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.
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.
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.
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.
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.
9472390
The interpretation and implication of the afterglow of GRB 060218
The nearby GRB 060216/SN 2006aj was an extremely long, weak and very soft GRB. It was peculiar in many aspects. We show here that the x-ray, ultraviolet/optical and radio afterglows of GRB 060218 have to be attributed to different physical processes arising from different emission regions. From the several components in this burst's afterglow only the radio afterglow can be interpreted in terms of the common external shock model. We infer from the radio that the blast wave's kinetic energy was ~1050 erg and the circumburst matter had a constant rather than a wind profile. The lack of a 'jet break' up to 22 days implies that the outflow was wide, θj>1. Even though the late x-ray afterglow decays normally, it cannot result from an external shock because of its very steep spectrum. Furthermore, the implied kinetic energy would have produced far too much radio. We suggest that this x-ray afterglow could be attributed to a continued activity of the central engine that within the collapsar scenario could arise from fall-back accretion. 'Central engine afterglow' may be common in under-luminous GRBs where the kinetic energy of the blast wave is small and the external shock does not dominate over this component. Such under-luminous GRBs might be very common but they are rarely recorded because they can be detected only from short distances.
15116637
An off-axis jet model for GRB 980425 and low-energy gamma-ray bursts
Using a simple off-axis jet model of gamma-ray bursts (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. 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.
12824571
Peak energy-isotropic energy relation in the off-axis gamma-ray burst model
Using a simple uniform jet model of prompt emissions of gamma-ray bursts (GRBs), we reproduce the observed peak energy-isotropic energy relation. A Monte Carlo simulation shows that the low isotropic energy part of the relation is dominated by events viewed from off-axis directions, and the number of the off-axis events is about one-third of the on-axis emissions. We also compute the observed event rates of the GRBs, the X-ray-rich GRBs, and the X-ray flashes detected by High Energy Transient Explorer 2, and we find that they are similar.
9208829
A New Measure for Weak-Lensing Flexion
We study the possibility of using the octopole moment of gravitationally lensed images as a direct measure of the third-order weak gravitational lensing effect, or the gravitational flexion. It turns out that there is a natural relation between flexion and certain combinations of octopole/higher multipole moments that we call the higher order lensing image's characteristics (HOLICs). This will allow one to measure directly flexion from observable octopole and higher multipole moments of background images. We show based on simulated observations how the use of HOLICs can improve the accuracy and resolution of a reconstructed mass map, in which we assume Gaussian uncertainties in the shape measurements estimated using deep i'-band data of blank fields observed with Suprime-Cam on the Subaru telescope.
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.
15565151
Gamma-ray bursts in 1.8 < z < 5.6 suggest that the time variation of the dark energy is small
We calibrated the peak energy–peak luminosity relation of gamma-ray bursts (GRBs; the so-called Yonetoku relation) using 33 events with redshift z < 1.62 without assuming any cosmological models. The luminosity distances to GRBs are estimated from those of large numbers of Type Ia supernovae with z < 1.755. This calibrated Yonetoku relation can be used as a new cosmic distance ladder towards higher redshifts. We determined the luminosity distances of 30 GRBs in 1.8 < z < 5.6 using the calibrated relation, and plotted the likelihood contour in the (Ωm, ΩΛ) plane. We obtained (Ωm, ΩΛ) = (0.37+0.14−0.11, 0.63+0.11−0.14) for a flat universe. Because our method is free from the circularity problem, we can say that our universe in 1.8 < z < 5.6 is compatible with the so-called concordance cosmological model derived for z < 1.8. This suggests that the time variation of the dark energy is small or zero up to z∼ 6.