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Search Results: 1 - 10 of 140302 matches for " Kristin K. Madsen "
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NuSTAR observations of the young, energetic radio pulsar PSR B1509-58
Ge Chen,Hongjun An,Victoria M. Kaspi,Fiona A. Harrison,Kristin K. Madsen,Daniel Stern
Physics , 2015,
Abstract: We report on Nuclear Spectroscopic Telescope Array (NuSTAR) hard X-ray observations of the young rotation-powered radio pulsar PSR B1509$-$58 in the supernova remnant MSH 15$-$52. We confirm the previously reported curvature in the hard X-ray spectrum, showing that a log parabolic model provides a statistically superior fit to the spectrum compared with the standard power law. The log parabolic model describes the NuSTAR data, as well as previously published gamma-ray data obtained with COMPTEL and AGILE, all together spanning 3 keV through 500 MeV. Our spectral modelling allows us to constrain the peak of the broadband high energy spectrum to be at 2.6$\pm$0.8 MeV, an improvement of nearly an order of magnitude in precision over previous measurements. In addition, we calculate NuSTAR spectra in 26 pulse phase bins and confirm previously reported variations of photon indices with phase. Finally, we measure the pulsed fraction of PSR B1509$-$58 in the hard X-ray energy band for the first time. Using the energy resolved pulsed fraction results, we estimate that the pulsar's off-pulse emission has a photon index value between 1.26 and 1.96. Our results support a model in which the pulsar's lack of GeV emission is due to viewing geometry, with the X-rays originating from synchrotron emission from secondary pairs in the magnetosphere.
Polarization studies with NuSTAR
Simone Lotti,Lorenzo Natalucci,Paolo Giommi,B. Grefenstette,Fiona A. Harrison,Kristin K. Madsen,Matteo Perri,Simonetta Puccetti,Andreas Zoglauer
Physics , 2012, DOI: 10.1117/12.925989
Abstract: The capability of NuSTAR to detect polarization in the Compton scattering regime (>50 keV) has been investigated. The NuSTAR mission, flown on June 2012 a Low Earth Orbit (LEO), provides a unique possibility to confirm the findings of INTEGRAL on the polarization of cosmic sources in the hard X-rays. Each of the two focal plane detectors are high resolution pixellated CZT arrays, sensitive in the energy range ~ 3 - 80 keV. These units have intrinsic polarization capabilities when the proper information on the double events is transmitted on ground. In this case it will be possible to detect polarization from bright sources on timescales of the order of 10^5s
In-flight PSF calibration of the NuSTAR hard X-ray optics
Hongjun An,Kristin K. Madsen,Niels J. Westergaard,Steven E. Boggs,Finn E. Christensen,William W. Craig,Charles J. Hailey,Fiona A. Harrison,Daniel K. Stern,William W. Zhang
Physics , 2014, DOI: 10.1117/12.2055481
Abstract: We present results of the point spread function (PSF) calibration of the hard X-ray optics of the Nuclear Spectroscopic Telescope Array (NuSTAR). Immediately post-launch, NuSTAR has observed bright point sources such as Cyg X-1, Vela X-1, and Her X-1 for the PSF calibration. We use the point source observations taken at several off-axis angles together with a ray-trace model to characterize the in-orbit angular response, and find that the ray-trace model alone does not fit the observed event distributions and applying empirical corrections to the ray-trace model improves the fit significantly. We describe the corrections applied to the ray-trace model and show that the uncertainties in the enclosed energy fraction (EEF) of the new PSF model is < 3% for extraction apertures of R > 60" with no significant energy dependence. We also show that the PSF of the NuSTAR optics has been stable over a period of ~300 days during its in-orbit operation.
On-demand sedation with propofol for colonoscopy—A prospective pilot study of the influence on short-term memory, psychomotor function and postural stability*  [PDF]
Mogens R?rb?k Madsen
Open Journal of Clinical Diagnostics (OJCD) , 2013, DOI: 10.4236/ojcd.2013.33023
Abstract: Background: After sedated colonoscopy, patients are discharged on the basis of their subjective judgment that they have recovered, corroborated by the nursing staff. The aim of this study was to assess objectively whether patients were in fact fully recovered at the time of discharge, and to demonstrate whether the methods of testing applied could detect any influence of sedation on short-term memory, psychomotor function and postural stability. Methods: Twenty-two patients were investigated. At the beginning of the procedure, a defined bolus of propofol/alfentanil was given intravenously. During the procedure, an additional bolus was injected one or more times as requested by the patient. After colonoscopy, the patients stayed in the recovery room until the patients judged that they had recovered completely, which was also the judgement of the nursing staff at that time. Before colonoscopy and again before discharge, tests were performed of short-term memory, psychomotor function and postural stability (balance). Results: A positive correlation was found between the duration of colonoscopy and the amount of sedative given (p < 0.03). No differences in short-term memory or postural control were found when measurements obtained before and after colonoscopy were compared. Reaction time was prolonged significantly after colonoscopy (p < 0.01), which was mainly due to prolongation of perception time (p < 0.003). No correlation was found between the observed reduction in psychomotor function and the amount of sedative given. Conclusions: The introduction of ultrashort-acting sedative and hypnotic agents has facilitated out-patient colonoscopy. However, although they feel that they have recovered fully, some patients are still affected by the sedative at the time of discharge, as demonstrated by tests of short-term memory, reaction time and postural stability.
Demonstration of an As2S3 Grating Coupler on Thin Film LiNbO3  [PDF]
Chen Zhang, Christi K. Madsen
Optics and Photonics Journal (OPJ) , 2018, DOI: 10.4236/opj.2018.84011
Abstract: We present both design and experimental results for an As2S3 grating coupler on a thin film LiNbO3 substrate. A basic grating coupler structure is designed with coupling efficiency of 53% to a single mode fiber. A maximum simulated coupling efficiency of 78.8% is achieved, assuming a polymer bonding process. The basic structure was fabricated, and the coupling efficiency was measured to be at least 23.4% at 1540 nm. Some of the loss may be attributable to non-grating sources, such as waveguide tapers and testing fiber tails. A grating cavity was then measured using the grating couplers. The cavity waveguide propagation loss was 2.0 dB/cm. For a 400 nm thick As2S3 on 500 nm thin film LiNbO3 on insulator, the confinement factor in the LiNbO3 crystal is 82.3% when the As2S3 waveguide width is 400 nm, showing that As2S3-on-thin film LiNbO3 is an excellent candidate for thin film electro-optic applications.
On the Spin of the Black Hole in IC 10 X-1
James F. Steiner,Dominic J. Walton,Javier A. Garcia,Jeffrey E. McClintock,Silas G. T. Laycock,Matthew J. Middleton,Robin Barnard,Kristin K. Madsen
Physics , 2015,
Abstract: The compact X-ray source in the eclipsing X-ray binary IC 10 X-1 has reigned for years as ostensibly the most massive stellar-mass black hole, with a mass estimated to be about twice that of its closest rival. However, striking results presented recently by Laycock et al. reveal that the mass estimate, based on emission-line velocities, is unreliable and that the mass of the X-ray source is essentially unconstrained. Using Chandra and NuSTAR data, we rule against a neutron-star model and conclude that IC 10 X-1 contains a black hole. The eclipse duration of IC 10 X-1 is shorter and its depth shallower at higher energies, an effect consistent with the X-ray emission being obscured during eclipse by a Compton-thick core of a dense wind. The spectrum is strongly disk-dominated, which allows us to constrain the spin of the black hole via X-ray continuum fitting. Three other wind-fed black-hole systems are known; the masses and spins of their black holes are high: M ~ 10-15 Msun and a*>0.8. If the mass of IC 10 X-1's black hole is comparable, then its spin is likewise high.
Analysis of As2S3-Ti: LiNbO3 Taper Couplers Using Supermode Theory  [PDF]
Xin Xia, Yifeng Zhou, Christi K. Madsen
Optics and Photonics Journal (OPJ) , 2012, DOI: 10.4236/opj.2012.24043

In this work, we develop a simulation method based on supermode theory and transfer matrix formalism, and then apply it to the analysis and design of taper couplers for vertically integrated As2S3 and Ti: LiNbO3 hybrid waveguides. Test structures based on taper couplers are fabricated and characterized. The experimental results confirm the validity of the modeling method, which in turn, is used to analyze the fabricated couplers.

Broadband X-ray Imaging and Spectroscopy of the Crab Nebula and Pulsar with NuSTAR
Kristin K. Madsen,Stephen Reynolds,Fiona Harrison,Hongjun An,Steven Boggs,Finn E. Christensen,William W. Craig,Chris L. Fryer,Brian W. Grefenstette,Charles J. Hailey,Craig Markwardt,Melania Nynka,Daniel Stern,Andreas Zoglauer,William Zhang
Physics , 2015, DOI: 10.1088/0004-637X/801/1/66
Abstract: We present broadband (3 -- 78 keV) NuSTAR X-ray imaging and spectroscopy of the Crab nebula and pulsar. We show that while the phase-averaged and spatially integrated nebula + pulsar spectrum is a power-law in this energy band, spatially resolved spectroscopy of the nebula finds a break at $\sim$9 keV in the spectral photon index of the torus structure with a steepening characterized by $\Delta\Gamma\sim0.25$. We also confirm a previously reported steepening in the pulsed spectrum, and quantify it with a broken power-law with break energy at $\sim$12 keV and $\Delta\Gamma\sim0.27$. We present spectral maps of the inner 100\as\ of the remnant and measure the size of the nebula as a function of energy in seven bands. These results find that the rate of shrinkage with energy of the torus size can be fitted by a power-law with an index of $\gamma = 0.094\pm 0.018$, consistent with the predictions of Kennel and Coroniti (1984). The change in size is more rapid in the NW direction, coinciding with the counter-jet where we find the index to be a factor of two larger. NuSTAR observed the Crab during the latter part of a $\gamma$-ray flare, but found no increase in flux in the 3 - 78 keV energy band.
NuSTAR discovery of a luminosity dependent cyclotron line energy in Vela X-1
Felix Fuerst,Katja Pottschmidt,Joern Wilms,John A. Tomsick,Matteo Bachetti,Steven E. Boggs,Finn E. Christensen,William W. Craig,Brian W. Grefenstette,Charles J. Hailey,Fiona Harrison,Kristin K. Madsen,Jon M. Miller,Daniel Stern,Dominic Walton,William Zhang
Physics , 2013, DOI: 10.1088/0004-637X/780/2/133
Abstract: We present NuSTAR observations of Vela X-1, a persistent, yet highly variable, neutron star high-mass X-ray binary (HMXB). Two observations were taken at similar orbital phases but separated by nearly a year. They show very different 3-79 keV flux levels as well as strong variability during each observation, covering almost one order of magnitude in flux. These observations allow, for the first time ever, investigations on kilo-second time-scales of how the centroid energies of cyclotron resonant scattering features (CRSFs) depend on flux for a persistent HMXB. We find that the line energy of the harmonic CRSF is correlated with flux, as expected in the sub-critical accretion regime. We argue that Vela X-1 has a very narrow accretion column with a radius of around 0.4 km that sustains a Coulomb interaction dominated shock at the observed luminosities of Lx ~ 3x10^36 erg/s. Besides the prominent harmonic line at 55 keV the fundamental line around 25 keV is clearly detected. We find that the strengths of the two CRSFs are anti-correlated, which we explain by photon spawning. This anti-correlation is a possible explanation for the debate about the existence of the fundamental line. The ratio of the line energies is variable with time and deviates significantly from 2.0, also a possible consequence of photon spawning, which changes the shape of the line. During the second observation, Vela X-1 showed a short off-state in which the power-law softened and a cut-off was no longer measurable. It is likely that the source switched to a different accretion regime at these low mass accretion rates, explaining the drastic change in spectral shape.
High-Energy X-ray Imaging of the Pulsar Wind Nebula MSH~15-52: Constraints on Particle Acceleration and Transport
Hongjun An,Kristin K. Madsen,Stephen P. Reynolds,Victoria M. Kaspi,Fiona A. Harrison,Steven E. Boggs,Finn E. Christensen,William W. Craig,Chris L. Fryer,Brian W. Grefenstette,Charles J. Hailey,Kaya Mori,Daniel Stern,William W. Zhang
Physics , 2014, DOI: 10.1088/0004-637X/793/2/90
Abstract: We present the first images of the pulsar wind nebula (PWN) MSH 15-52 in the hard X-ray band (>8 keV), as measured with the Nuclear Spectroscopic Telescope Array (NuSTAR). Overall, the morphology of the PWN as measured by NuSTAR in the 3-7 keV band is similar to that seen in Chandra high-resolution imaging. However, the spatial extent decreases with energy, which we attribute to synchrotron energy losses as the particles move away from the shock. The hard-band maps show a relative deficit of counts in the northern region towards the RCW 89 thermal remnant, with significant asymmetry. We find that the integrated PWN spectra measured with NuSTAR and Chandra suggest that there is a spectral break at 6 keV which may be explained by a break in the synchrotron-emitting electron distribution at ~200 TeV and/or imperfect cross calibration. We also measure spatially resolved spectra, showing that the spectrum of the PWN softens away from the central pulsar B1509-58, and that there exists a roughly sinusoidal variation of spectral hardness in the azimuthal direction. We discuss the results using particle flow models. We find non-monotonic structure in the variation with distance of spectral hardness within 50" of the pulsar moving in the jet direction, which may imply particle and magnetic-field compression by magnetic hoop stress as previously suggested for this source. We also present 2-D maps of spectral parameters and find an interesting shell-like structure in the NH map. We discuss possible origins of the shell-like structure and their implications.
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