also characterized by an X-ray power law which is substantially harder (Gamma~1.75) and with a cut-off at lower energies (E_c~130 keV); the iron line parameters are instead substantially similar to those measured in type 1 objects. There are only two possible solutions to this problem: to assume more reflection in Seyfert 2 galaxies than observed in Seyfert 1 or more complex absorption than estimated in the first instance. The first possibility is ruled out by the Seyfert 2 to Seyfert 1 ratio while the second provides an average Seyfert 2 intrinsic spectrum very similar to that of the Seyfert 1. The extra absorber is likely an artifact due to summing spectra with different amounts of absorption, althought we cannot exclude its presence in at least some individual sources. Our result argues strongly for a very similar central engine in both type of galaxies as expected under the unified theory., Comment: 12 pages, 4 figures, to be published in ApJ Letters

An unbiased estimate of the average intrinsic X-ray properties and column density distribution of Seyfert galaxies in the local Universe is crucial to validate unified models of active galactic nuclei (AGNs) and to synthesis models for the X-ray background. We present here preliminary results obtained from an on-going XMM-Newton study (~250 ks awarded in the EPIC GT) on a well-defined, statistically complete, and significant sample of nearby Seyfert galaxies., Comment: 4 pages, 3 figures, to appear in "Carnegie Observatories Astrophysics Series, Vol. 1: Coevolution of Black Holes and Galaxies," ed. L. C. Ho (Pasadena: Carnegie Observatories, http://www.ociw.edu/ociw/symposia/series/symposium1/proceedings.html)'

The nature of the ultraluminous X-ray sources (ULX) is still to be understood. Several hypotheses have been suggested to explain the high X-ray luminosity: stellar mass black holes with emission collimated by relativistic effects or anisotropies in the accretion disk, intermediate mass black holes, young supernovae remnants, background active galactic nuclei. To discriminate among the above hypotheses, the search for relativistic effects can be a valuable "pathfinder". Here we propose a list of ULX candidates suitable to show relativistic beaming, Comment: 5 pages, no figure. For the Proceedings of the 15th SIGRAV Conference on General Relativity and Gravitation. Corrected for a misspelled reference

We briefly report on an on-going spectroscopic study of hard X-ray sources selected serendipitously in 12 XMM-Newton fields. Results for the analysis of the 41 sources from the first seven EPIC observations have been discussed in a previous paper (Piconcelli et al. 2002, Paper I) where we found an absolute fraction of X-ray absorbed sources (~30%) lower than expected (~50%) by the predictions of popular CXB synthesis models at F(2-10)~5x10**(-14) erg cm**-2 s**-1. We present here the preliminary results concerning the whole sample including five new deeper XMM-Newton measurements increasing the sample to 90 sources. Even if still on-going, the present study appears to confirm and extend down to F(2-10)~10**(-14) erg cm**-2 s**-1 the above mismatch between observational data and theoretical expectations regarding the fraction of absorbed sources. Furthermore the sample average spectral index of 1.5-1.6 is steeper than the CXB slope indicating that the majority of obscured sources making the bulk of the CXB resides at even lower hard X-ray fluxes., Comment: Proceedings for the "X-ray Surveys: in the light of new observatories" Workshop (to be published in Astron. Nachrichten; includes anabs.cls style file). One page and two figures

We report the identification of the optical and radio counterparts of the ultraluminous X-ray (ULX) source XMMU J124825.9+083020 (NGC4698-ULX1). The optical spectrum taken with the VLT yields a redshift of z=0.43, which implies that the ULX is not associated with the nearby galaxy NGC4698. The spectral energy distribution calculated from the available data indicates that the source is likely to be a BL Lac object. The possible synchrotron peak at X-ray energies suggests that this source may be a gamma-ray emitter., Comment: 6 pages, 5 figures. Accepted for publication by Astronomy and Astrophysics

Little is presently known about the nature of ultraluminous X-ray sources (ULX). Different hypotheses have been proposed to explain their properties: intermediate-mass BHs, Kerr BHs, young SNR, or background AGN. Some of the current problems and open questions in this research field are here reviewed., Comment: 4 pages, no figures. Presented at AGN05: Inflows, Outflows and Reprocessing around black holes, Como (Italy), 11-14 June 2002. To appear in the Proceedings