P. Uttley

University of Amsterdam, Amsterdamo, North Holland, Netherlands

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Publications (156)574.65 Total impact

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    ABSTRACT: Timing of high-count rate sources with the NuSTAR Small Explorer Mission requires specialized analysis techniques. NuSTAR was primarily designed for spectroscopic observations of sources with relatively low count-rates rather than for timing analysis of bright objects. The instrumental dead time per event is relatively long (~2.5 msec), and varies by a few percent event-to-event. The most obvious effect is a distortion of the white noise level in the power density spectrum (PDS) that cannot be modeled easily with the standard techniques due to the variable nature of the dead time. In this paper, we show that it is possible to exploit the presence of two completely independent focal planes and use the cross power density spectrum to obtain a good proxy of the white noise-subtracted PDS. Thereafter, one can use a Monte Carlo approach to estimate the remaining effects of dead time, namely a frequency-dependent modulation of the variance and a frequency-independent drop of the sensitivity to variability. In this way, most of the standard timing analysis can be performed, albeit with a sacrifice in signal to noise relative to what would be achieved using more standard techniques. We apply this technique to NuSTAR observations of the black hole binaries GX 339-4, Cyg X-1 and GRS 1915+105.
    09/2014;
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    ABSTRACT: LOFT (Large Observatory for X-ray Timing) is one of the five candidates that were considered by ESA as an M3 mission (with launch in 2022-2024) and has been studied during an extensive assessment phase. It is specifically designed to perform fast X-ray timing and probe the status of the matter near black holes and neutron stars. Its pointed instrument is the Large Area Detector (LAD), a 10 m 2 -class instrument operating in the 2-30keV range, which holds the capability to revolutionise studies of variability from X-ray sources on the millisecond time scales. The LAD instrument has now completed the assessment phase but was not down-selected for launch. However, during the assessment, most of the trade-offs have been closed leading to a robust and well documented design that will be re- proposed in future ESA calls. In this talk, we will summarize the characteristics of the LAD design and give an overview of the expectations for the instrument capabilities.
    08/2014;
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    ABSTRACT: Lags measured from correlated X-ray/UV/optical monitoring of AGN allow us to determine whether UV/optical variability is driven by reprocessing of X-rays or X-ray variability is driven by UV/optical seed photon variations. We present the results of the largest study to date of the relationship between the X-ray, UV and optical variability in an AGN with 554 observations, over a 750d period, of the Seyfert 1 galaxy NGC 5548 with Swift. There is a good overall correlation between the X-ray and UV/optical bands, particularly on short timescales (tens of days). These bands lag the X-ray band with lags which are proportional to wavelength to the power 1.23+/-0.31. This power is very close to the power (4/3) expected if short timescale UV/optical variability is driven by reprocessing of X-rays by a surrounding accretion disc. The observed lags, however, are longer than expected from a standard Shakura-Sunyaev accretion disc with X-ray heating, given the currently accepted black hole mass and accretion rate values, but can be explained with a slightly larger mass and accretion rate, and a generally hotter disc. Some long term UV/optical variations are not paralleled exactly in the X-rays, suggesting an additional component to the UV/optical variability arising perhaps from accretion rate perturbations propagating inwards through the disc.
    07/2014;
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    ABSTRACT: Luminous accreting stellar mass and supermassive black holes produce power-law continuum X-ray emission from a compact central corona. Reverberation time lags occur due to light travel time-delays between changes in the direct coronal emission and corresponding variations in its reflection from the accretion flow. Reverberation is detectable using light curves made in different X-ray energy bands, since the direct and reflected components have different spectral shapes. Larger, lower frequency, lags are also seen and are identified with propagation of fluctuations through the accretion flow and associated corona. We review the evidence for X-ray reverberation in active galactic nuclei and black hole X-ray binaries, showing how it can be best measured and how it may be modelled. The timescales and energy-dependence of the high frequency reverberation lags show that much of the signal is originating from very close to the black hole in some objects, within a few gravitational radii of the event horizon. We consider how these signals can be studied in the future to carry out X-ray reverberation mapping of the regions closest to black holes.
    05/2014;
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    ABSTRACT: We use a simple one-dimensional parameterisation of timing properties to show that hard and hard-intermediate state transient black hole X-ray binaries with the same power-spectral shape have systematically harder X-ray power-law emission in higher-inclination systems. We also show that the power-spectral shape and amplitude of the broadband noise (with low-frequency quasi-periodic oscillations, QPOs, removed) is independent of inclination, confirming that it is well-correlated with the intrinsic structure of the emitting regions and that the "type C" QPO, which is inclination-dependent, has a different origin to the noise, probably geometric. Our findings suggest that the power-law emission originates in a corona which is flattened in the plane of the disc, and not in a jet-like structure which would lead to softer spectra at higher inclinations. However, there is tentative evidence that the inclination-dependence of spectral shape breaks down deeper into the hard state, suggesting a change in the coronal geometry.
    05/2014;
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    ABSTRACT: We demonstrate a new method of variability classification using observations of black hole X-ray binaries. Using `power colours' -- ratios of integrated power in different Fourier frequency bands -- we can clearly differentiate different canonical black hole states as the objects evolve during outburst. We analyse (~ 2400) Rossi X-ray Timing Explorer observations of 12 transient low mass black hole X-ray binaries and find that the path taken around the power colour-colour diagram as the sources evolve is highly consistent from object to object. We discuss how the consistency observed in the power colour-colour diagram between different objects allows for easy state classification based on only a few observations, and show how the power-spectral shapes can be simply classified using a single parameter, the power-spectral `hue'. To illustrate the benefits of our simple model-independent approach, we show that the persistent high mass X-ray binary Cyg X-1 shows very similar power-spectral evolution to the transient black hole sources, with the main difference being caused by a combination of a lack of quasi-periodic oscillations and an excess of low-frequency power-law noise in the Cyg X-1 power spectra during the transitional state. We also compare the transient objects to the neutron star atoll source Aquila X-1, demonstrating that it traces a different path in the power colour-colour plot. Thus, power-colours could be an effective method to classify newly discovered X-ray binaries.
    05/2014;
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    ABSTRACT: We demonstrate a new method of variability classification using observations of black hole X-ray binaries. Using `power colours' -- ratios of integrated power in different Fourier frequency bands -- we can clearly differentiate different canonical black hole states as the objects evolve during outburst. We analyse (~ 2400) Rossi X-ray Timing Explorer observations of 12 transient low mass black hole X-ray binaries and find that the path taken around the power colour-colour diagram as the sources evolve is highly consistent from object to object. We discuss how the consistency observed in the power colour-colour diagram between different objects allows for easy state classification based on only a few observations, and show how the power-spectral shapes can be simply classified using a single parameter, the power-spectral `hue'. To illustrate the benefits of our simple model-independent approach, we show that the persistent high mass X-ray binary Cyg X-1 shows very similar power-spectral evolution to the transient black hole sources, with the main difference being caused by a combination of a lack of quasi-periodic oscillations and an excess of low-frequency power-law noise in the Cyg X-1 power spectra during the transitional state. We also compare the transient objects to the neutron star atoll source Aquila X-1, demonstrating that it traces a different path in the power colour-colour plot. Thus, power-colours could be an effective method to classify newly discovered X-ray binaries.
    04/2014;
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    ABSTRACT: We present the use of continuous-time autoregressive moving average (CARMA) models as a method for estimating the variability features of a light curve, and in particular its power spectral density (PSD). CARMA models fully account for irregular sampling and measurement errors, making them valuable for quantifying variability, forecasting and interpolating light curves, and for variability-based classification. We show that the PSD of a CARMA model can be expressed as a sum of Lorentzian functions, which makes them extremely flexible and able to model a broad range of PSDs. We present the likelihood function for light curves sampled from CARMA processes, placing them on a statistically rigorous foundation, and we present a Bayesian method to infer the probability distribution of the PSD given the measured lightcurve. Because calculation of the likelihood function scales linearly with the number of data points, CARMA modeling scales to current and future massive time-domain data sets. We conclude by applying our CARMA modeling approach to light curves for an X-ray binary, two AGN, a long-period variable star, and an RR-Lyrae star, in order to illustrate their use, applicability, and interpretation.
    The Astrophysical Journal 02/2014; 788(1). · 6.73 Impact Factor
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    ABSTRACT: We present the most extensive analysis of Fourier-based X-ray timing properties of the black hole binary Cygnus X-1 to date, based on 12 years of bi-weekly monitoring with RXTE from 1999 to 2011. Our aim is a comprehensive study of timing behavior across all spectral states, including the elusive transitions and extreme hard and soft states. We discuss the dependence of the timing properties on spectral shape and photon energy, and study correlations between Fourier-frequency dependent coherence and time lags with features in the power spectra. Our main results are: (a) The fractional rms in the 0.125-256 Hz range in different spectral states shows complex behavior that depends on the energy range considered. It reaches its maximum not in the hard state, but in the soft state in the Comptonized tail above 10 keV. (b) The shape of power spectra in hard and intermediate states and the normalization in the soft state are strongly energy dependent in the 2.1-15 keV range. This emphasizes the need for an energy-dependent treatment of power spectra and a careful consideration of energy- and mass-scaling when comparing the variability of different source types, e.g., black hole binaries and AGN. PSDs during extremely hard and extremely soft states can be easily confused for energies above ~5 keV in the 0.125-256 Hz range. (c) The coherence between energy bands drops during transitions from the intermediate into the soft state but recovers in the soft state. (d) The time lag spectra in soft and intermediate states show distinct features at frequencies related to the frequencies of the main variability components seen in the power spectra and show the same shift to higher frequencies as the source softens. [...abridged] In particular, we discuss how the timing properties of Cyg X-1 can be used to assess the evolution of variability with spectral shape in other black hole binaries. [...abridged]
    02/2014;
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    ABSTRACT: LOFT, the Large Observatory For X-ray Timing, is one of five ESA M3 candidate missions. It will address the Cosmic Vision theme: "Matter under Extreme Conditions". By coupling for the first time a huge collecting area for the detection of X-ray photons with CCD-quality spectral resolution (15 times bigger in area than any previously flown X-ray instrument and >100 times bigger for spectroscopy than any similar-resolution instrument), the instruments on-board LOFT have been designed to (i) determine the properties of ultradense matter by reconstructing its Equation of State through neutron star mass and radius measurements of unprecedented accuracy; (ii) measure General Relativity effects in the strong field regime in the stationary spacetimes of neutron stars and black holes of all masses down to a few gravitational radii. Besides the above two themes, LOFT's observations will be devoted to "observatory science", providing new insights in a number of research fields in high energy astrophysics (e.g. Gamma-ray Bursts). The assessment study phase of LOFT, which ended in September 2013, demonstrated that the mission is low risk and the required Technology Readiness Level can be easily reached in time for a launch by the end of 2022.
    12/2013;
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    ABSTRACT: High-frequency iron K reverberation lags, where the red wing of the line responds before the line centroid, are a robust signature of relativistic reflection off the inner accretion disc. In this letter, we report the discovery of the Fe K lag in PG 1244+026 from ~120 ks of data (1 orbit of the XMM-Newton telescope). The amplitude of the lag with respect to the continuum is 1000 s at a frequency of ~1e-4 Hz. We also find a possible frequency-dependence of the line: as we probe higher frequencies (i.e. shorter timescales from a smaller emitting region) the Fe K lag peaks at the red wing of the line, while at lower frequencies (from a larger emitting region) we see the dominant reflection lag from the rest frame line centroid. The mean energy spectrum shows a strong soft excess, though interestingly, there is no indication of a soft lag. Given that this source has radio emission and it has little reported correlated variability between the soft excess and the hard band, we explore one possible explanation in which the soft excess in this source is dominated by the steep power-law like emission from a jet, and that a corona (or base of the jet) irradiates the inner accretion disc, creating the blurred reflection features evident in the spectrum and the lag. General Relativistic ray-tracing models fit the Fe K lag well, with the best-fit giving a compact X-ray source at a height of 5 gravitational radii and a black hole mass of 1.3e7 Msun.
    Monthly Notices of the Royal Astronomical Society 11/2013; · 5.52 Impact Factor
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    ABSTRACT: The recent detection of X-ray reverberation lags, especially in the Fe K$\alpha$ line region, around Active Galactic Nuclei (AGN) has opened up the possibility of studying the time-resolved response (reflection) of hard X-rays from the accretion disk around supermassive black holes. Here, we use general relativistic transfer functions for reflection of X-rays from a point source located at some height above the black hole to study the time lags expected as a function of frequency and energy in the Fe K$\alpha$ line region. We explore the models and the dependence of the lags on key parameters such as the height of the X-ray source, accretion disk inclination, black hole spin and black hole mass. We then compare these models with the observed frequency and energy dependence of the Fe K$\alpha$ line lag in NGC 4151. Assuming the optical reverberation mapping mass of $4.6\times10^7~M_\odot$ we get a best fit to the lag profile across the Fe K$\alpha$ line in the frequency range $(1-2)\times10^{-5}$ Hz for an X-ray source located at a height $h = 7^{+2.9}_{-2.6}~R_G$ with a maximally spinning black hole and an inclination $i < 30^\circ$.
    11/2013; 438(4).
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    ABSTRACT: We investigate the complex behavior of energy- and frequency-dependent time/phase lags in the plateau state and the radio-quiet hard state of GRS 1915+105. In our timing analysis, we find that when the source is faint in the radio, QPOs are observed above 2 Hz and typically exhibit soft lags (soft photons lag hard photons), whereas QPOs in the radio-bright plateau state are found below 2.2 Hz and consistently show hard lags. The phase lag at the QPO frequency is strongly anti-correlated with the QPO frequency, changing sign at 2.2 Hz. However, the phase lag at the frequency of the first harmonic is positive and nearly independent of frequency at at ~0.172 rad, regardless of the radio emission. The lag-energy dependence at the first harmonic is also independent of radio flux. However, the lags at the QPO frequency are negative at all energies during the radio-quiet state, but lags at the QPO frequency during the plateau state are positive at all energies and show a 'reflection-type' evolution of the lag-energy spectra with respect to the radio-quiet state. The lag-energy dependence is roughly logarithmic, but there is some evidence for a break around 4-6 keV. Finally, the Fourier frequency-dependent phase lag spectra are fairly flat during the plateau state, but increase from negative to positive during the radio-quiet state. We discuss the implications of our results in the light of some generic models.
    The Astrophysical Journal 10/2013; 778(2). · 6.73 Impact Factor
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    ABSTRACT: We present an X-ray spectral and temporal analysis of the variable active galaxy NGC 6814, observed with Suzaku during November 2011. Remarkably, the X-ray spectrum shows no evidence for the soft excess commonly observed amongst other active galaxies, despite its relatively low level of obscuration, and is dominated across the whole Suzaku bandpass by the intrinsic powerlaw-like continuum. Despite this, we clearly detect the presence of a low frequency hard lag of ~1600s between the 0.5-2.0 and 2.0-5.0 keV energy bands at greater than 6-sigma significance, similar to those reported in the literature for a variety of other AGN. At these energies, any additional emission from e.g. a very weak, undetected soft excess, or from distant reflection must contribute less than 3% of the observed countrates (at 90% confidence). Given the lack of any significant continuum emission component other than the powerlaw, we can rule out models that invoke distant reprocessing for the observed lag behavior, which must instead be associated with this continuum emission. These results are fully consistent with a propagating fluctuation origin for the low frequency hard lags, and with the interpretation of the high frequency soft lags - a common feature seen in the highest quality AGN data with strong soft excesses - as reverberation from the inner accretion disk.
    The Astrophysical Journal 10/2013; 777(2). · 6.73 Impact Factor
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    W. N. Alston, S. Vaughan, P. Uttley
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    ABSTRACT: We present an analysis of the X-ray time lags for the highly variable Seyfert 1 galaxy NGC 4051, based on a series of XMM-Newton observations taken in 2009. We investigate the Fourier frequency dependent time lags in the light curves between the 0.3--1.0 keV and 2.0--5.0 keV energy bands as a function of source flux, including simultaneous modelling of the resulting lag-frequency spectra. We find the shape of the lag-frequency spectra to vary significantly and systematically with source flux. We model the lag-frequency spectra using simple transfer functions, and find that two time lag components are required, one in each energy band. The simplest acceptable fits have only the relative contribution of the lagged component in the hard band varying with flux level, which can be associated with changes in the energy spectrum. We discuss the interpretation of these results in terms of the currently popular models for X-ray time lags.
    Monthly Notices of the Royal Astronomical Society 07/2013; 435(2). · 5.52 Impact Factor
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    ABSTRACT: We use archival XMM-Newton observations of Ark 564 and Mrk 335 to calculate the frequency dependent time-lags for these two well-studied sources. We discover high-frequency Fe K lags in both sources, indicating that the red wing of the line precedes the rest frame energy by roughly 100 s and 150 s for Ark 564 and Mrk 335, respectively. Including these two new sources, Fe K reverberation lags have been observed in seven Seyfert galaxies. We examine the low-frequency lag-energy spectrum, which is smooth, and shows no feature of reverberation, as would be expected if the low-frequency lags were produced by distant reflection off circumnuclear material. The clear differences in the low and high frequency lag-energy spectra indicate that the lags are produced by two distinct physical processes. Finally, we find that the amplitude of the Fe K lag scales with black hole mass for these seven sources, consistent with a relativistic reflection model where the lag is the light travel delay associated with reflection of continuum photons off the inner disc.
    Monthly Notices of the Royal Astronomical Society 06/2013; · 5.52 Impact Factor
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    ABSTRACT: In order to discern the physical nature of many gamma-ray sources in the sky, we must look not only in spectral and spatial dimensions, but also understand their temporal variability. However, timing analysis of sources with a highly transient nature, such as magnetar bursts, is difficult: standard Fourier techniques developed for long-term variability generally observed, for example, from AGN often do not apply. Here, we present newly developed timing methods applicable to transient events of all kinds, and show their successful application to magnetar bursts observed with Fermi/GBM. Magnetars are a prime subject for timing studies, thanks to the detection of quasi-periodicities in magnetar Giant Flares and their potential to help shed light on the structure of neutron stars. Using state-of-the art statistical techniques, we search for quasi-periodicities (QPOs) in a sample of bursts from Soft Gamma Repeater SGR J0501+4516 observed with Fermi/GBM and provide upper limits for potential QPO detections. Additionally, for the first time, we characterise the broadband variability behaviour of magnetar bursts and highlight how this new information could provide us with another way to probe these mysterious objects.
    03/2013;
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    ABSTRACT: We report the first study of Fourier-frequency-dependent coherence and phase/time lags at optical wavelengths of cataclysmic variables (MV Lyr and LU Cam) displaying typical flickering variability in white light. Observations were performed on the William Herschel Telescope using ULTRACAM. Lightcurves for both systems have been obtained with the SDSS filters $u'$, $g'$ and $r'$ simultaneously with cadences between $\approx0.5-2$ seconds, and allow us to probe temporal frequencies between ~10^{-3} Hz and ~1 Hz. We find high levels of coherence between the u', g' and r' lightcurves up to at least ~10^{-2} Hz. Furthermore we detect red/negative lags where the redder bands lag the bluer ones at the lowest observed frequencies. For MV Lyr time lags up to ~3 seconds are observed, whilst LU Cam displays larger time lags of ~10 seconds. Mechanisms which seek to explain red/negative lags observed in X-ray binaries and Active Galactic Nuclei involve reflection of photons generated close to the compact object onto the surface layers of the accretion disk, where the lag delay is simply the light-travel time from the emitting source to the reflecting accretion disk area. Although this could be a viable explanation for the lags observed in MV Lyr, the lags observed in LU Cam are too large to be explained by reflection from the disk and/or the donor star. We suggest reprocessing on the thermal timescale of boundary layer photons onto the accretion disk as a possible mechanism to explain the lags observed in accreting white dwarfs, or reverse (inside-out) shocks within the disk travelling through cooler disk regions as they move outwards.
    Monthly Notices of the Royal Astronomical Society 02/2013; 431(3). · 5.52 Impact Factor
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    ABSTRACT: IRAS 13224-3809 was observed in 2011 for 500 ks with the XMM-Newton observatory. We detect highly significant X-ray lags between soft (0.3 - 1 keV) and hard (1.2 - 5 keV) energies. The hard band lags the soft at low frequencies (i.e. hard lag), while the opposite (i.e. soft lag) is observed at high frequencies. In this paper, we study the lag during flaring and quiescent periods. We find that the frequency and absolute amplitude of the soft lag is different during high-flux and low-flux periods. During the low flux intervals, the soft lag is detected at higher frequencies and with smaller amplitude. Assuming that the soft lag is associated with the light travel time between primary and reprocessed emission, this behaviour suggests that the X-ray source is more compact during low-flux intervals, and irradiates smaller radii of the accretion disc (likely because of light bending effects). We continue with an investigation of the lag dependence on energy, and find that isolating the low-flux periods reveals a strong lag signature at the Fe K line energy, similar to results found using 1.3 Ms of data on another well known Narrow-Line Seyfert I galaxy, 1H0707-495.
    Monthly Notices of the Royal Astronomical Society 01/2013; 430(2). · 5.52 Impact Factor
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    ABSTRACT: The discovery of quasi-periodic oscillations (QPOs) in magnetar giant flares has opened up prospects for neutron star asteroseismology. However, with only three giant flares ever recorded, and only two with data of sufficient quality to search for QPOs, such analysis is seriously data limited. We set out a procedure for doing QPO searches in the far more numerous, short, less energetic magnetar bursts. The short, transient nature of these bursts requires the implementation of sophisticated statistical techniques to make reliable inferences. Using Bayesian statistics, we model the periodogram as a combination of red noise at low frequencies and white noise at high frequencies, which we show is a conservative approach to the problem. We use empirical models to make inferences about the potential signature of periodic and quasi-periodic oscillations at these frequencies. We compare our method with previously used techniques and find that although it is on the whole more conservative, it is also more reliable in ruling out false positives. We illustrate our Bayesian method by applying it to a sample of 27 bursts from the magnetar SGR J0501+4516 observed by the Fermi Gamma-ray Burst Monitor, and we find no evidence for the presence of QPOs in any of the bursts in the unbinned spectra, but do find a candidate detection in the binned spectra of one burst. However, whether this signal is due to a genuine quasi-periodic process, or can be attributed to unmodeled effects in the noise is at this point a matter of interpretation.
    The Astrophysical Journal 12/2012; 768(1). · 6.73 Impact Factor

Publication Stats

2k Citations
574.65 Total Impact Points

Institutions

  • 2006–2014
    • University of Amsterdam
      • Astronomical Institute Anton Pannekoek
      Amsterdamo, North Holland, Netherlands
  • 1998–2012
    • University of Southampton
      • • Department of Physics and Astronomy
      • • Faculty of Physical and Applied Sciences
      Southampton, England, United Kingdom
  • 2009
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, ENG, United Kingdom
  • 2005
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, MD, United States
    • University of Leicester
      • Department of Physics and Astronomy
      Leiscester, England, United Kingdom
  • 2003
    • University of New Hampshire
      Durham, New Hampshire, United States