S. B. Kraemer

The Catholic University of America, Washington, Washington, D.C., United States

Are you S. B. Kraemer?

Claim your profile

Publications (234)605.87 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Heavily saturated UV absorption lines, such as those detected in BAL QSOs and a few Seyfert 1s, are typically quite broad. This can be due to superposition of many individual components, i.e., velocity gradients, or the domination of the profiles by the damped wings, i.e., a "walking wall" effect. Nevertheless, even the weakest UV absorption lines detected in Seyfert 1s have widths that are significantly greater than thermal. Interestingly, the smoothness of broad emission line profiles and under-predictions of the strengths of BLR lines and coronal NLR lines by photo-ionization models suggest the importance of micro-turbulence and associated dissipative heating within the emission-line gas. Hence, it is also possible that turbulence is important for the absorbers. We will discuss the evidence for turbulence and its possible role in the structure of gas in the proximity of an AGN.
    No preview · Article · Aug 2012
  • [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the impact of feedback from outflowing UV and X-ray absorbers in nearby (z < 0.04) AGN. From studies of the kinematics, physical conditions, and variability of the absorbers in the literature, we calculate the possible ranges in total mass outflow rate (Ṁout) and kinetic luminosity (LK) for each AGN, summed over all of the absorbers. These calculations make use of values (or limits) for the radial locations of the absorbers determined from variability, excited-state absorption, or other considerations. From a sample of 10 Seyfert 1 galaxies with detailed photoionization models for their absorbers, we find that 7 have sufficient constraints on the absorber locations to determine Ṁout and LK. The 6 Seyfert 1s with moderate bolometric luminosities (Lbol = 1043 - 1045 ergs s-1) all have mass outflow rates that are 10 - 1000 times the mass accretion rates needed to generate their observed luminosities, indicating that most of the mass outflow originates from outside the inner accretion disk. Three of these (NGC 4051, NGC 3516, and NGC 3783) have LK in the range 0.5 - 5% Lbol, which is the range typically required by feedback models for efficient self-regulation of black-hole and galactic bulge growth. The other three (NGC 5548, NGC 4151, and NGC 7469) have LK > 0.1%Lbol, although these values may increase if radial locations can be determined for more of the absorbers. We conclude that the outflowing UV and X-ray absorbers in moderate-luminosity AGN have the potential to deliver significant feedback to their environments.
    No preview · Article · Aug 2012
  • Source
    D. M. Crenshaw · S. B. Kraemer
    [Show abstract] [Hide abstract]
    ABSTRACT: We present an investigation into the impact of feedback from outflowing UV and X-ray absorbers in nearby (z < 0.04) active galactic nuclei (AGNs). From studies of the kinematics, physical conditions, and variability of the absorbers in the literature, we calculate the possible ranges in the total mass outflow rate () and kinetic luminosity (L KE) for each AGN, summed over all of its absorbers. These calculations make use of values (or limits) for the radial locations of the absorbers determined from variability, excited-state absorption, and other considerations. From a sample of 10 Seyfert 1 galaxies with detailed photoionization models for their absorbers, we find that 7 have sufficient constraints on the absorber locations to determine and L KE. For the low-luminosity AGN NGC 4395, these values are low, although we do not have sufficient constraints on the X-ray absorbers to make definitive conclusions. At least five of the six Seyfert 1s with moderate bolometric luminosities (L bol = 1043 – 1045 erg s–1) have mass outflow rates that are 10-1000 times the mass accretion rates needed to generate their observed luminosities, indicating that most of the mass outflow originates from outside the inner accretion disk. Three of these (NGC 4051, NGC 3516, and NGC 3783) have L KE in the range 0.5%-5% L bol, which is the range typically required by feedback models for efficient self-regulation of black hole and galactic bulge growth. At least two of the other three (NGC 5548, NGC 4151, and NGC 7469) have L KE 0.1%L bol, although these values may increase if radial locations can be determined for more of the absorbers. We conclude that the outflowing UV and X-ray absorbers in moderate-luminosity AGNs have the potential to deliver significant feedback to their environments.
    Preview · Article · Jun 2012 · The Astrophysical Journal
  • Source
    D. M. Crenshaw · S. B. Kraemer
    [Show abstract] [Hide abstract]
    ABSTRACT: We present an investigation into the impact of feedback from outflowing UV and X-ray absorbers in nearby z < 0.04 AGN. From studies of the kinematics, physical conditions, and variability of the absorbers in the literature, we calculate the possible ranges in total mass outflow rate and kinetic luminosity for each AGN, summed over all of its absorbers. These calculations make use of values (or limits) for the radial locations of the absorbers determined from variability, excited-state absorption, and other considerations. From a sample of 10 Seyfert 1 galaxies with detailed photoionization models for their absorbers, we find that 7 have sufficient constraints on the absorber locations to determine feedback parameters. For the low-luminosity AGN NGC 4395, these values are low, although we do not have sufficient constraints on the X-ray absorbers to make definitive conclusions. At least 5 of the 6 Seyfert 1s with moderate bolometric luminosities have mass outflow rates that are 10 - 1000 times the mass accretion rates needed to generate their observed luminosities, indicating that most of the mass outflow originates from outside the inner accretion disk. Three of these (NGC 4051, NGC 3516, and NGC 3783) have kinetic luminosities in the range 0.5 to 5% bolometric, which is the range typically required by feedback models for efficient self-regulation of black-hole and galactic bulge growth. At least 2 of the other 3 (NGC 5548, NGC 4151, and NGC 7469) have kinetic luminosities > 0.1% bolometric, although these values may increase if radial locations can be determined for more of the absorbers. We conclude that the outflowing UV and X-ray absorbers in moderate-luminosity AGN have the potential to deliver significant feedback to their environments.
    Preview · Article · Apr 2012
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present new Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (COS) observations of the narrow-line Seyfert 1 galaxy NGC 4051. These data were obtained as part of a coordinated observing program including X-ray observations with the Chandra/High Energy Transmission Grating (HETG) spectrometer and Suzaku. We detected nine kinematic components of UV absorption, which were previously identified using the HST/Space Telescope Imaging Spectrograph (STIS). None of the absorption components showed evidence for changes in column density or profile within the ~10 yr between the STIS and COS observations, which we interpret as evidence of (1) saturation, for the stronger components, or (2) very low densities, i.e., n H < 1 cm–3, for the weaker components. After applying a +200 km s–1 offset to the HETG spectrum, we found that the radial velocities of the UV absorbers lay within the O VII profile. Based on photoionization models, we suggest that, while UV components 2, 5, and 7 produce significant O VII absorption, the bulk of the X-ray absorption detected in the HETG analysis occurs in more highly ionized gas. Moreover, the mass-loss rate is dominated by high-ionization gas which lacks a significant UV footprint.
    No preview · Article · Apr 2012 · The Astrophysical Journal
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a study of the structure and kinematics of outflowing winds in nearby active galactic nuclei (AGN). Variability and/or metastable absorption in the nuclear outflows allows us to determine their distances. Determination of the AGN inclinations based on narrow-line region (NLR) kinematics provides our viewing angle with respect to the obscuring torus. Photoionization models of the measured ionic column densities provide the physical conditions (ionization parameters, column densities) in the gas. Combining these parameters with the global covering factor and outflow velocities, we are able to determine the mass outflow rates and kinetic luminosities of the AGN winds. We find that the outflow rates are typically much larger than the accretion rates needed to sustain the AGN luminosities, indicating that most of the incoming fuel get blown out before reaching the inner accretion disk. The kinetic luminosities are often 0.5 to 5% of the bolometric luminosities, suggesting that AGN feedback is important in even moderate luminosity AGN.
    No preview · Article · Jan 2012
  • Source
    M. Meléndez · S. B. Kraemer · K. A. Weaver · R. F. Mushotzky
    [Show abstract] [Hide abstract]
    ABSTRACT: The shape of the spectral energy distribution (SED) of active galaxies in the extreme-ultraviolet (EUV)-soft X-ray band (13.6?eV-1?keV) is uncertain because obscuration by dust and gas can hamper our view of the continuum. To investigate the shape of the SED in this energy band, we have generated a set of photoionization models which reproduce the small dispersion found in correlations between high-ionization mid-infrared emission lines in a sample of hard X-ray-selected active galactic nuclei (AGNs). Our calculations show that a broken power-law continuum model is sufficient to reproduce the [Ne V]14.32 ?m/[Ne III], [Ne V]24.32 ?m/[O IV]25.89 ?m, and [O IV]25.89 ?m/[Ne III] ratios and does not require the addition of a "big bump" EUV model component. We constrain the EUV-soft X-ray slope, ?i , to be between 1.5 and 2.0 and derive a best fit of ?i ~ 1.9 for Seyfert 1 galaxies, consistent with previous studies of intermediate-redshift quasars. If we assume a blue bump model, most sources in our sample have derived temperatures between T BB = 105.18 K and 105.7 K, suggesting that the peak of this component spans a large range of energies extending from ~600 ? to 1900 ?. In this case, the best-fitting peak energy that matches the mid-infrared line ratios of Seyfert 1 galaxies occurs between ~700 and 1000 ?. Despite the fact that our results do not rule out the presence of an EUV bump, we conclude that our power-law model produces enough photons with energies >4?Ry to generate the observed amount of mid-infrared emission in our sample of Burst Alert Telescope AGNs.
    Full-text · Article · Jun 2011 · The Astrophysical Journal
  • Source
    T. J. Turner · L Miller · S. B. Kraemer · J. N. Reeves
    [Show abstract] [Hide abstract]
    ABSTRACT: We consider new Suzaku data for NGC 3516 taken during 2009, along with other recent X-ray observations of the source. The cumulative characteristics of NGC 3516 cannot be explained without invoking changes in the line-of-sight absorption. Contrary to many other well-studied Seyfert galaxies, NGC 3516 does not show a positive lag of hard X-ray photons relative to soft photons over the timescales sampled. In the context of reverberation models for the X-ray lags, the lack of such a signal in NGC 3516 is consistent with flux variations being dominated by absorption changes. The lack of any reverberation signal in such a highly variable source disfavors intrinsic continuum variability in this case. Instead, the colorless flux variations observed at high flux states for NGC 3516 are suggested to be a consequence of Compton-thick clumps of gas crossing the line-of-sight.
    Full-text · Article · Mar 2011 · The Astrophysical Journal
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the analysis of a new broad-band X-ray spectrum (0.6–180.0 keV) of the radio-quiet quasar MR 2251−178 which uses both Suzaku and Swift/Burst Alert Telescope data. In accordance with previous observations, we find that the general continuum can be well described by a power law with Γ= 1.6 and an apparent soft excess below 1 keV. Warm absorption is clearly present, and absorption lines due to the Fe unresolved transition array, Fe L (Fe xxiii–xxiv), S xv and S xvi are detected below 3 keV. At higher energies, Fe K absorption from Fe xxv–xxvi is detected and a relatively weak (EW = 25+12−8 eV) narrow Fe Kα emission line is observed (E= 6.44 ± 0.04 keV) which is well modelled by the presence of a mildly ionized (ξ≲ 30) reflection component with a low reflection fraction (R < 0.2). At least five ionized absorption components with 1020≲NH≲ 1023 cm−2 and 0 ≲ log ξ/erg cm s−1≲ 4 are required to achieve an adequate spectral fit. Alternatively, we show that the continuum can also be fit if a Γ∼ 2.0 power law is absorbed by a column of NH∼ 1023 cm−2 which covers ∼30 per cent of the source flux. Independent of which continuum model is adopted, the Fe L and Fe xxv Heα lines are well fit by a single absorber outflowing with vout∼ 0.14c. Such an outflow/disc-wind is likely to be substantially clumped (b∼ 10−3) in order to not vastly exceed the likely accretion rate of the source.
    Full-text · Article · Mar 2011 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the results of a deep 300 ks Chandra High Energy Transmission Grating (HETG) observation of the highly variable narrow-line Seyfert Type 1 galaxy NGC 4051. The HETG spectrum reveals 28 significant soft X-ray ionized lines in either emission or absorption; primarily originating from H-like and He-like K-shell transitions of O, Ne, Mg and Si (including higher order lines and strong forbidden emission lines from O vii and Ne ix) plus high-ionization L-shell transitions from Fe xvii to Fe xxii and lower ionization inner-shell lines (e.g. O vi). Modelling the data with xstar requires four distinct ionization zones for the gas, all outflowing with velocities <1000 km s−1. A selection of the strongest emission/absorption lines appear to be resolved with full width at half-maximum (FWHM) of ∼600 km s−1. We also present the results from a quasi-simultaneous 350 ks Suzaku observation of NGC 4051 where the X-ray Imaging Spectrometer (XIS) spectrum reveals strong evidence for blueshifted absorption lines at ∼6.8 and ∼7.1 keV, consistent with previous findings. Modelling with xstar suggests that this is the signature of a highly ionized, high-velocity outflow (log ξ= 4.1+0.2−0.1; vout∼−0.02c) which potentially may have a significant effect on the host galaxy environment via feedback. Finally, we also simultaneously model the broad-band 2008 XIS+HXD (Hard X-ray Detector) Suzaku data with archival Suzaku data from 2005 when the source was observed to have entered an extended period of low flux in an attempt to analyse the cause of the long-term spectral variability. We find that we can account for this by allowing for large variations in the normalization of the intrinsic power-law component which may be interpreted as being due to significant changes in the covering fraction of a Compton-thick partial-coverer obscuring the central continuum emission.
    Full-text · Article · Feb 2011 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a study of reddening and absorption toward the narrow line regions (NLRs) in active galactic nuclei (AGNs) selected from the Revised Shapley-Ames, 12 μm, and Swift/Burst Alert Telescope samples. For the sources in host galaxies with inclinations of b/a > 0.5, we find that the mean ratio of [O III] λ5007, from ground-based observations, and [O IV] 28.59 μm, from Spitzer/Infrared Spectrograph observations, is a factor of two lower in Seyfert 2s than Seyfert 1s. The combination of low [O III]/[O IV] and [O III] λ4363/λ5007 ratios in Seyfert 2s suggests more extinction of emission from the NLR than in Seyfert 1s. Similar column densities of dusty gas, N H~ several × 1021 cm–2, can account for the suppression of both [O III] λ5007 and [O III] λ4363, as compared to those observed in Seyfert 1s. Also, we find that the X-ray line O VII λ22.1 Å is weaker in Seyfert 2s, consistent with absorption by the same gas that reddens the optical emission. Using a Hubble Space Telescope/Space Telescope Imaging Spectrograph slitless spectrum of the Seyfert 1 galaxy NGC 4151, we estimate that only ~30% of the [O III] λ5007 comes from within 30 pc of the central source, which is insufficient to account for the low [O III]/[O IV] ratios in Seyfert 2s. If Seyfert 2 galaxies have similar intrinsic [O III] spatial profiles, the external dusty gas must extend further out along the NLR, perhaps in the form of nuclear dust spirals that have been associated with fueling flows toward the AGN.
    Preview · Article · Feb 2011 · The Astrophysical Journal
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Previous ground-based observations of the Seyfert 2 galaxy Mrk 78 revealed a double set of emission lines, similar to those seen in several active galactic nuclei (AGNs) from recent surveys. Are the double lines due to two AGNs with different radial velocities in the same galaxy, or are they due to mass outflows from a single AGN? We present a study of the outflowing ionized gas in the resolved narrow-line region (NLR) of Mrk 78 using observations from the Space Telescope Imaging Spectrograph (STIS) and Faint Object Camera aboard the Hubble Space Telescope as part of an ongoing project to determine the kinematics and geometries of AGN outflows. From the spectroscopic information, we determined the fundamental geometry of the outflow via our kinematics modeling program by recreating radial velocities to fit those seen in four different STIS slit positions. We determined that the double emission lines seen in ground-based spectra are due to an asymmetric distribution of outflowing gas in the NLR. By successfully fitting a model for a single AGN to Mrk 78, we show that it is possible to explain double emission lines with radial velocity offsets seen in AGN similar to Mrk 78 without requiring dual supermassive black holes.
    Preview · Article · Jan 2011 · The Astrophysical Journal
  • Source
    T. Fischer · M. D. Crenshaw · S. B. Kraemer · H. R. Schmitt

    Preview · Article · Jan 2011
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a study of the outflowing ionized gas in the resolved narrow-line region (NLR) of Mrk 78 using observations from Space Telescope Imaging Spectrograph (STIS) and Faint Object Camera (FOC) aboard the Hubble Space Telescope (HST). We determine that the double set of emission lines seen in ground based observations are due to an asymmetric distribution of outflowing gas in the NLR. In successfully fitting a single AGN model to Mrk 78, we show that it is possible to explain double emission lines seen in radial velocity offsets of AGN similar to Mrk 78 without the requirement of dual supermassive black holes.
    No preview · Article · Jan 2011
  • [Show abstract] [Hide abstract]
    ABSTRACT: Mass outflows of ionized gas, detected through Hubble Space Telescope observations of blueshifted UV absorption lines and spatially-resolved emission lines, are common in nearby AGN. We review the constraints that these observations place on the structure of AGN winds, and provide estimates of the mass outflow rates and kinematic luminosities needed for AGN feedback models.
    No preview · Article · Jan 2011
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a study of reddening and absorption towards the Narrow Line Regions (NLR) in active galactic nuclei (AGN) selected from the Revised Shapley-Ames, 12mu, and Swift/Burst Alert Telescope samples. For the sources in host galaxies with inclinations of b/a > 0.5, we find that mean ratio of [O III] 5007A, from ground-based observations, and [O IV] 28.59mu, from Spitzer/Infrared Spectrograph observations, is a factor of 2 lower in Seyfert 2s than Seyfert 1s. The combination of low [O III]/[O IV] and [O III] 4363/5007 ratios in Seyfert 2s suggests more extinction of emission from the NLR than in Seyfert 1s. Similar column densities of dusty gas, NH ~ several X 10^21 cm^-2, can account for the suppression of both [O III] 5007A and [O III] 4363A, as compared to those observed in Seyfert 1s. Also, we find that the X-ray line OVII 22.1A is weaker in Seyfert 2s, consistent with absorption by the same gas that reddens the optical emission. Using a Hubble Space Telescope/Space Telescope Imaging Spectrograph slitless spectrum of the Seyfert 1 galaxy NGC 4151, we estimate that only ~ 30% of the [O III] 5007A comes from within 30 pc of the central source, which is insufficient to account for the low [O III]/[OIV] ratios in Seyfert 2s. If Seyfert 2 galaxies have similar intrinsic [OIII] spatial profiles, the external dusty gas must extend further out along the NLR, perhaps in the form of nuclear dust spirals that have been associated with fueling flows towards the AGN. Comment: 33 pages, 7 figures. accepted for publication in the Astrophysical Journal
    Preview · Article · Nov 2010
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Previous ground based observations of the Seyfert 2 galaxy Mrk 78 revealed a double set of emission lines, similar to those seen in several AGN from recent surveys. Are the double lines due to two AGN with different radial velocities in the same galaxy, or are they due to mass outflows from a single AGN?We present a study of the outflowing ionized gas in the resolved narrow-line region (NLR) of Mrk 78 using observations from Space Telescope Imaging Spectrograph (STIS) and Faint Object Camera (FOC) aboard the Hubble Space Telescope(HST) as part of an ongoing project to determine the kinematics and geometries of active galactic nuclei (AGN) outflows. From the spectroscopic information, we deter- mined the fundamental geometry of the outflow via our kinematics modeling program by recreating radial velocities to fit those seen in four different STIS slit positions. We determined that the double emission lines seen in ground-based spectra are due to an asymmetric distribution of outflowing gas in the NLR. By successfully fitting a model for a single AGN to Mrk 78, we show that it is possible to explain double emission lines with radial velocity offsets seen in AGN similar to Mrk 78 without requiring dual supermassive black holes. Comment: 22 pages, 7 figures (2 color), accepted for publication in The Astrophysical Journal
    Preview · Article · Nov 2010
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We focus on determining the underlying physical cause of a Seyfert galaxy's appearance as type a 1.8 or 1.9. Are these "intermediate" Seyfert types typical Seyfert 1 nuclei with reddened broad-line regions? Or are they objects with intrinsically weak continua and broad emission lines? We compare measurements of the optical reddening of the narrow and broad-line regions with each other and with the X-ray column derived from XMM-Newton 0.5-10 keV spectra to determine the presence and location of dust in the line of sight. We also searched the literature to see if the objects showed evidence for broad-line variability, and determined if the changes were consistent with a change in reddening or a change in the intrinsic ionizing continuum flux. We find that 10 of 19 objects previously classified as Seyfert 1.8/1.9s received this designation due to their low continuum flux. In four objects the classification was due to BLR reddening, either by the torus or dust structures in the vicinity of the NLR; in the remaining five objects there is not sufficient evidence to favor one scenario over the other. These findings imply that, in general, samples of 1.8/1.9s are not suitable for use in studies of the gas and dust in the central torus. Comment: 85 pages, accepted by ApJ
    Full-text · Article · Oct 2010 · The Astrophysical Journal
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a study of the outflowing ionized gas in the resolved narrow-line region (NLR) of the Seyfert 2 galaxy Mrk 573, and its interaction with an in- ner dust/gas disk, based on Hubble Space Telescope (HST) WFPC2 and STIS observations. From the spectroscopic and imaging information, we determined the fundamental geometry of the outflow and inner disk, via two modeling pro- grams used to recreate the morphology of these regions imaged with HST. We also determined that the bicone of ionizing radiation from the Active Galactic Nucleus (AGN) intersects with the inner disk, illuminating a section of the disk including inner segments of spiral arms, fully seen through structure mapping, which appear to be outflowing and expanding. In addition, we see high velocities at projected distances of \geq 2'' (- 700 pc) from the nucleus, which could be due to rotation or to in situ acceleration of gas off the spiral arms. We find that the true half opening angle of the ionizing bicone (53 degrees) is much larger than the apparent half-opening angle (34 degrees) due to the above geometry, which may apply to a number of other Seyferts as well. Comment: 20 pages, 5 figures (1 color), to be published in The Astronomical Journal
    Preview · Article · Jun 2010 · The Astronomical Journal
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We compare mid-infrared emission-line properties, from high-resolution Spitzer spectra of a hard X-ray (14 -- 195 keV) selected sample of nearby (z < 0.05) AGN detected by the Burst Alert Telescope (BAT) aboard Swift. The luminosity distribution for the mid-infrared emission-lines, [O IV] 25.89 micron, [Ne II] 12.81 micron, [Ne III] 15.56 micron and [Ne V] 14.32/24.32 micron, and hard X-ray continuum show no differences between Seyfert 1 and Seyfert 2 populations, however six newly discovered BAT AGNs are under-luminous in [O IV], most likely the result of dust extinction in the host galaxy. The overall tightness of the mid-infrared correlations and BAT fluxes and luminosities suggests that the emission lines primarily arise in gas ionized by the AGN. We also compare the mid-infrared emission-lines in the BAT AGNs with those from published studies of ULIRGs, PG QSOs, star-forming galaxies and LINERs. We find that the BAT AGN sample fall into a distinctive region when comparing the [Ne III]/[Ne II] and the [O IV]/[Ne III] ratios. These line ratios are lower in sources that have been previously classified in the mid-infrared/optical as AGN than those found for the BAT AGN, suggesting that, in our X-ray selected sample, the AGN represents the main contribution to the observed line emission. These ratios represent a new emission line diagnostic for distinguishing between AGN and star forming galaxies. Comment: 54 pages, 9 Figures. Accepted for publication in The Astrophysical Journal.
    Full-text · Article · Apr 2010 · The Astrophysical Journal

Publication Stats

4k Citations
605.87 Total Impact Points

Institutions

  • 1997-2015
    • The Catholic University of America
      • Department of Physics
      Washington, Washington, D.C., United States
    • Rutgers, The State University of New Jersey
      • Department Physics and Astronomy
      New Brunswick, New Jersey, United States
  • 2002-2013
    • Georgia State University
      • Department of Physics and Astronomy
      Atlanta, Georgia, United States
  • 1998-2011
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States
  • 2010
    • NASA
      Вашингтон, West Virginia, United States
  • 2009
    • University of Oxford
      • Department of Physics
      Oxford, England, United Kingdom
  • 2004
    • University of Maryland, Baltimore County
      • Department of Physics
      Baltimore, Maryland, United States
  • 2001
    • The Ohio State University
      • Department of Astronomy
      Columbus, Ohio, United States