arXiv subjects

Tsvi Piran

Publications and source records attributed to Tsvi Piran.

At least 289 records · Page 16Linked to original sources

arXiv1999

A case devoid of bias: ORS voids vs. IRAS voids

We present a comparison between the voids in two nearly all sky redshift surveys: the ORS and the IRAS 1.2 Jy. While the galaxies in these surveys are selected differently and their populations are known to be biased relative to each other, the two void distributions are similar. We compare the spatial distribution of the two void populations and demonstrate the correlation between them. The voids also agree with regard to the overall void statistics -- a filling factor of ~0.45 of the volume, an average void diameter ~45/h Mpc and an average galaxy underdensity in the voids of approximately -0.9. Our measurements of the underdensities of the voids in the two surveys enable us to estimate the relative bias in the voids between optical and IRAS samples. We find b(opt) / b(IRAS) ~ 1, showing that on average there is little -- or no -- biasing between the two void populations.

astro-ph↗

arXiv1999

Gamma-ray Bursts - A Puzzle Being Resolved

For a few seconds a gamma-ray burst (GRB) becomes the brightest object in the Universe, over-shining the rest of the Universe combined! Clearly this reflects extreme conditions that are fascinating and worth exploring. The recent discovery of GRB afterglow have demonstrated that we are on the right track towards the resolution of this long standing puzzle. These observations have confirmed the relativistic fireball model (more specifically the internal-external shocks model). The prompt optical emission seen in GRB 990123 have demonstrated that GRBs involve ultra-relativistic motion. The breaks in the light curves of GRB 990123 and GRB 990510 and the peculiar light curves of GRB 980519 and GRB 980326 disclosed that these GRBs are beamed. I examine these recent developments and discuss their implications to the models of the source. I argue that the current understanding implies that GRBs signal the birth of stellar mass black holes.

astro-ph↗

arXiv1999

Jets in GRBs

In several GRBs afterglows, rapid temporal decay is observed which is inconsistent with spherical (isotropic) blast-wave models. In particular, GRB 980519 had the most rapidly fading of the well-documented GRB afterglows, with t^{-2.05\pm 0.04} in optical as well as in X-rays. We show that such temporal decay is more consistent with the evolution of a jet after it slows down and spreads laterally, for which t^{-p} decay is expected (where p is the index of the electron energy distribution). Such a beaming model would relax the energy requirements on some of the more extreme GRBs by a factor of several hundreds. It is likely that a large fraction of the weak (or no) afterglow observations are also due to the common occurrence of beaming in GRBs, and that their jets have already transitioned to the spreading phase before the first afterglow observations were made. With this interpretation, a universal value of p~2.5 is consistent with all data.

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arXiv1999

The Inner Structure of Black Holes

We study the gravitational collapse of a self-gravitating charged scalar-field. Starting with a regular spacetime, we follow the evolution through the formation of an apparent horizon, a Cauchy horizon and a final central singularity. We find a null, weak, mass-inflation singularity along the Cauchy horizon, which is a precursor of a strong, spacelike singularity along the $r=0$ hypersurface. The inner black hole region is bounded (in the future) by singularities. This resembles the classical inner structure of a Schwarzschild black hole and it is remarkably different from the inner structure of a charged static Reissner-Nordström or a stationary rotating Kerr black holes.

gr-qc↗

arXiv1999

GRB990123, The Optical Flash and The Fireball Model

We compare the ongoing observations of the remarkable burst GRB990123, the mother of all bursts, with the predictions of the afterglow theory. We show that the observations agree with the recent prediction that a reverse shock propagating into the ejecta would produce a very strong prompt optical flash. This reverse shock has also produced the 8.46GHz radio signal, observed after one day. The forward shock, which propagates into the ISM is the origin of the classical afterglow. It has produced the prompt X-ray signal as well as the late optical and IR emission. It would most likely produce a radio emission within the next few weeks. The observations suggest that the initial Lorentz factor of the ejecta was $\sim 200$. Within factors of order unity, this crude model explains all current observations of GRB990123.

astro-ph↗

arXiv1999

Predictions for The Very Early Afterglow and The Optical Flash

According to the internal-external shocks model for $γ$-ray bursts (GRBs), the GRB is produced by internal shocks within a relativistic flow while the afterglow is produced by external shocks with the ISM. We explore the early afterglow emission. For short GRBs the peak of the afterglow will be delayed, typically, by few dozens of seconds after the burst. For long GRBs the early afterglow emission will overlap the GRB signal. We calculate the expected spectrum and the light curves of the early afterglow in the optical, X-ray and $γ$-ray bands. These characteristics provide a way to discriminate between late internal shocks emission (part of the GRB) and the early afterglow signal. If such a delayed emission, with the characteristics of the early afterglow, will be detected it can be used both to prove the internal shock scenario as producing the GRB, as well as to measure the initial Lorentz factor of the relativistic flow. The reverse shock, at its peak, contains energy which is comparable to that of the GRB itself, but has a much lower temperature than that of the forward shock so it radiates at considerably lower frequencies. The reverse shock dominates the early optical emission, and an optical flash brighter than 15th magnitude, is expected together with the forward shock peak at x-rays or $γ$-rays. If this optical flash is not observed, strong limitations can be put on the baryonic contents of the relativistic shell deriving the GRBs, leading to a magnetically dominated energy density.

astro-ph↗

arXiv1999

The Early Afterglows

We calculate the expected spectrum and light curves of the early afterglow. For short GRBs the peak of the afterglow will be delayed, typically, by a few dozens of seconds after the burst. The x-ray and gamma-ray characteristics of this delayed emission provide a way to discriminate between late internal shocks emission (part of the GRB) and the early afterglow signal. Detection of this delayed emission will prove the internal shock scenario as producing the GRB, and will pinpoint the initial Lorentz factor gamma_0. In the optical band, the dominant emission arises from the reverse shock. This shock, carries an energy comparable to that of the forward shock. It radiates this energy at much lower frequencies, producing a short optical flash of 15th magnitude or brighter.

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arXiv1999

Images, Light Curves and Spectra of GRB Afterglow

We calculate the light curve and spectra near the peak and the self absorption break, for an adiabatic blast wave described by the Blandford-McKee solution, considering the emission from the whole region behind the shock front. The expected light curve and spectra are flat near the peak. This rules out the interpretation of the sharp peak observed in the optical afterglow of GRB970508 as the expected peak of the light curve. The observed image of an afterglow is calculated for a broad range of frequencies. We show that for frequencies below the self absorption frequency the image is rather homogeneous, as opposed to the bright ring at the outer edge and dim center, which appear at higher frequencies. We fit the observed spectra of GRB970508 to the detailed theory and obtain estimates of the physical parameters of this burst.

astro-ph↗

arXiv1998

Gamma-Ray Bursts and the Fireball Model

Gamma-ray bursts (GRBs) have puzzled astronomers since their accidental discovery in the late 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. The recent discovery of an X-ray afterglow by the Italian/Dutch satellite BeppoSAX has led to a detection of high red-shift absorption lines in the optical afterglow of GRB970508 and in several other bursts and to the identification of host galaxies to others. This has confirmed the cosmological origin. Cosmological GRBs release $\sim 10^{51}-10^{53}$ergs in a few seconds making them the most (electromagnetically) luminous objects in the Universe. The simplest, most conventional, and practically inevitable, interpretation of these observations is that GRBs result from the conversion of the kinetic energy of ultra-relativistic particles or possibly the electromagnetic energy of a Poynting flux to radiation in an optically thin region. This generic "fireball" model has also been confirmed by the afterglow observations. The "inner engine" that accelerates the relativistic flow is hidden from direct observations. Consequently it is difficult to infer its structure directly from current observations. Recent studies show, however, that this ``inner engine'' is responsible for the complicated temporal structure observed in GRBs. This temporal structure and energy considerations indicates that the ``inner engine'' is associated with the formation of a compact object - most likely a black hole.

astro-ph↗

arXiv1998

Late-Time Tails in Gravitational Collapse of a Self-Interacting (Massive) Scalar-Field and Decay of a Self-Interacting Scalar Hair

We study analytically the initial value problem for a self-interacting (massive) scalar-field on a Reissner-Nordström spacetime. Following the no-hair theorem we examine the dynamical physical mechanism by which the self-interacting (SI) hair decays. We show that the intermediate asymptotic behaviour of SI perturbations is dominated by an oscillatory inverse power-law decaying tail. We show that at late-times the decay of a SI hair is slower than any power-law. We confirm our analytical results by numerical simulations.

gr-qc↗

arXiv1998

Radiative Efficiencies of Continuously Powered Blast Waves

We use general arguments to show that a continuously powered radiative blast wave can behave self similarly if the energy injection and radiation mechanisms are self similar. In that case, the power-law indices of the blast wave evolution are set by only one of the two constituent physical mechanisms. If the luminosity of the energy source drops fast enough, the radiation mechanisms set the power-law indices, otherwise, they are set by the behavior of the energy source itself. We obtain self similar solutions for the Newtonian and the ultra-relativistic limits. Both limits behave self similarly if we assume that the central source supplies energy in the form of a hot wind, and that the radiative mechanism is the semi-radiative mechanism of Cohen, Piran & Sari (1998). We calculate the instantaneous radiative efficiencies for both limits and find that a relativistic blast wave has a higher efficiency than a Newtonian one. The instantaneous radiative efficiency depends strongly on the hydrodynamics and cannot be approximated by an estimate of local microscopic radiative efficiencies, since a fraction of the injected energy is deposited in shocked matter. These solutions can be used to calculate Gamma Ray Bursts afterglows, for cases in which the energy is not supplied instantaneously.

astro-ph↗

arXiv1998

Synchrotron Self Absorption in GRB Afterglow

GRB afterglow is reasonably described by synchrotron emission from relativistic blast waves at cosmological distances. We perform detailed calculations taking into account the effect of synchrotron self absorption. We consider emission from the whole region behind the shock front, and use the Blandford McKee self similar solution to describe the fluid behind the shock. We calculate the spectra and the observed image of a GRB afterglow near the self absorption frequency $ν_a$ and derive an accurate expression for $ν_a$. We show that the image is rather homogeneous for $ν<ν_a$, as opposed to the bright ring at the outer edge and dim center, which appear at higher frequencies. We compare the spectra we obtain to radio observations of GRB970508. We combine the calculations of the spectra near the self absorption frequency with other parts of the spectra and obtain revised estimates for the physical parameters of the burst: $E_{52}=0.53$, $ε_e=0.57$, $ε_B=0.0082$, $n_1=5.3$. These estimates are different by up to two orders of magnitude than the estimates based on an approximate spectrum.

astro-ph↗

arXiv1998

Gamma-Ray Bursts - The Second Revolution

Gamma-ray bursts GRBs are among the most mysterious astronomical phenomenon ever discovered. Unlike most astronomical discoveries which were explained within weeks or months after their initial discovery, GRBs remain a puzzle for more than thirty years. During the last decade our understanding of GRBs has undergone two major revolutions. First, BATSE discovered that GRBs are distributed isotropically over the sky and thereby demonstrated their cosmological origin. The second revolution tool place more recently when BeppoSAX discovered GRB afterglow. This confirmed the fireball model and led to a wealth of observational data, some of which has not been fully understood yet. The emerging picture is that GRBs are the most luminous objects and the most relativistic objects ever discovered: (i) GRBs involve relativistic motion at a velocity of 0.9999c or larger. (ii) Most current GRB models involve the formation of a black hole in one way or another. (iii) If binary neutron star mergers are the sources of GRBs then GRBs are also associated with gravitational radiation signals. Finally, (iv) as cosmological power-houses that are observed to high red-shift GRBs might be used to measure cosmological parameters and to teach us about the epoch of galaxy formation.

astro-ph↗

arXiv1998

Large Scale Fluctuations in the X-Ray Background

We present an attempt to measure the large angular scale fluctuations in the X-Ray Background (XRB) from the HEAO1-A2 data, expressed in terms of spherical harmonics. We model the harmonic coefficients assuming a power spectrum and an epoch-dependent bias parameter, and using a phenomenological scenario describing the evolution of the X-ray sources. From the few low-order multipoles detected above shot noise, we estimate the power-spectrum normalization on scales intermediate between those explored by local galaxy redshift surveys (~ 100 Mpc) and by the COBE Microwave Background measurements (~ 1000 Mpc). We find that the HEAO1 harmonics are consistent with present epoch rms fluctuations of the X-ray sources bx(0)sigma8 ~ 1-2 in 8 Mpc spheres. Therefore the observed fluctuations in the XRB are roughly as expected from interpolating between the local galaxy surveys and the COBE CMB experiment. We predict that an X-ray all-sky surface brightness survey resolving sources a factor of 10 fainter than HEAO1, may reveal fluctuations to significantly larger angular scales and therefore more strongly constrain the large scale structure of the Universe on scales of hundreds of Mpcs.

astro-ph↗

arXiv1998

Images and Spectra From the Interior of a Relativistic Fireball

The detection of an afterglow, following a gamma-ray burst (GRB), can be described reasonably well by synchrotron emission from a relativistic spherical expanding blast wave, driven by an expanding fireball. We perform detailed calculations considering the emission from the whole region behind the shock front. We use the Blandford McKee self similar adiabatic solution to describe the fluid behind the shock. Using this detailed model, we derive expressions for the peak flux, and the peak frequency at a given observed time. These expressions provide important numerical corrections to previous, more simplified models. We calculate the observed light curve and spectra for several magnetic field models. We show that both the light curve and the spectra are flat near the peak. This rules out the interpretation of the optical peak of GRB970508 as the peak of the light curve, predicted by the existing fireball models. We calculate the observed image of a GRB afterglow. The observed image is bright near the edge and dimmer at the center, thus creating a ring. The contrast between the edge and the center is larger at high frequencies and the width of the ring is smaller.

astro-ph↗

arXiv1998

Fluid dynamics of partially radiative blast waves

We derive a self similar solution for the propagation of an extreme relativistic (or Newtonian) radiative spherical blast wave into a surrounding cold medium. The solution is obtained under the assumption that the radiation process is fast, it takes place only in the vicinity of the shock and that it radiates away a fixed fraction of the energy generated by the shock. In the Newtonian regime these solutions generalize the Sedov-Taylor adiabatic solution and the pressure-driven fully radiative solution. In the extreme relativistic case these solutions generalize the Blandford-McKee adiabatic solution. They provide a new fully radiative extreme relativistic solution which is different from the Blandford-McKee fully radiative relativistic solution. This new solution develops a hot interior which causes it to cool faster than previous estimates. We find that the energy of the blast wave behaves as a power law of the location of the shock. The power law index depends on the fraction of the energy emitted by the shock. We obtain an analytic solution for the interior of the blast wave. These new solutions might be applicable to the study of GRB afterglow or SNRs.

astro-ph↗

arXiv1998

Hydrodynamics of a Relativistic Fireball: the Complete Evolution

We study numerically the evolution of an adiabatic relativistic fireball expanding into a cold uniform medium. We follow the stages of initial free expansion and acceleration, coasting and then deceleration and slowing down to a non-relativistic velocity. We compare the numerical results with simplified analytical estimates. We show that the relativistic self similar Blandford-McKee solution describes well the relativistic deceleration epoch. It is an excellent approximation throughout the relativistic deceleration stage, down to $γ\sim 5$, and a reasonable approximation even down to $γ\sim 2$ though the solution is rigorous only for $γ\gg 1$. We examine the transition into the Blandford-McKee solution, and the transition from the solution to the non-relativistic self similar Sedov-Taylor solution. These simulations demonstrate the attractive nature of the Blandford-McKee solution and its stability to radial perturbations.

astro-ph↗

arXiv1998

Mass-Inflation in Dynamical Gravitational Collapse of a Charged Scalar-Field

We study the inner-structure of a charged black-hole which is formed from the gravitational collapse of a self-gravitating charged scalar-field. Starting with a regular spacetime, we follow the evolution through the formation of an apparent horizon, a Cauchy horizon and a final central singularity. We find a null, weak, mass-inflation singularity along the Cauchy horizon, which is a precursor of a strong, spacelike singularity along the $r=0$ hypersurface.

gr-qc↗