K. Dodds-Eden

Max Planck Institute for Extraterrestrial Physics, Arching, Bavaria, Germany

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Publications (36)134.67 Total impact

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    ABSTRACT: Aims. We report on simultaneous observations and modeling of mid-infrared (MIR), near-infrared (NIR), and submillimeter (submm) emission of the source Sgr A* associated with the supermassive black hole at the center of our Galaxy. Our goal was to monitor the activity of Sgr A* at different wavelengths in order to constrain the emitting processes and gain insight into the nature of the close environment of Sgr A*. Methods. We used the MIR instrument VISIR in the BURST imaging mode, the adaptive optics assisted NIR camera NACO, and the sub-mm antenna APEX to monitor Sgr A* over several nights in July 2007. Results. The observations reveal remarkable variability in the NIR and sub-mm during the five nights of observation. No source was detected in the MIR, but we derived the lowest upper limit for a flare at 8.59 microns (22.4 mJy with A_8.59mu = 1.6+/- 0.5). This observational constraint makes us discard the observed NIR emission as coming from a thermal component emitting at sub-mm frequencies. Moreover, comparison of the sub-mm and NIR variability shows that the highest NIR fluxes (flares) are coincident with the lowest sub-mm levels of our five-night campaign involving three flares. We explain this behavior by a loss of electrons to the system and/or by a decrease in the magnetic field, as might conceivably occur in scenarios involving fast outflows and/or magnetic reconnection.
    Astronomy and Astrophysics 01/2013; · 5.08 Impact Factor
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    ABSTRACT: The X-ray and near-IR emission from Sgr A* is dominated by flaring, while a quiescent component dominates the emission at radio and sub-mm wavelengths. The spectral energy distribution of the quiescent emission from Sgr A* peaks at sub-mm wavelengths and is modeled as synchrotron radiation from a thermal population of electrons in the accretion flow, with electron temperatures ranging up to $\sim 5-20$\,MeV. Here we investigate the mechanism by which X-ray flare emission is produced through the interaction of the quiescent and flaring components of Sgr A*. The X-ray flare emission has been interpreted as inverse Compton, self-synchrotron-Compton, or synchrotron emission. We present results of simultaneous X-ray and near-IR observations and show evidence that X-ray peak flare emission lags behind near-IR flare emission with a time delay ranging from a few to tens of minutes. Our Inverse Compton scattering modeling places constraints on the electron density and temperature distributions of the accretion flow and on the locations where flares are produced. In the context of this model, the strong X-ray counterparts to near-IR flares arising from the inner disk should show no significant time delay, whereas near-IR flares in the outer disk should show a broadened and delayed X-ray flare.
    The Astronomical Journal 03/2012; 144(1). · 4.97 Impact Factor
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    ABSTRACT: Measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* at the Galactic Centre is a black hole four million times the mass of the Sun. With the exception of modest X-ray and infrared flares, Sgr A* is surprisingly faint, suggesting that the accretion rate and radiation efficiency near the event horizon are currently very low. Here we report the presence of a dense gas cloud approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations tightly constrain the cloud's orbit to be highly eccentric, with an innermost radius of approach of only ∼3,100 times the event horizon that will be reached in 2013. Over the past three years the cloud has begun to disrupt, probably mainly through tidal shearing arising from the black hole's gravitational force. The cloud's dynamic evolution and radiation in the next few years will probe the properties of the accretion flow and the feeding processes of the supermassive black hole. The kilo-electronvolt X-ray emission of Sgr A* may brighten significantly when the cloud reaches pericentre. There may also be a giant radiation flare several years from now if the cloud breaks up and its fragments feed gas into the central accretion zone.
    Nature 12/2011; 481(7379):51-4. · 38.60 Impact Factor
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    ABSTRACT: We present spatially resolved imaging and integral field spectroscopy data for 450 cool giant stars within 1\,pc from Sgr\,A*. We use the prominent CO bandheads to derive effective temperatures of individual giants. Additionally we present the deepest spectroscopic observation of the Galactic Center so far, probing the number of B9/A0 main sequence stars ($2.2-2.8\,M_\odot$) in two deep fields. From spectro-photometry we construct a Hertzsprung-Russell diagram of the red giant population and fit the observed diagram with model populations to derive the star formation history of the nuclear cluster. We find that (1) the average nuclear star-formation rate dropped from an initial maximum $\sim10$\,Gyrs ago to a deep minimum 1-2\,Gyrs ago and increased again during the last few hundred Myrs, and (2) that roughly 80% of the stellar mass formed more than 5\,Gyrs ago; (3) mass estimates within $\rm R\sim1\,pc$ from Sgr\,A* favor a dominant star formation mode with a 'normal' Chabrier/Kroupa initial mass function for the majority of the past star formation in the Galactic Center. The bulk stellar mass seems to have formed under conditions significantly different from the young stellar disks, perhaps because at the time of the formation of the nuclear cluster the massive black hole and its sphere of influence was much smaller than today.
    The Astrophysical Journal 10/2011; 741(2). · 6.73 Impact Factor
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    ABSTRACT: We derive the extinction curve toward the Galactic center (GC) from 1 to 19 μm. We use hydrogen emission lines of the minispiral observed by ISO-SWS and SINFONI. The extinction-free flux reference is the 2 cm continuum emission observed by the Very Large Array. Toward the inner 14'' × 20'', we find an extinction of A 2.166 μm = 2.62 ± 0.11, with a power-law slope of α = –2.11 ± 0.06 shortward of 2.8 μm, consistent with the average near-infrared slope from the recent literature. At longer wavelengths, however, we find that the extinction is grayer than shortward of 2.8 μm. We find that it is not possible to fit the observed extinction curve with a dust model consisting of pure carbonaceous and silicate grains only, and the addition of composite particles, including ices, is needed to explain the observations. Combining a distance-dependent extinction with our distance-independent extinction, we derive the distance to the GC to be R 0 = 7.94 ± 0.65 kpc. Toward Sgr A* (r < 05), we obtain AH = 4.21 ± 0.10, AKs = 2.42 ± 0.10, and A L' = 1.09 ± 0.13.
    The Astrophysical Journal 08/2011; 737(2):73. · 6.73 Impact Factor
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    ABSTRACT: We derive the extinction curve towards the Galactic Center from 1 to 19 micron. We use hydrogen emission lines of the minispiral observed by ISO-SWS and SINFONI. The extinction free flux reference is the 2 cm continuum emission observed by the VLA. Towards the inner 14" * 20" we find an extinction of A(2.166 micron)=2.62 +/- 0.11, with a power-law slope of alpha=-2.11 +/- 0.06 shortward of 2.8 micron, consistent with the average near infrared slope from the recent literature. At longer wavelengths, however, we find that the extinction is grayer than shortward of 2.8 micron. We find it is not possible to fit the observed extinction curve with a dust model consisting of pure carbonaceous and silicate grains only, and the addition of composite particles, including ices, is needed to explain the observations. Combining a distance dependent extinction with our distance independent extinction we derive the distance to the GC to be R_0=7.94 +/- 0.65 kpc. Towards Sgr A* (r<0.5") we obtain A_H=4.21 +/- 0.10, A_Ks=2.42 +/- 0.10 and A_L'=1.09 +/- 0.13.
    05/2011;
  • The Galactic Center: a Window to the Nuclear Environment of Disk Galaxies; 05/2011
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    ABSTRACT: We report preliminary results from a multiwavelength campaign aimed at the flaring activity of Sgr A*, which was carried out in early April 2009. Simultaneous data were collected in X-ray, near-infrared, and submillimeter wavebands with the XMM-Newton satellite, the VLT, and the APEX telescope, respectively. We present the detection of several flares and briefly discuss their properties in the context of the synchrotron self-Compton and plasmoid expansion models.
    05/2011;
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    ABSTRACT: We report the observations in the mid-infrared in the frame of the Galactic Center Large Program. We have determined color temperature of several point sources, around 300 K, and of the Minispiral, homogeneous around 170 K. Combined with observations in the submillimeter and the near-infrared the July 22nd 2007, the observations in mid-infrared have enable us to place a tight upper limit of 11.2 mJy in the de-reddened mid-infrared emission of Sgr A*. This upper limit in the mid-infrared domain enabled us to improve the model on emission process of Sgr A*.
    05/2011;
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    ABSTRACT: We summarize our latest observations of the nuclear star cluster in the central parsec of the Galaxy with the adaptive optics assisted, integral field spectrograph SINFONI on the ESO/VLT, which result in a total sample of 177 bona fide early-type stars. We find that most of these Wolf Rayet (WR), O- and B- stars reside in two strongly warped eccentric ( = 0.36±0.06) disks between 0.8'' and 12'' from SgrA*, as well as a central compact concentration (the S-star cluster) centered on SgrA*. The later type B stars (mK > 15) in the radial interval between 0.8'' and 12'' seem to be in a more isotropic distribution outside the disks. We observe a dearth of late-type stars in the central few arcseconds, which is puzzling. The stellar mass function of the disk stars is extremely top-heavy with a best fit power law of dN/dm α m-0.45±0.3. Since at least the WR/O-stars were formed in situ in a single star formation event ˜6 Myrs ago, this mass function probably reflects the initial mass function (IMF). The mass functions of the S-stars inside 0.8'' and of the early-type stars at distances beyond 12'' differ significantly from the disk IMF; they are compatible with a standard Salpeter/Kroupa IMF (best fit power law of dN/dm α m-2.15±0.3).
    05/2011;
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    ABSTRACT: GRAVITY is the second generation Very Large Telescope Interferometer instrument for precision narrow-angle astrometry and interferometric imaging. With its fibre-fed integrated optics, wavefront sensors, fringe tracker, beam stabilisation and a novel metrology concept, GRAVITY will push the sensitivity and accuracy of astrometry and interferometric imaging far beyond what is offered today. Providing precision astrometry of order 10 microarcseconds, and imaging with 4-milliarcsecond resolution, GRAVITY will revolutionise dynamical measurements of celestial objects: it will probe physics close to the event horizon of the Galactic Centre black hole; unambiguously detect and measure the masses of black holes in massive star clusters throughout the Milky Way; uncover the details of mass accretion and jets in young stellar objects and active galactic nuclei; and probe the motion of binary stars, exoplanets and young stellar discs. The instrument capabilities of GRAVITY are outlined and the science opportunities that will open up are summarised.
    The Messenger. 03/2011; 143:16-24.
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    ABSTRACT: The radiative counterpart of the supermassive black hole at the Galactic center (GC), Sgr A*, is subject to frequent flares visible simultaneously in X-rays and near-infrared (NIR). Often, enhanced radio variability from centimeter to sub-millimeter wavelengths is observed to follow these X-ray/NIR eruptions. We present here a multi-wavelength campaign carried out in April 2009, with the aim of characterizing this broadband flaring activity. Concurrent data from the XMM-Newton/EPIC (2-10 keV), VLT/NACO (2.1 microns, 3.8 microns), APEX/LABOCA (870 microns), and Fermi/LAT (0.1-200 GeV) instruments are employed to derive light curves and spectral energy distributions of new flares from Sgr A*. We detected two relatively bright NIR flares both associated with weak X-ray activity, one of which was followed by a strong sub-mm outburst 200 min later. Photometric spectral information on a NIR flare was obtained for the first time with NACO giving a power-law photon index alpha=-0.4\pm0.3. The first attempt to detect flaring activity from the Fermi GC source 1FGL J1745.6-2900 is also reported. NIR, X-ray, and sub-mm flares are finally modeled in the context of non-thermal emission processes. It is found that the simplest scenario involving a single expanding plasmoid releasing synchrotron NIR/sub-mm and synchrotron self-Compton X-ray radiation is inadequate to reproduce the data, but suggestions to reconcile the basic elements of the theory and the observations are proposed.
    Astronomy and Astrophysics 02/2011; · 5.08 Impact Factor
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    ABSTRACT: We have carried out multi-wavelength observations of Sgr A* in May and July of 2008 using the Chandra X-ray Observatory, APEX, ATCA, CARMA, CSO, IRAM, NMA, SMA, SMT, VLA, VLBA and VLT. Previous measurements have shown time delayed emission between near-IR and radio/submm. Here, we present two new aspects of the variability of Sgr A* across the electromagnetic spectrum. One is evidence for dimming of submm and radio flux during the peak of near-IR flare emission. This is consistent with recent finding that the variability of Sgr A* in radio and submm wavelengths is anti-correlated with near-IR flare emission. This newly recognized behavior is consistent with adiabatically cooling plasma blobs that are expanding but also partially eclipsing the background quiescent emission from Sgr A*. The second result is that cross correlation of X-rays and near-IR light curves suggests that X-ray flare emission lags behind near-IR flare emission by several minutes. This implies that X-ray emission is due to inverse Compton scattering of near-IR flare emission by submm emitting electrons.
    01/2011;
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    ABSTRACT: The emission from Sgr A*, the supermassive black hole in the Galactic Center, shows order of magnitude variability ("flares") a few times a day that is particularly prominent in the near-infrared (NIR) and X-rays. We present a time-dependent model for these flares motivated by the hypothesis that dissipation of magnetic energy powers the flares. We show that episodic magnetic reconnection can occur near the last stable circular orbit in time-dependent magnetohydrodynamic simulations of black hole accretion—the timescales and energetics of these events are broadly consistent with the flares from Sgr A*. Motivated by these results, we present a spatially one-zone time-dependent model for the electron distribution function in flares, including energy loss due to synchrotron cooling and adiabatic expansion. Synchrotron emission from transiently accelerated particles can explain the NIR/X-ray light curves and spectra of a luminous flare observed on 2007 April 4. A significant decrease in the magnetic field strength during the flare (coincident with the electron acceleration) is required to explain the simultaneity and symmetry of the simultaneous light curves. Our models predict that the NIR and X-ray spectral indices are related by Δα 0.5 (where νF ν να) and that there is only modest variation in the spectral index during flares. We also explore implications of this model for longer wavelength (radio-submillimeter) emission seemingly associated with X-ray and NIR flares; we argue that a few hour decrease in the submillimeter emission is a more generic consequence of large-scale magnetic reconnection than delayed radio emission from adiabatic expansion.
    The Astrophysical Journal 11/2010; 725(1):450. · 6.73 Impact Factor
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    ABSTRACT: In this paper we examine properties of the variable source Sgr A* in the near-infrared (NIR) using a very extensive Ks-band data set from NACO/VLT observations taken 2004 to 2009. We investigate the variability of Sgr A* with two different photometric methods and analyze its flux distribution. We find Sgr A* is continuously variable (meaning the source is always `on' and varying) in the near-infrared, and there also appears to be some medium-term variability on timescales of weeks to months. The flux distribution can be described by a lognormal distribution at low intrinsic fluxes (less than about 5 mJy, dereddened with A_{Ks}=2.5). The lognormal distribution has a median flux of ~1.6 mJy, but above 5 mJy the flux distribution is significantly flatter (high flux events are more common) than expected for the extrapolation of the lognormal distribution to high fluxes. We make a general identification of the low level emission above 5 mJy as flaring emission and of the low level emission as the quiescent state. We also report here the brightest Ks-band flare ever observed (from August 5th, 2008) which reached an intrinsic Ks-band flux of 27.5 mJy (m_{Ks}=13.5). This flare was a factor 27 increase over the median flux of Sgr A*, close to double the brightness of the star S2 in the Ks-band, and 40% brighter than the next brightest flare ever observed from Sgr A*. Comment: 16 pages, 9 figures, submitted to ApJ
    The Astrophysical Journal 08/2010; · 6.73 Impact Factor
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    ABSTRACT: To understand the correlation and the radiation mechanism of flare emission in different wavelength bands, we have coordinated a number of telescopes to observe SgrA* simultaneously. We focus only on one aspect of the preliminary results of our multi-wavelength observing campaigns, namely, the short time scale variability of emission from SgrA* in near-IR, X-ray and radio wavelengths. The structure function analysis indicate most of the power spectral density is detected on hourly time scales in all wavelength bands. We also report minute time scale variability at 7 and 13mm placing a strong constraint on the nature of the variable emission. The hourly time scale variability can be explained in the context of a model in which the peak frequency of emission shifts toward lower frequencies as a self-absorbed synchrotron source expands adiabatically near the acceleration site. The short time scale variability, on the other hand, places a strong constraint on the size of the emitting region. Assuming that rapid minute time scale fluctuations of the emission is optically thick in radio wavelength, light travel arguments requires relativistic particle energy, thus suggesting the presence of outflow from SgrA*. Comment: 9 pages, 4 figures, The Galactic Center: A Window on the Nuclear Environment of Disk Galaxies ASP Conference Series, 2010 eds: M. Morris, D. Q. Wang and F. Yuan
    04/2010;
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    ABSTRACT: We summarize recent observations and modeling of the brightest Sgr A* flare to be observed simultaneously in (near)-infrared and X-rays to date. Trying to explain the spectral characteristics of this flare through inverse Compton mechanisms implies physical parameters that are unrealistic for Sgr A*. Instead, a "cooling break" synchrotron model provides a more feasible explanation for the X-ray emission. In a magnetic field of about 5-30 Gauss the X-ray emitting electrons cool very quickly on the typical dynamical timescale while the NIR-emitting electrons cool more slowly. This produces a spectral break in the model between NIR and X-ray wavelengths that can explain the differences in the observed spectral indices. Comment: Proceedings of the Galactic Center Workshop 2009, Shanghai
    02/2010;
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    ABSTRACT: The center of the Milky Way hosts a massive black hole. The observational evidence for its existence is overwhelming. The compact radio source Sgr A* has been associated with a black hole since its discovery. In the last decade, high-resolution, near-infrared measurements of individual stellar orbits in the innermost region of the Galactic Center have shown that at the position of Sgr A* a highly concentrated mass of 4 x 10^6 M_sun is located. Assuming that general relativity is correct, the conclusion that Sgr A* is a massive black hole is inevitable. Without doubt this is the most important application of stellar orbits in the Galactic Center. Here, we discuss the possibilities going beyond the mass measurement offered by monitoring these orbits. They are an extremely useful tool for many scientific questions, such as a geometric distance estimate to the Galactic Center or the puzzle, how these stars reached their current orbits. Future improvements in the instrumentation will open up the route to testing relativistic effects in the gravitational potential of the black hole, allowing to take full advantage of this unique laboratory for celestial mechanics. Comment: Proceedings of the Galactic Center Workshop 2009, Shanghai
    02/2010;
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    ABSTRACT: We systematically investigate the error sources for high-precision astrometry from adaptive optics (AO) based near-infrared imaging data. We focus on the application in the crowded stellar field in the Galactic Centre. We show that at the level of ≲100 μas a number of effects are limiting the accuracy. Most important are the imperfectly subtracted seeing haloes of neighbouring stars, residual image distortions and unrecognized confusion of the target source with fainter sources in the background. Further contributors to the error budget are the uncertainty in estimating the point-spread function, the signal-to-noise ratio induced statistical uncertainty, coordinate transformation errors, the chromaticity of refraction in Earth's atmosphere, the post-AO differential tilt jitter and anisoplanatism. For stars as bright as mK= 14, residual image distortions limit the astrometry, for fainter stars the limitation is set by the seeing haloes of the surrounding stars. In order to improve the astrometry substantially at the current generation of telescopes, an AO system with high performance and weak seeing haloes over a relatively small field (r≲ 3 arcsec) is suited best. Furthermore, techniques to estimate or reconstruct the seeing halo could be promising.
    Monthly Notices of the Royal Astronomical Society 01/2010; 401(2):1177 - 1188. · 5.52 Impact Factor
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    ABSTRACT: We present a detailed analysis of high-resolution near-infrared imaging and spectroscopy of the potential star cluster IRS13E very close to the massive black hole in the Galactic center. We detect 19 objects in IRS13E from Ks-band images, 15 of which are also detected reliably in the H band. We derive consistent proper motions for these objects from the two bands. Most objects share a similar westward proper motion. We characterize the objects using spectroscopy (1.45-2.45 mum) and (narrowband) imaging from the H (1.66 mum) to the L' band (3.80 mum). Nine of the objects detected in both the Ks and H bands are very red, and we find that they are all consistent with being warm dust clumps. The dust emission may be caused by the colliding winds of the two Wolf-Rayet stars in the cluster. Three of the six detected stars do not share the motion or spectral properties of the three bright stars. This leaves only the three bright, early-type stars as potential cluster members. It is unlikely that these stars are a chance configuration. Assuming the presence of an intermediate mass black hole (IMBH), a mass of about 14,000 M sun follows from the velocities and positions of these three stars. However, our acceleration limits make such an IMBH nearly as unlikely as a chance occurrence of such a star association. Furthermore, there is no variable X-ray source in IRS13E despite the high density of dust and gas. Therefore, we conclude that is unlikely that IRS13E hosts a black hole massive enough to bind the three stars.
    The Astrophysical Journal 01/2010; 721(1):395-411. · 6.73 Impact Factor

Publication Stats

490 Citations
134.67 Total Impact Points

Institutions

  • 2007–2012
    • Max Planck Institute for Extraterrestrial Physics
      Arching, Bavaria, Germany
  • 2010
    • University of California, Berkeley
      • Department of Physics
      Berkeley, MO, United States