31,555 research outputs found
XMM-Newton Observations of High Redshift Quasars
We report on our XMM observations of the high redshift quasars BR 2237--0607
(z=4.558) and BR 0351--1034 (z=4.351), together with 14 other z>4 objects found
in the XMM public archive. Contrary to former reports, we do not find high
redshift radio-loud quasars to be more absorbed than their radio-quiet
counterparts. We find that the optical to X-ray spectral index alpha-ox is
correlated with the luminosity density at 2500 A, but does not show a
correlation with redshift. The mean 2-10 keV power-law slope of the 9 high
redshift radio-quiet quasars in our sample for which a spectral analysis can be
performed is alpha-x1.23+-0.48, similar to alpha-x=1.19 found from the ASCA
observations of low redshift Narrow-Line Seyfert 1 galaxies (NLS1s), and
significantly different from alpha-x=0.78 found for low redshift Broad-Line
Seyfert galaxies. While the optical/UV spectra of low to high redshift quasars
look remarkably similar, we find a first indication of a difference in their
X-ray spectrum. The steep X-ray spectral index suggests high Eddington ratios
L/L_Edd. These observations give credence to the hypothesis of Mathur (2000)
that NLS1s are low luminosity cousins of high redshift quasars, both likely to
be in their early evolutionary stage.Comment: 25 pages, AJ, in press (Jan 2006
On The Orbital Evolution of Jupiter Mass Protoplanet Embedded in A Self-Gravity Disk
We performed a series of hydro-dynamic simulations to investigate the orbital
migration of a Jovian planet embedded in a proto-stellar disk. In order to take
into account of the effect of the disk's self gravity, we developed and adopted
an \textbf{Antares} code which is based on a 2-D Godunov scheme to obtain the
exact Reimann solution for isothermal or polytropic gas, with non-reflecting
boundary conditions. Our simulations indicate that in the study of the runaway
(type III) migration, it is important to carry out a fully self consistent
treatment of the gravitational interaction between the disk and the embedded
planet. Through a series of convergence tests, we show that adequate numerical
resolution, especially within the planet's Roche lobe, critically determines
the outcome of the simulations. We consider a variety of initial conditions and
show that isolated, non eccentric protoplanet planets do not undergo type III
migration. We attribute the difference between our and previous simulations to
the contribution of a self consistent representation of the disk's self
gravity. Nevertheless, type III migration cannot be completely suppressed and
its onset requires finite amplitude perturbations such as that induced by
planet-planet interaction. We determine the radial extent of type III migration
as a function of the disk's self gravity.Comment: 19 pages, 13 figure
Non-damping oscillations at flaring loops
Context. QPPs are usually detected as spatial displacements of coronal loops
in imaging observations or as periodic shifts of line properties in
spectroscopic observations. They are often applied for remote diagnostics of
magnetic fields and plasma properties on the Sun. Aims. We combine imaging and
spectroscopic measurements of available space missions, and investigate the
properties of non-damping oscillations at flaring loops. Methods. We used the
IRIS to measure the spectrum over a narrow slit. The double-component Gaussian
fitting method was used to extract the line profile of Fe XXI 1354.08 A at "O
I" window. The quasi-periodicity of loop oscillations were identified in the
Fourier and wavelet spectra. Results. A periodicity at about 40 s is detected
in the line properties of Fe XXI, HXR emissions in GOES 1-8 A derivative, and
Fermi 26-50 keV. The Doppler velocity and line width oscillate in phase, while
a phase shift of about Pi/2 is detected between the Doppler velocity and peak
intensity. The amplitudes of Doppler velocity and line width oscillation are
about 2.2 km/s and 1.9 km/s, respectively, while peak intensity oscillate with
amplitude at about 3.6% of the background emission. Meanwhile, a quasi-period
of about 155 s is identified in the Doppler velocity and peak intensity of Fe
XXI, and AIA 131 A intensity. Conclusions. The oscillations at about 40 s are
not damped significantly during the observation, it might be linked to the
global kink modes of flaring loops. The periodicity at about 155 s is most
likely a signature of recurring downflows after chromospheric evaporation along
flaring loops. The magnetic field strengths of the flaring loops are estimated
to be about 120-170 G using the MHD seismology diagnostics, which are
consistent with the magnetic field modeling results using the flux rope
insertion method.Comment: 9 pages, 9 figures, 1 table, accepted by A&
Measuring topic network centrality for identifying technology and technological development in online communities
Online communities are a rapidly growing knowledge repository that provides scholarly research, technical discussion, and social interactivity. This abundance of online information increases the difficulty of keeping up with new developments difficult for researchers and practitioners. Thus, we introduced a novel method that analyses both knowledge and social sentiment within the online community to discover the topical coverage of emerging technology and trace technological trends. The method utilizes the Weibull distribution and Shannon entropy to measure and link social sentiment with technological topics. Based on question-and-answer and social sentiment data from Zhihu, which is an online question and answer (Q&A) community with high-profile entrepreneurs and public intellectuals, we built an undirected weighting network and measured the centrality of nodes for technology identification. An empirical study on artificial intelligence technology trends supported by expert knowledge-based evaluation and cognition provides sufficient evidence of the method's ability to identify technology. We found that the social sentiment of hot technological topics presents a long-tailed distribution statistical pattern. High similarity between the topic popularity and emerging technology development trends appears in the online community. Finally, we discuss the findings in various professional fields that are widely applied to discover and track hot technological topics
Ground states of hard-core bosons in one dimensional periodic potentials
With Girardeau's Fermi-Bose mapping, we find the exact ground states of
hard-core bosons residing in a one dimensional periodic potential. The analysis
of these ground states shows that when the number of bosons is commensurate
with the number of wells in the periodic potential, the boson system is a
Mott insulator whose energy gap, however, is given by the single-particle band
gap of the periodic potential; when is not commensurate with , the
system is a metal (not a superfluid). In fact, we argue that there may be no
superfluid phase for any one-dimensional boson system in terms of Landau's
criterion of superfluidity. The Kronig-Penney potential is used to illustrate
our results.Comment: 6 pages, 6 figure
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