L. Ostorero

University of Maryland, College Park, Maryland, United States

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Publications (63)92.05 Total impact

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    ABSTRACT: We report Fermi/LAT observations and broad-band spectral modeling of the radio-loud active galaxy 4C +55.17 (z=0.896), formally classified as a flat-spectrum radio quasar. Using 19 months of all-sky survey Fermi/LAT data, we detect a gamma-ray continuum extending up to an observed energy of 145 GeV, and furthermore we find no evidence of gamma-ray variability in the source over its observed history. We illustrate the implications of these results in two different domains. First, we investigate the origin of the steady gamma-ray emission, where we re-examine the common classification of 4C +55.17 as a quasar-hosted blazar and consider instead its possible nature as a young radio source. We analyze and compare constraints on the source physical parameters in both blazar and young radio source scenarios by means of a detailed multiwavelength analysis and theoretical modeling of its broad-band spectrum. Secondly, we show that the gamma-ray spectrum may be formally extrapolated into the very-high energy (VHE; >= 100 GeV) range at a flux level detectable by the current generation of ground-based Cherenkov telescopes. This enables us to place constraints on models of extragalactic background light (EBL) within LAT energies and features the source as a promising candidate for VHE studies of the Universe at an unprecedented redshift of z=0.896.
    The Astrophysical Journal 07/2011; · 6.73 Impact Factor
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    ABSTRACT: LambdaCDM, for the currently preferred cosmological density Omega_0 and cosmological constant Omega_Lambda, predicts that the Universe expansion decelerates from early times to redshift z~0.9 and accelerates at later times. On the contrary, the cosmological model based on conformal gravity predicts that the cosmic expansion has always been accelerating. To distinguish between these two very different cosmologies, we resort to gamma-ray bursts (GRBs), which have been suggested to probe the Universe expansion history at z>1, where identified type Ia supernovae (SNe) are rare. We use the full Bayesian approach to infer the cosmological parameters and the additional parameters required to describe the GRB data available in the literature. For the first time, we use GRBs as cosmological probes without any prior information from other data. In addition, when we combine the GRB samples with SNe, our approach neatly avoids all the inconsistencies of most numerous previous methods that are plagued by the so-called circularity problem. In fact, when analyzed properly, current data are consistent with distance moduli of GRBs and SNe that can respectively be, in a variant of conformal gravity, ~15 and ~3 magnitudes fainter than in LambdaCDM. Our results indicate that the currently available SN and GRB samples are accommodated equally well by both LambdaCDM and conformal gravity and do not exclude a continuous accelerated expansion. We conclude that GRBs are currently far from being effective cosmological probes, as they are unable to distinguish between these two very different expansion histories.
    Journal of Cosmology and Astroparticle Physics 03/2011; 10(10). · 6.04 Impact Factor
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    ABSTRACT: In a dynamical-radiative model we recently developed to describe the physics of compact, GHz-peaked-spectrum (GPS) sources, the relativistic jets propagate across the inner, kpc-sized region of the host galaxy, while the electron population of the expanding lobes evolves and emits synchrotron and inverse-Compton (IC) radiation. Interstellar-medium gas clouds engulfed by the expanding lobes, and photoionized by the active nucleus, are responsible for the radio spectral turnover through free-free absorption (FFA) of the synchrotron photons. The model provides a description of the evolution of the spectral energy distribution (SED) of GPS sources with their expansion, predicting significant and complex high-energy emission, from the X-ray to the gamma-ray frequency domain. Here, we test this model with the broadband SEDs of a sample of 11 X-ray-emitting GPS galaxies with compact-symmetric-object morphology, and show that (1) the shape of the radio continuum at frequencies lower than the spectral turnover is indeed well accounted for by the FFA mechanism and (2) the observed X-ray spectra can be interpreted as non-thermal radiation produced via IC scattering of the local radiation fields off the lobe particles, providing a viable alternative to the thermal, accretion-disk-dominated scenario. We also show that the relation between the hydrogen column densities derived from the X-ray (N H) and radio (N H I ) data of the sources is suggestive of a positive correlation, which, if confirmed by future observations, would provide further support to our scenario of high-energy emitting lobes.
    The Astrophysical Journal 01/2010; 715:1071-1093. · 6.73 Impact Factor
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    L. Ostorero
    01/2010;
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    ABSTRACT: The study of the broad-band emission of GHz-Peaked-Spectrum (GPS) radio galaxies is a powerful tool to investigate the physical processes taking place in the central, kpc-sized region of their active hosts, where the jets propagate and the lobes expand, interacting with the surrounding interstellar medium (ISM). We recently developed a new dynamical-radiative model to describe the evolution of the GPS phenomenon (Stawarz et al. 2008): as the relativistic jets propagate through the ISM, gradually engulfing narrow-line emitting gas clouds along their way, the electron population of the expanding lobes evolves, emitting synchrotron light, as well as inverse-Compton radiation via up-scattering of the photon fields from the host galaxy and its active nucleus. The model, which successfully reproduces the key features of the GPS radio sources as a class, provides a description of the evolution of their spectral energy distribution (SED) with the lobes' expansion, predicting significant and complex X-ray to γ -ray emission.We apply here the model to the broad-band SED's of a sample of known, X-ray emitting GPS galaxies, and show that (i) the free-free absorption mechanism enables us to reproduce the radio continuum at frequencies below the turnover; (ii) the lobes' non-thermal, inverse-Compton emission can account for the observed X-ray spectra, providing a viable alternative to the thermal, accretion-dominated scenario. We also show that, in our sample, the relationship between the X-ray and radio hydrogen column densities, NH and NHI, is suggestive of a positive correlation, which, if confirmed, would support the scenario of high-energy emitting lobes (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Astronomische Nachrichten 01/2009; 330(2‐3):275 - 278. · 1.40 Impact Factor
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    ABSTRACT: Light curves of 3C 279 are presented in optical (R band), X-rays (RXTE/PCA), and γ rays (CGRO/EGRET) for 1999 January-February and 2000 January-March. During both of those epochs the γ-ray levels were high and all three observed bands demonstrated substantial variation, on timescales as short as 1 day. Correlation analyses provided no consistent pattern, although a rather significant optical/γ-ray correlation was seen in 1999, with a γ-ray lag of ~2.5 days, and there are other suggestions of correlations in the light curves. For comparison, correlation analysis is also presented for the γ-ray and X-ray light curves during the large γ ray flare in 1996 February and the two γ-bright weeks leading up to it; the correlation at that time was strong, with a γ-ray/X-ray offset of no more than 1 day.
    The Astrophysical Journal 12/2008; 558(2):583. · 6.73 Impact Factor
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    ABSTRACT: Of the blazars detected by EGRET in GeV γ-rays, 3C 279 is not only the best observed by EGRET but also one of the best monitored at lower frequencies. We have assembled 11 spectra, from GHz radio through GeV γ-rays, from the time intervals of EGRET observations. Although some of the data have appeared in previous publications, most are new, including data taken during the high states in early 1999 and early 2000. All of the spectra show substantial γ-ray contribution to the total luminosity of the object; in a high state, the γ-ray luminosity dominates over that at all other frequencies by a factor of more than 10. There is no clear pattern of time correlation; different bands do not always rise and fall together, even in the optical, X-ray, and γ-ray bands. The spectra are modeled using a leptonic jet, with combined synchrotron self-Compton plus external Compton γ-ray production. Spectral variability of 3C 279 is consistent with variations of the bulk Lorentz factor of the jet, accompanied by changes in the spectral shape of the electron distribution. Our modeling results are consistent with the UV spectrum of 3C 279 being dominated by accretion disk radiation during times of low γ-ray intensity.
    The Astrophysical Journal 12/2008; 553(2):683. · 6.73 Impact Factor
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    ABSTRACT: The BL Lac object S5 0716+71 was observed in a global multi-frequency campaign to search for rapid and correlated flux density variability and signatures of an inverse-Compton (IC) catastrophe during the states of extreme apparent brightness temperatures. The observing campaign involved simultaneous monitoring at radio to IR/optical wavelengths centered around a 500-ks INTEGRAL pointing (November 10-17, 2003). We present the combined analysis and results of the cm- to sub-mm observations including a detailed study of the inter- to intra-day variability and spectral characteristics of 0716+714. We further constrain the variability brightness temperatures (T_B) and Doppler factors (delta) comparing the radio-bands with the hard X-ray emission (3-200 keV). 0716+714 was in an exceptionally high state (outburst) and different (slower) phase of short-term variability. The flux density variability in the cm- to mm-bands is dominated by a correlated, ~4 day time scale amplitude increase of up to ~35% systematically more pronounced towards shorter wavelengths. This contradicts expectations from standard interstellar scintillation (ISS) and suggests a source-intrinsic origin of the variability. The derived lower limits to T_B exceed the 10^12 K IC-limit by up to 3-4 orders of magnitude. Assuming relativistic boosting, we obtain robust and self-consistent lower limits of delta >= 5-33, in good agreement with delta_VLBI obtained from VLBI studies and the IC-Doppler factors delta_IC > 14-16 obtained from the INTEGRAL data. Since a strong contribution from ISS can be excluded and a simultaneous IC catastrophe was not observed, we conclude that relativistic Doppler boosting naturally explains the apparent violation of the theoretical limits within standard synchrotron-self-Compton (SSC) jet models of AGN. Comment: 20 pages, accepted for publication in A&A
    Astronomy and Astrophysics 09/2008; · 5.08 Impact Factor
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    Antonaldo Diaferio, Luisa Ostorero
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    ABSTRACT: We run adiabatic N-body/hydrodynamical simulations of isolated self-gravitating gas clouds to test whether conformal gravity, an alternative theory to General Relativity, is able to explain the properties of X-ray galaxy clusters without resorting to dark matter. We show that the gas clouds rapidly reach equilibrium with a density profile which is well fit by a beta-model whose normalization and slope are in approximate agreement with observations. However, conformal gravity fails to yield the observed thermal properties of the gas cloud: (i) the mean temperature is at least an order of magnitude larger than observed; (ii) the temperature profiles increase with the square of the distance from the cluster center, in clear disagreement with real X-ray clusters. These results depend on a gravitational potential whose parameters reproduce the velocity rotation curves of spiral galaxies. However, this parametrization stands on an arbitrarily chosen conformal factor. It remains to be seen whether a different conformal factor, specified by a spontaneous breaking of the conformal symmetry, can reconcile this theory with observations. Comment: 10 pages, 5 figures, MNRAS in press. A few clarifications included, according to referee's suggestions
    Monthly Notices of the Royal Astronomical Society 08/2008; · 5.52 Impact Factor
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    ABSTRACT: Here we propose a simple dynamical description of young and compact (< kpc) radio galaxies, consisting of relativistic jet propagation into a uniform gaseous medium in the central parts of an elliptical host. In the framework of the proposed model, we follow the evolution of ultrarelativistic electrons injected from a terminal hotspot of a jet to expanding lobes, taking into account their adiabatic energy losses as well as radiative cooling. This allows us to discuss the broad-band lobe emission of young radio sources. In particular, we find a relatively strong and complex high-energy emission component produced by inverse-Compton up-scattering of various surrounding photon fields by the lobe electrons. We argue that such high-energy radiation is strong enough to account for several observed properties of GHz-peaked-spectrum (GPS) radio galaxies at UV and X-ray frequencies. In addition, it is expected to extend up to GeV (or even TeV) photon energies and can thus be probed by several modern gamma-ray instruments. In particular, we suggest that GPS radio galaxies should constitute a relatively numerous class of extragalactic sources detected by GLAST.
    06/2008;
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    ABSTRACT: Here we discuss evolution and broad-band emission of compact (< kpc) lobes in young radio sources. We propose a simple dynamical description for these objects, consisting of a relativistic jet propagating into a uniform gaseous medium in the central parts of an elliptical host. In the framework of the proposed model, we follow the evolution of ultrarelativistic electrons injected from a terminal hotspot of a jet to expanding lobes, taking into account their adiabatic energy losses as well as radiative cooling. This allows us to discuss the broad-band lobe emission of young radio sources. In particular, we argue that the observed spectral turnover in the radio synchrotron spectra of these objects cannot originate from the synchrotron self-absorption process but is most likely due to free-free absorption effects connected with neutral clouds of interstellar medium engulfed by the expanding lobes and photoionized by active centers. We also find a relatively strong and complex high-energy emission component produced by inverse-Compton up-scattering of various surrounding photon fields by the lobes' electrons. We argue that such high energy radiation is strong enough to account for several observed properties of GHz-peaked-spectrum (GPS) radio galaxies at UV and X-ray frequencies. In addition, this emission is expected to extend up to GeV (or possibly even TeV) photon energies and can thus be probed by several modern gamma-ray instruments. In particular, we suggest that GPS radio galaxies should constitute a relatively numerous class of extragalactic sources detected by GLAST. Comment: 32 pages, 3 figures included. Revised version, accepted for publication in ApJ
    The Astrophysical Journal 12/2007; · 6.73 Impact Factor
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    ABSTRACT: Two guest-observer XMM-Newton pointings of the blazar OJ 287 in 2005 are introduced, along with part of the radio, mm, near-IR, and optical data obtained during a coordinated and intensive WEBT campaign, during longer-term monitoring observations performed by teams of the ENIGMA network, and during other independent observing programs (like VLBA observations). In that year OJ 287 showed an interesting variable behavior in the optical band. An optical outburst, well matched by our WEBT observations, is claimed in the period Oct.-Nov. 2005, and the XMM-Newton X-ray observations are performed in correspondence with two active optical states (an intermediate flare and such outburst). X-ray data indicates different flux levels, spectral slopes, and emission components, and VLBA radio maps are consistent with a jet precession model. This appreciable observing effort is still ongoing (a further XMM-Newton pointing is planned in 2008), joined with further parallel/multi-monitoring observing programmes devoted to this interesting object.
    LI Congresso Nazionale della Società Astronomica Italiana, Firenze, Italy; 04/2007
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    ABSTRACT: Two guest-observer XMM-Newton observations of the peculiar blazar OJ 287 in 2005 are briefly presented, along with the multiwavelength data obtained during a coordinated intensive WEBT campaign, and data obtained within longer-term independent monitor programs, performed also by other facilities. During that year OJ 287 showed an interesting variability trend in the optical band. The X-ray observations, performed in correspondence with two active optical states (a flare and an outburst), indicate different flux levels, spectral slopes, and emission components, while VLBA radio maps are consistent with a jet precession model. A further XMM-Newton observation of OJ 287 is granted and foreseen in spring 2008, providing the opportunity of a multifrequency campaign to be performed in conjunction with GLAST.
    The First GLAST Symposium, Stanford, CA, USA; 02/2007
  • Bollettino della Societa Paleontologica Italiana 01/2007; 78:741. · 0.70 Impact Factor
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    ABSTRACT: The possibility to detect simultaneously in the X-ray band the synchrotron and Inverse Compton (IC)emission of intermediate BL Lac objects offers the unique opportunity to study contemporaneously the low- and high-energy tails of the electron distribution in the jets of these sources. We attempted to disentangle the X-ray spectral variability properties of both the low- and high-energy ends of the synchrotron and Inverse Compton emission of the intermediate BL Lac object S5 0716+71. We carried out spectral, temporal and cross-correlation analyses of the data from a long XMM-Newton pointing of S5 0716+71 and we compared our findings with previous results from past X-ray observations. Strong variability was detected during the XMM exposure.Both the synchrotron and Inverse Compton components were found to vary on time scales of hours, implying a size of the emitting region of $R\la 0.7\delta /(1+z)$ light-hours. The synchrotron emission was discovered to become dominant during episodes of flaring activity, following a harder-when-brighter trend. Tight correlations were observed between variations in different energy bands. Upper limits on time lags between the soft and hard X-ray light curves are of the order of a few hundred seconds. Comment: 14 pages, 9 figures, 5 tables, A&A in press, minor corrections to match the version to be published
    Astronomy and Astrophysics 07/2006; · 5.08 Impact Factor
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    ABSTRACT: We report on a densely time sampled polarimetric flux density monitoring of the BL Lac object S5 0716+71 at 86 GHz and 229 GHz with the IRAM 30 m telescope within a coordinated broad spectral band campaign, centred around an INTEGRAL observation during November 10 to 16, 2003. Our aim was to search for signatures of inverse-Compton "catastrophes". At 86 GHz, making use of a new calibration strategy, we reach a relative rms accuracy of the flux density measurements of 1.2%. At this frequency, S5 0716+71 showed no intra-day variability, but showed remarkable inter-day variability with a flux density increase of 34% during the first four observing days, which can not be explained by source extrinsic causes. The 86 GHz linear polarization fraction of S5 0716+71 was unusually large 15.0+-1.8%. Inter-day variability in linear polarization at 86 GHz, with significance level >~95%; sigma_P/ =15% and sigma_chi=6 deg., was also observed. From the emission variations at the synchrotron turnover frequency (~86 GHz) we compute an apparent brightness temperature T_B,app>1.4x10^14K at a redshift of 0.3, which exceeds by two orders of magnitude the inverse-Compton limit. A relativistic correction for T_B,app with a Doppler factor delta > 7.8 brings the observed brightness temperature down to the inverse Compton limit. A more accurate lower limit of delta > 14.0, is obtained from the comparison of the 86 GHz synchrotron flux density and the upper limits for the synchrotron self-Compton flux density obtained from the INTEGRAL observations. The relativistic beaming of the emission by this high Doppler factor explains the non-detection of "catastrophic" inverse-Compton avalanches by INTEGRAL. Comment: Accepted for publication in A&A. 16 pages, 11 figures
    Astronomy and Astrophysics 06/2006; · 5.08 Impact Factor
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    ABSTRACT: Some intra-day variable, compact extra-galactic radio sources show brightness temperatures severely exceeding 10^{12} K, the limit set by catastrophic inverse-Compton (IC) cooling in sources of incoherent synchrotron radiation. The violation of the IC limit, possible under non-stationary conditions, would lead to IC avalanches in the soft-gamma-ray energy band during transient periods. For the first time, broadband signatures of possible IC catastrophes were searched for in S5 0716+71. A multifrequency observing campaign targetting S5 0716+71 was carried out in November 2003 under the framework of the European Network for the Investigation of Galactic nuclei through Multifrequency Analysis (ENIGMA) together with a campaign by the Whole Earth Blazar Telescope (WEBT), involving a pointing by the soft-gamma-ray satellite INTEGRAL, optical, near-infrared, sub-millimeter, millimeter, radio, and Very Long Baseline Array (VLBA) monitoring. S5 0716+71 was very bright at radio frequencies and in a rather faint optical state during the INTEGRAL pointing; significant inter-day and low intra-day variability was recorded in the radio regime, while typical fast variability features were observed in the optical band. No correlation was found between the radio and optical emission. The source was not detected by INTEGRAL, neither by the X-ray monitor JEM-X nor by the gamma-ray imager ISGRI, but upper limits to the source emission in the 3-200 keV energy band were estimated. A brightness temperature Tb>2.1x10^{14} K was inferred from the radio variability, but no corresponding signatures of IC avalanches were recorded at higher energies. The absence of IC-catastrophe signatures provides either a lower limit delta>8 to the Doppler factor affecting the radio emission or strong constraints for modelling of the Compton catastrophes in S5 0716+71. Comment: 15 pages, 3 EPS figures, 3 tables, to appear in A&A
    02/2006;
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    ABSTRACT: Multicolor (BV RIJHK) observations of the BL Lac object S5 0716+714 carried out irom 2001–2004 indicate that the spectral energy distribution of the slowly varying component remained constant over the entire wavelength interval and over the three-year observation period. The distribution can be represented by the power law F v ∼ v −1.12, providing evidence that this component has a synchrotron nature. The color characteristics of very rapid variability detected on several nights are the same as those for the slowly varying component. An analysis of published data on the color variability of the object for previous years indicates that the color characteristics of the slowly varying component are stable, independent of the time intervals considered and the characteristic variability time scales.
    Astronomy Reports 01/2006; 50(6):458-467. · 0.76 Impact Factor
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    ABSTRACT: The southern gamma-ray blazar PKS 2155-304 is one of the brightest and most intensively studied prototypes of BL Lac object. PKS 2155-304 has recently aroused the interest of Cherenkov telescope projects like HESS and MAGIC, the former having already observed the source in 2002 and 2003. This blazar was monitored with the KVA optical telescope (R-band intranight photometry and unfiltered polarization observations), in the frame of a new HESS multiwavelength campaign performed in August-September 2004.
    10/2005;
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    ABSTRACT: A multiwavelength campaign to observe the BL Lac object AO 0235+16 (z=0.94) was set up by the Whole Earth Blazar Telescope (WEBT) collaboration during the observing seasons 2003-2004 and 2004-2005, involving radio, near-IR and optical photometric monitoring, VLBA monitoring, optical spectral monitoring, and three pointings by the XMM-Newton satellite. Here we report on the results of the first season, which involved the participation of 24 optical and near-IR telescopes and 4 radio telescopes, as well as the first XMM-Newton pointing, which occurred on January 18-19, 2004. Unpublished data from previous epochs were also collected (from 5 optical-NIR and 3 radio telescopes), in order to fill the gap between the end of the period presented in Raiteri et al. (2001) and the start of the WEBT campaign. The contribution of the southern AGN, 2 arcsec distant from the source, is taken into account. It is found to especially affect the blue part of the optical spectrum when the source is faint. In the optical and near-IR the source has been very active in the last 3 years, although it has been rather faint most of the time, with noticeable variations of more than a magnitude over a few days. In contrast, in the radio bands it appears to have been ``quiescent'' since early 2000. The major radio (and optical) outburst predicted to peak around February-March 2004 (with a six month uncertainty) has not occurred yet. When comparing our results with the historical light curves, two different behaviours seem to characterize the optical outbursts: only the major events present a radio counterpart. The X-ray spectra obtained by the three EPIC detectors are well fitted by a power law with extra-absorption at z=0.524; the energy index in the 0.2-10 keV range is well constrained: alpha=0.645 ± 0.028 and the 1 keV flux density is 0.311 ± 0.008~mu Jy. The analysis of the X-ray light curves reveals that no significant variations occurred during the pointing. In contrast, simultaneous dense radio monitoring with the 100 m telescope at Effelsberg shows a ~2-3% flux decrease in 6-7 h, which, if intrinsic, would imply a brightness temperature well above the Compton limit and hence a lower limit to the Doppler factor delta &gap; 46. We construct the broad-band spectral energy distribution of January 18-19, 2004 with simultaneous radio data from Effelsberg, optical data from the Nordic Optical Telescope (NOT), optical-UV data from the Optical Monitor onboard XMM-Newton, and X-ray data by the EPIC instruments. Particular care is taken to correct data for extinction due to both the Milky Way and the z=0.524 absorber. The resulting SED suggests the existence of a bump in the UV spectral region.
    åp. 07/2005; 438:39-53.

Publication Stats

366 Citations
92.05 Total Impact Points

Institutions

  • 2011
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
  • 2009
    • INFN - Istituto Nazionale di Fisica Nucleare
      Frascati, Latium, Italy
  • 2007–2009
    • Jagiellonian University
      • Obserwatorium Astronomiczne
      Cracovia, Lesser Poland Voivodeship, Poland
  • 2000–2008
    • Università degli Studi di Torino
      Torino, Piedmont, Italy
  • 2005–2006
    • University of Turku
      • Department of Physics and Astronomy
      Turku, Province of Western Finland, Finland
  • 2003
    • Francis Marion University
      • Department of Physics and Astronomy
      Florence, South Carolina, United States