arXiv subjects

Tsvi Piran

Publications and source records attributed to Tsvi Piran.

At least 235 records · Page 13Linked to original sources

arXiv2005

Neutrino-Dominated Accretion and Supernovae

We suggest that part of the infalling material during the core-collapse of a massive star goes into orbit around the compact core to form a hot, dense, centrifugally-supported accretion disk whose evolution is strongly influenced by neutrino interactions. Under a wide range of conditions, this neutrino-dominated accretion flow will be advection-dominated and will develop a substantial outflowing wind. We estimate the energy carried out in the wind and find that it exceeds $10^{50}$ erg for a wide range of parameters and even exceeds 10^{51} erg for reasonable parameter choices. We propose that the wind energy will revive a stalled shock and will help produce a successful supernova explosion. We discuss the role of the disk wind in both prompt and delayed explosions. While both scenarios are feasible, we suggest that a delayed explosion is more likely, and perhaps even unavoidable. Finally, we suggest that the disk wind may be a natural site for r-process nucleosynthesis.

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arXiv2005

A Note on "Probing the existence of the $E_{peak}$-$E_{iso}$ correlation in long Gamma Ray Bursts" by Ghirlanda et al. - astro-ph/0502186

In Nakar & Piran (2004) we have shown that a large fraction of BATSE bursts cannot satisfy the Amati relation between the isotropic energy - peak energy of GRBs. Our results have been corroborated by a further analysis carried out by Band and Preece (2005). Recently Ghirlanda et al. (2005) claimed that when the exact distribution of the scatter around the Amati relation is taken into account, the BATSE sample is consistent with the Amati relation. We show that the probability that the BATSE sample is consistent with the Amati relation (including the scattering), as found by Ghirlanda et al. (2005) for a sample of 23 bursts with known redshift, is vanishingly small.

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arXiv2005

The Beaming Factor and Other Open Questions In GRB Jets

I review several central open questions concerning GRB Jets. 1) I discuss a new estimates of the beaming correction for the rate of GRBs ~ 75 +- 20. 2) I discuss the universal structured jet (USJ) model and conclude that while jets might be structured they are less likely to be universal. 3) I discuss recent observations of a sideways expansion of a GRB afterglow and compare these with current hydrodynamics simulations of jet evolution. 4) I discuss the implications of resent outliers to the energy-angle relation.

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arXiv2005

Magnetic Fields in Gamma-Ray Bursts: A Short Review

Magnetic fields play a crucial role in the physics of Gamma-Ray Bursts (GRBs). Strong observational evidence indicates that the observed afterglow and most likely the prompt emission arise from synchrotron emission. It is possible that Poynting flux plays an important or even dominant role in the relativistic outflow from the inner engine, but like in other astronomical relativistic jets this suggestion is controversial. Finally, it is likely that magnetic fields larger than $10^{15}$ G occur within GRBs' inner engines and contribute to the acceleration and collimation of the relativistic jets. I review here the GRB fireball model and discuss the role that magnetic fields play in its various components. I suggest that the early afterglow, that reflects the initial interaction of the relativistic jet with its surrounding matter is the best available tool to explore the nature of relativistic outflow in astronomical relativistic jets.

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arXiv2005

Early GRB afterglow from a reverse shock as a tracer of the prompt gamma-ray light curve

We discuss the optical and radio early afterglow emission of the reverse shock that crosses a baryonic ejecta as it interacts with the external interstellar medium (ISM). We show that the peak of the optical flash divides the light curve of the reverse shock into two distinctive phases. The emission after the peak depends weakly on the initial conditions of the ejecta and therefore it can be used as an identifiable signature of a reverse shock emission. On the other hand, the emission before the optical peak is highly sensitive to the initial conditions and therefore can be used to investigate the initial hydrodynamic profile of the ejecta. In particular, if the prompt $γ$-ray emission results from internal shocks, the early reverse shock emission should resemble a smoothed version of the prompt $γ$-ray light curve.

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arXiv2005

The Luminosity and Redshift Distributions of Short-Duration GRB

Using the BATSE peak flux distribution we rederive the short GRBs luminosity function and compare it with the observed redshift distribution of long bursts. We show that both distributions are compatible with the assumption that short as well as long bursts follow the star formation rate. In this case the difference between the two observed distributions can be interpreted as arising mostly from differences in the detector's sensitivity to long and short bursts, while the local rate of short bursts is 0.1 h_70^3 Gpc^-3 yr^-1 . We also consider the possibility that short GRBs may be associated with binary neutron star mergers and estimate the effect of the merging time delay on the luminosity function and redshift distribution. We find that in this case the local rate of short GRBs is ~0.8 h_70^3 Gpc^-3 yr^-1. Assuming that all binary merging systems lead to short GRBs, we find a typical jet opening angle of 1.6^o

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arXiv2004

The Origin of the Binary Pulsar J0737-3039B

Evolutionary scenarios suggest that the progenitor of the new binary pulsar J0737-3039B \cite{ref1,ref2} was a He-star with $M > 2.1-2.3~\Ms$ \cite{ref3,ref4}. We show that this case implies that the binary must have a large ($>120$ km/s) center of mass velocity. However, the location, $\sim 50$ pc from the Galactic plane, suggests that the system has, at high likelihood, a significantly smaller center of mass velocity and a progenitor more massive than 2.1~$\Ms$ is ruled out (at 97% c.l.). A progenitor mass around 1.45~$\Ms$, involving a new previously unseen gravitational collapse, is kinematically favored. The low mass progenitor is consistent with the recent scintillations based velocity measurement of 66$\pm 15$ km/s \cite{ref12new} (and which also rules out the high mass solution at 99% c.l.) and inconsistent with the higher earlier estimates of 141$\pm 8.5$ km/s \cite{ref11new}. Direct proper motion measurements, that should be available within a year or so, should better help to distinguish between the two scenarios.

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arXiv2004

Gamma Ray Bursts as Probes of Quantum Gravity

Gamma ray bursts (GRBs) are short and intense pulses of $γ$-rays arriving from random directions in the sky. Several years ago Amelino-Camelia et al. pointed out that a comparison of time of arrival of photons at different energies from a GRB could be used to measure (or obtain a limit on) possible deviations from a constant speed of light at high photons energies. I review here our current understanding of GRBs and reconsider the possibility of performing these observations.

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arXiv2004

Early Afterglow Emission from a Reverse Shock as a Diagnostic Tool for GRB Outflows

The Gamma-Ray burst (GRB) - afterglow transition is one of the most interesting and least studied GRB phases. During this phase the relativistic ejecta begins interacting with the surrounding matter. A strong short lived reverse shock propagates into the ejecta (provided that it is baryonic) while the forward shock begins to shape the surrounding matter into a Blandford-McKee profile. We suggest a parametrization of the early afterglow light curve and we calculate (analytically and numerically) the observed parameters that results from a reverse shock emission (in an interstellar medium [ISM] environment). We present a new fingerprint of the reverse shock emission that is added to the well known $t^{-2}$ optical decay. Observation of this signature would indicate that the reverse shock dominates the emission during the early afterglow. The existence of a reverse shock will in turn imply that the relativistic ejecta contains a significant baryonic component. This signature would also imply that the surrounding medium is an ISM. We further show that: (i) The reverse shock optical flash depends strongly on initial conditions of the relativistic ejecta. (ii) Previous calculations have generally overestimated the strength of this optical flash. (iii) If the reverse shock dominates the optical flash then detailed observations of the early afterglow light curve would possibly enable us to determine the initial physical conditions within the relativistic ejecta and specifically to estimate its Lorentz factor and its width.

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arXiv2004

The apparent size of GRB afterglows as a test of the Fireball model

Taylor et al. (2004) reported recently on the first direct measurement of the apparent size of a GRB afterglow. Here we confront these observations with the predictions of the fireball model. We use a simple model to study numerically the evolution of the fireball and determine its apparent size, starting at the early spherical expansion phase through the jet break and into the Newtonian regime. We perform these calculations on the background of a uniform interstellar medium and a massive stellar wind environment. We find that the calculated apparent size at the time of the measurements taken by Taylor et al. depends only weakly on the chosen parameters (the jet opening angle, the energy, and the external density profile). Thus it is reassuring that the only possible outcome of the model, within the plausible range of parameters, agrees very well with the data. These measurements therefore present us with a strong test of possible GRB models, which the fireball model passes successfully.

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arXiv2004

The Physics of Gamma-Ray Bursts

Gamma-Ray Bursts (GRBs), short and intense pulses of low energy gamma-rays, have fascinated astronomers and astrophysicists since their unexpected discovery in the late sixties. During the last decade, several space missions: BATSE (Burst and Transient Source Experiment) on Compton Gamma-Ray Observatory, BeppoSAX and now HETE II (High-Energy Transient Explorer), together with ground optical, infrared and radio observatories have revolutionized our understanding of GRBs showing that they are cosmological, that they are accompanied by long lasting afterglows and that they are associated with core collapse Supernovae. At the same time a theoretical understanding has emerged in the form of the fireball internal-external shocks model. According to this model GRBs are produced when the kinetic energy of an ultra-relativistic flow is dissipated in internal collisions. The afterglow arises when the flow is slowed down by shocks with the surrounding circum-burst matter. This model has numerous successful predictions like the prediction of the afterglow itself, the prediction of jet breaks in the afterglow light curve and of an optical flash that accompanies the GRBs themselves. In this review I focus on theoretical aspects and on physical processes believed to take place in GRBs.

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arXiv2004

GRB 990123 revisited: Further Evidence for a Reverse Shock

Recently we have presented a new theoretical analysis of the reverse shock emission. We use this analysis here to revisit GRB 990123. We find new and compelling evidences that the optical flash and the radio flare of GRB 990123 resulted from a reverse shock. This suggests that a significant fraction of the energy of the relativistic ejecta must have been carried by baryons. It also suggests that the external medium is an ISM and that in this burst the reverse shock emission dominated at early time over other possible processes. We use the early optical emission to constrain the physical parameters of the original ejecta and the microscopic parameters in the emitting reverse shocked region.

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arXiv2004

On the Low Progenitor Mass of the Binary Pulsar J0737-3039B

The observed orbital parameters of the recently discovered binary pulsar J0737-3039 are used to constrain the progenitor system. In particular, the small observed eccentricity and the small inferred peculiar velocity imply that during the formation of the second NS in the system, a very small mass Delta M <~ 0.15 Msun, was ejected. A progenitor more massive than 1.4 Msun is unlikely and a progenitor more massive than 2.3 Msun is practically ruled out. We therefore argue that the companion removed most of the progenitor envelope prior to its collapse. The second collapse may have been a SN of Type Ib, or more probably, of Type Ic. Alternatively, it could have been a bare collapse, an option which is kinematically favored. Future constraints on the peculiar motion could solidify this constraint on the mass loss. A future determination of the pulsars' spin vectors and proper motion could allow us the complete determination of the conditions during the second collapse.

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arXiv2004

Caged Black Holes: Black Holes in Compactified Spacetimes I -- Theory

In backgrounds with compact dimensions there may exist several phases of black objects including the black-hole and the black-string. The phase transition between them raises puzzles and touches fundamental issues such as topology change, uniqueness and Cosmic Censorship. No analytic solution is known for the black hole, and moreover, one can expect approximate solutions only for very small black holes, while the phase transition physics happens when the black hole is large. Hence we turn to numerical solutions. Here some theoretical background to the numerical analysis is given, while the results will appear in a forthcoming paper. Goals for a numerical analysis are set. The scalar charge and tension along the compact dimension are defined and used as improved order parameters which put both the black hole and the black string at finite values on the phase diagram. Predictions for small black holes are presented. The differential and the integrated forms of the first law are derived, and the latter (Smarr's formula) can be used to estimate the ``overall numerical error''. Field asymptotics and expressions for physical quantities in terms of the numerical ones are supplied. Techniques include ``method of equivalent charges'', free energy, dimensional reduction, and analytic perturbation for small black holes.

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arXiv2004

Caged Black Holes: Black Holes in Compactified Spacetimes II - 5d Numerical Implementation

We describe the first convergent numerical method to determine static black hole solutions (with S^3 horizon) in 5d compactified spacetime. We obtain a family of solutions parametrized by the ratio of the black hole size and the size of the compact extra dimension. The solutions satisfy the demanding integrated first law. For small black holes our solutions approach the 5d Schwarzschild solution and agree very well with new theoretical predictions for the small corrections to thermodynamics and geometry. The existence of such black holes is thus established. We report on thermodynamical (temperature, entropy, mass and tension along the compact dimension) and geometrical measurements. Most interestingly, for large masses (close to the Gregory-Laflamme critical mass) the scheme destabilizes. We interpret this as evidence for an approach to a physical tachyonic instability. Using extrapolation we speculate that the system undergoes a first order phase transition.

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arXiv2003

Some Recent Peculiarities of the Early Afterglow

We consider some recent developments in GRB/afterglow observations: (i) the appearance of a very hard prompt component in GRB 941017, and (ii) variability in the early afterglow light curves of GRB 021004 and GRB 030329. We show that these observations fit nicely within the internal-external shocks model. The observed variability indicates that the activity of the inner engine is more complicated than was thought earlier and that it involves patchy shells and refreshed shocks. We refute the claims of Berger et al. and of Sheth et al. that the radio and mm observations of GRB 030329 are inconsistent with refreshed shocks.

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arXiv2003

Implications of the $γ$-ray Polarization of GRB 021206

We compare two possible scenarios for the producing of high level of polarization within the prompt emission of a GRB: synchrotron emission from a relativistic jet with a uniform (in space and time) magnetic field and synchrotron emission from a jet with a random magnetic field in the plane of the shock. Somewhat surprisingly we find that both scenarios can produce a comparable level of polarization ($\sim 45-50$% for the uniform field and $\sim 30-35$% for a random field). Uniform time independent field most naturally arises by expansion of the field from the compact object. It requires a $10^{12}$G field at the source and a transport of the field as $\propto R^{-1}$. It {\it does not} imply Poynting flux domination of the energy of the wind. There is a serious difficulty however, within this scenario, accounting for particle acceleration (which requires random magnetic fields) both for Poynting flux and non-Poynting flux domination. Significant polarization can also arise from a random field provided that the observer is located within $1/Γ$ orientation from a narrow ($θ_j \sim 1/Γ$) jet. While most jets are wider, the jet of GRB 021206 from which strong polarization was recently observed, was most likely very narrow. GRB 021206 is among the strongest bursts ever. Adopting the energy-angle relation we find an estimated angle of $<1/40$rad or even smaller. Thus, for this particular burst the required geometry is not unusual. We conclude that the RHESSI observations suggest that the prompt emission results from synchrotron radiation. However, in view of the comparable levels of polarizations predicted by both the random field and the homogeneous field scenarios these observations are insufficient to rule out or confirm either one.

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arXiv2003

Implications of Spacetime Quantization for the Bahcall-Waxman Neutrino Bound

There is growing interest in quantum-spacetime models in which small departures from Lorentz symmetry are governed by the Planck scale. In particular, several studies have considered the possibility that these small violations of Lorentz symmetry may affect various astrophysical observations, such as the evaluation of the GZK limit for cosmic rays, the interaction of TeV photons with the Far Infrared Background and the arrival time of photons with different energies from cosmological sources. We show that the same Planck-scale departures from Lorentz symmetry that lead to a modification of the GZK limit which would be consistent with the observations reported by AGASA, also have significant implications for the evaluation of the Bahcall-Waxman bound on the flux of high-energy neutrinos produced by photo-meson interactions in sources of size not much larger than the proton photo-meson mean free path.

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