41,629 research outputs found
The z>4 Quasar Population Observed by Chandra and XMM-Newton
The current status of our Chandra and XMM-Newton project on high-redshift
(z>4) quasars is briefly reviewed. We report the main results obtained in the
last few years for the detected quasars, along with a few (~10%) intriguing
cases where no detection has been obtained with Chandra snapshot observations.Comment: 4 pages, to appear in the proceedings of 'Multiwavelength AGN
surveys' (Cozumel, December 8-12 2003), ed. R. Maiolino and R. Mujic
Entry corridor definition and SM/RCS deorbit requirements for Apollo block 1 earth orbit missions. Project Apollo
Entry corridor definition and SM reaction control system for Apollo Block 1 earth orbit mission
Integrating hot and cool intelligences: Thinking Broadly about Broad Abilities
Although results from factor-analytic studies of the broad, second-stratum abilities of human intelligence have been fairly consistent for decades, the list of broad abilities is far from complete, much less understood. We propose criteria by which the list of broad abilities could be amended and envision alternatives for how our understanding of the hot intelligences (abilities involving emotionally-salient information) and cool intelligences (abilities involving perceptual processing and logical reasoning) might be integrated into a coherent theoretical framework
Pressure and isotope effect on the anisotropy of MgB
We analyze the data for the pressure and boron isotope effect on the
temperature dependence of the magnetization near . Invoking the
universal scaling relation for the magnetization at fixed magnetic field it is
shown that the relative shift of , induced by pressure or boron isotope
exchange, mirrors essentially that of the anisotropy. This uncovers a novel
generic property of anisotropic type II superconductors, inexistent in the
isotropic case. For MgB it implies that the renormalization of the Fermi
surface topology due to pressure or isotope exchange is dominated by a
mechanism controlling the anisotropy.Comment: 7 pages, 3 figure
The X-ray Properties of the Most-Luminous Quasars from the Sloan Digital Sky Survey
Utilizing 21 new Chandra observations as well as archival Chandra, ROSAT, and
XMM-Newton data, we study the X-ray properties of a representative sample of 59
of the most optically luminous quasars in the Universe (M_i~~-29.3 to -30.2)
spanning a redshift range of z~~1.5-4.5. Our full sample consists of 32 quasars
from the Sloan Digital Sky Survey (SDSS) Data Release 3 (DR3) quasar catalog,
two additional objects in the DR3 area that were missed by the SDSS selection
criteria, and 25 comparably luminous quasars at z>~4. This is the largest X-ray
study of such luminous quasars to date. By jointly fitting the X-ray spectra of
our sample quasars, excluding radio-loud and broad absorption line (BAL)
objects, we find a mean X-ray power-law photon index of
Gamma=1.92^{+0.09}_{-0.08} and constrain any neutral intrinsic absorbing
material to have a mean column density of N_H<~2x10^{21} cm^{-2}. We find,
consistent with other studies, that Gamma does not change with redshift, and we
constrain the amount of allowed Gamma evolution for the most-luminous quasars.
Our sample, excluding radio-loud and BAL quasars, has a mean X-ray-to-optical
spectral slope of a_ox=-1.80+/-0.02, as well as no significant evolution of
a_ox with redshift. We also comment upon the X-ray properties of a number of
notable quasars, including an X-ray weak quasar with several strong narrow
absorption-line systems, a mildly radio-loud BAL quasar, and a well-studied
gravitationally lensed quasar.Comment: 18 pages (emulateapj), 11 figures. Accepted for publication in The
Astrophysical Journa
Excitation Induced Dephasing in Semiconductor Quantum Dots
A quantum kinetic theory is used to compute excitation induced dephasing in
semiconductor quantum dots due to the Coulomb interaction with a continuum of
states, such as a quantum well or a wetting layer. It is shown that a frequency
dependent broadening together with nonlinear resonance shifts are needed for a
microscopic explanation of the excitation induced dephasing in such a system,
and that excitation induced dephasing for a quantum-dot excitonic resonance is
different from quantum-well and bulk excitons.Comment: 6 pages, 4 figures. Extensively revised text, two figures change
Comment on "Fano Resonance for Anderson Impurity Systems"
In a recent Letter, Luo et al. (Phys. Rev. Lett. 92, 256602 (2004)) analyze
the Fano line shapes obtained from scanning tunneling spectroscopy (STS) of
transition metal impurities on a simple metal surface, in particular of the
Ti/Au(111) and Ti/Ag(100) systems. As the key point of their analysis, they
claim that there is not only a Fano interference effect between the impurity
d-orbital and the conduction electron continuum, as derived in Ujsaghy et al.
(Phys. Rev. Lett. 85, 2557 (2000)), but that the Kondo resonance in the
d-electron spectral density has by itself a second Fano line shape, leading to
the experimentally observed spectra. In the present note we point out that this
analysis is conceptually incorrect. Therefore, the quantitative agreement of
the fitted theoretical spectra with the experimental results is meaningless.Comment: 1 page, no figures. Accepted for publication in PRL; revised version
uploaded on November 18th, 200
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