9,449 research outputs found
Reverse Shock Emission in Gamma-ray Bursts Revisited
A generic synchrotron external shock model is the widely preferred paradigm
used to interpret the broad-band afterglow data of gamma-ray bursts (GRBs),
including predicted observable signatures from a reverse shock which have been
confirmed by observations. Investigations of the nature of the reverse shock
emission can provide valuable insights into the intrinsic properties of the GRB
ejecta. Here we briefly review the standard and the extended models of the
reverse shock emission, discussing the connection between the theory and
observations, including the implications of the latest observational advances.Comment: Invited review, to be published in special issue on "GRB in Swift and
Fermi Era" in Journal of Advances in Astronom
Non-Gaussianity excess problem in classical bouncing cosmologies
The simplest possible classical model leading to a cosmological bounce is
examined in the light of the non-Gaussianities it can generate. Concentrating
solely on the transition between contraction and expansion, and assuming
initially purely Gaussian perturbations at the end of the contracting phase, we
find that the bounce acts as a source such that the resulting value for the
post-bounce may largely exceed all current limits, to the
point of potentially casting doubts on the validity of the perturbative
expansion. We conjecture that if one can assume that the non-Gaussianity
production depends only on the bouncing behavior of the scale factor and not on
the specifics of the model examined, then many realistic models in which a
nonsingular classical bounce takes place could exhibit a generic
non-Gaussianity excess problem that would need to be addressed for each case.Comment: 7 pages, 2 figures. Inclusion of additional results (equations and
plots) and more detailed discussion in the introduction and main body.
Accepted for publication in Physical Review
Production of non-gaussianities in a bouncing phase
We compute the level of non-gaussianities produced by a cosmological bouncing
phase in the minimal non-singular setup that lies within the context of General
Relativity when the matter content consists of a simple scalar field with a
standard kinetic term. Such a bouncing phase is obtained by requiring that the
spatial sections of the background spacetime be positively curved. We restrict
attention to the close vicinity of the bounce by Taylor expanding the scale
factor, the scalar field and its potential in powers of the conformal time
around the bounce. We find that possibly large non-gaussianities are
generically produced at the bounce itself and also discuss which shapes of
non-gaussianities are mostly likely to be produced.Comment: Matches published versio
Semiparametric Estimation in Multivariate Nonstationary Time Series Models
A system of multivariate semiparametric nonlinear time series models is studied with possible dependence structures and nonstationarities in the parametric and nonparametric components. The parametric regressors may be endogenous while the nonparametric regressors are assumed to be strictly exogenous. The parametric regressors may be stationary or nonstationary and the nonparametric regressors are nonstationary integrated time series. Semiparametric least squares (SLS) estimation is considered and its asymptotic properties are derived. Due to endogeneity in the parametric regressors, SLS is not consistent for the parametric component and a semiparametric instrumental variable (SIV) method is proposed instead. Under certain regularity conditions, the SIV estimator of the parametric component is shown to have a limiting normal distribution. The rate of convergence in the parametric component depends on the properties of the regressors. The conventional ân rate may apply even when nonstationarity is involved in both sets of regressors.Endogeneity; integrated process, nonstationarity; partial linear model; simultaneity; vector semiparametric regression.
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