G. C. Perola

Università Degli Studi Roma Tre, Roma, Latium, Italy

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Publications (247)698.43 Total impact

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    ABSTRACT: We present preliminary results from two observations of the radio galaxy Centaurus A performed by the BeppoSAX satellite on 1997 February 20–21 and on 1998 January 6–7. In the second pointing the source was brighter by a factor 1.3. We did not detect any spectral variation of the nuclear continuum in spite of the long-term flux change between the two observations. At both epochs, the nuclear point-like emission was well fitted with a strongly absorbed (NH ∼ 1023cm−2) power law with an exponential cutoff at high energies (Ecut > 200keV). We also observed a significant flux variation of the iron line between the two observations. The flux of the line and of the continuum changed in the opposite sense. The line is more intense at the first epoch, when the nuclear source was at the lower intensity level. The implied delay between the continuum and line variations strongly suggests that the cold material responsible for the iron line production is not located very near to the primary X-ray source. There is also evidence that the line profile changed between the two epochs, being broader and slightly blueshifted when the source was fainter. It is possible that the emission feature is a blend of cold and ionized iron lines produced in separate regions surrounding the nuclear source.
    Advances in Space Research 07/2013; 25(s 3–4):485–488. DOI:10.1016/S0273-1177(99)00784-X · 1.24 Impact Factor
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    ABSTRACT: The New Hard X-ray Mission (NHXM) has been designed to provide a real breakthrough on a number of hot astrophysical issues that includes: black holes census, the physics of accretion, the particle acceleration mechanisms, the effects of radiative transfer in highly magnetized plasmas and strong gravitational fields. NHXM combines fine imaging capability up to 80 keV, today available only at E<10 keV, with sensitive photoelectric imaging polarimetry. It consists of four identical mirrors, with a 10 m focal length, achieved after launch by means of a deployable structure. Three of the four telescopes will have at their focus identical spectral-imaging cameras, while a X-ray imaging polarimeter will be placed at the focus of the fourth. In order to ensure a low and stable background, NHXM will be placed in a low Earth equatorial orbit. Here we will provide an overall description of this mission and of the developments that are currently occurring in Italy. In the meanwhile we are forming an international collaboration, with the goal to have a consortium of leading Institutes and people that are at the forefront of the scientific and technological developments that are relevant for this mission.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
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    ABSTRACT: The Italian New Hard X-ray Mission (NHXM) has been designed to provide a real breakthrough on a number of hot astrophysical issues that includes: black holes census, the physics of accretion, the particle acceleration mechanisms, the effects of radiative transfer in highly magnetized plasmas and strong gravitational fields. NHXM is an evolution of the HEXIT-Sat concept and it combines fine imaging capability up to 80 keV, today available only at E<10 keV, with sensitive photoelectric imaging polarimetry. It consists of four identical mirrors, with a 10 m focal length, achieved after launch by means of a deployable structure. Three of the four telescopes will have at their focus identical spectral-imaging cameras, while X-ray imaging polarimetric cameras will be placed at the focus of the fourth. In order to ensure a low and stable background, NHXM will be placed in a low Earth equatorial orbit. In this paper we provide an overall description of this mission that is currently in phase B. Comment: 9 pages, 5 figures. Accepted for publication on PoS, proceedings of "The Extreme sky: Sampling the Universe above 10 keV", held in Otranto (Italy), 13-17 October 2009
    04/2010; DOI:10.1063/1.3475341
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    ABSTRACT: The Italian New Hard X-ray Mission (NHXM) is an evolution of the HEXIT-Sat concept, extending up to 80 keV the fine imaging capability today available only at E<10 keV, with the further addition of photoelectric imaging polarimetry. NHXM consists of four identical mirrors, with a 10 m focal length, achieved after launch by means of a deployable structure. Three of the four telescopes will have at their focus three identical spectro-imaging cameras, while a X-ray imaging polarimeter will be placed at the focus of the fourth. In order to ensure a low and stable background, NHXM will be place on a low Earth equatorial orbit. NHXM will provide a real breakthrough on a number of hot astrophysical issues, broadly falling under two main topics: i) censing the black holes in the Universe and probing the physics of accretion in the most diverse conditions; ii) investigating the particle acceleration mechanisms at work in different contexts, and the effects of radiative transfer in highly magnetized plasmas and strong gravitational fields.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2010; DOI:10.1117/12.857032 · 0.20 Impact Factor
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    ABSTRACT: There have been recent claims that a significant fraction of type 2 AGN accrete close or even above the Eddington limit. In type 2 AGN the bolometric luminosity (L_b) is generally inferred from the [OIII] emission line luminosity (L_OIII). The key issue, in order to estimate the bolometric luminosity in these AGN, is therefore to know the bolometric correction to be applied to L_OIII. A complication arises from the fact that the observed L_OIII is affected by extinction, likely due to dust within the narrow line region. The extinction-corrected [OIII] luminosity (L^c_OIII) is a better estimator of the nuclear luminosity than L_OIII. However, so far only the bolometric correction to be applied to the uncorrected L_OIII has been evaluated. This paper is devoted to estimate the bolometric correction C_OIII=L_b/L^c_OIII in order to derive the Eddington ratios for the type 2 AGN in a sample of SDSS objects. We have collected from the literature 61 sources with reliable estimate of both L^c_OIII and X-ray luminosities (L_X). To estimate C_OIII, we combined the observed correlation between L^c_OIII and L_X with the X-ray bolometric correction. We found, contrary to previous studies, a linear correlation between L^c_OIII and L_X. We estimated C_OIII using the luminosity-dependent X-ray bolometric correction of Marconi et al. (2004), and we found a mean value of C_OIII in the luminosity ranges log L_OIII=38-40, 40-42, and 42-44 of 87, 142 and 454 respectively. We used it to calculate the Eddington ratio distribution of type 2 SDSS AGN at 0.3<z<0.4 and we found that these sources are not accreting near their Eddington limit, contrary to previous claims. Comment: 8 pages, 4 figures, accepted for publication in Astronomy and Astrophysics
    Astronomy and Astrophysics 05/2009; DOI:10.1051/0004-6361/200912023 · 4.48 Impact Factor
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    ABSTRACT: We present two ROSAT Position Sensitive Proportional Counter (PSPC) observations of the radio-loud, lobe-dominated quasar 3C 351, which shows an "ionized absorber" in its X-ray spectrum. The factor of 1.7 change in flux in the ~2 years between the observations allows a test of models for this ionized absorber. The absorption feature at ~0.7 keV (quasar frame) is present in both spectra but with a lower optical depth when the source intensity—and hence the ionizing flux at the absorber—is higher, in accordance with a simple, single-zone, equilibrium photoionization model. Detailed modeling confirms this agreement quantitatively. The maximum response time of 2 years allows us to limit the gas density: ne>2×104 cm-3; and the distance R of the ionized gas from the central source is less than 19 pc. This produces a strong test for a photoionized absorber in 3C 351: a factor of 2 flux change in ~1 week in this source must show nonequilibrium effects in the ionized absorber.
    The Astrophysical Journal 01/2009; 512(1):136. DOI:10.1086/306741 · 6.28 Impact Factor
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    ABSTRACT: We report on the 0.5-200 keV spectral properties of Cyg X-1 observed at different epochs with the Narrow Field Instruments of the BeppoSAX satellite. The source was in its soft state during the first observation of 1996 June. In the second observation of 1996 September, the source had parameters characteristic to its hard state. A soft X-ray excess, a broad Fe Kα line and Compton reflection are clearly detected in both states. The soft-state broadband continuum is well modeled by a disk blackbody (accounting for the soft excess) and Compton upscattering of the disk photons by a hybrid, thermal/nonthermal plasma, probably forming a corona above the disk (also giving rise to the Compton-reflection component). In the hard state, the primary hard X-ray spectrum can be well modeled by Compton upscattering of a weak blackbody emission by a thermal plasma at a temperature of ~60 keV. The soft excess is then explained by thermal Comptonization of the same blackbody emission by another hot plasma cloud characterized by a low value of its Compton parameter. Finally, we find the characteristic ratio of the bolometric flux in the soft state to that in the hard state to be about 3. This value is much more compatible with theories of state transitions than the previously reported (and likely underestimated) value of 1.5.
    The Astrophysical Journal 12/2008; 546(2):1027. DOI:10.1086/318304 · 6.28 Impact Factor
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    ABSTRACT: We have determined the cosmological evolution of the density of active galactic nuclei (AGNs) and of their NH distribution as a function of the unabsorbed 2-10 keV luminosity up to redshift 4. We used the HELLAS2XMM sample combined with other published catalogs, yielding a total of 508 AGNs. Our best fit is obtained with a luminosity-dependent density evolution (LDDE) model where low-luminosity (LX ~ 1043 ergs s-1) AGNs peak at z ~ 0.7, while high-luminosity AGNs (LX > 1045 ergs s-1) peak at z ~ 2.0. A pure luminosity evolution model (PLE) can instead be rejected. There is evidence that the fraction of absorbed (NH > 1022 cm-2) AGNs decreases with the intrinsic X-ray luminosity and increases with the redshift. Our best-fit solution provides a good fit to the observed counts, the cosmic X-ray background, and to the observed fraction of absorbed AGNs as a function of the flux in the 10-15 ergs cm-2 s-1 < S2-10 < 10-10 ergs cm-2 s-1 range. We find that the absorbed, high-luminosity (LX > 1044 ergs s-1) AGNs have a density of 267 deg-2 at fluxes S2-10 > 10-15 ergs cm-2 s-1. Using these results, we estimate a density of supermassive black holes in the local universe of ρBH = 3.2 h × 105 M☉ Mpc-3, which is consistent with the recent measurements of the black hole mass function in the local galaxies.
    The Astrophysical Journal 12/2008; 635(2):864. DOI:10.1086/497586 · 6.28 Impact Factor
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    ABSTRACT: We present BeppoSAX observations of the southern S1 region in the European Large-Area Infrared Space Observatory (ISO) Survey (ELAIS). These observations cover an area of ~1.7 deg2 and reach an on-axis (~0.7 deg2) 2-10 keV (hard X-ray, or HX) sensitivity of ~10-13 ergs s-1 cm-2. This is the first HX analysis of an ISOCAM survey. We detect nine sources with a signal-to-noise ratio SNRHX > 3, four additional sources with a 1.3-10 keV (total X-ray, or T) SNRT > 3, and two additional sources that seem to be associated with QSOs having SNRT > 2.9. The number densities of the SNRHX > 3 sources are consistent with the ASCA and BeppoSAX log N-log S functions. Six BeppoSAX sources have reliable ISOCAM 15 μm counterparts within ~60''. All these ISOCAM sources have optical counterparts of R < 20 mag. Five of these sources have been previously optically classified, four as QSOs and one as a broad absorption line (BAL) QSO at z = 2.2. The remaining unclassified source has X-ray and photometric properties consistent with those of a nearby Seyfert galaxy. One further HX source has a 2.6 σ ISOCAM counterpart associated with a galaxy at z = 0.325. If this ISOCAM source is real, the HX/MIR properties suggest either an unusual QSO or a cD cluster galaxy. We have constructed MIR and HX spectral energy distributions to compute the expected HX/MIR ratios for these classes of objects up to z = 3.2 and assess the HX/MIR survey depth. The BAL QSO has an observed X-ray softness ratio and HX/MIR flux ratio similar to those of QSOs but different from those found for low-redshift BAL QSOs. This difference can be explained in terms of absorption, and it suggests that high-redshift BAL QSOs should be comparatively easy to detect in the HX band, allowing their true fraction in the high-redshift QSO population to be determined. The QSOs cover a wide redshift range (0.4 < z < 2.6) and have HX/MIR flux ratios consistent with those found for nearby IRAS and optically selected Palomar-Green QSOs. This suggests that MIR-selected QSOs of R < 20 mag come from the same population as optically selected QSOs. We confirm this with a comparison of the B/MIR flux ratios of MIR and blue-band-selected QSOs.
    The Astrophysical Journal 12/2008; 554(1):18. DOI:10.1086/321351 · 6.28 Impact Factor
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    A. Lamastra, G. C. Perola, G. Matt
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    ABSTRACT: The evidence of a decrease with increasing luminosity of the fraction f_{abs} of absorbed and Compton-thin among X-ray (2-10 keV) selected AGN is observationally rather well supported, while that of an increase of f_{abs} with redshift is rather controversial. In Lamastra, Perola & Matt (2006) the gravitational effect of the SMBH on the molecular interstellar gas, in the central region of the host galaxy, was shown to predict an anti-correlation between f_{abs} and M_{BH}. The most recent findings on the distribution of the Eddington ratio \lambda=L_b/L_E as a function of M_{BH} and z are used to convert that relationship into one between f_{abs} and both bolometric (L_b) and X-ray (L_X) luminosities at various values of z. The findings on \lambda(M_{BH},z) are properly treated in order to ensure completeness in the prediction of f_{abs} above a certain luminosity, at values of z=0.1, 0.35, 0.7 and >1. To verify the consequence of these findings alone, we adopted in a first istance a distribution of gas surface density \Sigma, observed in a sample of local spiral galaxies, irrespective of the galaxy morphological type and z. Assuming in the \lambda(M_{BH},z) distribution the Eddington limit, \lambda=1, as a ``natural'' cut-off, the predictions are consistent with the existence of an anti-correlation between f_{abs} and L_X, but fail to reproduce an increase of f_{abs} with z. Because the early type galaxies on average are much poorer in molecular gas than late type ones, a quantitative agreement with the local value of f_{abs} requires the existence of a correlation between \Sigma and the central activity. An increase of typical values of \Sigma with z, correlated with the activity, might explain an increase of f_{abs} with z. However, at the highest luminosities f_{abs} could hardly exceed about 0.3. Comment: 12 pages, 12 figures, accepted for publication in A&A
    Astronomy and Astrophysics 05/2008; 487(1). DOI:10.1051/0004-6361:20079162 · 4.48 Impact Factor
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    ABSTRACT: The development of micropixel gas detectors, capable to image tracks produced in a gas by photoelectrons, makes possible to perform polarimetry of X-ray celestial sources in the focus of grazing incidence X-ray telescopes. HXMT is a mission by the Chinese Space Agency aimed to survey the Hard X-ray Sky with Phoswich detectors, by exploitation of the direct demodulation technique. Since a fraction of the HXMT time will be spent on dedicated pointing of particular sources, it could host, with moderate additional resources a pair of X-ray telescopes, each with a photoelectric X-ray polarimeter in the focal plane. We present the design of the telescopes and the focal plane instrumentation and discuss the performance of this instrument to detect the degree and angle of linear polarization of some representative sources. Notwithstanding the limited resources the proposed instrument can represent a breakthrough in X-ray Polarimetry. Comment: 10 pages, 9 figures
    Proceedings of SPIE - The International Society for Optical Engineering 09/2007; DOI:10.1117/12.734536 · 0.20 Impact Factor
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    ABSTRACT: The effective content of cosmic rays (CR) in galaxy clusters remains elusive. This paper aims to estimate a maximum production of both secondary relativistic electrons, SRE, and gamma rays, GR, from the relativistic protons, RP, that have supposedly accumulated throughout the entire history of a cluster. The production rate is normalized by adopting a reference value of 0.3 for the ratio of RP to thermal pressure. The SRE content which obtains, when constrained to reproduce the observed radio brightness profile, yields univocally B(r), if the presence of primary RE were negligible. This procedure is applied to four radio halo clusters (Coma, A2163, A2255, A2319). In these objects, the central value B_0 required is consistent with typical, albeit rather uncertain, values derived from FR. On the other hand, B(r) typically increases beyond the thermal core, a hardly acceptable condition. This problem is alleviated by assuming a mix of SRE and of primary RE, with the latter becoming the dominant component beyond the thermal core. These results suggest that in clusters without a radio halo detected so far a diffuse radio emission should also be observable due to SRE alone, and therefore more centrally condensed. To encourage deeper radio observations of such clusters, some examples were selected that seem rather promising. Efforts in this direction, if accompanied by FR measurements, could provide highly significant constraints on the CR content in clusters, even before the GLAST mission will have accomplished the hard task of detecting the GR. A complementary result concerns the excess far UV in the Coma cluster, that some authors have attributed to IC emission from SRE. It is shown that this hypothesis can be excluded, because it requires a RP energy content in excess of the thermal one. Comment: 12 pages, 12 figures; to be published in A&A
    Astronomy and Astrophysics 01/2007; DOI:10.1051/0004-6361:20065977 · 4.48 Impact Factor
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    ABSTRACT: We present broad band timing and spectral analysis of Beppo-SAX data of the Seyfert galaxy NGC 4151, and propose multi-component models to explain the complex spectrum of this source and the observed, energy dependent, spectral variability. Many different components are needed to model SAX NGC 4151 spectra. The intrinsic continuum is well described by a simple power law with energy spectral index slightly correlated with intensity (α ∼ 0.4–0.5) up to ∼ 15 keV. At higher energies an exponential cutoff is clearly detected in the PDS spectra (c = 50 ± 5 keV). There is also an evidence for a flattening of the spectrum above ∼ 15 keV which is well described by a reflection model. A non-variable, complex soft component becomes dominant below ∼ 1 keV. Between 1 keV and ∼ 5 keV, a complicate absorption structure reveals the presence of either a cold absorber partially covering the nuclear continuum or a very thick warm absorber, or both. Finally, a strong Iron line at 6.4 keV is clearly detected, with varying equivalent width, becoming stronger as continuum intensity drops.
    Nuclear Physics B - Proceedings Supplements 01/2007; 69:481-485. DOI:10.1016/S0920-5632(98)00266-7 · 0.88 Impact Factor
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    ABSTRACT: We present an analysis of all SAX observations of NGC 4151. This source was observed 5 times from 1996 to 2001 with durations ranging from a day to four days. The intrinsic continuum, is absorbed at low energies by a complex system: a cold patchy absorber plus a warm uniform screen photoionized by the central continuum. We find that this dual absorber is the main driver of the observed variability, up to a factor of eight, at 3 keV. In particular the covering fraction of the cold absorber changes on time scales of the order of a day, supporting its association with the Broad Line Region. The column density of the warm gas varies on a longer time scale (months to year). Some of the small amplitude spectral variability above 10 keV can be explained with an intrinsic variation (\Delta\Gamma~0.2).The flux below 1 keV remains constant confirming an extended origin. Its spectrum is reproduced by a combination of a thermal component and a power law with the same slope as the intrinsic continuum but with an intensity a few per cent. A Compton reflection component is significantly detected in 1996 (averaged value of \Omega/2\pi ~0.4), with intensity decreasing on time scale of year, and it desappears in 2000 and 2001. The long time scale of variations argues for an association with an optically thick torus at a distance of few light years. An iron line was detected in all spectra. Its energy is consistent with fluorescence by cold iron. We find that the line is variable. Its behaviour is reproduced by a variable component proportional to the level of the reflection flux plus a constant component. The flux of the latter is consistent with the extended line emission observed by Chandra. We conclude that the first component is likely arising from the torus and the second is produced in the extended Narrow Line Region. Comment: Accepted for publication in A&A
    Astronomy and Astrophysics 11/2006; DOI:10.1051/0004-6361:20065028 · 4.48 Impact Factor
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    ABSTRACT: We report on the BeppoSAX observation of the X-ray afterglow of GRB 000214 (“Valentine’s Day Burst”). A strong emission line is detected (3 s\sigma significance level) in the X-ray spectrum with a centroid energy of 4.7 ±0.24.7 \pm 0.2 keV. This feature, if interpreted as Ka emission from hydrogen-like iron, corresponds to a redshift of z = 0.47. The intensity (EW ~ 2\sim 2 keV) and duration (tens of hours) of the line give information on the distance of the emitting material ( R ³ 3 1015R \ge 3 \times 10^{15} cm) and its mass ( M ³ 1.4M\odotM \ge 1.4M_\odot ). These results are not easily reconciled with the binary merger and hypernova models for gamma ray bursts, and rather point to a SupraNova scenario.
    11/2006: pages 112-114;
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    ABSTRACT: X-Ray Polarimetry can be now performed by using a Micro Pattern Gas Chamber in the focus of a telescope. It requires large area optics for most important scientific targets. But since the technique is additive a dedicated mission with a cluster of small telescopes can perform many important measurements and bridge the 40 year gap between OSO-8 data and future big telescopes such as XEUS. POLARIX has been conceived as such a pathfinder. It is a Small Satellite based on the optics of JET-X. Two telescopes are available in flight configuration and three more can be easily produced starting from the available superpolished mandrels. We show the capabilities of such a cluster of telescopes each equipped with a focal plane photoelectric polarimeter and discuss a few alternative solutions. Comment: 9 pages, 5 figures
    Proceedings of SPIE - The International Society for Optical Engineering 09/2006; DOI:10.1117/12.672951 · 0.20 Impact Factor
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    ABSTRACT: XEUS is a large area telescope aiming to rise X-ray Astronomy to the level of Optical Astronomy in terms of collecting areas. It will be based on two satellites, locked on a formation flight, one with the optics, one with the focal plane. The present design of the focal plane foresees, as an auxiliary instrument, the inclusion of a Polarimeter based on a Micropattern Chamber. We show how such a device is capable to solve open problems on many classes of High Energy Astrophysics objects and to use X-ray sources as a laboratory for a substantial progress on Fundamental Physics. Comment: 12 pages, 7 figures
    Proceedings of SPIE - The International Society for Optical Engineering 09/2006; DOI:10.1117/12.671693 · 0.20 Impact Factor
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    Alessandra Lamastra, Giorgio Matt, G. Cesare Perola
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    ABSTRACT: The BH mass (and the related Eddington ratio) in broad line AGN is usually evaluated by combining estimates (often indirect) of the BLR radius and of the FWHM of the broad lines, under the assumption that the BLR clouds are in Keplerian motion around the BH. Such an evaluation depends on the geometry of the BLR. There are two major options for the BLR configuration: spherically symmetric or ``flattened''. In the latter case the inclination to the line of sight becomes a relevant parameter. This paper is devoted to evaluate the bias on the estimate of the Eddington ratio when a spherical geometry is assumed (more generally when inclination effects are ignored), while the actual configuration is ``flattened'', as some evidence suggests. This is done as a function of luminosity and redshift, on the basis of recent results which show the existence of a correlation between the fraction of obscured AGN and these two parameters up to at least z=2.5. The assumed BLR velocity field is akin to the ``generalized thick disk'' proposed by Collin et al. (2006). Assuming an isotropic orientation in the sky, the mean value of the bias is calculated as a function of luminosity and redshift. It is demonstrated that, on average, the Eddington ratio obtained assuming a spherical geometry is underestimated for high luminosities, and overestimated for low luminosities. This bias converges for all luminosities at z about 2.7, while nothing can be said on this bias at larger redshifts due to the lack of data. The effects of the bias, averaged over the luminosity function of broad line AGN, have been calculated. The results imply that the bias associated with the a-sphericity of the BLR make even worse the discrepancy between the observations and the predictions of evolutionary models. Comment: 6 pages, 3 figures, accepted for publication in A&A
    Astronomy and Astrophysics 09/2006; DOI:10.1051/0004-6361:20065905 · 4.48 Impact Factor
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    ABSTRACT: Development of multi-layer optics makes feasible the use of X-ray telescope at energy up to 60-80 keV: in this paper we discuss the extension of photoelectric polarimeter based on Micro Pattern Gas Chamber to high energy X-rays. We calculated the sensitivity with Neon and Argon based mixtures at high pressure with thick absorption gap: placing the MPGC at focus of a next generation multi-layer optics, galatic and extragalactic X-ray polarimetry can be done up till 30 keV. Comment: 12 pages, 7 figures
    Proceedings of SPIE - The International Society for Optical Engineering 06/2006; 6266:62662X. DOI:10.1117/12.671975 · 0.20 Impact Factor
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    ABSTRACT: An extensive theoretical literature predicts that X-ray Polarimetry can directly determine relevant physical and geometrical parameters of astrophysical sources, and discriminate between models further than allowed by spectral and timing data only. X-ray Polarimetry can also provide tests of Fundamental Physics. A high sensitivity polarimeter in the focal plane of a New Generation X-ray telescope could open this new window in the High Energy Sky.

Publication Stats

4k Citations
698.43 Total Impact Points

Institutions

  • 5–2013
    • Università Degli Studi Roma Tre
      • Department of Mathematics and Physics
      Roma, Latium, Italy
  • 2006–2007
    • Università degli Studi Europea di Roma
      Roma, Latium, Italy
  • 1988–2006
    • Sapienza University of Rome
      • Department of Physics
      Roma, Latium, Italy
  • 2005
    • The Astronomical Observatory of Brera
      Merate, Lombardy, Italy
  • 1993–2003
    • University of Rome Tor Vergata
      • Dipartimento di Fisica
      Roma, Latium, Italy
  • 2000
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 1999
    • University of Maryland, Baltimore County
      Baltimore, Maryland, United States
  • 1994
    • Columbia University
      New York, New York, United States
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States
  • 1975–1994
    • National Research Council
      Roma, Latium, Italy
  • 1991
    • University of Tuebingen
      Tübingen, Baden-Württemberg, Germany
  • 1990
    • NASA
      • Goddard Space Flight Centre
      Washington, West Virginia, United States
  • 1986
    • Università degli Studi di Palermo
      Palermo, Sicily, Italy
  • 1981
    • The Ecological Society of America
      Florida, United States
  • 1978–1979
    • University of Milan
      • Department of Physics
      Milano, Lombardy, Italy
  • 1977–1978
    • University of Leicester
      • Department of Physics and Astronomy
      Leiscester, England, United Kingdom