1,908 research outputs found
Electromagnetic signatures of supermassive black hole binaries resolved by PTAs
Pulsar timing arrays (PTAs) may eventually be able to detect not only the
stochastic gravitational-wave (GW) background of SMBH binaries, but also
individual, particularly massive binaries whose signals stick out above the
background. In this contribution, we discuss the possibility of identifying and
studying such `resolved' binaries through their electromagnetic emission. The
host galaxies of such binaries are themselves expected to be also very massive
and rare, so that out to redshifts z~2 a unique massive galaxy may be
identified as the host. At higher redshifts, the PTA error boxes are larger and
may contain as many as several hundred massive-galaxy interlopers. In this
case, the true counterpart may be identified, if it is accreting gas
efficiently, as an active galactic nucleus (AGN) with a peculiar spectrum and
variable emission features. Specifically, the binary's tidal torques expel the
gas from the inner part of the accretion disk, making it unusually dim in X-ray
and UV bands and in broad optical emission lines. The tails of the broad wings
of any FeK-alpha emission line may also be `clipped' and missing. The binary's
orbital motion, as well as the gas motions it induces, may trigger
quasiperiodic variations. These include coherent flux variability, such as
luminous, multi-wavelength flares, as well as Doppler shifts of broad emission
lines and `see-saw' oscillations in the FeK-alpha line. Additional features,
such as evidence for a recent major merger or dual collimated jets, could also
corroborate the counterpart. These properties would make resolved PTA sources
stand out among AGN with similar overall luminosities and allow their
identification.Comment: 18 pages, 5 figures, accepted for publication in a focus issue on
PTAs in Classical and Quantum Gravit
The effect of baryonic streaming motions on the formation of the first supermassive black holes
Observations of quasars at redshifts z > 6 reveal that 10^9 Msol supermassive
black holes (SMBHs) had already formed when the Universe was < 0.9 Gyr old. One
hypothesis for the origins of these SMBHs is that they grew from the remnants
of the first generation of massive stars, which formed in low-mass (~ 10^5 to
10^6 Msol) dark matter minihaloes at z > 20. This is the regime where baryonic
streaming motions--the relative velocities of baryons with respect to dark
matter in the early Universe--most strongly inhibit star formation by
suppressing gas infall and cooling. We investigate the impact of this effect on
the growth of the first SMBHs using a suite of high-fidelity,
ellipsoidal-collapse Monte Carlo merger-tree simulations. We find that the
suppression of seed BH formation by the streaming motions significantly reduces
the number density of the most massive BHs at z > 15, but the residual effect
at lower redshifts is essentially negligible. The streaming motions can reduce
by a factor of few the number density of the most luminous quasars at z ~
10-11, where such objects could be detected by the James Webb Space Telescope.
We conclude, with minor theoretical caveats, that baryonic streaming motions
are unlikely to pose a significant additional obstacle to the formation of the
observed high-redshift quasar SMBHs. Nor do they appreciably affect the heating
and reionization histories of the Universe or the merger rates of nuclear BHs
in the mass and redshift ranges of interest for proposed gravitational-wave
detectors.Comment: 9 pages, 5 figures, accepted for publication in MNRA
A simple Havel-Hakimi type algorithm to realize graphical degree sequences of directed graphs
One of the simplest ways to decide whether a given finite sequence of
positive integers can arise as the degree sequence of a simple graph is the
greedy algorithm of Havel and Hakimi. This note extends their approach to
directed graphs. It also studies cases of some simple forbidden edge-sets.
Finally, it proves a result which is useful to design an MCMC algorithm to find
random realizations of prescribed directed degree sequences.Comment: 11 pages, 1 figure submitted to "The Electronic Journal of
Combinatorics
Database management and implementation for coal fire detection and monitoring in the Ruijgou Coalfield, Northwest China
Coal fire monitoring by aerospace remote sensing and GIS techniques is an advanced approach, which has been implemented in the
Rujigou coalfield, northwest China. A database of several different types of data, including remote sensing images, results of
geological surveys, etc., has been collected and assembled in a PC-based information system, which is being developed by the
International Institute for Aerospace Survey and Earth Sciences (ITC). This coal fire monitoring and management information
system (CoalMan) comprises of two main parts: one is the database with its management tools, and the tools for data processing
and analysis form the other. This paper first introduces the structure of the CoalMan information system, presents the data types
and contents of CoalMan database, then shows the data management methods. A complex data analysis package is outlined too, as
the core of the information system
The three-dimensional carrier-envelope-phase map of focused few-cycle pulsed Gaussian beams
We derive an analytical expression that describes the complete
three-dimensional carrier-envelope phase (CEP) distribution of in the focal
volume of ultrashort pulsed Gaussian beams focused by spherical mirrors or
lenses. The focal CEP map depends on the so-called factor specifying the
frequency-dependence of the beam width of the source few-cycle pulse, on its
chirp and on the small chromatic aberration introduced by a lens without
appreciably distorting or broadening the few-cycle pulse. We show how to tailor
the CEP map of mirror-focused and lens-focused few-cycle pulses in order to
produce negligible transversal and axial CEP variations in specific regions of
the focal volume for phase-sensitive interactions of light with matter taking
place in a volume or on a surface. We propose a quasi-achromatic doublet lens
that can implement in practice these tailored CEP distributions.Comment: 9 pages, 6 figure
Parametric farm performance and efficiency methodology: Stochastic Frontier Analysis
There is a continuously growing literature on the agricultural transformation in Central and Eastern European countries (see some surveys in Brooks and Nash 2002; Rozelle and Swinnen 2004). The research has focused on various aspects of transition, including land reform, farm restructuring, price and trade liberalisation, but even though Farm Accountancy Data Network (FADN) data are now available for some years, there are only a few studies (e.g. Bakucs et al. 2010, Fogarasi and Latruffe, 2007, Baráth et al., 2009) focusing on Hungarian farm performance. The objective of this paper is to shed light on some methodological issues that are needed to study Hungarian farm performance. Here we consider one aspect of farm performance, namely technical efficiency. This measure refers to whether farmers are capable of using existing technology to its full potential by producing the most possible from a given set of production factor quantities
Modified DMRG algorithm for the zigzag spin-1/2 chain with frustrated antiferromagnetic exchange: Comparison with field theory at large
A modified density matrix renormalization group (DMRG) algorithm is applied
to the zigzag spin-1/2 chain with frustrated antiferromagnetic exchange ,
between first and second neighbors. The modified algorithm yields
accurate results up to for the magnetic gap to the
lowest triplet state, the amplitude of the bond order wave (BOW) phase, the
wavelength of the spiral phase, and the spin correlation length
. The dependences of , , and provide
multiple comparisons to field theories of the zigzag chain. The twist angle of
the spiral phase and the spin structure factor yield additional comparisons
between DMRG and field theory. Attention is given to the numerical accuracy
required to obtain exponentially small gaps or exponentially long correlations
near a quantum phase transition.Comment: 7 pages, 8 figures, submitted to PR
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