arXiv subjects

Tsvi Piran

Publications and source records attributed to Tsvi Piran.

At least 307 records · Page 17Linked to original sources

arXiv1998

The Hardness Distribution of Gamma-Ray Bursts

It is often stated that gamma-ray bursts (GRBs) have typical energies of several hundreds $\keV$, where the typical energy may be characterized by the hardness H, the photon energy corresponding to the peak of $νF_ν$. Among the 54 BATSE bursts analyzed by Band et al. (1993), and 136 analyzed by us, more then 60% have 50 keV < H < 300 keV. Is the narrow range of H a real feature of GRBs or is it due to an observational difficulty to detect harder bursts? We consider a population of standard candle bursts with a hardness distribution: rho(H) d log H \propto H^gamma d log H and no luminosity - hardness correlation. We model the detection algorithm of BATSE as a function of H, including cosmological effects, detector characteristics and triggering procedure, and we calculate the expected distribution of H in the observed sample for various values of gamma. Both samples shows a paucity of soft (X-ray) bursts, which may be real. However, we find that the observed samples are consistent with a distribution above H=120 keV with gamma \sim -0.5 (a slowly decreasing numbers of GRBs per decade of hardness). Thus, we suggest that a large population of unobserved hard gamma-ray bursts may exist.

astro-ph↗

arXiv1998

Late-Time Evolution of Charged Gravitational Collapse and Decay of Charged Scalar Hair - III. Nonlinear Analysis

We study the nonlinear gravitational collapse of a charged massless scalar-field. We confirm the existence of oscillatory inverse power-law tails along future timelike infinity, future null infinity and along the future outer-horizon. The nonlinear dumping exponents are in excellent agreement with the analytically predicted ones. Our results prove the analytic conjecture according to which a charged hair decays slower than a neutral one and also suggest the occurrence of mass-inflation along the Cauchy horizon of a dynamically formed charged black-hole.

gr-qc↗

arXiv1998

Gamma-Ray Bursts and Related Phenomena

Gamma-ray bursts (GRBs) have puzzled astronomers since their accidental discovery in the sixties. The BATSE detector on the COMPTON-GRO satellite has been detecting one burst per day for the last six years. Its findings have revolutionized our ideas about the nature of these objects. They have shown that GRBs are at cosmological distances. This idea was accepted with difficulties at first. However, the recent discovery of an x-ray afterglow by the Italian/Dutch satellite BeppoSAX led to a detection of high red-shift absorption lines in the optical afterglow of GRB970508 and to a confirmation of its cosmological origin. The simplest and practically inevitable interpretation of these observations is that GRBs result from the conversion of the kinetic energy of ultra-relativistic particles flux to radiation in an optically thin region. The "inner engine" that accelerates the particles or generates the Poynting flux is hidden from direct observations. Recent studies suggest the ``internal-external'' model: internal shocks that take place within the relativistic flow produce the GRB while the subsequent interaction of the flow with the external medium produce the afterglow. The ``inner engine'' that produces the flow is, however, hidden from direct observations. We review this model with a specific emphasis on its implications to underground physics.

astro-ph↗

arXiv1998

Late-Time Evolution of Charged Gravitational Collapse and Decay of Charged Scalar Hair - II

We study analytically the initial value problem for a charged massless scalar-field on a Reissner-Nordström spacetime. Using the technique of spectral decomposition we extend recent results on this problem. Following the no-hair theorem we reveal the dynamical physical mechanism by which the charged hair is radiated away. We show that the charged perturbations decay according to an inverse power-law behaviour at future timelike infinity and along future null infinity. Along the future outer horizon we find an oscillatory inverse power-law relaxation of the charged fields. We find that a charged black hole becomes ``bald'' slower than a neutral one, due to the existence of charged perturbations. Our results are also important to the study of mass-inflation and the stability of Cauchy horizons during a dynamical gravitational collapse of charged matter in which a charged black-hole is formed.

gr-qc↗

arXiv1997

Late-Time Evolution of Charged Gravitational Collapse and Decay of Charged Scalar Hair - I

We study analytically the asymptotic evolution of charged fields around a Reissner-Nordström black-hole. Following the no-hair theorem we focus attention on the dynamical mechanism by which the charged hair is radiated away. We find an inverse power-law relaxation of the charged fields at future timelike infinity, along future null infinity and an oscillatory inverse power-law relaxation along the future outer horizon. We show that a charged hair is shedd slower than a neutral one. Our results are also of importance to the study of mass inflation and the stability of Cauchy horizons during a dynamical gravitational collapse of charged matter to form a charged black-hole

gr-qc↗

arXiv1997

Spectra and Light Curves of Gamma-Ray Burst Afterglows

The recently discovered GRB afterglow is believed to be described reasonably well by synchrotron emission from a slowing down relativistic shell that collides with an external medium. To compare theoretical models with afterglow observations we calculate here the broad band spectrum and corresponding light curve of synchrotron radiation from a power-law distribution of electrons in an expanding relativistic shock. Both the spectrum and the light curve consist of several power-law segments. The light curve is constructed under two limiting models for the hydrodynamical evolution of the shock: fully adiabatic and fully radiative. We compare the results with observations of $γ$-ray burst afterglows.

astro-ph↗

arXiv1997

A catalogue of the voids in the IRAS 1.2-Jy survey

Using the VOID FINDER algorithm we have compiled a catalogue of voids in the IRAS 1.2-Jy sample. The positions of the voids correspond well to underdense regions seen in the IRAS smoothed density map. However, since in our analysis no smoothing is used, all structures appear much sharper: walls are not smeared and the voids are not artificially reduced by them. Therefore the current method based on the galaxy point distribution is better suited to determine the diameter of voids in the galaxy distribution. We have identified 24 voids, covering more than 30% of the volume considered. By comparing the results with equivalent random catalogues we have determined that 12 voids are significant at a 0.95 confidence level, having an average diameter of 40+-6 h^{-1} Mpc. Our results serve not only for charting the cosmography of the nearby Universe, but also to give support to the results recently obtained with the SSRS2 sample, suggesting a void-filled Universe. Moreover, our results indicate that the voids detected have a similar scale, demonstrating that both optically and IRAS-selected galaxies delineate the same large-scale structures.

astro-ph↗

arXiv1997

On Gravitational Repulsion

The concepts of negative gravitational mass and gravitational repulsion are alien to general relativity. Still, we show here that small negative fluctuations - small dimples in the primordial density field - that act as if they have an effective negative gravitational mass, play a dominant role in shaping our Universe. These initially tiny perturbations repel matter surrounding them, expand and grow to become voids in the galaxy distribution. These voids - regions with a diameter of $40 \h$ Mpc which are almost devoid of galaxies - are the largest object in the Universe.

gr-qc↗

arXiv1997

The Implications of Direct Red-Shift Measurement of $γ$-ray Bursts

The recent discoveries of X-ray and optical counterparts for GRBs, and a possible discovery of a host galaxy, implies that a direct measurement of the red-shift of some GRBs host galaxies is eminent. We discuss the implications of such measurements. They could enable us to determine the GRBs luminosity distribution, the variation of the rate of GRBs with cosmic time, and even, under favorable circumstances, to estimate $Ω$. Using GRB970508 alone, and assuming standard candles, we constrain the intrinsic GRB evolution to $ρ(z)=(1+z)^{-0.5\pm0.7}$.

astro-ph↗

arXiv1997

Can internal shocks produce the variability in GRBs?

We discuss the possibility that gamma-ray bursts result from internal shocks in an ultra-relativistic matter. Using a simple model we calculate the temporal structure and we estimate the efficiency of this process. In this model the ultra-relativistic matter flow is represented by a succession of shells with random values of the Lorentz factor. We calculate the shocks that take place between those shells and we estimate the resulting emission. Internal shocks can produce the highly variable temporal structure observed in most of the bursts provided that the source emitting the relativistic flow is highly variable. The observed peaks are in almost one to one correlation to the activity of the emitting source. A large fraction of the kinetic energy is converted to radiation. The most efficient case is when an inner engine produces shells with comparable energy but very different Lorentz factors. It also gives the most preferable temporal structure.

astro-ph↗

arXiv1997

Spherical Collapse of a Mass-Less Scalar Field With Semi-Classical Corrections

We investigate numerically spherically symmetric collapse of a scalar field in the semi-classical approximation. We first verify that our code reproduces the critical phenomena (the Choptuik effect) in the classical limit and black hole evaporation in the semi classical limit. We then investigate the effect of evaporation on the critical behavior. The introduction of the Planck length by the quantum theory suggests that the classical critical phenomena, which is based on a self similar structure, will disappear. Our results show that when quantum effects are not strong enough, critical behavior is observed. In the intermediate regime, evaporation is equivalent to a decrease of the initial amplitude. It doesn't change the echoing structure of near critical solutions. In the regime where black hole masses are low and the quantum effects are large, the semi classical approximation breaks down and has no physical meaning.

gr-qc↗

arXiv1997

Voids in the Large-Scale Structure

Voids are the most prominent feature of the LSS of the universe. Still, they have been generally ignored in quantitative analysis of it, essentially due to the lack of an objective tool to identify and quantify the voids. To overcome this, we present the Void-Finder algorithm, a novel tool for objectively quantifying galaxy voids. The algorithm classifies galaxies as either wall- or field-galaxies. Then it identifies voids in the wall-galaxy distribution. Voids are defined as continuous volumes that do not contain any wall-galaxies. The voids must be thicker than an adjustable limit, which is refined in successive iterations. We test the algorithm using Voronoi tessellations. By appropriate scaling of the parameters we apply it to the SSRS2 survey and to the IRAS 1.2 Jy. Both surveys show similar properties: ~50% of the volume is filled by the voids, which have a scale of at least 40 Mpc, and a -0.9 under-density. Faint galaxies populate the voids more than bright ones. These results suggest that both optically and IRAS selected galaxies delineate the same LSS. Comparison with the recovered mass distribution further suggests that the observed voids in the galaxy distribution correspond well to under-dense regions in the mass distribution. This confirms the gravitational origin of the voids.

astro-ph↗

arXiv1997

Implication of Temporal Structure in GRB

The highly variable temporal structure observed in most GRBs provides us with unexpected clues. We show that variable GRBs cannot be produced by external shocks models and consequently cannot be produced by an ``explosive'' inner engine. The observed temporal structure must reflect the activity of the inner engine that must be producing unsteady and irregular ``wind'' which is converted to radiation via internal shocks.

astro-ph↗

arXiv1997

Variability in GRBs - A Clue

We show that external shocks cannot produce a variable GRB, unless they are produced by an extremely narrow jets (angular opening of < ~10^{-4}) or if only a small fraction of the shell emits the radiation and the process is very inefficient. Internal shocks can produce the observed complex temporal structure provided that the source itself is variable. In this case, the observed temporal structure reflects the activity of the ``inner engine'' that drives the bursts. This sets direct constraints on it.

astro-ph↗

arXiv1996

The X-Ray Background as a Probe of Density Fluctuations at High Redshift

The X-Ray Background (XRB) probes structure on scales intermediate between those explored by local galaxy redshift surveys and by the COBE Microwave Background measurements. We predict the large scale angular fluctuations in the XRB, expressed in terms of spherical harmonics for a range of assumed power-spectra and evolution scenarios. The dipole is due to large scale structure as well as to the observer's motion (the Compton-Getting effect). For a typical observer the two effects turn out to be comparable in amplitude. The coupling of the two effects makes it difficult to use the XRB for independent confirmation of the CMB dipole being due to the observer's motion. The large scale structure dipole (rms per component) relative to the monopole is in the range $a_{1m}/a_{00} \sim (0.5-9.0) \times 10^{-3} $. The spread is mainly due to the assumed redshift evolution scenarios of the X-ray volume emissivity $ρ_x(z)$. The dipole's prediction is consistent with a measured dipole in the HEAO1 XRB map. Typically, the harmonic spectrum drops with $l$ like $a_{lm} \sim l^{-0.4}$. This behaviour allows us to discriminate a true clustering signal against the flux shot noise, which is constant with $l$, and may dominate the signal unless bright resolved sources are removed from the XRB map. We also show that Sachs-Wolfe and Doppler (due to the motion of the sources) effects in the XRB are negligible. Although our analysis focuses on the XRB, the formalism is general and can be easily applied to other cosmological backgrounds.

astro-ph↗

arXiv1996

Cosmological GRBs: Internal vs. External Shocks

Internal (IS) or external (ES) shocks in relativistic expanding shells are currently the best known mechanism for producing GRBs. We calculate the hydrodynamic conditions and the cooling processes in internal shocks, that occur when one layer overtakes another. We compare these conditions with the conditions in external shocks that occur when the shell encounters the ISM. We find that the complicated temporal structure observed in GRBs could be easily explained in IS scenario. Unlike ES, the observed temporal structure simply reflects the temporal behavior of the internal engine that produces the shell. We also find that opacity and in particular the opacity for pair production poses strong constraints on the parameter space and consequently bursts with very narrow peaks are expected to be optically thick and would not contain GeV photons. Finally we find that as in ES synchrotron is the most likely radiation process.

astro-ph↗

arXiv1996

Critical Behaviour and Universality in Gravitational Collapse of a Charged Scalar Field

We summarize results from a study of spherically symmetric collapse of a {\it charged} (complex) massless scalar-field \cite{Hod}. We present an analytic argument which conjecture the generalization of the mass-scaling relation and echoing phenomena, originally discovered by Choptuik, for the {\it charged} case. Furthermore, we study the behaviour of the self-similar critical solution under {\it external} perturbations -- addition of a cosmological constant $Λ$ and a charge-conjugation $e$. Finally, we study the scaling-relation of the black-hole charge. Using an analytic argument we conjecture that black-holes of infinitesimal mass are neutral or obey the relation $Q_{BH} \ll M_{BH}$. We verify our predictions with numerical results.

gr-qc↗

arXiv1996

Spherical Harmonic Expansion of Gamma Ray Burst Distributions: Probing Large Scale Structure?

We have laid down the formalism and techniques necessary for computing the multipole components in a spherical harmonic expansion for bursting sources given any specific power spectrum of density perturbations. Using this formalism we have explicitly computed and tabulated the expected first few multipole components for the GRB distribution using a Cold Dark Matter power spectrum. Unfortunately, our analysis leads us to expect that an anisotropy signal for this model of structure formation will be below the shot noise level for the foreseeable future. Earlier studies by others had claimed that it may be possible within a decade or so to probe structure formation using the angular distribution of GRBs. We find that while it may be possible to probe the dipole due to our motion with respect to the cosmic rest-frame, it does not seem feasible to probe the multipole components due to intrinsic fluctuations if the actual power spectrum is fixed to within an order of magnitude by the Cold Dark Matter model of structure formation.

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