E. Hays

Hiroshima University, Hirosima, Hiroshima, Japan

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Publications (269)1461.63 Total impact

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    ABSTRACT: The Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope, has detected a transient gamma-ray source in the Galactic Plane starting on 15 March 2012. The preliminary LAT position is (J2000.0): RA = 135.56 deg, Dec = -46.41 deg, (l, b = 267.47 deg, 0.07 deg) with a 68% confidence error circle radius 0.23 deg (statistical uncertainty only). Preliminary analysis of the Fermi-LAT data indicates that on 15 March 2012 the source was detected with a flux (E >100 MeV) of 1.2+/-0.3 x 10^-6 ph cm^-2 s^-1.
    The Astronomer's Telegram. 03/2012;
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    ABSTRACT: The Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope, has observed a gamma-ray flare from a new gamma-ray source. The preliminary best-fit location of the gamma-ray source (RA = 159.604 deg, Decl. = -53.242 deg, J2000) is +4.6 deg above the Galactic plane, and has a 95% containment radius of 7.2 arcmin (statistical errors only). The bright flat spectrum radio source, PMN J1038-5311 (RA = 10 38 40.56, Decl.
    The Astronomer's Telegram. 03/2012; 3978:1.
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    ABSTRACT: We report on a systematic investigation of the γ-ray properties of 120 hard X-ray-selected Seyfert galaxies classified as "radio-quiet" objects, utilizing the three-year accumulation of Fermi Large Area Telescope (LAT) data. Our sample of Seyfert galaxies is selected using the Swift Burst Alert Telescope 58 month catalog, restricting the analysis to the bright sources with average hard X-ray fluxes F 14 – 195 keV ≥ 2.5 × 10–11 erg cm–2 s–1 at high Galactic latitudes (|b| > 10°). In order to remove "radio-loud" objects from the sample, we use the "hard X-ray radio loudness parameter," R rX, defined as the ratio of the total 1.4 GHz radio to 14-195 keV hard X-ray energy fluxes. Among 120 X-ray bright Seyfert galaxies with R rX <10–4, we did not find a statistically significant γ-ray excess (TS > 25) positionally coincident with any target Seyferts, with possible exceptions of ESO 323-G077 and NGC 6814. The mean value of the 95% confidence level γ-ray upper limit for the integrated photon flux above 100 MeV from the analyzed Seyferts is 4 × 10–9 photons cm–2 s–1 , and the upper limits derived for several objects reach 1 × 10–9 photons cm–2 s–1 . Our results indicate that no prominent γ-ray emission component related to active galactic nucleus activity is present in the spectra of Seyferts around GeV energies. The Fermi-LAT upper limits derived for our sample probe the ratio of γ-ray to X-ray luminosities L γ/L X < 0.1, and even <0.01 in some cases. The obtained results impose novel constraints on the models for high-energy radiation of "radio-quiet" Seyfert galaxies.
    The Astrophysical Journal 02/2012; 747(2):104. · 6.73 Impact Factor
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    ABSTRACT: Gamma-ray binaries are stellar systems containing a neutron star or black hole, with gamma-ray emission produced by an interaction between the components. These systems are rare, even though binary evolution models predict dozens in our Galaxy. A search for gamma-ray binaries with the Fermi Large Area Telescope (LAT) shows that 1FGL J1018.6-5856 exhibits intensity and spectral modulation with a 16.6-day period. We identified a variable x-ray counterpart, which shows a sharp maximum coinciding with maximum gamma-ray emission, as well as an O6V((f)) star optical counterpart and a radio counterpart that is also apparently modulated on the orbital period. 1FGL J1018.6-5856 is thus a gamma-ray binary, and its detection suggests the presence of other fainter binaries in the Galaxy.
    Science 01/2012; 335(6065):189-93. · 31.20 Impact Factor
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    ABSTRACT: The Geostationary Operational Environmental Satellite (GOES) M2-class solar flare, SOL2010-06-12T00:57, was modest in many respects yet exhibited remarkable acceleration of energetic particles. The flare produced an ~50 s impulsive burst of hard X- and γ-ray emission up to at least 400 MeV observed by the Fermi Gamma-ray Burst Monitor and Large Area Telescope experiments. The remarkably similar hard X-ray and high-energy γ-ray time profiles suggest that most of the particles were accelerated to energies 300 MeV with a delay of ~10 s from mildly relativistic electrons, but some reached these energies in as little as ~3 s. The γ-ray line fluence from this flare was about 10 times higher than that typically observed from this modest GOES class of X-ray flare. There is no evidence for time-extended >100 MeV emission as has been found for other flares with high-energy γ-rays.
    The Astrophysical Journal 01/2012; 21222314141238142242(21). · 6.73 Impact Factor
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    ABSTRACT: We report on the detection of high-energy γ-ray emission from the Moon during the first 24 months of observations by the Fermi Large Area Telescope (LAT). This emission comes from particle cascades produced by cosmic-ray (CR) nuclei and electrons interacting with the lunar surface. The differential spectrum of the Moon is soft and can be described as a log-parabolic function with an effective cutoff at 2-3 GeV, while the average integral flux measured with the LAT from the beginning of observations in 2008 August to the end of 2010 August is F (> ##IMG## [http://ej.iop.org/images/0004-637X/758/2/140/apj445159ieqn1.gif] $100 rm MeV) =(1.04pm 0.01,rm [statistical error]pm 0.1,rm [systematic error])times 10^-6$ cm –2 s –1 . This flux is about a factor 2-3 higher than that observed between 1991 and 1994 by the EGRET experiment on board the Compton Gamma Ray Observatory , F (>100 MeV) ##IMG## [http://ej.iop.org/icons/Entities/ap.gif] ≈ 5 × 10 –7 cm –2 s –1 , when solar activity was relatively high. The higher γ-ray flux measured by Fermi is consistent with the deep solar minimum conditions during the first 24 months of the mission, which reduced effects of heliospheric modulation, and thus increased the heliospheric flux of Galactic CRs. A detailed comparison of the light curve with McMurdo Neutron Monitor rates suggests a correlation of the trends. The Moon and the Sun are so far the only known bright emitters of γ-rays with fast celestial motion. Their paths across the sky are projected onto the Galactic center and high Galactic latitudes as well as onto other areas crowded with high-energy γ-ray sources. Analysis of the lunar and solar emission may thus be important for studies of weak and transient sources near the ecliptic.
    The Astrophysical Journal 01/2012; 758(2):140. · 6.73 Impact Factor
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    ABSTRACT: We present a detailed analysis of the GeV gamma-ray emission toward the supernova remnant (SNR) G8.7–0.1 with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. An investigation of the relationship between G8.7–0.1 and the TeV unidentified source HESS J1804–216 provides us with an important clue on diffusion process of cosmic rays if particle acceleration operates in the SNR. The GeV gamma-ray emission is extended with most of the emission in positional coincidence with the SNR G8.7–0.1 and a lesser part located outside the western boundary of G8.7–0.1. The region of the gamma-ray emission overlaps spatially connected molecular clouds, implying a physical connection for the gamma-ray structure. The total gamma-ray spectrum measured with LAT from 200 MeV-100 GeV can be described by a broken power-law function with a break of 2.4 ± 0.6 (stat) ± 1.2 (sys) GeV, and photon indices of 2.10 ± 0.06 (stat) ± 0.10 (sys) below the break and 2.70 ± 0.12 (stat) ± 0.14 (sys) above the break. Given the spatial association among the gamma rays, the radio emission of G8.7–0.1, and the molecular clouds, the decay of π0s produced by particles accelerated in the SNR and hitting the molecular clouds naturally explains the GeV gamma-ray spectrum. We also find that the GeV morphology is not well represented by the TeV emission from HESS J1804–216 and that the spectrum in the GeV band is not consistent with the extrapolation of the TeV gamma-ray spectrum. The spectral index of the TeV emission is consistent with the particle spectral index predicted by a theory that assumes energy-dependent diffusion of particles accelerated in an SNR. We discuss the possibility that the TeV spectrum originates from the interaction of particles accelerated in G8.7–0.1 with molecular clouds, and we constrain the diffusion coefficient of the particles.
    The Astrophysical Journal 12/2011; 744(1):80. · 6.73 Impact Factor
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    ABSTRACT: Satellite galaxies of the Milky Way are among the most promising targets for dark matter searches in gamma rays. We present a search for dark matter consisting of weakly interacting massive particles, applying a joint likelihood analysis to 10 satellite galaxies with 24 months of data of the Fermi Large Area Telescope. No dark matter signal is detected. Including the uncertainty in the dark matter distribution, robust upper limits are placed on dark matter annihilation cross sections. The 95% confidence level upper limits range from about 10(-26)  cm3  s(-1) at 5 GeV to about 5×10(-23)   cm3  s(-1) at 1 TeV, depending on the dark matter annihilation final state. For the first time, using gamma rays, we are able to rule out models with the most generic cross section (∼3×10(-26)  cm3  s(-1) for a purely s-wave cross section), without assuming additional boost factors.
    Physical Review Letters 12/2011; 107(24):241302. · 7.73 Impact Factor
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    ABSTRACT: The origin of Galactic cosmic rays is a century-long puzzle. Indirect evidence points to their acceleration by supernova shockwaves, but we know little of their escape from the shock and their evolution through the turbulent medium surrounding massive stars. Gamma rays can probe their spreading through the ambient gas and radiation fields. The Fermi Large Area Telescope (LAT) has observed the star-forming region of Cygnus X. The 1- to 100-gigaelectronvolt images reveal a 50-parsec-wide cocoon of freshly accelerated cosmic rays that flood the cavities carved by the stellar winds and ionization fronts from young stellar clusters. It provides an example to study the youth of cosmic rays in a superbubble environment before they merge into the older Galactic population.
    Science 11/2011; 334(6059):1103-7. · 31.20 Impact Factor
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    ABSTRACT: The high-frequency peaked BL Lac object PKS 2005-489 was the target of a multi-wavelength campaign with simultaneous observations in the TeV gamma-ray (H.E.S.S.), GeV gamma-ray (Fermi/LAT), X-ray (RXTE, Swift), UV (Swift) and optical (ATOM, Swift) bands. This campaign was carried out during a high flux state in the synchrotron regime. The flux in the optical and X-ray bands reached the level of the historical maxima. The hard GeV spectrum observed with Fermi/LAT connects well to the very high energy (VHE, E>100GeV) spectrum measured with H.E.S.S. with a peak energy between ~5 and 500 GeV. Compared to observations with contemporaneous coverage in the VHE and X-ray bands in 2004, the X-ray flux was ~50 times higher during the 2009 campaign while the TeV gamma-ray flux shows marginal variation over the years. The spectral energy distribution during this multi-wavelength campaign was fit by a one zone synchrotron self-Compton model with a well determined cutoff in X-rays. The parameters of a one zone SSC model are inconsistent with variability time scales. The variability behaviour over years with the large changes in synchrotron emission and small changes in the inverse Compton emission does not warrant an interpretation within a one-zone SSC model despite an apparently satisfying fit to the broadband data in 2009.
    Astronomy and Astrophysics 11/2011; 533:110. · 5.08 Impact Factor
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    ABSTRACT: We investigate the Fermi Large Area Telescope γ-ray and 15 GHz Very Long Baseline Array radio properties of a joint γ-ray and radio-selected sample of active galactic nuclei (AGNs) obtained during the first 11 months of the Fermi mission (2008 August 4-2009 July 5). Our sample contains the brightest 173 AGNs in these bands above declination –30° during this period, and thus probes the full range of γ-ray loudness (γ-ray to radio band luminosity ratio) in the bright blazar population. The latter quantity spans at least 4 orders of magnitude, reflecting a wide range of spectral energy distribution (SED) parameters in the bright blazar population. The BL Lac objects, however, display a linear correlation of increasing γ-ray loudness with synchrotron SED peak frequency, suggesting a universal SED shape for objects of this class. The synchrotron self-Compton model is favored for the γ-ray emission in these BL Lac objects over external seed photon models, since the latter predict a dependence of Compton dominance on Doppler factor that would destroy any observed synchrotron SED-peak-γ-ray-loudness correlation. The high-synchrotron peaked (HSP) BL Lac objects are distinguished by lower than average radio core brightness temperatures, and none display large radio modulation indices or high linear core polarization levels. No equivalent trends are seen for the flat-spectrum radio quasars (FSRQs) in our sample. Given the association of such properties with relativistic beaming, we suggest that the HSP BL Lac objects have generally lower Doppler factors than the lower-synchrotron peaked BL Lac objects or FSRQs in our sample.
    The Astrophysical Journal 11/2011; 742(1):27. · 6.73 Impact Factor
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    ABSTRACT: The Crab Nebula was detected with the Milagro experiment at a statistical significance of 17 standard deviations over the lifetime of the experiment. The experiment was sensitive to approximately 100 GeV - 100 TeV gamma ray air showers by observing the particle footprint reaching the ground. The fraction of detectors recording signals from photons at the ground is a suitable proxy for the energy of the primary particle and has been used to measure the photon energy spectrum of the Crab Nebula between ~1 and ~100 TeV. The TeV emission is believed to be caused by inverse-Compton up-scattering scattering of ambient photons by an energetic electron population. The location of a TeV steepening or cutoff in the energy spectrum reveals important details about the underlying electron population. We describe the experiment and the technique for distinguishing gamma-ray events from the much more-abundant hadronic events. We describe the calculation of the significance of the excess from the Crab and how the energy spectrum is fit. The fit is consistent with values measured by IACTs between 1 and 20 TeV. Fixing the spectral index to values that have been measured below 1 TeV by IACT experiments (2.4 to 2.6), the fit to the Milagro data suggests that Crab exhibits a spectral steepening or cutoff between about 20 to 40 TeV.
    The Astrophysical Journal 10/2011; · 6.73 Impact Factor
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    ABSTRACT: We present the sensitivity of HAWC to Gamma Ray Bursts (GRBs). HAWC is a very high-energy gamma-ray observatory currently under construction in Mexico at an altitude of 4100 m. It will observe atmospheric air showers via the water Cherenkov method. HAWC will consist of 300 large water tanks instrumented with 4 photomultipliers each. HAWC has two data acquisition (DAQ) systems. The main DAQ system reads out coincident signals in the tanks and reconstructs the direction and energy of individual atmospheric showers. The scaler DAQ counts the hits in each photomultiplier tube (PMT) in the detector and searches for a statistical excess over the noise of all PMTs. We show that HAWC has a realistic opportunity to observe the high-energy power law components of GRBs that extend at least up to 30 GeV, as it has been observed by Fermi LAT. The two DAQ systems have an energy threshold that is low enough to observe events similar to GRB 090510 and GRB 090902b with the characteristics observed by Fermi LAT. HAWC will provide information about the high-energy spectra of GRBs which in turn could help to understanding about e-pair attenuation in GRB jets, extragalactic background light absorption, as well as establishing the highest energy to which GRBs accelerate particles.
    08/2011;
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    ABSTRACT: We present a detailed statistical analysis of the correlation between radio and gamma-ray emission of the Active Galactic Nuclei (AGN) detected by Fermi during its first year of operation, with the largest datasets ever used for this purpose. We use both archival interferometric 8.4 GHz data (from the VLA and ATCA, for the full sample of 599 sources) and concurrent single-dish 15 GHz measurements from the Owens Valley Radio Observatory (OVRO, for a sub sample of 199 objects). Our unprecedentedly large sample permits us to assess with high accuracy the statistical significance of the correlation, using a surrogate-data method designed to simultaneously account for common-distance bias and the effect of a limited dynamical range in the observed quantities. We find that the statistical significance of a positive correlation between the cm radio and the broad band (E>100 MeV) gamma-ray energy flux is very high for the whole AGN sample, with a probability <1e-7 for the correlation appearing by chance. Using the OVRO data, we find that concurrent data improve the significance of the correlation from 1.6e-6 to 9.0e-8. Our large sample size allows us to study the dependence of correlation strength and significance on specific source types and gamma-ray energy band. We find that the correlation is very significant (chance probability <1e-7) for both FSRQs and BL Lacs separately; a dependence of the correlation strength on the considered gamma-ray energy band is also present, but additional data will be necessary to constrain its significance.
    Astrophysical Journal - ASTROPHYS J. 08/2011; 741.
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    ABSTRACT: We investigate the Fermi LAT gamma-ray and 15 GHz VLBA radio properties of a joint gamma-ray- and radio-selected sample of AGNs obtained during the first 11 months of the Fermi mission (2008 Aug 4 - 2009 Jul 5). Our sample contains the brightest 173 AGNs in these bands above declination -30 deg. during this period, and thus probes the full range of gamma-ray loudness (gamma-ray to radio band luminosity ratio) in the bright blazar population. The latter quantity spans at least four orders of magnitude, reflecting a wide range of spectral energy distribution (SED) parameters in the bright blazar population. The BL Lac objects, however, display a linear correlation of increasing gamma-ray loudness with synchrotron SED peak frequency, suggesting a universal SED shape for objects of this class. The synchrotron self-Compton model is favored for the gamma-ray emission in these BL Lacs over external seed photon models, since the latter predict a dependence of Compton dominance on Doppler factor that would destroy any observed synchrotron SED peak - gamma-ray loudness correlation. The high-synchrotron peaked (HSP) BL Lac objects are distinguished by lower than average radio core brightness temperatures, and none display large radio modulation indices or high linear core polarization levels. No equivalent trends are seen for the flat-spectrum radio quasars (FSRQ) in our sample. Given the association of such properties with relativistic beaming, we suggest that the HSP BL Lacs have generally lower Doppler factors than the lower-synchrotron peaked BL Lacs or FSRQs in our sample.
    Astrophysical Journal - ASTROPHYS J. 07/2011; 742.
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    ABSTRACT: We describe a dedicated cosmic-ray telescope that explores a new method for detecting Cerenkov radiation from high-energy primary cosmic rays and the large particle air shower they induce upon entering the atmosphere. Using a camera comprising 16 multi-anode photomultiplier tubes for a total of 256 pixels, the Track Imaging Cerenkov Experiment (TrICE) resolves substructures in particle air showers with 0.086 degree resolution. Cerenkov radiation is imaged using a novel two-part optical system in which a Fresnel lens provides a wide-field optical trigger and a mirror system collects delayed light with four times the magnification. TrICE records well-resolved cosmic-ray air showers at rates ranging between 0.01-0.1 Hz.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 07/2011; 659(1). · 1.14 Impact Factor
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    ABSTRACT: The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, previously reported a new gamma-ray flare from the direction of the Crab Nebula beginning on the 9th of April (ATel #3276). The increased emission was afterwards confirmed by the AGILE satellite (ATel #3282).The Crab Nebula is currently also being monitored by Chandra, which observed a bright knot east of the pulsar (ATel #3283), similar to previous observations in the September 2010 flare.
    The Astronomer's Telegram. 04/2011;
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    ABSTRACT: A young and energetic pulsar powers the well-known Crab Nebula. Here, we describe two separate gamma-ray (photon energy greater than 100 mega-electron volts) flares from this source detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. The first flare occurred in February 2009 and lasted approximately 16 days. The second flare was detected in September 2010 and lasted approximately 4 days. During these outbursts, the gamma-ray flux from the nebula increased by factors of four and six, respectively. The brevity of the flares implies that the gamma rays were emitted via synchrotron radiation from peta-electron-volt (10(15) electron volts) electrons in a region smaller than 1.4 × 10(-2) parsecs. These are the highest-energy particles that can be associated with a discrete astronomical source, and they pose challenges to particle acceleration theory.
    Science 02/2011; 331(6018):739-42. · 31.20 Impact Factor
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    ABSTRACT: We report on observations of BL Lacertae during the first 18 months of Fermi LAT science operations and present results from a 48 day multifrequency coordinated campaign from 2008 August 19 to 2008 October 7. The radio to gamma-ray behavior of BL Lac is unveiled during a low-activity state thanks to the coordinated observations of radio-band (Metsähovi and VLBA), near-IR/optical (Tuorla, Steward, OAGH, and MDM), and X-ray (RXTE and Swift) observatories. No variability was resolved in gamma rays during the campaign, and the brightness level was 15 times lower than the level of the 1997 EGRET outburst. Moderate and uncorrelated variability has been detected in UV and X-rays. The X-ray spectrum is found to be concave, indicating the transition region between the low- and high-energy components of the spectral energy distribution (SED). VLBA observation detected a synchrotron spectrum self-absorption turnover in the innermost part of the radio jet appearing to be elongated and inhomogeneous, and constrained the average magnetic field there to be less than 3 G. Over the following months, BL Lac appeared variable in gamma rays, showing flares (in 2009 April and 2010 January). There is no evidence for the correlation of gamma rays with the optical flux monitored from the ground in 18 months. The SED may be described by a single-zone or a two-zone synchrotron self-Compton (SSC) model, but a hybrid SSC plus external radiation Compton model seems to be preferred based on the observed variability and the fact that it provides a fit closest to equipartition.
    The Astrophysical Journal 01/2011; 730(2). · 6.73 Impact Factor
  • Astrophysical Journal. 01/2011; 731(1).

Publication Stats

2k Citations
1,461.63 Total Impact Points

Institutions

  • 2013
    • Hiroshima University
      • Division of Physical Sciences
      Hirosima, Hiroshima, Japan
  • 2012–2013
    • Deutsches Elektronen-Synchrotron
      Hamburg, Hamburg, Germany
    • Los Alamos National Laboratory
      • Physics Division
      Los Alamos, California, United States
    • Max Planck Institute for Gravitational Physics (Albert-Einstein-Institute)
      Potsdam, Brandenburg, Germany
  • 2009–2013
    • INFN - Istituto Nazionale di Fisica Nucleare
      Frascati, Latium, Italy
    • University College Dublin
      • School of Physics
      Dublin, Leinster, Ireland
    • Universidad Nacional Autónoma de México
      • Institute of Astronomy
      Ciudad de México, The Federal District, Mexico
  • 2009–2012
    • Politecnico di Bari
      Bari, Apulia, Italy
  • 2011
    • CUNY Graduate Center
      New York City, New York, United States
    • Università degli Studi di Perugia
      • Department of Physics
      Perugia, Umbria, Italy
  • 2008–2011
    • National Academy of Sciences
      Washington, Washington, D.C., United States
    • University of Leeds
      • School of Physics and Astronomy
      Leeds, England, United Kingdom
  • 2006–2011
    • University of Chicago
      • Enrico Fermi Institute
      Chicago, IL, United States
  • 2010
    • Laboratoire Leprince-Ringuet
      Paliseau, Île-de-France, France
    • Stanford University
      • Department of Physics
      Palo Alto, California, United States
    • National Institute of Astrophysics
      • Institute of Space Astrophysics and Cosmic Physics IASF - Rome
      Roma, Latium, Italy
  • 2009–2010
    • McGill University
      • Department of Physics
      Montréal, Quebec, Canada
    • Agenzia Spaziale Italiana
      Roma, Latium, Italy
  • 2003–2010
    • University of Maryland, College Park
      • • Department of Astronomy
      • • Department of Physics
      Maryland, United States
  • 2005
    • University of Wisconsin, Madison
      • Department of Physics
      Madison, MS, United States