2,947 research outputs found
On the Shape of Pulse Spectra in Gamma-Ray Bursts
The discovery (Liang & Kargatis 1996), that the peak energy of time-resolved
spectra of gamma-ray burst (GRB) pulses decays exponentially with fluence, is
analytically shown to imply that the time-integrated photon number spectrum of
a pulse should have a unique shape, given by an underlying E^-1 behavior. We
also show that the asymptotic low energy normalization of the time-integrated
spectrum is equal to the exponential decay constant. We study analytically how
this general behavior is modified in more realistic situations and show that
diversity is then introduced in the properties of time-integrated GRB pulse
spectra. We argue that further diversity will occur in time-integrated
multi-pulse (complex) GRB spectra. The total energy received per cm^2 is
approximately the decay constant times the maximum peak energy of the pulse.
Our analytical results connect the properties of the time-integrated pulse
spectrum with those of the time-resolved spectra, and can thus be used when
studying observed GRB pulse spectra. We illustrate with the bright burst GRB
910807 and comment on GRB 910525 and GRB 921207.Comment: 7 pages, 6 postscript figures, accepted by the Astrophysical Journa
On the Time Evolution of Gamma-Ray Burst Pulses: A Self-Consistent Description
For the first time, the consequences of combining two well-established
empirical relations, describing different aspects of the spectral evolution of
observed gamma-ray burst (GRB) pulses, are explored. These empirical relations
are: i) the hardness-intensity correlation, and ii) the hardness-photon fluence
correlation. From these we find a self-consistent, quantitative, and compact
description for the temporal evolution of pulse decay phases within a GRB light
curve. In particular, we show that in the case of the two empirical relations
both being valid, the instantaneous photon flux (intensity) must behave as
1/(1+ t/\tau) where \tau is a time constant that can be expressed in terms of
the parameters of the two empirical relations. The time evolution is fully
defined by two initial constants, and two parameters. We study a complete
sample of 83 bright GRB pulses observed by the Compton Gamma-Ray Observatory
and identify a major subgroup of GRB pulses (~45 %), which satisfy the
spectral-temporal behavior described above. In particular, the decay phase
follows a reciprocal law in time. It is unclear what physics causes such a
decay phase.Comment: 4 pages, 1 figure, 2 tables, to appear in ApJ
Testing the predictability and efficiency of securitized real estate markets
This paper conducts tests of the random walk hypothesis and market efficiency for 14 national public real estate markets. Random walk properties of equity prices influence the return dynamics and determine the trading strategies of investors. To examine the stochastic properties of local real estate index returns and to test the hypothesis that public real estate stock prices follow a random walk, the single variance ratio tests of Lo and MacKinlay (1988) as well as the multiple variance ratio test of Chow and Denning (1993) are employed. Weak-form market efficiency is tested directly using non-parametric runs tests. Empirical evidence shows that weekly stock prices in major securitized real estate markets do not follow a random walk. The empirical findings of return predictability suggest that investors might be able to develop trading strategies allowing them to earn excess returns compared to a buy-and-hold strategy. --Securitized real estate,weak-form market efficiency,random walk hypothesis,variance ratio tests,runs test,trading strategies
Complex-Path Prediction of Resonance-Assisted Tunneling in Mixed Systems
We present a semiclassical prediction of regular-to-chaotic tunneling in
systems with a mixed phase space, including the effect of a nonlinear resonance
chain. We identify complex paths for direct and resonance-assisted tunneling in
the phase space of an integrable approximation with one nonlinear resonance
chain. We evaluate the resonance-assisted contribution analytically and give a
prediction based on just a few properties of the classical phase space. For the
standard map excellent agreement with numerically determined tunneling rates is
observed. The results should similarly apply to ionization rates and quality
factors.Comment: 6 pages, 2 figure
Stochastic Database Cracking: Towards Robust Adaptive Indexing in Main-Memory Column-Stores
Modern business applications and scientific databases call for inherently
dynamic data storage environments. Such environments are characterized by two
challenging features: (a) they have little idle system time to devote on
physical design; and (b) there is little, if any, a priori workload knowledge,
while the query and data workload keeps changing dynamically. In such
environments, traditional approaches to index building and maintenance cannot
apply. Database cracking has been proposed as a solution that allows on-the-fly
physical data reorganization, as a collateral effect of query processing.
Cracking aims to continuously and automatically adapt indexes to the workload
at hand, without human intervention. Indexes are built incrementally,
adaptively, and on demand. Nevertheless, as we show, existing adaptive indexing
methods fail to deliver workload-robustness; they perform much better with
random workloads than with others. This frailty derives from the inelasticity
with which these approaches interpret each query as a hint on how data should
be stored. Current cracking schemes blindly reorganize the data within each
query's range, even if that results into successive expensive operations with
minimal indexing benefit. In this paper, we introduce stochastic cracking, a
significantly more resilient approach to adaptive indexing. Stochastic cracking
also uses each query as a hint on how to reorganize data, but not blindly so;
it gains resilience and avoids performance bottlenecks by deliberately applying
certain arbitrary choices in its decision-making. Thereby, we bring adaptive
indexing forward to a mature formulation that confers the workload-robustness
previous approaches lacked. Our extensive experimental study verifies that
stochastic cracking maintains the desired properties of original database
cracking while at the same time it performs well with diverse realistic
workloads.Comment: VLDB201
- âŠ