15830405
Cooling Timescales and Temporal Structure of Gamma-Ray Bursts
A leading mechanism for producing cosmological gamma-ray bursts (GRBs) is via ultrarelativistic particles in an expanding fireball. The kinetic energy of the particles is converted into thermal energy in two shocks, a forward shock and a reverse shock, when the outward flowing particles encounter the interstellar medium. The thermal energy is then radiated via synchrotron emission and Comptonization. We estimate the synchrotron cooling timescale of the shocked material in the forward and reverse shocks 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 energy density of the magnetic field in the shocked material must be greater than about 1% of the thermal energy density. In addition, the electrons must be nearly in equipartition with the protons, since otherwise we do not have reasonable radiative efficiencies of GRBs. 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, they provide a plausible explanation for several observations. 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 presence of a characteristic break energy in GRB spectra.
17494376
Hydrodynamic time scales and temporal structure of 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’ durations are related to the deceleration time, the variations are 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.
119377297
GRB 990123: The Optical Flash and the Fireball Model
We show that the ongoing observations of the remarkable burst GRB 990123 agree with the predictions of the fireball model. Specifically, the observations confirm the recent prediction that the reverse shock propagating into the ejecta would produce a very strong prompt optical flash. This reverse shock has also produced the 8.46 GHz radio signal, observed after 1 day. The forward shock, which is the origin of the classical afterglow, has produced the prompt X-ray signal as well as the late optical and IR emission. The observations suggest that the initial Lorentz factor of the ejecta was ~200. Within factors of order unity, this crude model explains all current observations of GRB 990123.
119538851
Planck scale deformation of Lorentz symmetry as a solution to the UHECR and the TeV gamma paradoxes
One of the most puzzling current experimental physics paradoxes is the arrival on Earth of ultrahigh energy cosmic rays (UHECRs) with energies above the Greisen-Zatsepin-Kuzmin threshold $(5\ifmmode\times\else\texttimes\fi{}{10}^{19} \mathrm{eV}).$ Photopion production by cosmic microwave background radiation photons should reduce the energy of these protons below this level. The recent observation of 20 TeV photons from Mk 501 (a BL Lac object at a distance of 150 Mpc) is another somewhat similar paradox. These high energy photons should have disappeared due to pair production with IR background photons. A common feature of these two paradoxes is that they can both be seen as ``threshold anomalies'': energies corresponding to an expected threshold (pion production or pair creation) are reached but the threshold is not observed. Several (relatively speculative) models have been proposed for the UHECR paradox. No solution has yet been proposed for the $\mathrm{TeV}\ensuremath{-}\ensuremath{\gamma}$ paradox. Remarkably, the single drastic assumption of the violation of ordinary Lorentz invariance would resolve both paradoxes. We present here a formalism for the systematic description of the type of Lorentz-invariance deformation (LID) that could be induced by the nontrivial short-distance structure of space-time, and we show that this formalism is well suited for comparison of experimental data with LID predictions. We use the UHECR and $\mathrm{TeV}\ensuremath{-}\ensuremath{\gamma}$ data, as well as upper bounds on time-of-flight differences between photons of different energies, to constrain the parameter space of the LID. A model with only two free parameters, an energy scale and a dimensionless parameter characterizing the functional dependence on the energy scale, is shown to be sufficient to solve both the UHECR and the $\mathrm{TeV}\ensuremath{-}\ensuremath{\gamma}$ threshold anomalies while satisfying the time-of-flight bounds. The allowed region of the two-parameter space is relatively small, but, remarkably, it fits perfectly the expectations of the quantum-gravity-motivated space-time models known to support such deformations of Lorentz invariance: an integer value of the dimensionless parameter and a characteristic energy scale constrained to a narrow interval in the neighborhood of the Planck scale.
119206775
Reconciling the Gamma-Ray Burst Rate and Star Formation Histories
While there are numerous indications that gamma-ray bursts (GRBs) arise from the deaths of massive stars, the GRB rate does not follow the global cosmic star formation rate and, within their hosts, GRBs are more concentrated in regions of very high star formation. We explain both puzzles here. Using the publicly available VESPA database of the Sloan Digital Sky Survey (SDSS) Data Release 7 spectra, we explore a multi-parameter space in galaxy properties such as stellar mass, metallicity, and dust to find the subset of galaxies that reproduces the GRB rate recently obtained by Wanderman & Piran. We find that only galaxies with present stellar masses below <1010 M ☉ and low metallicity reproduce the observed GRB rate. This is consistent with direct observations of GRB hosts and provides an independent confirmation of the nature of GRB hosts. Because of the significantly larger sample of SDSS galaxies, we compute their correlation function and show that they are anti-biased with respect to dark matter: they are in filaments and voids. Using recent observations of massive stars in local dwarfs we show how the fact that GRB host galaxies are dwarfs can explain the observation that GRBs are more concentrated in regions of high star formation than are supernovae. Finally, we explain these results using new theoretical advances in the field of star formation.
15784559
The early afterglow
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 few dozens of seconds after the burst. The X-ray and γ -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 . In the optical band, the dominant emission arises from the reverse shock. This shock, carries a comparable amount of energy to the forward shock. It radiates this energy at much lower frequencies, producing a short optical flash of 15th magnitude or brighter.
18067084
Statistics of Weak Gravitational Lensing in Cold Dark Matter Models: Magnification Bias on Quasar Luminosity Functions
We compute statistical properties of weak gravitational lensing by large-scale structure in three cold dark matter (CDM) models: two —at models with 0) and (0.3, 0.7) and one open model () 0 , j 0 ) \ (1, with 0), where and are the density parameter and cosmological constant, respec- () 0 , j 0 ) \ (0.3, ) 0 j 0 tively. We use a particle-particle/particle-mesh (P3M) N-body code to simulate the formation and evolu- tion of large-scale structure in the universe. We perform 1.1 ) 107 ray-tracing experiments for each model, by computing the Jacobian matrix along random lines of sight, using the multiple lensplane algorithm. From the results of these experiments, we calculate the probability distribution functions of the convergences, shears, and magni—cations, and their rms values. We —nd that the rms values of the convergence and shear agree with the predictions of a nonlinear analytical model. We also —nd that the probability distribution functions of the magni—cations k have a peak at values slightly smaller than k \ 1 and are strongly skewed toward large magni—cations. In particular, for the high-density () 0 \ 1) model, a power-law tail appears in the distribution function at large magni—cations for sources at red- shifts The rms values of the magni—cations essentially agree with the nonlinear analytical predic- z s ( 2. tions for sources at low redshift but exceed these predictions for high redshift sources, once the power-law tail appears. We study the eUect of magni—cation bias on the luminosity functions of high- redshift quasars, using the calculated probability distribution functions of the magni—cations. We show that the magni—cation bias is moderate in the absence of the power-law tail in the magni—cation dis- tribution but depends strongly on the value of the density parameter In presence of the power-law ) 0 . tail, the bias becomes considerable, especially at the bright end of the luminosity functions where its logarithmic slope steepens. We present a speci—c example that demonstrates that the bias —attens the bright-side logarithmic slope of a double power-law luminosity function. Subject headings: cosmology: observationscosmology: theorydark matter ¨ gravitational lensinglarge-scale structure of universequasars: general
16735826
GRB 980425 in the Off‐Axis Jet Model of the Standard GRBs
Using a simple off‐axis jet model of GRBs, we can reproduce the observed unusual properties of the prompt emission of GRB 980425, such as the extremely low isotropic equivalent γ‐ray energy, the low peak energy, the high fluence ratio, and the long spectral lag when the jet with the standard energy of ∼ 1051 ergs and the opening half‐angle of 10°⩽Δθ⩽30° is seen from the off‐axis viewing angle θv ∼ Δθ + 10γ−1, where γ is a Lorentz factor of the jet. For our adopted fiducial parameters, if the jet that caused GRB 980425 is viewed from the on‐axis direction, the intrinsic peak energy Ep(1 + z) is ∼2.0–4.0 MeV, which corresponds to those of GRB 990123 and GRB 021004. Our model might be able to explain the other unusual properties of this event. We also discuss the connection of GRB 980425 in our model with the X‐ray flash, and the origin of a class of GRBs with small Eγ such as GRB 030329.
18307664
Cosmological X‐Ray Flashes from Off‐Axis Jets
The 〈V/Vmax〉 of the cosmological X‐ray flashes detected by WFC/BeppoSAX is calculated theoretically in a simple jet model. The total emission energy from the jet is assumed to be constant. We find that if the jet opening half‐angle is smaller than 0.03 radians, off‐axis emission from sources at z⩽4 can be seen. The theoretical 〈V/Vmax〉 is less than 0.4, which is consistent with the observational result of 0.27 ± 0.16 at the 1 σ level. This suggests that the off‐axis GRB jet with the small opening half‐angle at the cosmological distance can be identified as the cosmological X‐ray flash.
14610341
A Method for Weak-Lensing Flexion Analysis by the HOLICs Moment Approach*
We have developed a method for measuring higher order weak-lensing distortions of faint background galaxies, namely, the weak gravitational flexion, by fully extending the Kaiser, Squires, and Broadhurst method to include higher order lensing image characteristics (HOLICs) introduced by Okura, Umetsu, and Futamase. We take into account explicitly the weight function in calculations of noisy shape moments and the effect of higher order point-spread function (PSF) anisotropy, as well as isotropic PSF smearing. Our HOLICs formalism allows accurate measurements of flexion from practical observational data in the presence of a noncircular, anisotropic PSF. We test our method using mock observations of simulated galaxy images and actual, ground-based Subaru observations of the massive galaxy cluster A1689 (z = 0.183). From the high-precision measurements of the spin-1 first flexion, we obtain a high-resolution mass map in the central region of A1689. The reconstructed mass map shows a bimodal feature in the central 4' × 4' region of the cluster. The major, pronounced peak is associated with the brightest cluster galaxy and central cluster members, while the secondary mass peak is associated with a local concentration of bright galaxies. The refined, high-resolution mass map of A1689 demonstrates the power of the generalized weak-lensing analysis techniques for quantitative and accurate measurements of the weak gravitational lensing signal.