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261 results on '"Walton, D.J."'

5. The Broadband Spectral Variability of Holmberg IX X-1

Author

Walton, D.J, Furst, F, Harrison, F.A, Middleton, M.J, Fabian, A. C, Bachetti, M, Barret, D, Miller, J. M, Ptak, A, Rana, V, Stern, D, and Tao, L

Subjects
Astrophysics
Abstract

We present results from four new broadband X-ray observations of the extreme ultraluminous X-ray source Holmberg IX X-1 (L (sub X) greater than 10 (sup 40) ergs per second), performed by Suzaku and NuSTAR in coordination. Combined with the archival data, we now have broadband observations of this remarkable source from six separate epochs. Two of these new observations probe lower fluxes than seen previously, allowing us to extend our knowledge of the broadband spectral variability exhibited. The spectra are well fit by two thermal blackbody components that dominate the emission below 10 kiloelectronvolts, as well as a steep (Gamma approximately equal to 3.5) power-law tail thatdominates above approximately 15 kiloelectronvolts. Remarkably, while the 0.3-10.0 kiloelectronvolts flux varies by a factor of approximately 3 between all these epochs, the 15-40 kiloelectronvolts flux varies by only approximately 20 percent. Although the spectral variability is strongest in the approximately 1-10 kiloelectronvolts band, both of the thermal components are required to vary when all epochs are considered. We also revisit the search for iron absorption features by leveraging the high-energy NuSTAR data to improve our sensitivity to extreme velocity outflows in light of the ultra-fast outflow recently detected in NGC 1313 X-1. Iron absorption from a similar outflow along our line of sight can be ruled out in this case. We discuss these results in the context of super-Eddington accretion models that invoke a funnel-like geometry for the inner flow, and propose a scenario in which we have an almost face-on view of a funnel that expands to larger radii with increasing flux, resulting in an increasing degree of geometrical collimation for the emission from intermediate-temperature regions.

Published
2017
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6. An ultraluminous X-ray source powered by an accreting neutron star

Author

Bachetti, M., Harrison, F.A., Walton, D.J., Grefenstette, B.W., Chakrabarty, D., Furst, F., Barret, D., Beloborodov, A., Boggs, S.E., Christensen, F.E., Craig, W.W., Fabian, A.C., Hailey, C.J., Hornschemeier, A., Kaspi, V., Kulkarni, S.R., Maccarone, T., Miller, J.M., Rana, V., Stern, D., Tendulkar, S.P., Tomsick, J., Webb, N.A., and Zhang, W.W.

Subjects
Galaxies -- Observations, Galactic center -- Observations, Environmental issues, Science and technology, Zoology and wildlife conservation
Abstract

The majority of ultraluminous X-ray sources are point sources that are spatially offset from the nuclei of nearby galaxies and whose X-ray luminosities exceed the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass black holes (1,2). Their X-ray luminosities in the 0.5-10 kiloelectronvolt energy band range from [10.sup.39] to [10.sup.41] ergs per second (3). Because higher masses imply less extreme ratios of the luminosity to the isotropic Eddington limit, theoretical models have focused on black hole rather than neutron star systems (1,2). The most challenging sources to explain are those at the luminous end of the range (more than [10.sub.40] ergs per second), which require black hole masses of 50-100 times the solar value or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries, or both. Here we report broadband X-ray observations of the nuclear region of the galaxy M82 that reveal pulsations with an average period of 1.37 seconds and a 2.5-day sinusoidal modulation. The pulsations result from the rotation of a magnetized neutron star, and the modulation arises from its binary orbit. The pulsed flux alone corresponds to an X-ray luminosity in the 3-30 kiloelectronvolt range of 4.9 x [10.sup.39] ergs per second. The pulsating source is spatially coincident with a variable source (4) that can reach an X-ray luminosity in the 0.3-10 kiloelectronvolt range of 1.8 x [10.sup.40] ergs per second (1). This association implies a luminosity of about 100 times the Eddington limit for a 1.4-solar-mass object, or more than ten times brighter than any known accreting pulsar. This implies that neutron stars may not be rare in the ultraluminous X-ray population, and it challenges physical models for the accretion of matter onto magnetized compact objects., The brightest accretion-powered X-ray pulsars, A0538-66 (5), SMCX-1 (6) and GRO J1744-28 (7), are variable, with reported X-ray luminosities up to [L.sub.X] (2-20 keV) ≅ [10.sup.39] erg [s.sup.-1], which are [...]

Published
2014

7. Reflection from the strong gravity regime in a lensed quasar at redshift z = 0.658

Author

Reis, R.C., Reynolds, M.T., Miller, J.M., and Walton, D.J.

Subjects
Gravity -- Research, Black holes (Astronomy) -- Observations, Astronomical research, Quasars -- Properties, Environmental issues, Science and technology, Zoology and wildlife conservation
Abstract

The co-evolution of a supermassive black hole with its host galaxy (1) through cosmic time is encoded in its spin (2-4). At z > 2, supermassive black holes are thought [...]

Published
2014

14. NuSTAR reveals the hidden nature of SS433

Author

Middleton, M.J., Walton, D.J., Alston, W., Dauser, T., Eikenberry, S., Jiang, Y.F., Fabian, A.C., Fuerst, F., Brightman, M., Marshall, H., Parker, M., Pinto, C., Harrison, F.A., Bachetti, M., Altamirano, D., Bird, A.J., Perez, G., Miller-Jones, James, Charles, P., Boggs, S., Christensen, F., Craig, W., Forster, K., Grefenstette, B., Hailey, C., Madsen, K., Stern, D., Zhang, W., Middleton, M.J., Walton, D.J., Alston, W., Dauser, T., Eikenberry, S., Jiang, Y.F., Fabian, A.C., Fuerst, F., Brightman, M., Marshall, H., Parker, M., Pinto, C., Harrison, F.A., Bachetti, M., Altamirano, D., Bird, A.J., Perez, G., Miller-Jones, James, Charles, P., Boggs, S., Christensen, F., Craig, W., Forster, K., Grefenstette, B., Hailey, C., Madsen, K., Stern, D., and Zhang, W.

Abstract

SS433 is the only Galactic binary system known to persistently accrete at highly super-critical (or hyper-critical) rates, similar to those in tidal disruption events, and likely needed to explain the rapid growth of those very high redshift quasars containing massive SMBHs. Probing the inner regions of SS433 in the X-rays is crucial to understanding this system, and super-critical accretion in general, but is highly challenging due to obscuration by the surrounding wind, driven from the accretion flow. NuSTAR observed SS433 in the hard X-ray band across multiple phases of its 162 d superorbital precession period. Spectral-timing tools allow us to infer that the hard X-ray emission from the inner regions is likely being scattered towards us by the walls of the wind-cone. By comparing to numerical models, we determine an intrinsic X-ray luminosity of ≥ 2 × 1037 erg s-1 and that, if viewed face on, we would infer an apparent luminosity of >1 × 1039 erg s-1, confirming SS433's long-suspected nature as an ultraluminous X-ray source (ULX). We present the discovery of a narrow, ∼100 s lag due to atomic processes occurring in outflowing material travelling at least 0.14-0.29c, which matches absorption lines seen in ULXs and - in the future - will allow us to map a super-critical outflow for the first time.

Published
2021

17. A dynamic black hole corona in an active galaxy through X-ray reverberation mapping

Author

Alston, W.N., Fabian, Andrew C., Kara, E., Parker, M.L., Dovciak, M., Pinto, C., Jiang, J., Middleton, M.J., Miniutti, Giovanni, Walton, D.J., Wilkins, D.R., Buisson, D.J.K., Caballero-García, María Dolores, Cackett, E.M., De Marco, B., Gallo, Luigi C., Lohfink, A.M., Reynolds, C.S., Uttley, P., Young, A.J., Zogbhi, A., Alston, W.N., Fabian, Andrew C., Kara, E., Parker, M.L., Dovciak, M., Pinto, C., Jiang, J., Middleton, M.J., Miniutti, Giovanni, Walton, D.J., Wilkins, D.R., Buisson, D.J.K., Caballero-García, María Dolores, Cackett, E.M., De Marco, B., Gallo, Luigi C., Lohfink, A.M., Reynolds, C.S., Uttley, P., Young, A.J., and Zogbhi, A.

Abstract

X-ray reverberation echoes are assumed to be produced in the strongly distorted spacetime around accreting supermassive black holes. This signal allows us to spatially map the geometry of the inner accretion flow—a region that cannot yet be spatially resolved by any telescope—and provides a direct measure of the black hole mass and spin. The reverberation timescale is set by the light travel path between the direct emission from a hot X-ray corona and the reprocessed emission from the inner edge of the accretion disk. However, there is an inherent degeneracy in the reverberation signal between black hole mass, inner disk radius and height of the illuminating corona above the disk. Here we use a long X-ray observation of the highly variable active galaxy IRAS 13224−3809 to track the reverberation signal as the system evolves on timescales of a day. With the inclusion of all the relativistic effects, modelling reveals that the height of the X-ray corona increases with increasing luminosity, providing a dynamic view of the inner accretion region. This simultaneous modelling allows us to break the inherent degeneracies and obtain an independent timing-based estimate for the mass and spin of the black hole. The uncertainty on black hole mass is comparable to the leading optical reverberation method, making X-ray reverberation a powerful technique, particularly for sources with low optical variability.

Published
2020

19. Reconstitution of nicotinic acetylcholine receptors into gel-protected lipid membranes

Author

Beddow, J.A., Peterson, I.R., Heptinstall, J., and Walton, D.J.

Subjects
Chemistry, Analytic -- Research, Chemistry
Abstract

We report the functional reconstitution of nicotinic acetylcholine receptors into gel-protected bilayer lipid membranes using two different methods. In the first case, reconstitution was achieved by direct membrane formation from an emulsion of glycerol monooleate, hexane, and a membrane receptor extract. In the second case, incorporation was achieved via the fusion of vesicles from a preparation of membrane-bound receptors into preformed membranes after diffusion through the protective front gel layer. Measurement of the dc conductivity of the membranes in the presence of either acetylcholine or [alpha]-bungarotoxin was used to test for the functional activity of incorporated receptors.

Published
2004

24. A rapidly spinning supermassive black hole at the centre of NGC 1365

Author

Risaliti, G., Harrison, F.A., Madsen, K.K., Walton, D.J., Boggs, S.E., Christensen, F.E., Craig, W.W., Grefenstette, B.W., Hailey, C.J., Nardini, E., Stern, Daniel, and Zhang, W.W.

Subjects
Black holes (Astronomy) -- Observations, Spiral galaxies -- Observations -- Spectra, Environmental issues, Science and technology, Zoology and wildlife conservation
Abstract

Broad X-ray emission lines from neutral and partially ionized iron observed in active galaxies have been interpreted as fluorescence produced by the reflection of hard X-rays off the inner edge [...]

Published
2013
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30. Discovery and analysis of a ULX nebula in NGC 3521

Author

López, K.M., Jonker, P.G., Heida, M., Torres, M.A.P., Roberts, T.P., Walton, D.J., Moon, D.-S., Harrison, F.A., López, K.M., Jonker, P.G., Heida, M., Torres, M.A.P., Roberts, T.P., Walton, D.J., Moon, D.-S., and Harrison, F.A.

Abstract

Contains fulltext : 207533.pdf (publisher's version ) (Open Access), We present Very Large Telescope/X-shooter and Chandra X-ray observatory/ACIS observations of the ULX [SST2011] J110545.62+000016.2 in the galaxy NGC 3521. The source identified as a candidate near-infrared counterpart to the ULX in our previous study shows an emission line spectrum of numerous recombination and forbidden lines in the visible and near-infrared spectral regime. The emission from the candidate counterpart is spatially extended (∼34 pc) and appears to be connected with an adjacent H ii region, located ∼138 pc to the NE. The measured velocities of the emission lines confirm that both the candidate counterpart and H ii region reside in NGC 3521. The intensity ratios of the emission lines from the ULX counterpart show that the line emission originates from the combined effect of shock and photoionisation of low metallicity (12 + log (O/H) = 8.19 ± 0.11) gas. Unfortunately, there is no identifiable spectral signature directly related to the photosphere of the mass-donor star in our spectrum. From the archival Chandra data, we derive the X-ray luminosity of the source in the 0.3–7 keV range to be (1.9 ± 0.8) × 1040 erg cm−2 s−1, almost a factor of four higher than what is previously reported.

Published
2019

31. Axis location of a worn cylindrical mechanical object

Author

Walton, D.J. and Wu, C.

Subjects
Computer-Aided Design, Approximation, Scientific Research, New Technique, Mechanical Engineering, Least Squares Approximation, Spline, Smoothing, Curve Fitting, Simulation, Computer-Aided Engineering, Parameter Estimation
Published
1992

35. Turning point preserving planar interpolation

Author

Walton, D.J. and Xu, R.

Subjects
Computer Graphics, Curve Fitting, Algorithm, Approximation, Computer-Aided Design, Spline, Computational Complexity, Modeling
Published
1991

37. Erratum for the Report 'A precise measurement of the magnetic field in the corona of the black hole binary V404 Cygni'

Author

Dallilar, Y., Eikenberry, S.S., Garner, A., Stelter, D., Gottlieb, A., Gandhi, P., Casella, P., Dhillon, V.S., Marsh, T.R., Littlefair, S.P., Hardy, L., Fender, R., Mooley, K., Walton, D.J., Fuerst, F., Bachetti, M., Castro-Tirado, A.J., Charcos, M., Edwards, M.L., Lasso-Cabrera, N.M., Marin-Franch, A., Raines, S.N., Ackley, K., Bennett, J.G., Cenarro, A.J., Chinn, B., Donoso, H.V., Frommeyer, R., Hanna, K., Herlevich, M.D., Julian, J., Miller, P., Mullin, S., Murphey, C.H., Packham, C., Varosi, F., Vega, C., Warner, C., Ramaprakash, A.N., Burse, M., Punnadi, S., Chordia, P., Gerarts, A., Martin, H.D.P., Calero, M.M., Scarpa, R., Acosta, S.F., Sanchez, W.M.H., Siegel, B., Perez, F.F., Martin, H.D.V., Losada, J.A.R., Nunez, A., Tejero, A., Gonzalez, C.E.M., Rodriguez, C.C., Molgo, J., Rodriguez, J.E., Caceres, J.I.F., Garcia, L.A.R., Lopez, M.H., Dominguez, R., Gaggstatter, T., Lavers, A.C., Geier, S., Pessev, P., and Sarajedini, A.

Abstract

In the Report “A precise measurement of the magnetic field in the corona of the black hole binary V404 Cygni,” a calculation error led to values of the magnetic field that were about 14 times too high. The mathematical expressions given in the Report were correct, but the code used to calculate the numerical values included an extraneous factor, which led to incorrect results. The magnetic fields calculated from the observations at different wavelengths were all scaled by the same factor, so after this is removed they remain consistent with each other. The corrected value of the magnetic field is lower than previously calculated, making the field in V404 Cygni even more unlike those estimated for other systems. However, the lower magnetic field is no longer consistent with the value predicted from the equipartition model. The text, materials and methods, Table S1, and Figure S3 have been updated to reflect the corrected magnetic field values and to state that the system was not in equipartition. No other results or conclusions of the study were affected. The authors thank J. Malzac (Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse) for alerting them to this error.

Published
2018

45. A growth-rate indicator for Compton-thick active galactic nuclei

Author

Brightman, Murray, Masini, A., Ballantyne, D., Brandt, W.N., Chen, C.-T., Comastri, A., Farrah, D., Gandhi, P., Ricci, C., Stern, D., and Walton, D.J.

Subjects
Astrophysics::High Energy Astrophysical Phenomena, Active Galactic Nuclei, Astrophysics::Cosmology and Extragalactic Astrophysics, Astrophysics::Galaxy Astrophysics
Abstract

The physical conditions of the X-ray emitting corona in AGN, parameterised by the X-ray spectral index, Gamma, is dependent on the Eddington ratio. This has been shown from studies of broad-lined unobscured AGN and offers a way to estimate the Eddington ratio from X-ray spectra alone. We test this relationship for a sample of megamaser AGN that have typically lower-mass black holes, are highly inclined and heavily obscured, using NuSTAR data to account for the heavy obscuration. We find that the same relationship between Gamma and Eddington ratio exists for these sources, which has implications the isotropic emission of the corona, and hence AGN unification, and for estimating growth rates of heavily obscured AGN.

Published
2016
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46. Relativistic Iron Line Fits

Author

Fink, M., Dauser, T., Beuchert, T., Jeffreson, S., Tawabutr, J., Wilms, J., García, J., and Walton, D.J.

Subjects
Astrophysics::High Energy Astrophysical Phenomena, Active Galactic Nuclei, Astrophysics::Cosmology and Extragalactic Astrophysics, Astrophysics::Galaxy Astrophysics
Abstract

The 6.4\,keV Iron reflection line possesses strong diagnostic potential for AGN-systems. In the rare case of unobscured AGN, this line receives a contribution from the very center of the accretion flow close to the event horizon that is subject to strong relativistic effects. The shape of this line distortion can be used infer important parameters of the central accretion region, especially the black hole spin parameter $a*$ and the accretion disk inclination $i$.\\ We analyze several (nine?) bare AGN spectra from the sample of Walton et al. 2012 using high resolution spectra from the XMM and NuStar archives. The relativistic reflection is modeled using the \textsc{Relxill} code (Dauser 20XX). The newest iteration of the \textsc{Relxill} model also supports a lamp post geometry for the irradiation of the accretion disk.\\ By combining these detailed models with the wide spectral range of NuStar and XMM/NuStar joint observations we can put tight constraints on the aforementioned parameters and we can constrain the height of the source $h$ in a possible lamp post geometry.

Published
2016
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49. Keck/MOSFIRE spectroscopy of five ULX counterparts

Author

Heida, M., Jonker, P.G., Torres, M.A.P., Roberts, T.P., Walton, D.J., Moon, D.-S., Stern, D., Harrison, F.A., Heida, M., Jonker, P.G., Torres, M.A.P., Roberts, T.P., Walton, D.J., Moon, D.-S., Stern, D., and Harrison, F.A.

Abstract

Contains fulltext : 157927.pdf (preprint version ) (Open Access)

Published
2016

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