30,773 research outputs found
Gamma-Ray Burst Afterglows with Energy Injection: Homogeneous Versus Wind External Media
Assuming an adiabatic evolution of a gamma-ray burst (GRB) fireball
interacting with an external medium, we calculate the hydrodynamics of the
fireball with energy injection from a strongly magnetic millisecond pulsar
through magnetic dipole radiation, and obtain the light curve of the optical
afterglow from the fireball by synchrotron radiation. Results are given both
for a homogeneous external medium and for a wind ejected by GRB progenitor. Our
calculations are also available in both ultra-relativistic and non-relativistic
phases. Furthermore, the observed R-band light curve of GRB{000301C} can be
well fitted in our model, which might provide a probe of the properties of GRB
progenitors.Comment: revised version for publication in Chin. Phys. Let
Computation over multi-access channels : multi-hop implementation and resource allocation
For future wireless networks, enormous numbers of interconnections are required, creating a multi-hop topology and leading to a great challenge on data aggregation. Instead of collecting data individually, a more efficient technique, computation over multi-access channels (CoMAC), has emerged to compute functions by exploiting the signal-superposition property of wireless channels. However, it is still an open problem on the implementation of CoMAC in multi-hop wireless networks considering fading channel and resource allocation. In this paper, we propose multi-layer CoMAC (ML-CoMAC) by combining CoMAC and orthogonal communication to compute functions in the multi-hop network. Firstly, to make the multi-hop network more tractable, we reorganize it into a hierarchical network with multiple layers that consists of subgroups and groups. Then, in the hierarchical network, the implementation of ML-CoMAC is given by computing and communicating subgroup and group functions over layers, where CoMAC is applied to compute each subgroup function and orthogonal communication is adopted for each group to obtain the group function. The general computation rate is derived and the performance is further improved through time allocation and power control. The closed-form solutions to optimization problems are obtained, which suggests that orthogonal communication and existing CoMAC schemes are generalized
The Evolution of Universe with th B-I Type Phantom Scalar Field
We considered the phantom cosmology with a lagrangian ,
which is original from the nonlinear Born-Infeld type scalar field with the
lagrangian . This cosmological model can explain the
accelerated expansion of the universe with the equation of state parameter
. We get a sufficient condition for a arbitrary potential to admit a
late time attractor solution: the value of potential at the critical
point should be maximum and large than zero. We study a specific
potential with the form of
via phase plane
analysis and compute the cosmological evolution by numerical analysis in
detail. The result shows that the phantom field survive till today (to account
for the observed late time accelerated expansion) without interfering with the
nucleosynthesis of the standard model(the density parameter
at the equipartition epoch), and also avoid the
future collapse of the universe.Comment: 17 pages, 10 figures,typos corrected, references added,figures added
and enriched, title changed, main result remaine
Determinants of immediate price impacts at the trade level in an emerging order-driven market
The common wisdom argues that, in general, large trades cause large price
changes, while small trades cause small price changes. However, for extremely
large price changes, the trade size and news play a minor role, while the
liquidity (especially price gaps on the limit order book) is a more influencing
factor. Hence, there might be other influencing factors of immediate price
impacts of trades. In this paper, through mechanical analysis of price
variations before and after a trade of arbitrary size, we identify that the
trade size, the bid-ask spread, the price gaps and the outstanding volumes at
the bid and ask sides of the limit order book have impacts on the changes of
prices. We propose two regression models to investigate the influences of these
microscopic factors on the price impact of buyer-initiated partially filled
trades, seller-initiated partially filled trades, buyer-initiated filled
trades, and seller-initiated filled trades. We find that they have
quantitatively similar explanation powers and these factors can account for up
to 44% of the price impacts. Large trade sizes, wide bid-ask spreads, high
liquidity at the same side and low liquidity at the opposite side will cause a
large price impact. We also find that the liquidity at the opposite side has a
more influencing impact than the liquidity at the same side. Our results shed
new lights on the determinants of immediate price impacts.Comment: 21 IOP tex pages including 5 figures and 5 tables. Accepted for
publication in New Journal of Physic
A New Spin-Orbit Induced Universality Class in the Quantum Hall Regime ?
Using heuristic arguments and numerical simulations it is argued that the
critical exponent describing the localization length divergence at the
quantum Hall transition is modified in the presence of spin-orbit scattering
with short range correlations. The exponent is very close to , the
percolation correlation length exponent, the prediction of a semi-classical
argument. In addition, a region of weakly localized regime, where the
localization length is exponentially large, is conjectured.Comment: 4 two-column pages including 4 eps figure
A Comprehensive Analysis of Swift/XRT Data: III. Jet Break Candidates in X-ray and Optical Afterglow Lightcurves
The Swift/XRT data of 179 GRBs (from 050124 to 070129) and the optical
afterglow data of 57 pre- and post-Swift GRBs are analyzed, in order to
systematically investigate the jet-like breaks in the X-ray and optical
afterglow lightcurves. We find that not a single burst can be included in the
``Platinum'' sample, in which the data satisfy all the criteria of a jet break.
By releasing one or more requirements to define a jet break, some candidates of
various degrees could be identified. In the X-ray band, 42 out of the 103
well-sampled X-ray lightcurves have a decay slope of the post-break segment
>1.5 (``Bronze'' sample), and 27 of them also satisfy the closure relations of
the forward models (``Silver'' sample). The numbers of the ``Bronze'' and
``Silver'' candidates in the optical lightcurves are 27 and 23, respectively.
Thirteen bursts have well-sampled optical and X-ray lightcurves, but only seven
cases are consistent with an achromatic break, but even in these cases only one
band satisfies the closure relations (``Gold'' sample). The observed break time
in the XRT lightcurves is systematically earlier than that in the optical
bands. All these raise great concerns in interpreting the jet-like breaks as
jet breaks and further inferring GRB energetics from these breaks. By assuming
that these breaks are jet breaks, we perform a similar analysis as previous
work to calculate the jet opening angle (theta_j) and energetics (E_k) with the
``Silver'' and ``Gold'' jet break candidates. The derived E_K distribution
reveals a much larger scatter than the pre-Swift sample. A tentative
anti-correlation between theta_j and E_{K,iso} is found for both the pre-Swift
and Swift GRBs, indicating that the E_K could still be quasi-universal, if the
breaks in discussion are indeed jet breaks(abridge).Comment: 48 pages, including 5 tables and 8 figures. Accepted for publication
in ApJ. This is the third paper of a series. Paper I and II see
astro-ph/0612246 (ApJ, 2007, 666,1002) and arXiv:0705.1373 (ApJ, 2007, 669,
n2,in press
Emergence of long memory in stock volatility from a modified Mike-Farmer model
The Mike-Farmer (MF) model was constructed empirically based on the
continuous double auction mechanism in an order-driven market, which can
successfully reproduce the cubic law of returns and the diffusive behavior of
stock prices at the transaction level. However, the volatility (defined by
absolute return) in the MF model does not show sound long memory. We propose a
modified version of the MF model by including a new ingredient, that is, long
memory in the aggressiveness (quantified by the relative prices) of incoming
orders, which is an important stylized fact identified by analyzing the order
flows of 23 liquid Chinese stocks. Long memory emerges in the volatility
synthesized from the modified MF model with the DFA scaling exponent close to
0.76, and the cubic law of returns and the diffusive behavior of prices are
also produced at the same time. We also find that the long memory of order
signs has no impact on the long memory property of volatility, and the memory
effect of order aggressiveness has little impact on the diffusiveness of stock
prices.Comment: 6 pages, 6 figures and 1 tabl
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