Publish in OALib Journal

ISSN: 2333-9721

APC: Only $99


Any time

2019 ( 343 )

2018 ( 609 )

2017 ( 643 )

2016 ( 660 )

Custom range...

Search Results: 1 - 10 of 331261 matches for " H. L. Marshall "
All listed articles are free for downloading (OA Articles)
Page 1 /331261
Display every page Item
Early Results from the Chandra X-ray Observatory
H. L. Marshall
Physics , 2000, DOI: 10.1016/S1387-6473(00)00098-1
Abstract: We present some early results on AGN from the Chandra X-ray Observatory, highlighting high resolution spectroscopy using the High Energy Transmission Grating Spectrometer (HETGS). The quasar PKS 0637-752 was found to have a very bright X-ray jet whose shape is remarkably similar to that of the radio jet on a size scale of 100 kpc, but the X-ray emission is still inexplicably bright. Two BL Lac objects, PKS 2155-304 and Mrk 421, observed with the spectrometer were found to have no strong absorption or emission features. Other radio loud AGN observed with the HETGS show simple power law spectra without obvious features.
Using the High Resolution X-ray Spectrum of PSR B0656+14 to Constrain the Chemical Composition of the Neutron Star Atmosphere
H. L. Marshall,N. S. Schulz
Physics , 2002, DOI: 10.1086/340835
Abstract: Observations of PSR B0656+14 using the Chandra Low Energy Transmission Grating Spectrometer are presented. The zeroth order events are pulsed at an amplitude of 10 +/- 2% and the image may be slightly extended. The extended emission is modelled as a Gaussian with a FWHM of about 0.75", for a linear size (at a distance of 760 pc) of 8.5e15 cm. In the absence of systematic errors in the detector point spread function, the extended emission comprises <~ 50% of the observed flux in the 0.2-2.0 keV band, for a luminosity of <~ 3e32 erg/s. The spectrum is well modelled by a dominant blackbody with T = 8.0e5 +/- 3e4 K and a size of 22.5 +/- 2.1 km in addition to a harder component that is modelled as a hotter and much smaller blackbody. No significant absorption features are found in the spectrum that might be expected from ionization edges of H or He or bound-bound transitions of Fe in magnetized atmospheres. Such features are expected to be deep but could vary in position or strength with rotation phase. There are no strong absorption features in the pulse-phased spectra, however, so we conclude that the atmosphere is not dominated by Fe or other heavy elements that would be partially ionized at a temperature of 1e6 K.
Flax shive as a sources of activated carbon for metals remediation
Marshall, W. E.,Wartelle, L. H.,Akin, D. E.
BioResources , 2007,
Abstract: Flax shive constitutes about 70% of the flax stem and has limited use. Because shive is a lignocellulosic by-product, it can potentially be pyrolyzed and activated to produce an activated carbon. The objective of this study was to create an activated carbon from flax shive by chemical activation in order to achieve significant binding of selected divalent cations (cadmium, calcium, copper, magnesium, nickel, zinc). Shive carbons activated by exposure to phosphoric acid and com-pressed air showed greater binding of cadmium, copper, nickel or zinc than a sulfuric acid-activated flax shive carbon reported in the literature and a commercial, wood-based carbon. Uptake of calcium from a drinking water sample by the shive carbon was similar to commercial drinking water filters that contained cation exchange resins. Magnesium removal by the shive carbon was greater than a commercial drinking water filtration carbon but less than for filters containing cation exchange resins. The results indicate that chemically activated flax shive carbon shows considerable promise as a component in industrial and residential water filtration systems for removal of divalent cations.
An Extreme Ultraviolet Explorer Atlas of Seyfert Galaxy Light Curves: Search for Periodicity
J. P. Halpern,K. M. Leighly,H. L. Marshall
Physics , 2002, DOI: 10.1086/346106
Abstract: The Deep Survey instrument on the Extreme Ultraviolet Explorer satellite (EUVE) obtained long, nearly continuous soft X-ray light curves of 5-33 days duration for 14 Seyfert galaxies and QSOs. We present a uniform reduction of these data, which account for a total of 231 days of observation. Several of these light curves are well suited to a search for periodicity or QPOs in the range of hours to days that might be expected from dynamical processes in the inner accretion disk around ~10^8 M_sun black holes. Light curves and periodograms of the three longest observations show features that could be transient periods: 0.89 days in RX J0437.4-4711, 2.08 days in Ton S180, and 5.8 days in 1H 0419-577. The statistical significance of these signals is estimated using the method of Timmer & Konig (1995), which carefully takes into account the red-noise properties of Seyfert light curves. The result is that the signals in RX J0437.4-4711 and Ton S180 exceed 95% confidence with respect to red noise, while 1H 0419-577 is only 64% significant. These period values appear unrelated to the length of the observation, which is similar in the three cases, but they do scale roughly as the luminosity of the object, which would be expected in a dynamical scenario if luminosity scales with black hole mass.
X-ray Timing of the Enigmatic Neutron Star RX J0720.4-3125
D. L. Kaplan,S. R. Kulkarni,M. H. van Kerkwijk,H. L. Marshall
Physics , 2002, DOI: 10.1086/341102
Abstract: RX J0720.4-3125 is the third brightest neutron star in the soft X-ray sky and has been a source of mystery since its discovery, as its long 8-s period separates it from the population of typical radio pulsars. Three models were proposed for this source: a neutron star accreting from the interstellar medium, an off-beam radio pulsar, or an old, cooling magnetar. Using data from Chandra, ROSAT, and BeppoSAX we are able to place an upper limit to the period derivative, $|\dot{P}| < 3.6\times10^{-13}{s s}^{-1}$ (3-$\sigma$). While our upper limit on $\dot P$ allows for the accretion model, this model is increasingly untenable for another similar but better studied neutron star, RX J1856.5-3754, and we therefore consider the accretion model unlikely for RX J0720.4-3125. We constrain the initial magnetic field of RX J0720.4-3125 to be $\lsim 10^{14}$ G based on cooling models, suggesting that it is not and never was a magnetar, but is instead middle-aged neutron star. We propose that it is either a long-period high-magnetic field pulsar with $\dot P\sim 10^{-13}{s s}^{-1}$ similar to PSR J1814-1744, or a neutron star born with an initial period of $\approx 8.3$ s and $\dot P\sim 10^{-15}{s s}^{-1}$. The proximity of RX J0720.4-3125 is strongly suggestive of a large population of such objects; if so, radio pulsar surveys must have missed many of these sources.
The Photoionized Accretion Disk in Her X-1
L. Ji,N. Schulz,M. Nowak,H. L. Marshall,T. Kallman
Physics , 2009, DOI: 10.1088/0004-637X/700/2/977
Abstract: We present an analysis of several high-resolution Chandra grating observations of the X-ray binary pulsar Her X-1. With a total exposure of 170 ks, the observations are separated by years and cover three combinations of orbital and super-orbital phases. Our goal is to determine distinct properties of the photoionized emission and its dependence on phase-dependent variations of the continuum. We find that the continua can be described by a partial covering model which above 2 keV is consistent with recent results from \rxte studies and at low energies is consistent with recent \xmm and \sax studies. Besides a powerlaw with fixed index, an additional thermal blackbody of 114 eV is required to fit wavelengths above 12 \AA ($\sim$ 1 keV). We find that likely all the variability is caused by highly variable absorption columns in the range (1 -- 3)$\times 10^{23}$ cm$^{-2}$. Strong Fe K line fluorescence in almost all observations reveals that dense, cool material is present not only in the outer regions of the disk but interspersed throughout the disk. Most spectra show strong line emission stemming from a photoionized accretion disk corona. We model the line emission with generic thermal plasma models as well as with the photoionization code XSTAR and investigate changes of the ionization balance with orbital and superorbital phases. Most accretion disk coronal properties such as disk radii, temperatures, and plasma densities are consistent with previous findings for the low state. We find that these properties change negligibly with respect to orbital and super-orbital phases. A couple of the higher energy lines exhibit emissivities that are significantly in excess of expectations from a static accretion disk corona.
The Quiescent Counterpart of the Soft Gamma-ray Repeater SGR 0526-66
S. R. Kulkarni,D. L. Kaplan,H. L. Marshall,D. A. Frail,T. Murakami,D. Yonetoku
Physics , 2002, DOI: 10.1086/346110
Abstract: It is now commonly believed that Soft gamma-ray repeaters (SGRs) and Anomalous X-ray pulsars (AXPs) are magnetars -- neutron stars powered by their magnetic fields. However, what differentiates these two seemingly dissimilar objects is, at present, unknown. We present Chandra observations of RX J052600.3-660433, the quiescent X-ray counterpart of SGR 0526-66. Restricting to a period range around 8-s, the period noted in the afterglow of the burst of 5 March 1979, we find evidence for a similar periodicity in two epochs of data, obtained 20 months apart, implying a period derivative of $6.6(5)e-11$ s/s which is similar to the period derivatives of the magnetars. The spectrum is best fitted by a combination of a black body and a power law. However, the photon index of the power law component, $\Gamma\sim 3$ -- intermediate to those of AXPs and SGRs. This continuum of $\Gamma$ leads us to suggest that the underlying physical parameter which differentiates SGRs from AXPs is manifested in the power law component. Two decades ago, SGR 0526-66 was a classical SGR whereas now it behaves like an AXP. Thus it is possible that the same object cycles between SGR and AXP state. We speculate that the main difference between AXPs and SGRs is the geometry of the $B$-fields and this geometry is time dependent. [Abstract truncated]
A Multiwavelength Study of Three Hybrid Blazars
E. C. Stanley,P. Kharb,M. L. Lister,H. L. Marshall,C. O'Dea,S. Baum
Physics , 2015,
Abstract: We present multiwavelength imaging observations of PKS 1045-188, 8C 1849+670, and PKS 2216-038, three radio-loud active galactic nuclei from the MOJAVE-Chandra Sample that straddle the Fanaroff-Riley (FR) boundary between low- and high-power jets. These hybrid sources provide an excellent opportunity to study jet emission mechanisms and the influence of the external environment. We used archival VLA observations, and new Hubble and Chandra observations to identify and study the spectral properties of five knots in PKS 1045-188, two knots in 8C 1849+670, and three knots in PKS 2216-038. For the seven X-ray visible knots, we constructed and fit the broadband spectra using synchrotron and inverse Compton/cosmic microwave background (IC/CMB) emission models. In all cases, we found that the lack of detected optical emission ruled out the X-ray emission from the same electron population that produces radio emission. All three sources have high total extended radio power, similar to that of FR II sources. We find this is in good agreement with previously studied hybrid sources, where high-power hybrid sources emit X-rays via IC/CMB and the low-power hybrid sources emit X-rays via synchrotron emission. This supports the idea that it is total radio power rather than FR morphology that determines the X-ray emission mechanism. We found no significant asymmetries in the diffuse X-ray emission surrounding the host galaxies. Sources PKS 1045-188 and 8C 1849+670 show significant differences in their radio and X-ray termination points, which may result from the deceleration of highly relativistic bulk motion.
Chandra Observations of Two High-Redshift Quasars
T. Fang,H. L. Marshall,G. L. Bryan,C. R. Canizares
Physics , 2001, DOI: 10.1086/321473
Abstract: We report the first high-resolution X-ray spectra of two high-redshift quasars, S5 0836+710 and PKS 2149-306, obtained with the Chandra High Energy Transmission Grating Spectrometer (HETGS). The primary goal of this observation is to use the high spectral resolving power of the HETGS to detect X-ray absorption produced by a hot intergalactic medium. The continuum of both quasars can be fitted by absorbed power laws. The power law photon index (Gamma) of S5 0836+710 is consistent with that found in previous observations with ROSAT and ASCA, while the power law of PKS 2149-306 is harder than found in a previous observation by ASCA. Excess continuum absorption above the Galactic value is found in S5 0836+710, as evidenced in ASCA and ROSAT observations. No significant emission or absorption feature is detected in either source at 3 sigma level. Based on the detection limits we constrain the properties of possible emitters and absorbers. The upper limit of the equivalent width of Fe K emission lines could be as low as ~ 10 eV. Absorbers with a column density higher than 8e16 cm^{-2} for O VIII or 5e16 cm^{-2} for Si XIV would have been detected. We propose a method to constrain the cosmological parameters (namely Omega_0 and Omega_b) via the X-ray forest theory, but current data do not give significant constraints. However, we estimate that observing ~ 7 bright quasars should give at least one O VIII and more O VII absorption lines at 95% possibility. We also find that combined with the constraints from the distortion of the CMB spectrum, the X-ray Gunn-Peterson test can marginally constrain a uniform, enriched IGM.
Tissue Specific Deletion of Inhibitor of Kappa B Kinase 2 with OX40-Cre Reveals the Unanticipated Expression from the OX40 Locus in Skin Epidermis
Georgina H. Cornish, Sim L. Tung, Daniel Marshall, Steve Ley, Benedict P. Seddon
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0032193
Abstract: NF-κB signalling plays an essential role in T cell activation and generation of regulatory and memory populations in vivo. In the present study, we aimed to investigate the role of NF-κB signalling in post-activation T cells using tissue specific ablation of inhibitor of kappa-B kinase 2 expression, an important component of the inhibitor of kappa-B kinase complex in canonical NF-κB signalling. The OX40 antigen is expressed on activated T cells. Therefore, we used previously described mouse strain expressing Cre recombinase from the endogenous OX40 locus. Ablation of IKK2 expression using OX40Cre mice resulted in the development of an inflammatory response in the skin epidermis causing wide spread skin lesions. The inflammatory response was characterised by extensive leukocytic infiltrate in skin tissue, hyperplasia of draining lymph nodes and widespread activation in the T cell compartment. Surprisingly, disease development did not depend on T cells but was rather associated with an unanticipated expression of Cre in skin epidermis, and activation of the T cell compartment did not require Ikbk2 deletion in T cells. Employment of Cre reporter strains revealed extensive Cre activity in skin epidermis. Therefore, development of skin lesions was rather more likely explained by deletion of Ikbk2 in skin keratinocytes in OX40Cre mice.
Page 1 /331261
Display every page Item

Copyright © 2008-2017 Open Access Library. All rights reserved.