2,816 research outputs found
Exact versus approximate solutions in Gamma-Ray Burst afterglows
We have recently obtained the exact analytic solutions of the relativistic
equations relating the radial and time coordinate of a relativistic thin
uniform shell expanding in the interstellar medium in the fully radiative and
fully adiabatic regimes. We here re-examine the validity of the constant-index
power-law relations between the Lorentz gamma factor and its radial coordinate,
usually adopted in the current Gamma-Ray Burst (GRB) literature on the grounds
of an "ultrarelativistic" approximation. Such expressions are found to be
mathematically correct but only approximately valid in a very limited range of
the physical and astrophysical parameters and in an asymptotic regime which is
reached only for a very short time, if any, and are shown to be not applicable
to GRBs.Comment: 4 pages, 2 figures, to appear on ApJ Letter
On the exact analytic expressions for the equitemporal surfaces in Gamma-Ray Burst afterglows
We have recently shown (see Bianco & Ruffini 2004) that marked differences
exist between the EQuiTemporal Surfaces (EQTSs) for the Gamma-Ray Burst (GRB)
afterglows numerically computed by the full integration of the equations of
motion and the ones found in the current literature expressed analytically on
the grounds of various approximations. In this Letter the exact analytic
expressions of the EQTSs are presented both in the case of fully radiative and
adiabatic regimes. The new EQTS analytic solutions validate the numerical
results obtained in Bianco & Ruffini (2004) and offer a powerful tool to
analytically perform the estimates of the physical observables in GRB
afterglows.Comment: 3 pages, 1 figure, to appear on ApJ Letter
Theoretical interpretation of GRB 011121
GRB011121 is analyzed as a prototype to understand the ``flares'' recently
observed by Swift in the afterglow of many GRB sources. Detailed theoretical
computation of the GRB011121 light curves in selected energy bands are
presented and compared and contrasted with observational BeppoSAX data.Comment: 2 pages, 1 figure, to appear in the proceedings of "Swift and GRBs",
Venice, 2006, Il Nuovo Cimento, in pres
Exact versus approximate beaming formulas in Gamma-Ray Burst afterglows
We present the exact analytic expressions to compute, assuming the emitted
Gamma-Ray Burst (GRB) radiation is not spherically symmetric but is confined
into a narrow jet, the value of the detector arrival time at which we start to
"see" the sides of the jet, both in the fully radiative and adiabatic regimes.
We obtain this result using our exact analytic expressions for the EQuiTemporal
Surfaces (EQTSs) in GRB afterglows. We re-examine the validity of three
different approximate formulas currently adopted for the adiabatic regime in
the GRB literature. We also present an empirical fit of the numerical solutions
of the exact equations, compared and contrasted with the three above
approximate formulas. The extent of the differences is such as to require a
reassessment on the existence and entity of beaming in the cases considered in
the current literature, as well as on its consequences on the GRB energetics.Comment: 4 pages, 4 figures, to appear on ApJ Let
Decaying Majoron Dark Matter and Neutrino Masses
We review the recent proposal by Lattanzi & Valle of the majoron as a
suitable warm dark matter candidate. The majoron is the Goldstone boson
associated to the spontaneous breaking of ungauged lepton number, one of the
mechanisms proposed to give rise to neutrino masses. The majoron can acquire a
mass through quantum gravity effects, and can possibly account for the observed
dark matter component of the Universe. We present constraints on the majoron
lifetime, mass and abundance obtained by the analysis of the cosmic microwave
background data. We find that, in the case of thermal production, the limits
for the majoron mass read 0.12 keV<m_J<0.17 keV, and discuss how these limits
are modified in the non-thermal case. The majoron lifetime is constrained to be
larger than 250 Gyrs. We also apply this results to a given seesaw model for
the generation of neutrino masses, and find that this constraints the energy
scale for the lepton number breaking phase transition to be above 10^6 GeV. We
thus find that the majoron decaying dark matter (DDM) scenario fits nicely in
models where neutrino masses arise "a la seesaw" and may lead to other possible
cosmological implications.Comment: 7 pages, 3 figures. Contribution to proceedings of the 4th
Sino-Italian Workshop on Relativistic Astrophysics, Pescara, 20-30 July 200
Boost symmetry in the Quantum Gravity sector
We perform a canonical quantization of gravity in a second-order formulation,
taking as configuration variables those describing a 4-bein, not adapted to the
space-time splitting. We outline how, neither if we fix the Lorentz frame
before quantizing, nor if we perform no gauge fixing at all, is invariance
under boost transformations affected by the quantization.Comment: 8 pages, to appear in the proceedings of'' 4th Italian-Sino Workshop
on Relativistic Astrophysics'', AIP Conference Serie
Gravitational Stability and Bulk Cosmology
We present a discussion of the effects induced by bulk viscosity either on
the very early Universe stability and on the dynamics associated to the extreme
gravitational collapse of a gas cloud. In both cases the viscosity coefficient
is related to the energy density via a power-law of the form
(where ) and the behavior of the
density contrast in analyzed.
In the first case, matter filling the isotropic and homogeneous background is
described by an ultra-relativistic equation of state. The analytic expression
of the density contrast shows that its growth is suppressed forward in time as
soon as overcomes a critical value. On the other hand, in such a
regime, the asymptotic approach to the initial singularity admits an unstable
collapsing picture.
In the second case, we investigate the top-down fragmentation process of an
uniform and spherically symmetric gas cloud within the framework of a Newtonian
approach, including the negative pressure contribution associated to the bulk
viscous phenomenology. In the extreme regime toward the singularity, we show
that the density contrast associated to an adiabatic-like behavior of the gas
(which is identified by a particular range of the politropic index) acquire,
for sufficiently large viscous contributions, a vanishing behavior which
prevents the formation of sub-structures. Such a feature is not present in the
isothermal-like collapse. We also emphasize that in the adiabatic-like case
bulk viscosity is also responsible for the appearance of a threshold scale
(equivalent to a Jeans length) beyond which perturbations begin to increase.Comment: 13 pages, no figur
The luminosity evolution over the EQuiTemporal Surfaces in the prompt emission of Gamma-Ray Bursts
Due to the ultrarelativistic velocity of the expanding "fireshell" (Lorentz
gamma factor \gamma \sim 10^2 - 10^3), photons emitted at the same time from
the fireshell surface do not reach the observer at the same arrival time. In
interpreting Gamma-Ray Bursts (GRBs) it is crucial to determine the properties
of the EQuiTemporal Surfaces (EQTSs): the locus of points which are source of
radiation reaching the observer at the same arrival time. In the current
literature this analysis is performed only in the latest phases of the
afterglow. Here we study the distribution of the GRB bolometric luminosity over
the EQTSs, with special attention to the prompt emission phase. We analyze as
well the temporal evolution of the EQTS apparent size in the sky. We use the
analytic solutions of the equations of motion of the fireshell and the
corresponding analytic expressions of the EQTSs which have been presented in
recent works and which are valid for both the fully radiative and the adiabatic
dynamics. We find the novel result that at the beginning of the prompt emission
the most luminous regions of the EQTSs are the ones closest to the line of
sight. On the contrary, in the late prompt emission and in the early afterglow
phases the most luminous EQTS regions are the ones closest to the boundary of
the visible region. This transition in the emitting region may lead to specific
observational signatures, i.e. an anomalous spectral evolution, in the rising
part or at the peak of the prompt emission. We find as well an expression for
the apparent radius of the EQTS in the sky, valid in both the fully radiative
and the adiabatic regimes. Such considerations are essential for the
theoretical interpretation of the prompt emission phase of GRBs.Comment: 11 pages, 5 figures, in the Proceedings of the 1st Galileo-Xu GuangQi
Meeting, October 26-30, 2009, Shangha
Equations of motion, initial and boundary conditions for GRB
We compare and contrast the different approaches to the optically thick
adiabatic phase of GRB all the way to the transparency. Special attention is
given to the role of the rate equation to be self consistently solved with the
relativistic hydrodynamic equations. The works of Shemi and Piran (1990),
Piran, Shemi and Narayan (1993), Meszaros, Laguna and Rees (1993) and Ruffini,
Salmonson, Wilson and Xue (1999,2000) are compared and contrasted. The role of
the baryonic loading in these three treatments is pointed out. Constraints on
initial conditions for the fireball produced by electro-magnetic black hole are
obtained.Comment: 10 pages, 8 figure
Baryonic loading and e^+e^- rate equation in GRB sources
The expansion of the electron-positron plasma in the GRB phenomenon is
compared and contrasted in the treatments of Meszaros, Laguna and Rees, of
Shemi, Piran and Narayan, and of Ruffini et al. The role of the correct
numerical integration of the hydrodynamical equations, as well as of the rate
equation for the electron-positron plasma loaded with a baryonic mass, are
outlined and confronted for crucial differences.Comment: 5 pages, 3 figures, to appear in the proceedings of "Relativistic
Astrophysics and Cosmology - Einstein's Legacy" meeting, November 7-11, 2005,
Munich, Germany, edited by B. Aschenbach, V. Burwitz, G. Hasinger, and B.
Leibundgu
- …