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  • 1
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    Keck/MOSFIRE spectroscopy of five ULX counterparts (2016)
    Oxford University Press
    Details
    Publication Date: 2016-04-20
    Description: We present H -band spectra of the candidate counterparts of five ultraluminous X-ray sources (ULXs; two in NGC 925, two in NGC 4136 and Holmberg II X-1) obtained with Keck/MOSFIRE (Multi-Object Spectrometer for Infra-Red Exploration). The candidate counterparts of two ULXs (J022721+333500 in NGC 925 and J120922+295559 in NGC 4136) have spectra consistent with (M-type) red supergiants (RSGs). We obtained two epochs of spectroscopy of the candidate counterpart to J022721+333500, separated by 10 months, but discovered no radial velocity variations with a 2 upper limit of 40 km s –1 . If the RSG is the donor star of the ULX, the most likely options are that either the system is seen at low inclination (〈40°) or the black hole mass is less than 100 M , unless the orbital period is longer than 6 years, in which case the obtained limit is not constraining. The spectrum of the counterpart to J120922+295559 shows emission lines on top of its stellar spectrum, and the remaining three counterparts do not show absorption lines that can be associated with the atmosphere of a star; their spectra are instead dominated by emission lines. Those counterparts with RSG spectra may be used in the future to search for radial velocity variations, and, if those are present, determine dynamical constraints on the mass of the accretor.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 2
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    Bright radio emission from an ultraluminous stellar-mass microquasar in M 31 (2012)
    Nature Publishing Group (NPG)
    Details
    Publication Date: 2012-12-14
    Description: A subset of ultraluminous X-ray sources (those with luminosities of less than 10(40) erg s(-1); ref. 1) are thought to be powered by the accretion of gas onto black holes with masses of approximately 5-20M cicled dot, probably by means of an accretion disk. The X-ray and radio emission are coupled in such Galactic sources; the radio emission originates in a relativistic jet thought to be launched from the innermost regions near the black hole, with the most powerful emission occurring when the rate of infalling matter approaches a theoretical maximum (the Eddington limit). Only four such maximal sources are known in the Milky Way, and the absorption of soft X-rays in the interstellar medium hinders the determination of the causal sequence of events that leads to the ejection of the jet. Here we report radio and X-ray observations of a bright new X-ray source in the nearby galaxy M 31, whose peak luminosity exceeded 10(39) erg s(-1). The radio luminosity is extremely high and shows variability on a timescale of tens of minutes, arguing that the source is highly compact and powered by accretion close to the Eddington limit onto a black hole of stellar mass. Continued radio and X-ray monitoring of such sources should reveal the causal relationship between the accretion flow and the powerful jet emission.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Middleton, Matthew J -- Miller-Jones, James C A -- Markoff, Sera -- Fender, Rob -- Henze, Martin -- Hurley-Walker, Natasha -- Scaife, Anna M M -- Roberts, Timothy P -- Walton, Dominic -- Carpenter, John -- Macquart, Jean-Pierre -- Bower, Geoffrey C -- Gurwell, Mark -- Pietsch, Wolfgang -- Haberl, Frank -- Harris, Jonathan -- Daniel, Michael -- Miah, Junayd -- Done, Chris -- Morgan, John S -- Dickinson, Hugh -- Charles, Phil -- Burwitz, Vadim -- Della Valle, Massimo -- Freyberg, Michael -- Greiner, Jochen -- Hernanz, Margarita -- Hartmann, Dieter H -- Hatzidimitriou, Despina -- Riffeser, Arno -- Sala, Gloria -- Seitz, Stella -- Reig, Pablo -- Rau, Arne -- Orio, Marina -- Titterington, David -- Grainge, Keith -- England -- Nature. 2013 Jan 10;493(7431):187-90. doi: 10.1038/nature11697. Epub 2012 Dec 12.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Physics Department, University of Durham, Durham DH1 3LE, UK. m.j.middleton@durham.ac.uk〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/23235823" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Published by Nature Publishing Group (NPG)
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  • 3
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    VLT/FORS2 observations of four high-luminosity ULX candidates (2013)
    Oxford University Press
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    Publication Date: 2013-06-30
    Description: We obtained Very Large Telescope/FOcal Reducer and low dispersion Spectrograph spectra of the optical counterparts of four high-luminosity ( L X  ≥ 10 40 erg s –1 ) Ultraluminous X-ray sources (ULX) candidates from the catalogue of Walton et al. We first determined accurate positions for the X-ray sources from archival Chandra observations and identified counterparts in archival optical observations that are sufficiently bright for spectroscopy with an 8 m telescope. From the spectra we determine the redshifts to the optical counterparts and emission line ratios. One of the candidate ULXs, in the spiral galaxy ESO 306–003, appears to be a bona fide ULX in an H ii region. The other three sources, near the elliptical galaxies NGC 533 and NGC 741 and in the ring galaxy AM 0644–741, turn out to be background active galactic nuclei (AGN) with redshifts of 1.85, 0.88 or 1.75 and 1.40, respectively. Our findings confirm the trend of a high probability of finding background AGN for systems with a ratio of log( F X /F opt ) in the range of –1–1.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
    Electronic Resource
    Electronic Resource
    Nuclear magnetic resonance imaging of simulated voids in cement slurries (1991)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 37 (1991), S. 1895-1899 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690371213
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Dynamic NMR imaging of rapid depth filtration of clay in porous media (1991)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 37 (1991), S. 1900-1903 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690371214
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    Paper (German National Licenses)
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  • 6
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    Functional MRI at 1.5 tesla: A comparison of the blood oxygenation level-dependent signal and electrophysiology (2000)
    National Academy of Sciences
    Details
    Publication Date: 2000-08-08
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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  • 7
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    NIR counterparts to ULXs (III): completing the photometric survey and selected spectroscopic results★ (2020)
    Oxford University Press
    Details
    Publication Date: 2020-07-07
    Description: We present results from the remaining sources in our search for near-infrared (NIR) candidate counterparts to ultraluminous X-ray sources (ULXs) within ≃10 Mpc. We observed 23 ULXs in 15 galaxies and detected NIR candidate counterparts to 6 of them. Two of these have an absolute magnitude consistent with a single red supergiant (RSG). Three counterparts are too bright for an RSG and spatially extended, and thus we classify them as stellar clusters. The other candidate is too faint for an RSG. Additionally, we present the results of our NIR spectroscopic follow-up of five sources: four originally classified as RSG and one as a stellar cluster on the basis of previous photometry. The stellar cluster candidate is actually a nebula. Of the four RSG candidates, one source has a broad H α emission line redshifted by ∼z = 1, making it a background active galactic nucleus (AGN). Two other sources show stellar spectra consistent with them being RSGs. The final RSG candidate is too faint to classify, but does not show strong (nebular) emission lines in its spectrum. After our search for NIR counterparts to 113 ULXs, where we detected a candidate counterpart for 38 ULXs, we have spectroscopically confirmed the nature of 12: 5 sources are nebulae, 1 source is not classified, 1 source is an AGN, and 5 are RSGs. These possible five ULX–RSG binary systems would constitute ${simeq} (4 pm 2){{ m per cent}}$ of the observed ULXs, a fraction almost four times larger than what was predicted by binary evolution simulations.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 8
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    Discovery of a red supergiant counterpart to RX J004722.4-252051, a ULX in NGC 253 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-11
    Description: We present two epochs of near-infrared spectroscopy of the candidate red supergiant counterpart to RX J004722.4–252051, a ULX in NGC 253. We measure radial velocities of the object and its approximate spectral type by cross-correlating our spectra with those of known red supergiants. Our VLT/X-shooter spectrum is best matched by that of early M-type supergiants, confirming the red supergiant nature of the candidate counterpart. The radial velocity of the spectrum, taken on 2014 August 23, is 417 ± 4 km s –1 . This is consistent with the radial velocity measured in our spectrum taken with Magellan/MMIRS on 2013 June 28, of 410 ± 70 km s –1 , although the large error on the latter implies that a radial velocity shift expected for a black hole of tens of M can easily be hidden. Using nebular emission lines we find that the radial velocity due to the rotation of NGC 253 is 351 ± 4 km s –1 at the position of the ULX. Thus the radial velocity of the counterpart confirms that the source is located in NGC 253, but also shows an offset with respect to the local bulk motion of the galaxy of 66 ± 6 km s –1 . We argue that the most likely origin for this displacement lies either in a SN kick, requiring a system containing a  50 M black hole, and/or in orbital radial velocity variations in the ULX binary system, requiring a  100 M black hole. We therefore conclude that RX J004722.4–252051 is a strong candidate for a ULX containing a massive stellar black hole.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    The ultraluminous state revisited: fractional variability and spectral shape as diagnostics of super-Eddington accretion (2013)
    Oxford University Press
    Details
    Publication Date: 2013-09-26
    Description: Although we are nearing a consensus that most ultraluminous X-ray sources (ULXs) below 10 41 erg s –1 represent stellar mass black holes accreting in a super-Eddington ‘ultraluminous’ accretion state, little is yet established of the physics of this extreme accretion mode. Here, we use a combined X-ray spectral and timing analysis of an XMM–Newton sample of ULXs to investigate this new accretion regime. We start by suggesting an empirical classification scheme that separates ULXs into three classes based on the spectral morphologies observed by Gladstone et al.: a singly peaked broadened disc class, and two-component hard ultraluminous and soft ultraluminous regimes, with the spectra of the latter two classes dominated by the harder and softer component, respectively. We find that at the lowest luminosities ( L X  〈 3 10 39 erg s –1 ) the ULX population is dominated by sources with broadened disc spectra, whilst ULXs with two-component spectra are seen almost exclusively at higher luminosities, suggestive of a distinction between ~Eddington and super-Eddington accretion modes. We find high levels of fractional variability are limited to ULXs with soft ultraluminous spectra, and a couple of the broadened disc sources. Furthermore, the variability in these sources is strongest at high energies, suggesting it originates in the harder of the two spectral components. We argue that these properties are consistent with current models of super-Eddington emission, where a massive radiatively driven wind forms a funnel-like geometry around the central regions of the accretion flow. As the wind provides the soft spectral component this suggests that inclination is the key determinant in the observed two-component X-ray spectra, which is very strongly supported by the variability results if this originates due to clumpy material at the edge of the wind intermittently obscuring our line-of-sight to the spectrally hard central regions of the ULX. The pattern of spectral variability with luminosity in two ULXs that straddle the hard/soft ultraluminous regime boundary is consistent with the wind increasing at higher accretion rates, and thus narrowing the opening angle of the funnel. Hence, this work suggests that most ULXs can be explained as stellar mass black holes accreting at and above the Eddington limit, with their observed characteristics dominated by two variables: accretion rate and inclination.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    The X-ray spectral evolution of the ultraluminous X-ray source Holmberg IX X-1 (2016)
    Oxford University Press
    Details
    Publication Date: 2016-06-30
    Description: We present a new analysis of X-ray spectra of the archetypal ultraluminous X-ray source (ULX) Holmberg IX X-1 obtained by the Swift , XMM–Newton and NuSTAR observatories. This ULX is a persistent source, with a typical luminosity of ~10 40  erg s –1 , that varied by a factor of 4–5 over eight years. We find that its spectra tend to evolve from relatively flat or two-component spectra in the medium energy band (1–6 keV), at lower luminosities, to a spectrum that is distinctly curved and disc-like at the highest luminosities, with the peak energy in the curved spectrum tending to decrease with increased luminosity. We argue that the spectral evolution of the ULX can be explained by super-Eddington accretion models, where in this case we view the ULX down the evacuated funnel along its rotation axis, bounded by its massive radiatively driven wind. The spectral changes then originate in enhanced geometric beaming as the accretion rate increases and wind funnel narrows, causing the scattered flux from the central regions of the supercritical flow to brighten faster than the isotropic thermal emission from the wind, and so the curved hard spectral component to dominate at the highest luminosities. The wind also Compton down-scatters photons at the edge of the funnel, resulting in the peak energy of the spectrum decreasing. We also confirm that Holmberg IX X-1 displays spectral degeneracy with luminosity, and suggest that the observed differences are naturally explained by precession of the black hole rotation axis for the suggested wind geometry.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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