N. Rea

IEEC Institute of Space Studies of Catalonia, Barcino, Catalonia, Spain

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Publications (259)1064.41 Total impact

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    ABSTRACT: Diffuse radio emission was detected around the soft gamma-ray repeater SGR 1806-20, after its 2004 powerful giant flare. We study the possible extended X-ray emission at small scales around SGR 1806-20, in two observations by the High Resolution Camera Spectrometer (HRC-S) on board of the Chandra X-ray Observatory: in 2005, 115 days after the giant flare, and in 2013, during quiescence. We compare the radial profiles extracted from data images and PSF simulations, carefully considering various issues related with the uncertain calibration of the HRC PSF at sub-arcsecond scales. We do not see statistically significant excesses pointing to an extended emission on scales of arcseconds. As a consequence, SGR 1806-20 is compatible with being point-like in X-rays, months after the giant flare, as well as in quiescence.
    Journal of High Energy Astrophysics. 11/2014;
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    ABSTRACT: AX J1745.6-2901 is a high-inclination (eclipsing) neutron star Low Mass X-ray Binary (LMXB) located less than ~1.5 arcmin from Sgr A*. Ongoing monitoring campaigns have targeted Sgr A* frequently and these observations also cover AX J1745.6-2901. We present here an X-ray analysis of AX J1745.6-2901 using a large dataset of 38 XMM-Newton observations, including eleven which caught AX J1745.6-2901 in outburst. Fe K absorption is clearly seen when AX J1745.6-2901 is in the soft state, but disappears during the hard state. The variability of these absorption features does not appear to be due to changes in the ionizing continuum. The small Kalpha/Kbeta ratio of the equivalent widths of the Fe xxv and Fe xxvi lines suggests that the column densities and turbulent velocities of the absorbing ionised plasma are in excess of N_H ~ 10^23 cm^-2 and v_turb >~ 500 km s^-1. These findings strongly support a connection between the wind (Fe K absorber) and the accretion state of the binary. These results reveal strong similarities between AX J1745.6-2901 and the eclipsing neutron star LMXB, EXO 0748-676, as well as with high-inclination black hole binaries, where winds (traced by the same Fe K absorption features) are observed only during the accretion-disc-dominated soft states, and disappear during the hard states characterised by jet emission.
    09/2014;
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    ABSTRACT: The origin of the strong magnetic fields measured in magnetars is one of the main uncertainties in the neutron star field. On the other hand, the recent discovery of a large number of such strongly magnetized neutron stars, is calling for more investigation on their formation. The first proposed model for the formation of such strong magnetic fields in magnetars was through alpha-dynamo effects on the rapidly rotating core of a massive star. Other scenarios involve highly magnetic massive progenitors that conserve their strong magnetic moment into the core after the explosion, or a common envelope phase of a massive binary system. In this work, we do a complete re-analysis of the archival X-ray emission of the Supernova Remnants (SNR) surrounding magnetars, and compare our results with all other bright X-ray emitting SNRs, which are associated with Compact Central Objects (CCOs; which are proposed to have magnetar-like B-fields buried in the crust by strong accretion soon after their formation), high-B pulsars and normal pulsars. We find that emission lines in SNRs hosting highly magnetic neutron stars do not differ significantly in elements or ionization state from those observed in other SNRs, neither averaging on the whole remnants, nor studying different parts of their total spatial extent. Furthermore, we find no significant evidence that the total X-ray luminosities of SNRs hosting magnetars, are on average larger than that of typical young X-ray SNRs. Although biased by a small number of objects, we found that for a similar age, there is the same percentage of magnetars showing a detectable SNR than for the normal pulsar population.
    09/2014;
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    ABSTRACT: The magnetic field strength at birth is arguably one of the most important properties to determine the evolutionary path of a neutron star. Objects with very high fields, collectively known as magnetars, are characterized by high X-ray quiescent luminosities, occurrence of outbursts, and, for some of them, sporadic giant flares. While the magnetic field strength is believed to drive their collective behaviour, however, the diversity of their properties, and, especially, the observation of magnetar-like bursts from “low-field” pulsars, has been a theoretical puzzle. In this review, we discuss results of long-term simulations following the coupled evolution of the X-ray luminosity and the timing properties for a large, homogeneous sample of X-ray emitting isolated neutron stars, accounting for a range of initial magnetic field strengths, envelope compositions, and neutron star masses. In addition, by following the evolution of magnetic stresses within the neutron star crust, we can also relate the observed magnetar phenomenology to the physical properties of neutron stars, and in particular to their age and magnetic field strength and topology. The dichotomy of “high-B” field pulsars versus magnetars is naturally explained, and occasional outbursts from old, low B-field neutron stars are predicted. We conclude by speculating on the fate of old magnetars, and by presenting observational diagnostics of the neutron star crustal field topology. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Astronomische Nachrichten 08/2014; 335(6‐7). · 1.40 Impact Factor
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    Monthly Notices of the Royal Astronomical Society 06/2014; · 5.52 Impact Factor
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    ABSTRACT: The thermal X-ray spectra of several isolated neutron stars display deviations from a pure blackbody. The accurate physical interpretation of these spectral features bears profound implications for our understanding of the atmospheric composition, magnetic field strength and topology, and equation of state of dense matter. With specific details varying from source to source, common explanations for the features have ranged from atomic transitions in the magnetized atmospheres or condensed surface, to cyclotron lines generated in a hot ionized layer near the surface. Here we quantitatively evaluate the X-ray spectral distortions induced by inhomogeneous temperature distributions of the neutron star surface. To this aim, we explore several surface temperature distributions, we simulate their corresponding general relativistic X-ray spectra (assuming an isotropic, blackbody emission), and fit the latter with a single blackbody model. We find that, in some cases, the presence of a spurious 'spectral line' is required at a high significance level in order to obtain statistically acceptable fits, with central energy and equivalent width similar to the values typically observed. We also perform a fit to a specific object, RX J0806.4-4123, finding several surface temperature distributions able to model the observed spectrum. The explored effect is unlikely to work in all sources with detected lines, but in some cases it can indeed be responsible for the appearance of such lines. Our results enforce the idea that surface temperature anisotropy can be an important factor that should be considered and explored also in combination with more sophisticated emission models like atmospheres.
    Monthly Notices of the Royal Astronomical Society 06/2014; 443(1). · 5.52 Impact Factor
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    ABSTRACT: We report observations using the Swift/XRT, NuSTAR, and Chandra X-ray telescopes of the transient X-ray source CXOGC J174540.0-290005, during its 2013 outburst. Due to its location in the field of multiple observing campaigns targeting Sgr A*, this is one of the best-studied outbursts of a very faint X-ray binary (VFXB; peak LX < 1036 erg/s) yet recorded, with detections in 173 ks of X-ray observations over 50 days. VFXBs are of particular interest, due to their unusually low outburst luminosities and time-averaged mass transfer rates, which are hard to explain within standard accretion physics and binary evolution. The 2013 outburst of CXOGC J174540.0-290005 peaked at Lx (2-10 keV)=5.0 × 1035 erg/s, and all data above 1034 ergs/s were well-fit by an absorbed power-law of photon index ∼ 1.7, extending from 2 keV out to ~70 keV. We discuss the implications of these observations for the accretion state of CXOGC J174540.0-290005.
    Monthly Notices of the Royal Astronomical Society 05/2014; · 5.52 Impact Factor
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    ABSTRACT: We report observations using the Swift/XRT, NuSTAR, and Chandra X-ray telescopes of the transient X-ray source CXOGC J174540.0-290005, during its 2013 outburst. Due to its location in the field of multiple observing campaigns targeting Sgr A*, this is one of the best-studied outbursts of a very faint X-ray binary (VFXB; peak $L_X<10^{36}$ erg/s) yet recorded, with detections in 173 ks of X-ray observations over 50 days. VFXBs are of particular interest, due to their unusually low outburst luminosities and time-averaged mass transfer rates, which are hard to explain within standard accretion physics and binary evolution. The 2013 outburst of CXOGC J174540.0-290005 peaked at $L_X$(2-10 keV)=$5.0\times10^{35}$ erg/s, and all data above $10^{34}$ ergs/s were well-fit by an absorbed power-law of photon index $\sim1.7$, extending from 2 keV out to $\sim$70 keV. We discuss the implications of these observations for the accretion state of CXOGC J174540.0-290005.
    Monthly Notices of the Royal Astronomical Society 04/2014; 442(1). · 5.52 Impact Factor
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    ABSTRACT: We present a long-term phase-coherent timing analysis and pulse-phase resolved spectroscopy for the two outbursts observed from the transient anomalous X-ray pulsar CXOU J164710.2-45521. For the first outburst we used 11 Chandra and XMM-Newton observations between September 2006 to August 2009, the longest baseline yet for this source. We obtain a coherent timing solution with $P=10.61065583(4)$ s, $\dot{P} = 9.72(1) \times 10^{-13}\;$s s$^{-1}$ and $\ddot{P} = -1.05(5)\times10^{-20}\; $s s$^{-2}$. Under the standard assumptions this implies a surface dipolar magnetic field of $\sim 10^{14}$ G, confirming this source as a standard-$B$ magnetar. We also study the evolution of the pulse profile (shape, intensity and pulsed fraction) as a function of time and energy. Using the phase-coherent timing solution we perform a phase-resolved spectroscopy analysis, following the spectral evolution of pulse-phase features, which hints at the physical processes taking place on the star. The results are discussed from the perspective of magneto-thermal evolution models and the untwisting magnetosphere model. Finally, we present similar analysis for the second, less intense, 2011 outburst. For the timing analysis we used Swift data together with 2 XMM-Newton and Chandra pointings. The results inferred for both outbursts are compared and briefly discussed in a more general framework.
    03/2014; 441(2).
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    ABSTRACT: Rotation-powered millisecond radio pulsars are assumed to have been spun up to their present period by a $10^8$ - $10^9$ yr long X-ray bright phase of accretion of matter and angular momentum, in a low-to-intermediate mass binary system. Recently, the discovery of sources in a transitional phase, alternating cyclically between accretion and rotation-powered states on time scales of few years or less, have been reported. Here, we present a thorough statistical analysis of the spin distributions of the various classes of millisecond pulsars to assess the evolution of their spin period between the different stages. Accreting sources which showed oscillations exclusively during thermonuclear type-I X-ray bursts (nuclear-powered millisecond pulsars) are found to be significantly faster than rotation-powered sources, while accreting sources which possess a magnetosphere and show coherent pulsations (accreting millisecond pulsars) are not. On the other hand, if accreting millisecond pulsars and eclipsing rotation-powered millisecond pulsars form a common class of transitional pulsars, these are shown to have an intermediate spin distribution between the faster nuclear-powered millisecond pulsars and the slower non-eclipsing rotation-powered millisecond pulsars. We interpret these findings in terms of a spin down due to the decreasing mass accretion rate during the latest stages of the accretion phase, and of the different orbital evolutionary channels mapped by the various classes of pulsars. Instrumental selection effects are expected to have a small effect on the observed samples.
    Astronomy and Astrophysics 03/2014; 566. · 5.08 Impact Factor
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    ABSTRACT: An absorption feature, the properties of which strongly depend on the pulse phase, has been recently discovered in the X-ray spectrum of the soft gamma repeater SGR0418+5729. If interpreted as a proton cyclotron line, its energy implies a magnetic field ranging from 2× 1014 G to more than 1015 G, which confirms the magnetar interpretation for this source and provides us with the most direct measurement of the magnetic field intensity of an isolated neutron star. The lower value of the dipole field inferred from the timing parameters for SGR 0418+5729 (B = 6× 1012 G) requires that the high magnetic field responsible for the observed feature resides in a strong multi-polar component located close to the neutron star surface, in agreement with the predictions of the magnetar model. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Astronomische Nachrichten 03/2014; 335(3). · 1.40 Impact Factor
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    ABSTRACT: We have observed SGR J1745-29 (Kennea et al. 2013, ApJ 770, L24; Mori et al. 2013, ApJ 770, L23; Rea et al. 2013, ApJ 775, L34) with the ACIS-S camera onboard Chandra for about 47ks starting on 2014-02-21 (ObsID 16508), as part of the on-going Chandra X-ray monitoring of the Galactic Center region (PIs: Haggard, Baganoff, Rea).
    01/2014;
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    ABSTRACT: We study the outburst of the newly discovered X-ray transient 3XMM J185246.6+003317, re-analyzing all available XMM-Newton observations of the source to perform a phase-coherent timing analysis, and derive updated values of the period and period derivative. We find the source rotating at P = 11.55871346(6) s (90% confidence level; at epoch MJD 54728.7) but no evidence for a period derivative in the seven months of outburst decay spanned by the observations. This translates to a 3σ upper limit for the period derivative of \dot{P}< 1.4\times 10^{-13} s s–1, which, assuming the classical magneto-dipolar braking model, gives a limit on the dipolar magnetic field of B dip < 4.1 × 1013 G. The X-ray outburst and spectral characteristics of 3XMM J185246.6+003317 confirm its identification as a magnetar, but the magnetic field upper limit we derive defines it as the third "low-B" magnetar discovered in the past 3 yr, after SGR 0418+5729 and Swift J1822.3–1606. We have also obtained an upper limit to the quiescent luminosity (<4 × 1033 erg s–1), in line with the expectations for an old magnetar. The discovery of this new low field magnetar reaffirms the prediction of about one outburst per year from the hidden population of aged magnetars.
    The Astrophysical Journal 01/2014; 781(1):L17-. · 6.73 Impact Factor
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    ABSTRACT: The recent discovery of a dense, cold cloud (dubbed "G2") approaching Sgr A* offers an opportunity to test models of black hole accretion and its associated feedback. G2's orbit is eccentric and the cloud shows signs of tidal disruption by the black hole. High-energy emission from the Sgr A*/G2 encounter may rise toward pericenter (mid-to-late 2013, or early 2014) and continue over the next several years as the material circularizes. This encounter is also likely to enhance Sgr A*'s flare activity across the electromagnetic spectrum. We present preliminary results from our 2013 joint Chandra/XMM/VLA monitoring campaigns. Our programs aim to study the radiation properties of Sgr A* as G2 breaks up and feeds the accretion flow, to constrain the rates and emission mechanisms of faint X-ray flares, and to detect G2 itself as it is shocked and heated. We discuss the constraints these data place on theoretical models for the Sgr A*/G2 encounter and outline plans for continued monitoring with Chandra, XMM, HST, and VLA in 2014.
    01/2014;
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    ABSTRACT: We report on the quiescent state of the Soft Gamma Repeater SGR 0501+4516 observed by XMM-Newton on 2009 August 30. The source exhibits an absorbed flux ~75 times lower than that measured at the peak of the 2008 outburst, and a rather soft spectrum, with the same value of the blackbody temperature observed with ROSAT back in 1992. This new observation is put into the context of all existing X-ray data since its discovery in August 2008, allowing us to complete the study of the timing and spectral evolution of the source from outburst until its quiescent state. The set of deep XMM-Newton observations performed during the few-years timescale of its outburst allows us to monitor the spectral characteristics of this magnetar as a function of its rotational period, and their evolution along these years. After the first ~10 days, the initially hot and bright surface spot progressively cooled down during the decay. We discuss the behaviour of this magnetar in the context of its simulated secular evolution, inferring a plausible dipolar field at birth of 3x10^14 G, and a current (magneto-thermal) age of ~10 kyr.
    12/2013; 438(4).
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    ABSTRACT: X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017 but not selected. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus and two additional GPDs filled with pressurized Ar-DME facing the sun. The Minimum Detectable Polarization is 14 % at 1 mCrab in 10E5 s (2-10 keV) and 0.6 % for an X10 class flare. The Half Energy Width, measured at PANTER X-ray test facility (MPE, Germany) with JET-X optics is 24 arcsec. XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil).
    Experimental Astronomy 12/2013; 36(3):523-567. · 2.97 Impact Factor
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    ABSTRACT: Several binary systems, composed of a star and a compact object, have been detected in the GeV-TeV range. Several systems have been observed but only a handful of sources have shown emission at those energies. Here, we present the observations conducted by MAGIC of different {\gamma}-ray binary systems. On one hand, we show the latest studies on the binary system LS I +61 303, which displays variability on different timescales. With the latest MAGIC observations, we will try to shed light on our understanding of this source, by presenting super-orbital and multi-wavelength studies. On the other hand, we show the observational results on the binary system HD 215227. This source has been proposed as a new {\gamma}-ray binary for being spatially coincident with the gamma-ray source AGL J2241+4454 detected by AGILE at E >100 GeV.
    11/2013;
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    ABSTRACT: We study the outburst of the newly discovered magnetar 3XMM J1852+0033 (Zhou et al. 2013), re-analysing all available XMM-Newtonobservations of the source to perform a phase-coherent timing analysis, and derive updated values of the period and period derivative. We find the source rotating at P=11.55871346(6)s (90% confidence level; at epoch MJD 54728.7) but no evidence for a period derivative in the 7 months of outburst decay spanned by the observations. This translates in a 3sigma upper limit for the period derivative of Pdot<1.4x10^{-13} s/s , which assuming the classical magneto-dipolar braking gives a limit on the dipolar magnetic field of B_dip<4.1x10^{13} G . This limit makes 3XMM J1852+0033 the third "low-B" magnetar discovered in the past three years, after with SGR 0418+5729 (Rea et al. 2010), and Swift J1822.3-1606 (Rea et al. 2012, Scholz et al. 2012). The luminosity and temperature evolution are similar to other magnetar outbursts. We have also obtained an upper limit to the quiescent luminosity (< 4 x10^{33} erg/s), in line with the expectations for an old magnetar. The discovery of this new low field magnetar reaffirms the prediction of about one outburst per year from the hidden population of aged magnetars.
    11/2013;
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    ABSTRACT: We present the discovery of IGR J18245-2452, the first millisecond pulsar observed to swing between a rotation-powered, radio pulsar state, and an accretion-powered X-ray pulsar state (Papitto et al. 2013, Nature, 501, 517). This transitional source represents the most convincing proof of the evolutionary link shared by accreting neutron stars in low mass X-ray binaries, and radio millisecond pulsars. It demonstrates that swings between these two states take place on the same time-scales of luminosity variations of X-ray transients, and are therefore most easily interpreted in terms of changes in the rate of mass in-flow. While accreting mass, the X-ray emission of IGR J18245-2452 varies dramatically on time-scales ranging from a second to a few hours. We interpret a state characterised by a lower flux and pulsed fraction, and by sudden increases of the hardness of the X-ray emission, in terms of the onset of a magnetospheric centrifugal inhibition of the accretion flow. Prospects of finding new members of the newly established class of transitional pulsars are also briefly discussed.
    10/2013;
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    ABSTRACT: IGR J18245-2452 is the fifteenth discovered accreting millisecond X-ray pulsar and the first source of this class showing direct evidence for transition between accretion and rotational powered emission states. These "swings" provided the strongest confirmation of the pulsar recycling scenario available so far. During the two XMM-Newton observations that were carried out while the source was in outburst in April 2013, IGR J18245-2452 displayed a unique and peculiar variability of its X-ray emission. In this work, we report on a detailed analysis of the XMM- Newton data and focus in particular on the timing and spectral variability of the source. IGR J18245-2452 continuously switches between lower and higher intensity states, with typical variations in flux up to a factor of about 100 in time scales as short as few seconds. These variations in the source intensity are sometimes associated to a dramatic spectral hardening, during which the power-law photon index of the source changes from Gamma=1.7 to Gamma=0.7. The pulse profiles extracted at different count rates and energies show a complex variability. These phenomena are not usually observed in accreting millisecond X-ray pulsars, at least not on such a short time scale. Fast variability was also found in the ATCA radio observations carried out for about 6 hours during the outburst at a frequency of 5.5 and 9 GHz. We interpret the variability observed from IGR J18245-2452 in terms of a "hiccup" accretion phase, during which the accretion of material from the inner boundary of the Keplerian disk is reduced by the onset of centrifugal inhibition of accretion, possibly causing the launch of strong outflows. Changes across accretion and propeller regimes have been long predicted and reproduced by MHD simulations of accreting millisecond X-ray pulsars but never observed to produce an extreme variability as that shown by IGR J18245-2452.
    Astronomy and Astrophysics 10/2013; 567. · 5.08 Impact Factor

Publication Stats

2k Citations
1,064.41 Total Impact Points

Institutions

  • 2013–2014
    • IEEC Institute of Space Studies of Catalonia
      Barcino, Catalonia, Spain
    • Université Bordeaux 1
      • UMR CENBG - Centre D'Études Nucléaires de Bordeaux-Gradignan
      Talence, Aquitaine, France
  • 2006–2014
    • University of Amsterdam
      • Astronomical Institute Anton Pannekoek
      Amsterdamo, North Holland, Netherlands
  • 2012–2013
    • Institut Marqués, Spain, Barcelona
      Barcino, Catalonia, Spain
  • 2011–2013
    • Catalan Institution for Research and Advanced Studies
      Barcino, Catalonia, Spain
    • University of Padova
      Padua, Veneto, Italy
    • The Astronomical Observatory of Brera
      Merate, Lombardy, Italy
  • 2009–2013
    • Institut de Ciències de l'Espai
      Catalonia, Spain
    • Istituto Universitario di Studi Superiori di Pavia
      Ticinum, Lombardy, Italy
  • 2010
    • George Mason University
      • Center for Earth Observing and Space Research
      Fairfax, Virginia, United States
    • Autonomous University of Barcelona
      • Facultat de Ciències
      Cerdanyola del Vallès, Catalonia, Spain
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2009–2010
    • INFN - Istituto Nazionale di Fisica Nucleare
      Frascati, Latium, Italy
  • 2008
    • University of Southampton
      • Department of Physics and Astronomy
      Southampton, ENG, United Kingdom
  • 2005–2008
    • Netherlands Institute for Space Research, Utrecht
      Utrecht, Utrecht, Netherlands
  • 2003–2008
    • University of Rome Tor Vergata
      • Dipartimento di Fisica
      Roma, Latium, Italy
  • 2007
    • University of Sydney
      Sydney, New South Wales, Australia
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
  • 2004
    • Università Degli Studi Roma Tre
      Roma, Latium, Italy