13,750 research outputs found
On the energy of gamma-ray bursts
We show that gamma-ray burst (GRB) afterglow observations strongly suggest,
within the fireball model framework, that radiating electrons are shock
accelerated to a power-law energy distribution, with universal index p \approx
2.2, and that the fraction of shock energy carried by electrons, \xi_e, is
universal and close to equipartition, \xi_e ~ 1/3. For universal p and \xi_e, a
single measurement of the X-ray afterglow flux on the time scale of a day
provides a robust estimate of the fireball energy per unit solid angle,
\epsilon, averaged over a conical section of the fireball of opening angle
\theta ~ 0.1.
Applying our analysis to BeppoSAX afterglow data we find that: (i) Fireball
energies are in the range of 4\pi\epsilon=10^{51.5} to 10^{53.5} erg; (ii) The
ratio of observed -ray to total fireball energy per unit solid angle,
\epsilon_\gamma / \epsilon, is of order unity, satisfying
abs[log10(\epsilon_\gamma/\epsilon)]<0.5; (iii) If fireballs are jet like,
their opening angle should satisfy \theta>=0.1.
Our results imply that if typical opening angles are \theta ~ 0.1, a value
consistent with our analysis, the total energy associated with a GRB event is
in the range of 10^{50} erg to 10^{51.5} erg.Comment: 16 pages; Submitted to Ap
A simple empirical redshift indicator for gamma-ray bursts
We propose a new empirical redshift indicator for gamma-ray bursts. This
indicator is easily computed from the gamma-ray burst spectral parameters, and
its duration, and it provides ``pseudo-redshifts'' accurate to a factor two.
Possible applications of this redshift indicator are briefly discussed.Comment: Final version, 4 pages, 3 figures, A&A, vol. 407, L
X-ray afterglow detection of the short gamma-ray burst 991014
GRB 991014 is one of the shortest gamma-ray bursts detected so far with the
Wide Field Cameras aboard BeppoSAX, both in gamma-rays and X-rays. The duration
is 9.6 sec in 2-28 keV and 3.2 sec in 40 to 700 keV (as measured between the
times when 5 and 95% of the burst photons have been accumulated). We refine the
InterPlanetary Network annulus of the burst, present the detection of the X-ray
afterglow of GRB 991014 within this refined annulus, and discuss X-ray and
gamma-ray observations of the prompt and afterglow emission. Except for the
briefness of the prompt event, no other unusual aspects were found in the
prompt and afterglow observations as compared to such measurements in previous
gamma-ray bursts.Comment: accepted for publication in Astrophysical Journa
GRB990510: on the possibility of a beamed X-ray afterglow
We discuss the prompt emission of the gamma-ray burst (GRB) 990510 and its
subsequent X-ray afterglow from 8.0 to 44.3 hrs after the prompt emission,
using observations with the Gamma-ray Burst Monitor and Narrow Field
Instruments on BeppoSAX. In the 40-700 keV band, GRB990510 had a fluence of
\~1.9x10^{-5}erg cm^{-2}, whereas it reached a peak flux of ~2.4x10^{-6}erg
cm^{-2} s^{-1}. The X-ray afterglow decay light curve can be satisfactorily
described by a single power law with index of -1.42+/-0.07. Both the X-ray and
optical behaviour of the afterglow can be explained by gamma-ray burst debris
expanding as a jet; we find that the cooling frequency is (fixed) between the
optical and X-ray wavelength bands.Comment: 16 pages, 4 figures, accepted for publication in the Astrophysical
Journa
Weak solutions to problems involving inviscid fluids
We consider an abstract functional-differential equation derived from the
pressure-less Euler system with variable coefficients that includes several
systems of partial differential equations arising in the fluid mechanics. Using
the method of convex integration we show the existence of infinitely many weak
solutions for prescribed initial data and kinetic energy
BeppoSAX measurements of the bright gamma-ray burst 010222
We analyze the BeppoSAX measurements of the prompt and afterglow emission of
the gamma-ray burst GRB010222. Among 45 GRBs detected with the Wide Field
Cameras on BeppoSAX, the 40-700 keV fluence of (9.3+/-0.3)E-5 erg cm-2 is only
surpassed by GRB990123. In terms of the isotropic 20-2000 keV energy output of
7.8E53 erg, it ranks third of all GRBs with measured distances. Since this
burst is so bright, the data provide complete and valuable coverage up to 65 hr
after the event, except for a gap between 3.5 and 8.0 hr. The 2-10 keV flux
history shows clear signs of a break which is consistent with a break seen in
the optical, and provides supporting evidence for the achromatic nature of the
break. An explanation for the break in the context of a collimated expansion is
not straightforward. Rather, a model is favored whereby the fireball is braked
to the non-relativistic regime quickly (within a fraction of day) by a dense
1E6 cm-3 circumburst medium. This implies that, after a mild beaming
correction, GRB010222 may be the most energetic burst observed thus far. The
X-ray decay index after the break is 1.33+/-0.04, the spectral index
0.97+/-0.05. The decay is, with unprecedented accuracy, identical to that
observed in the optical.Comment: Accepted on June 6 for publication in ApJ part I. Publication due in
October 2001. Accepted version has only minor modification
Positronium Decay : Gauge Invariance and Analyticity
The construction of positronium decay amplitudes is handled through the use
of dispersion relations. In this way, emphasis is put on basic QED principles:
gauge invariance and soft-photon limits (analyticity).
A firm grounding is given to the factorization approaches, and some
ambiguities in the spin and energy structures of the positronium wavefunction
are removed. Non-factorizable amplitudes are naturally introduced. Their
dynamics is described, especially regarding the enforcement of gauge invariance
and analyticity through delicate interferences. The important question of the
completeness of the present theoretical predictions for the decay rates is then
addressed. Indeed, some of those non-factorizable contributions are unaccounted
for by NRQED analyses. However, it is shown that such new contributions are
highly suppressed, being of order alpha^3.
Finally, a particular effective form factor formalism is constructed for
parapositronium, allowing a thorough analysis of binding energy effects and
analyticity implementation.Comment: 34 pages, 13 figure
Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts
We present the main results of a study of spectral and energetics properties
of twelve gamma-ray bursts (GRBs) with redshift estimates. All GRBs in our
sample were detected by BeppoSAX in a broad energy range (2-700 keV). From the
redshift estimates and the good-quality BeppoSAX time-integrated spectra we
deduce the main properties of GRBs in their cosmological rest frames. All
spectra in our sample are satisfactorily represented by the Band model with no
significant soft X-ray excesses or spectral absorptions. We find a positive
correlation between the estimated total (isotropic) energies in the 1-10000 keV
energy range (E_rad) and redshifts z. Interestingly, more luminous GRBs are
characterized also by larger peak energies E_p of their EF(E) spectra.
Furthermore, more distant GRBs appear to be systematically harder in the X-ray
band compared to GRBs with lower redshifts. We discuss how selection and data
truncation effects could bias our results and give possible explanations for
the correlations that we found.Comment: 10 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
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