10 research outputs found
FERMI constraints on the high energy, ~1 GeV, emission of long GRBs
We investigate the constraints imposed on the luminosity function (LF) of
long duration Gamma Ray Bursts (LGRBs) by the flux distribution of bursts
detected by the GBM at ~1 MeV, and the implications of the non detection of the
vast majority, ~95%, of the LGRBs at higher energy, ~1 GeV, by the LAT
detector. We find a LF that is consistent with those determined by BATSE and
Swift. The non detections by LAT set upper limits on the ratio R of the prompt
fluence at ~1 GeV to that at ~1 MeV. The upper limits are more stringent for
brighter bursts, with R<{0.1,0.3,1} for {5,30,60}% of the bursts. This implies
that for most bursts the prompt ~1 GeV emission may be comparable to the ~1 MeV
emission, but can not dominate it. The value of R is not universal, with a
spread of (at least) an order of magnitude around R~10^(-1). For several bright
bursts with reliable determination of the photon spectral index at ~1 MeV, the
LAT non detection implies an upper limit to the ~100 MeV flux which is <0.1 of
the flux obtained by extrapolating the ~1 MeV flux to high energy. For the
widely accepted models, in which the ~1 MeV power-law photon spectrum reflects
the power-law energy distribution of fast cooling electrons, this suggests that
either the electron energy distribution does not follow a power-law over a wide
energy range, or that the high energy photons are absorbed. Requiring an order
unity pair production optical depth at ~100 MeV sets an upper limit for the
Lorentz factor, Gamma<=10^(2.5).Comment: 12 pages, 6 figures. Submitted to A&
The energy budget of GRBs based on updated prompt \& afterglow observations
We compare the isotropic equivalent 15-2000 keV gamma-ray energy, E_gamma,
emitted by a sample of 91 swift Gamma-Ray Bursts (GRBs) with known redshifts,
with the isotropic equivalent fireball energy, E_fb, as estimated within the
fireball model framework from X-ray afterglow observations of these bursts. The
uncertainty in E_gamma, which spans the range of ~10^51 erg to ~10^53.5 erg, is
approximately 25% on average, due mainly to the extrapolation from the BAT
detector band to the 15-2000 keV band. The uncertainty in E_fb is approximately
a factor of 2, due mainly to the X-ray measurements' scatter. We find E_gamma
and E_fb to be tightly correlated. The average(std) of {\eta}^11hr_gamma is
approximately log_10(E_gamma/(3{\epsilon} _eE^11hr_fb)) are -0.34(0.60), and
the upper limit on the intrinsic spread of {\eta}_gamma is approximately 0.5
({\epsilon}_e is the fraction of shocked plasma energy carried by electrons and
E^x hr_fb is inferred from the X-ray flux at x hours). We also find that E_fb
inferred from X-ray observations at 3 and 11 hours are similar, with an
average(std) of log_10(E^3hr_fb/E^11hr_fb) of 0.04(0.28). The small variance of
{\eta}_gamma implies that burst-to-burst variations in {\epsilon}_e and in the
efficiency of fireball energy conversion to gamma-rays are small, and suggests
that both are of order unity. The small variance of {\eta}_gamma and the
similarity of E^3hr_fb and E^11hr_fb further imply that {\epsilon}_e does not
vary significantly with shock Lorentz factor, and that for most bursts the
modification of fireball energy during the afterglow phase, by processes such
as radiative losses or extended duration energy injection, are not significant.
Finally, our results imply that if fireballs are indeed jets, then the jet
opening angle satisfies {\theta}>0.1 for most cases. [abridged]Comment: An expanded discussion of the absence of energy injection and 'flat
X-ray phases' in the majority of burst
Intracoronary Injection of In Situ Forming Alginate Hydrogel Reverses Left Ventricular Remodeling After Myocardial Infarction in Swine
ObjectivesThis study sought to determine whether alginate biomaterial can be delivered effectively into the infarcted myocardium by intracoronary injection to prevent left ventricular (LV) remodeling early after myocardial infarction (MI).BackgroundAlthough injectable biomaterials can improve infarct healing and repair, the feasibility and effectiveness of intracoronary injection have not been studied.MethodsWe prepared a calcium cross-linked alginate solution that undergoes liquid to gel phase transition after deposition in infarcted myocardium. Anterior MI was induced in swine by transient balloon occlusion of left anterior descending coronary artery. At 4 days after MI, either alginate solution (2 or 4 ml) or saline was injected selectively into the infarct-related coronary artery. An additional group (n = 19) was treated with incremental volumes of biomaterial (1, 2, and 4 ml) or 2 ml saline and underwent serial echocardiography studies.ResultsExamination of hearts harvested after injection showed that the alginate crossed the infarcted leaky vessels and was deposited as hydrogel in the infarcted tissue. At 60 days, control swine experienced an increase in left ventricular (LV) diastolic area by 44%, LV systolic area by 45%, and LV mass by 35%. In contrast, intracoronary injection of alginate (2 and 4 ml) prevented and even reversed LV enlargement (p < 0.01). Post-mortem analysis showed that the biomaterial (2 ml) increased scar thickness by 53% compared with control (2.9 ± 0.1 mm vs. 1.9 ± 0.3 mm; p < 0.01) and was replaced by myofibroblasts and collagen.ConclusionsIntracoronary injection of alginate biomaterial is feasible, safe, and effective. Our findings suggest a new percutaneous intervention to improve infarct repair and prevent adverse remodeling after reperfused MI
Constraints on the Local Sources of Ultra High-Energy Cosmic Rays
Ultra high-energy cosmic rays (UHECRs) are believed to be protons accelerated
in magnetized plasma outflows of extra-Galactic sources. The acceleration of
protons to ~10^{20} eV requires a source power L>10^{47} erg/s. The absence of
steady sources of sufficient power within the GZK horizon of 100 Mpc, implies
that UHECR sources are transient. We show that UHECR "flares" should be
accompanied by strong X-ray and gamma-ray emission, and that X-ray and
gamma-ray surveys constrain flares which last less than a decade to satisfy at
least one of the following conditions: (i) L>10^{50} erg/s; (ii) the power
carried by accelerated electrons is lower by a factor >10^2 than the power
carried by magnetic fields or by >10^3 than the power in accelerated protons;
or (iii) the sources exist only at low redshifts, z<<1. The implausibility of
requirements (ii) and (iii) argue in favor of transient sources with L>10^{50}
erg/s.Comment: 7 pages, 1 figure, submitted to JCA
Neutrino signatures of the supernova - gamma ray burst relationship
We calculate the TeV-PeV neutrino fluxes of gamma-ray bursts associated with
supernovae, based on the observed association between GRB 030329 and supernova
SN 2003dh. The neutrino spectral flux distributions can test for possible
delays between the supernova and the gamma-ray burst events down to much
shorter timescales than what can be resolved with photons. As an illustrative
example, we calculate the probability of neutrino induced muon and electron
cascade events in a km scale under-ice detector at the South Pole, from the GRB
030329. Our calculations demonstrate that km scale neutrino telescopes are
expected to detect signals that will allow to constrain supernova-GRB models.Comment: 7 pages, 2 figures. Accepted for publication in Phys. Rev.