849 research outputs found
Afterglows from precursors in Gamma Ray Bursts. Application to the optical afterglow of GRB 091024
About 15% of Gamma Ray Bursts have precursors, i.e. emission episodes
preceding the main event, whose spectral and temporal properties are similar to
the main emission. We propose that precursors have their own fireball,
producing afterglow emission due to the dissipation of the kinetic energy via
external shock. In the time lapse between the precursor and the main event, we
assume that the central engine is not completely turned off, but it continues
to eject relativistic material at a smaller rate, whose emission is below the
background level. The precursor fireball generates a first afterglow by the
interaction with the external circumburst medium. Matter injected by the
central engine during the "quasi-quiescent" phase replenishes the external
medium with material in relativistic motion. The fireball corresponding to the
main prompt emission episode crashes with this moving material, producing a
second afterglow, and finally catches up and merges with the first precursor
fireball. We apply this new model to GRB 091024, an event with a precursor in
the prompt light curve and two well defined bumps in the optical afterglow,
obtaining an excellent agreement with the existing data.Comment: 11 pages, 6 figures, 3 tables. Accepted for publication in MNRAS,
Main Journa
The faster the narrower: characteristic bulk velocities and jet opening angles of Gamma Ray Bursts
The jet opening angle theta_jet and the bulk Lorentz factor Gamma_0 are
crucial parameters for the computation of the energetics of Gamma Ray Bursts
(GRBs). From the ~30 GRBs with measured theta_jet or Gamma_0 it is known that:
(i) the real energetic E_gamma, obtained by correcting the isotropic equivalent
energy E_iso for the collimation factor ~theta_jet^2, is clustered around
10^50-10^51 erg and it is correlated with the peak energy E_p of the prompt
emission and (ii) the comoving frame E'_p and E'_gamma are clustered around
typical values. Current estimates of Gamma_0 and theta_jet are based on
incomplete data samples and their observed distributions could be subject to
biases. Through a population synthesis code we investigate whether different
assumed intrinsic distributions of Gamma_0 and theta_jet can reproduce a set of
observational constraints. Assuming that all bursts have the same E'_p and
E'_gamma in the comoving frame, we find that Gamma_0 and theta_jet cannot be
distributed as single power-laws. The best agreement between our simulation and
the available data is obtained assuming (a) log-normal distributions for
theta_jet and Gamma_0 and (b) an intrinsic relation between the peak values of
their distributions, i.e theta_jet^2.5*Gamma_0=const. On average, larger values
of Gamma_0 (i.e. the "faster" bursts) correspond to smaller values of theta_jet
(i.e. the "narrower"). We predict that ~6% of the bursts that point to us
should not show any jet break in their afterglow light curve since they have
sin(theta_jet)<1/Gamma_0. Finally, we estimate that the local rate of GRBs is
~0.3% of all local SNIb/c and ~4.3% of local hypernovae, i.e. SNIb/c with
broad-lines.Comment: 15 pages, 8 figures, 1 table. Accepted for publication in MNRA
Bulk Lorentz factors of Gamma-Ray Bursts
Knowledge of the bulk Lorentz factor of GRBs allows us to
compute their comoving frame properties shedding light on their physics. Upon
collisions with the circumburst matter, the fireball of a GRB starts to
decelerate, producing a peak or a break (depending on the circumburst density
profile) in the light curve of the afterglow. Considering all bursts with known
redshift and with an early coverage of their emission, we find 67 GRBs with a
peak in their optical or GeV light curves at a time . For another
106 GRBs we set an upper limit . We show that
is due to the dynamics of the fireball deceleration and not to the passage of a
characteristic frequency of the synchrotron spectrum across the optical band.
Considering the of 66 long GRBs and the 85 most constraining upper
limits, using censored data analysis methods, we reconstruct the most likely
distribution of . All are larger than the time when the prompt emission peaks, and are much larger than the time when the fireball becomes transparent. The reconstructed distribution of
has median value 300 (150) for a uniform (wind) circumburst
density profile. In the comoving frame, long GRBs have typical isotropic
energy, luminosity, and peak energy erg, erg s ,
and keV in the homogeneous (wind) case. We
confirm that the significant correlations between and the rest frame
isotropic energy (), luminosity () and peak energy
() are not due to selection effects. Assuming a typical opening
angle of 5 degrees, we derive the distribution of the jet baryon loading which
is centered around a few .Comment: 19 pages, 11 figures, 6 tables. Accepted for publication on Astronomy
& Astrophysic
A biotic support during pregnancy to strengthen the gastrointestinal performance in puppies
Up to 60% of neonates can be affected by gastroenteritis due to specific pathogens or aspecific polymicrobial interactions. The present study evaluated if a dietary supplementation with MOS, FOS, E. faecium and L. acidophilus in pregnancy may reduce gastroenteritis in puppies. Fifteen Great Danes were divided in 3 groups. The control group (CG) ate a standard diet. In 2 study groups, the diet was supplemented with pre-and probiotics during the last (1WG) and the last 4 pregnancy weeks (4WG). Up to 9 weeks, puppies were checked daily to identify first-or second-presentation gastroenteritis. Data were processed by χ2 (P < 0.05). First-presentation gastroenteritis was more frequent in CG than in 1WG than in 4WG. Second-presentation gastroenteritis was more frequent in CG than in 1 and 4WG. Puppies from pre-and probiotics supplemented bitches were less prone to gastroenteritis. 1 or 4WG equally reduced second-presentation gastroenteritis in puppies, but 4WG was better than 1WG on first-presentation gastroenteritis. By entero-mammary link, supplemented bitches produced higher immune quality colostrum, thus puppies faced immunitary challenges better; moreover, maternal microbiota, positively altered by supplementation, was transferred to newborns, becoming more resistant to gastroenteritis. This information can be useful in clinical practice with the goal of preventing gastroenteritis in puppies and reducing its prevalence and severity
There is a short gamma-ray burst prompt phase at the beginning of each long one
We compare the prompt intrinsic spectral properties of a sample of short
Gamma--ray Burst (GRB) with the first 0.3 seconds (rest frame) of long GRBs
observed by Fermi/GBM. We find that short GRBs and the first part of long GRBs
lie on the same E_p--E_iso correlation, that is parallel to the relation for
the time averaged spectra of long GRBs. Moreover, they are indistinguishable in
the E_p--L_iso plane. This suggests that the emission mechanism is the same for
short and for the beginning of long events, and both short and long GRBs are
very similar phenomena, occurring on different timescales. If the central
engine of a long GRB would stop after ~0.3 * (1+z) seconds the resulting event
would be spectrally indistinguishable from a short GRB.Comment: 14 pages, 6 figures, MNRAS accepte
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