243 research outputs found
Imaging Large Scale Structure in the X-ray Sky
We present the first results from a wide solid angle, moderately deep {\it
Chandra} survey of the Lockman Hole North-West region. Our 9 ACIS-I fields
cover an effective solid angle of 0.4 deg and reach a depth of \ergpcmsqps in the 0.4--2 keV band and
\ergpcmsqps in the 2--8 keV band. The best fit logN-logS for the entire field,
the largest contiguous {\it Chandra} field yet observed, matches well onto that
of the {\it Chandra} Deep Field North. We show that the full range of the
`cosmic variance' previously seen in different {\it Chandra} fields is
reproduced in this small region of the sky. Counts-in-cells analysis shows that
the hard band sources are more strongly correlated than the soft band sources.Comment: To be published in ApJL v58
Biases in Virial Black Hole Masses: An SDSS Perspective
We compile black hole (BH) masses for quasars in the redshift
range included in the Fifth Data Release of the
Sloan Digital Sky Survey (SDSS), using virial BH mass estimators based on the
\hbeta, \MgII, and \CIV emission lines. We find that: (1) within our sample,
the widths of the three lines follow log-normal distributions, with means and
dispersions that do not depend strongly on luminosity or redshift;(2) the
\MgII- and \hbeta-estimated BH masses are consistent with one another; and (3)
the \CIV BH mass estimator may be more severely affected by a disk wind
component than the \MgII and \hbeta estimators, giving a positive bias in mass
correlated with the \CIV-\MgII blueshift. Most SDSS quasars have virial BH
masses in the range . There is a clear upper mass limit of
for active BHs at , decreasing at lower
redshifts. Making the reasonable assumptions that the underlying BH mass
distribution decreases with mass and that the Eddington ratio distribution at
fixed BH mass has non-zero width, we show that the measured virial BH mass
distribution and Eddington ratio distribution are subject to Malmquist bias. A
radio quasar subsample (with ) has mean virial BH
mass larger by dex than the whole sample. A broad absorption line
(BAL) quasar subsample (with ) has identical virial
mass distribution as the nonBAL sample, with no mean offset. (Abridged)Comment: Updated virial mass measurements; improved presentation of the MC
simulation; added new discussion sections; conclusions unchanged. The full
table1 is available at
http://www.astro.princeton.edu/~yshen/BH_mass/datafile1.txt.tar.g
Constraining the Lifetime of Quasars from their Spatial Clustering
The lifetime t_Q of the luminous phase of quasars is constrained by current
observations to be between 10^6 and 10^8 years, but is otherwise unkown. We
model the quasar luminosity function in detail in the optical and X-ray bands
using the Press-Schechter formalism, and show that the expected clustering of
quasars depends strongly on their assumed lifetime. We quantify this
dependence, and find that existing measurements of the correlation length of
quasars are consistent with the range 10^6 < t_Q < 10^8 years. We then show
that future measurements of the power spectrum of quasars out to z=3, from the
2dF or Sloan Digital Sky Survey, can significantly improve this constraint, and
in principle allow a precise determination of t_Q. We estimate the systematic
errors introduced by uncertainties in the modeling of the quasar-halo
relationship, as well as by the possible existence of obscured quasars.Comment: ApJ, in press (emulateapj
Quantum phase transition to unconventional multi-orbital superfluidity in optical lattices
Orbital physics plays a significant role for a vast number of important
phenomena in complex condensed matter systems such as high-T
superconductivity and unconventional magnetism. In contrast, phenomena in
superfluids -- especially in ultracold quantum gases -- are commonly well
described by the lowest orbital and a real order parameter. Here, we report on
the observation of a novel multi-orbital superfluid phase with a {\it complex}
order parameter in binary spin mixtures. In this unconventional superfluid, the
local phase angle of the complex order parameter is continuously twisted
between neighboring lattice sites. The nature of this twisted superfluid
quantum phase is an interaction-induced admixture of the p-orbital favored by
the graphene-like band structure of the hexagonal optical lattice used in the
experiment. We observe a second-order quantum phase transition between the
normal superfluid (NSF) and the twisted superfluid phase (TSF) which is
accompanied by a symmetry breaking in momentum space. The experimental results
are consistent with calculated phase diagrams and reveal fundamentally new
aspects of orbital superfluidity in quantum gas mixtures. Our studies might
bridge the gap between conventional superfluidity and complex phenomena of
orbital physics.Comment: 5 pages, 4 figure
On the Cosmological Evolution of the Luminosity Function and the Accretion Rate of Quasars
We consider a class of models for the redshift evolution (between 0\lsim z
\lsim 4) of the observed optical and X-ray quasar luminosity functions (LFs),
with the following assumptions: (i) the mass-function of dark matter halos
follows the Press-Schechter theory, (ii) the black hole (BH) mass scales
linearly with the halo mass, (iii) quasars have a constant universal lifetime,
and (iv) a thin accretion disk provides the optical luminosity of quasars,
while the X-ray/optical flux ratio is calibrated from a sample of observed
quasars. The mass accretion rate onto quasar BHs is a free parameter
of the models, that we constrain using the observed LFs. The accretion rate
inferred from either the optical or X-ray data under these assumptions
generally decreases as a function of cosmic time from to . We find that a comparable accretion rate is inferred from the X-ray and
optical LF only if the X-ray/optical flux ratio decreases with BH mass. Near
, drops to substantially sub-Eddington values at which
advection-dominated accretion flows (ADAFs) exist. Such a decline of ,
possibly followed by a transition to radiatively inefficient ADAFs, could
explain both the absence of bright quasars in the local universe and the
faintness of accreting BHs at the centers of nearby galaxies. We argue that a
decline of the accretion rate of the quasar population is indeed expected in
cosmological structure formation models.Comment: Latex, 23 pages, 9 figures, accepted for publication in Ap
Creating, moving and merging Dirac points with a Fermi gas in a tunable honeycomb lattice
Dirac points lie at the heart of many fascinating phenomena in condensed
matter physics, from massless electrons in graphene to the emergence of
conducting edge states in topological insulators [1, 2]. At a Dirac point, two
energy bands intersect linearly and the particles behave as relativistic Dirac
fermions. In solids, the rigid structure of the material sets the mass and
velocity of the particles, as well as their interactions. A different, highly
flexible approach is to create model systems using fermionic atoms trapped in
the periodic potential of interfering laser beams, a method which so far has
only been applied to explore simple lattice structures [3, 4]. Here we report
on the creation of Dirac points with adjustable properties in a tunable
honeycomb optical lattice. Using momentum-resolved interband transitions, we
observe a minimum band gap inside the Brillouin zone at the position of the
Dirac points. We exploit the unique tunability of our lattice potential to
adjust the effective mass of the Dirac fermions by breaking inversion symmetry.
Moreover, changing the lattice anisotropy allows us to move the position of the
Dirac points inside the Brillouin zone. When increasing the anisotropy beyond a
critical limit, the two Dirac points merge and annihilate each other - a
situation which has recently attracted considerable theoretical interest [5-9],
but seems extremely challenging to observe in solids [10]. We map out this
topological transition in lattice parameter space and find excellent agreement
with ab initio calculations. Our results not only pave the way to model
materials where the topology of the band structure plays a crucial role, but
also provide an avenue to explore many-body phases resulting from the interplay
of complex lattice geometries with interactions [11, 12]
Topological orbital ladders
We unveil a topological phase of interacting fermions on a two-leg ladder of
unequal parity orbitals, derived from the experimentally realized double-well
lattices by dimension reduction. topological invariant originates simply
from the staggered phases of -orbital quantum tunneling, requiring none of
the previously known mechanisms such as spin-orbit coupling or artificial gauge
field. Another unique feature is that upon crossing over to two dimensions with
coupled ladders, the edge modes from each ladder form a parity-protected flat
band at zero energy, opening the route to strongly correlated states controlled
by interactions. Experimental signatures are found in density correlations and
phase transitions to trivial band and Mott insulators.Comment: 12 pages, 5 figures, Revised title, abstract, and the discussion on
Majorana numbe
Increase Productivity Through Knowledge Management
Increase in competition level requires companies to improve the efficiency of work force use characterized by labor productivity. Professional knowledge of staff and its experience play the key role in it. The results of Extrusion Line operator's working time analysis are performed in this article. The analysis revealed that the reasons of working time ineffective use connected with inadequate information exchange and knowledge management in the company. Authors suggest the way to solve this problem: the main sources of knowledge in engineering enterprise have been defined, the conditions of success and the stages of knowledge management control have been stated
The Cosmic Evolution of Hard X-ray Selected Active Galactic Nuclei
We use highly spectroscopically complete deep and wide-area Chandra surveys
to determine the cosmic evolution of hard X-ray-selected AGNs. We determine
hard X-ray luminosity functions (HXLFs) for all spectral types and for
broad-line AGNs (BLAGNs) alone. At z<1.2, both are well described by pure
luminosity evolution. Thus, all AGNs drop in luminosity by almost an order of
magnitude over this redshift range. We show that this observed drop is due to
AGN downsizing. We directly compare our BLAGN HXLFs with the optical QSO LFs
and find that the optical QSO LFs do not probe faint enough to see the downturn
in the BLAGN HXLFs. We rule out galaxy dilution as a partial explanation for
the observation that BLAGNs dominate the number densities at the higher X-ray
luminosities, while optically-narrow AGNs (FWHM<2000 km/s) dominate at the
lower X-ray luminosities by measuring the nuclear UV/optical properties of the
Chandra sources using the HST ACS GOODS-North data. The UV/optical nuclei of
the optically-narrow AGNs are much weaker than expected if they were similar to
the BLAGNs. We therefore postulate the need for a luminosity dependent unified
model. Alternatively, the BLAGNs and the optically-narrow AGNs could be
intrinsically different source populations. We cover both interpretations by
constructing composite spectral energy distributions--including long-wavelength
data from the MIR to the submillimeter--by spectral type and by X-ray
luminosity. We use these to infer the bolometric corrections (from hard X-ray
luminosities to bolometric luminosities) needed to map the accretion history.
We determine the accreted supermassive black hole mass density for all spectral
types and for BLAGNs alone using the observed evolution of the hard X-ray
energy density production rate and our inferred bolometric corrections.Comment: 36 pages, Accepted by The Astronomical Journal (scheduled for Feb
2005), Figure 15a-c greyscale images can be found at
http://www.astro.wisc.edu/~barger/barger.fig15a.jpeg et
- …