arXiv subjects

Tsvi Piran

Publications and source records attributed to Tsvi Piran.

At least 325 records · Page 18Linked to original sources

arXiv1996

Relativity at Action or Gamma-Ray Bursts

Gamma ray Bursts (GRBs) - short bursts of few hundred keV $γ$-rays - have fascinated astronomers since their accidental discovery in the sixties. GRBs were ignored by most relativists who did not expect that they are associated with any relativistic phenomenon. The recent observations of the BATSE detector on the Compton GRO satellite have revolutionized our ideas on these bursts and the picture that emerges shows that GRBs are the most relativistic objects discovered so far.

gr-qc↗

arXiv1996

Fine-Structure of Choptuik's Mass-Scaling Relation

We conjecture (analytically) and demonstrate (numerically) the existence of a fine-structure above the power-law behavior of the mass of black-holes that form in gravitational collapse of spherical massless scalar field. The fine-structure is a periodic function of the critical-separation $(p-p^*)$. We predict that the period $\varpi$ is {\it universal} and that it depends on the previous universal parameters, the critical exponent, $β$, and the echoing period $Δ$ as $\varpi = Δ/β$.

gr-qc↗

arXiv1996

New Evidence for The Cosmological Origin of Gamma-Ray Bursts

We find that Gamma-ray bursts (GRBs) at the 3B catalog are correlated (at 95\% confidence level) with Abell clusters. This is the first known correlation of GRBs with any other astronomical population. It confirms the cosmological origin of GRBs. Comparison of the rich clusters auto-correlation with the cross-correlation found here suggests that \sim 26 \pm 15\% of an accurate (δ<2.3 deg.) position GRBs sub-sample members are located within 600 \hmpc.

astro-ph↗

arXiv1996

Cooling Time Scales and Temporal Structure of Gamma-Ray Bursts

A leading mechanism for producing cosmological gamma-ray bursts (GRBs) is via ultra-relativistic particles in an expanding fireball. The kinetic energy of the particles is converted into thermal energy in a forward shock and a reverse shock, when the outward flowing particles encounter the interstellar medium. The thermal energy is radiated via synchrotron emission and Comptonization. We estimate the synchrotron cooling time scale of the shocked regions for electrons of various Lorentz factors, focusing in particular on those electrons whose radiation falls within the energy detection range of the BATSE detectors. We find that in order to produce the rapid variability observed in most bursts the magnetic energy density in the shocked material must be greater than about 1\% of the thermal energy density. Additionally, the electrons must be nearly in equipartition with the protons, otherwise the radiative efficiency is low. Inverse Compton scattering can increase the cooling rate of the relevant electrons but the Comptonized emission itself is never within the BATSE range. These arguments allow us to pinpoint the conditions within the radiating regions in GRBs and to determine the important radiation processes. In addition, the model predicts that the duty cycle of intensity variations in GRB light curves should be nearly independent of burst duration, and should scale inversely as the square root of the observed photon energy. Both correlations are in agreement with observations. The model also provides a plausible explanation for the bimodal distribution of burst durations. There is no explanation, however, for the characteristic break energy in GRB spectra.

astro-ph↗

arXiv1996

The Signature of a Correlation between $>10^{19}{\rm eV}$ Cosmic Ray Sources and Large Scale Structure

We analyze the anisotropy signature expected if the high energy (above $10^{19}$eV) cosmic ray (CR) sources are extra-Galactic and trace the distribution of luminous matter on large scales. We investigate the dependence of the anisotropy on both the relative bias between the CR sources and the galaxy distribution and on the (unknown) intrinsic CR source density. We find that the expected anisotropy associated with the large scale structure (LSS) should be detected once the number of CR events observed above $10^{19}{\rm eV}$ is increased by a factor of $\sim10$. This would require $\sim30$ observation-years with existing experiments, but less then $1$ year with the proposed $\sim5000\ {\rm km}^2$ Auger detectors. We find that the recently reported concentration of the Haverah Park CR events towards the super-galactic plane is not consistent with the known LSS. If real, the Haverah Park result suggests that the CR sources are much more concentrated towards the super-galactic plane than the known LSS. Our results are not sensitive to the number density of CR sources. We show that once the number of detected events is increased by a factor of $\sim10$, the number density would be strongly constrained by considering the probability for having repeating sources.

astro-ph↗

arXiv1996

Automated Detection of Voids in Redshift Surveys

We present a new void search algorithm for automated detection of voids in three-dimensional redshift surveys. Based on a model in which the main features of the LSS of the Universe are voids and walls, we classify the galaxies into wall galaxies and field galaxies and we define voids as continuous volumes that are devoid of any wall galaxies. Field galaxies are allowed within the voids. The algorithm makes no assumptions regarding the shapes of the voids and the only constraint that is imposed is that the voids are always thicker than a preset limit, thus eliminating connections between adjacent voids through smal breaches in the walls. By appropriate scaling of the parameters with the selection function this algorithm can be used to analyze flux-limited surveys. We test the algorithm on Voronoi tessellations and apply it to the SSRS2 redshift survey to derive the spectrum of void sizes and other void properties. We find that the average diameter of a void is $37\pm 8 \h$ Mpc. We suggest the usage of this fully automated algorithm to derive a quantitative description of the voids, as another tool in describing the large scale structure of the Universe and for comparison with numerical simulations.

astro-ph↗

arXiv1995

Hydrodynamic Time Scales and Temporal Structure in GRBs

We calculate the hydrodynamic time scales for a spherical ultra-relativistic shell that is decelerated by the ISM and discuss the possible relations between these time scales and the observed temporal structure in $γ$-ray bursts. We suggest that the bursts' duration is related to the deceleration time, the variability is related to the ISM inhomogeneities and precursors are related to internal shocks within the shell. Good agreement can be achieved for these quantities with reasonable, not fined tuned, astrophysical parameters. The difference between Newtonian and relativistic reverse shocks may lead to the observed bimodal distribution of bursts' durations.

astro-ph↗

arXiv1995

A Simultaneous Spectral Invariant Analysis of the GRB Count Distribution and Time Dilation

The analysis of the BATSE's count distribution within cosmological models suffers from observational uncertainties due to the variability of the bursts' spectra: when BATSE observes bursts from different redshifts at a fixed energy band it detects photons from different energy bands at the source. This adds a spectral dependence to the count distribution $N(C)$. Similarly variation of the duration as a function of energy at the source complicates the time dilation analysis. We describe here a new statistical formalism that performs the required ``blue shifting" of the count number and the burst duration in a statistical manner. This formalism allows us to perform a combined best fit (maximal likelihood) to the count distribution, $N(C)$, and the duration distribution simultaneously. The outcome of this analysis is a single best fit value for the redshift of the observed bursts.

astro-ph↗

arXiv1995

Are There MeV Gamma-Ray Bursts?

It is often stated that gamma-ray bursts (GRBs) have typical energies of several hundred keV. Is this a real feature of GRBs or is it due to an observational bias? We consider the possibility that bursts of a given bolometric luminosity occur with a hardness distribution $p(H)d \log H \propto H^γd \log H$. We model the detection efficiency of BATSE as a function of $H$ and calculate the expected distribution of $H$ in the observed sample for various values of $γ$. We show that because the detection efficiency of BATSE falls steeply with increasing $H$, the paucity of hard bursts need not be real. We find that the observed sample is consistent with a distribution above $H = 100$ keV with $γ\approx 0$ or even $γ=0.5$. Thus, a large population of unobserved hard gamma-ray bursts may exist. It is important to extend the present analysis to a larger sample of BATSE bursts and to include the OSSE and COMPTEL limits. If the full sample is consistent with $γ \sgreat\ 0$, then it would be interesting to look for MeV bursts in the future.

astro-ph↗

arXiv1995

Hydrodynamic Time Scales and Temporal Structure of GRBs

astro-ph↗

arXiv1995

Towards Understanding Gamma-Ray Bursts

$γ$-ray bursts (GRBs) have puzzled astronomers since their accidental discovery in the sixties. The BATSE detector on COMPTON-GRO satellite has been detecting GRBs for the last four years at a rate of one burst per day. Its findings has revolutionized our ideas about the nature of these objects. In this lecture I show that the simplest, most conventional and practically inevitable, interpretation of the observations is that GRBs form during the conversion of the kinetic energy of ultra-relativistic particles to radiation. The inner ``engine" that accelerates these particles is well hidden from direct observations and its origin might remain mysterious for a long time.

astro-ph↗

arXiv1995

Void Sizes and CMBR Fluctuations

Following Blumenthal \etl \cite{blu} and Piran \etl \cite{Pi} I discuss a simple model for the gravitational formation of voids. This model enables us to estimate the primordial perturbation required on the scale of voids. A comparison of this condition with the constraints on the spectrum of fluctuations derived from CMBR observations reveals that gravitational growth of large voids require more power on the scale of $\approx 1500$\kms than is generally accepted in CDM models. For a generic power law primordial spectrum $P(k) \propto k^n$ gravitational growth of voids requires $n \gsim 1.25$, which is marginally compatible with the observed CMBR fluctuations. If future measurements will show that the amplitude of fluctuations required to produce the voids is too large they will imply that the voids did not grow gravitationally and that galaxies do not trace matter on very large scale.

astro-ph↗

arXiv1995

Dipole Anisotropy from an Entropy Gradient

It is generally accepted that the observed CMBR dipole arises from the motion of the local group relative to the CMBR frame. An alternative interpretation is that the dipole results from an ultra-large scale ($λ> 100 c/H_0)$ isocurvature perturbation. Recently it was argued that this alternative possibility is ruled out. We examine the growth of perturbations on scales larger than the Hubble radius and in view of this analysis, we show that the isocurvature interpretation is still a viable explanation. If the dipole is due to peculiar motion then it should appear in observations of other background sources provided that they are distant enough.

astro-ph↗

arXiv1995

The Distribution Of Cosmological $γ$-ray Bursts

We compare the burst distribution of the new (2B) BATSE catalogue to a cosmological distribution. The observed distribution agrees well with a cosmological one, however, it is insensitive to cosmological parameters such as $Ø$ and $Λ$. The bursts are not necessarily standard candles and their luminosity can vary by up to a factor of ten. The maximal red shift, $\zm$, of bursts longer than 2 sec is $2.1^{+1.}_{-.7}$ (assuming no evolution). The present data is insufficient to determine maximal red shift, $\zm$, of bursts shorter than 2 sec.

astro-ph↗

arXiv1994

Gamma-Ray Bursts and Binary Neutron Star Mergers

Neutron star binaries, such as the one observed in the famous binary pulsar PSR 1916+13, end their life in a catastrophic merge event (denoted here NS$^2$M). The merger releases $\approx 5 \cdot 10^{53}$ergs, mostly as neutrinos and gravitational radiation. A small fraction of this energy suffices to power $γ$-ray bursts (GRBs) at cosmological distances. Cosmological GRBs must pass, however, an optically thick fireball phase and the observed $γ$-rays emerge only at the end of this phase. Hence, it is difficult to determine the nature of the source from present observations (the agreement between the rates of GRBs and NS$^2$Ms being only an indirect evidence for this model). In the future a coinciding detection of a GRB and a gravitational radiation signal could confirm this model.

astro-ph↗

arXiv1994

New Evidence for the Cosmological Origin of Gama-Ray Bursts

We compare the burst distribution of the new (2B) BATSE catalogue to a cosmological distribution. We find that the distribution is insensitive to cosmological parameters such as Omega and Lambda and to the width of the bursts luminosity function. The maximal red shift of the long bursts is ~2.1 (assuming no evolution) while Zm(long) of the short bursts is significantly lower Zm(short) =~ 0.5 In agreement with this relatively nearby origin of the short burst we find an indication that these bursts are correlated ( >=2 sigma level at 10 degrees) with Abell clusters. This is the first known correlation of the bursts with any other astrophysical population and if confirmed by further observations it will provides additional evidence for the cosmological origin of those bursts.

astro-ph↗

arXiv1994

Stability of Fireballs and $γ$-Ray Bursts

Fireballs are an essential part of any cosmological $γ$-ray burst. We drive here a stability criterion for fireballs and show that fireballs are Rayleigh-Taylor unstable in any region in which the entropy decreases outward. The instability begins to operate when the fireball becomes matter dominated. Among the possible implication of the instability are: (i) Conversion of a fraction of the radiation energy to a convective energy expressed in the motion of bubbles relative to each other. (ii) Penetration of fast bubbles through slower ones and creation of high $γ$ regimes which are essential for efficient conversion of the energy to $γ$-rays. (iii) Formation of rapid variation (of the scale of the bubbles) in the observed $γ$-rays.

astro-ph↗

arXiv1994

The Internal Geometry of an Evaporating Black Hole

We present a semi-classical model for the formation and evaporation of a four dimensional black hole. We solve the equations numerically and obtain solutions describing the entire the space-time geometry from the collapse to the end of the evaporation. The solutions satisfy the evaporation law: $\dot M \propto -M^{-2}$ which confirms dynamically that black holes do evaporate thermally. We find that the evaporation process is in fact the shrinking of a throat that connects a macroscopic interior ``universe" to the asymptotically flat exterior. It ends either by pinching off the throat leaving a closed universe and a Minkowskian exterior or by freezing up when the throat's radius approaches a Planck size. In either case the macroscopic inner universe is the region where the information lost during the evaporation process is hidden.

hep-th↗