16519092
Analytic Light Curves of Gamma-Ray Burst Afterglows: Homogeneous versus Wind External Media
Assuming an adiabatic evolution of a gamma-ray burst (GRB) remnant interacting with an external medium, we calculate the injection, cooling, and absorption break frequencies and the afterglow flux for plausible orderings of the break and observing frequencies. The analytical calculations are restricted to a relativistic remnant and, in the case of collimated ejecta, to the phase where there is an insignificant lateral expansion. Results are given for both a homogeneous external medium and a wind ejected by the GRB progenitor. We compare the afterglow emission at different observing frequencies, for each type of external medium. It is found that observations at submillimeter frequencies during the first day provide the best way of discriminating between the two models. By taking into account the effect of inverse Compton (IC) scatterings on the electron cooling, a new possible time dependence of the cooling break is identified. The signature of the upscattering losses could be seen in the optical synchrotron emission from a GRB remnant interacting with a preejected wind, as a temporary mild flattening of the afterglow decay. The upscattered radiation itself should be detected in the soft X-ray emission from GRB remnants running into denser external media, starting a few hours after the main event.
14393282
Scattered emission from a relativistic outflow and its application to gamma-ray bursts
We investigate a scenario of photon scattering by electrons within a relativistic outflow. The outflow is composed of discrete shells with different speeds. One shell emits radiation for a short duration. Some of this radiation is scattered by the shell(s) behind. We calculate in a simple two-shell model the observed scattered flux density as a function of the observed primary flux density, the normalized arrival time delay between the two emission components, the Lorentz factor ratio of the two shells and the scattering shell's optical depth. Thomson scattering in a cold shell and inverse Compton scattering in a hot shell are both considered. The results of our calculations are applied to the gamma-ray bursts and the afterglows. We find that the scattered flux from a cold slower shell is small and likely to be detected only for those bursts with very weak afterglows. A hot scattering shell could give rise to a scattered emission as bright as the X-ray shallow decay component detected in many bursts, on a condition that the isotropically equivalent total energy carried by the hot electrons is large, ∼10 52 -10 56 erg. The scattered emission from a faster shell could appear as a late short γ-ray/MeV flash or become part of the prompt emission depending on the delay of the ejection of the shell.
53358770
Evidence for mild deviation from power-law distribution of electrons in relativistic shocks: GRB 090902B
Many previous studies have determined that the long lasting emission at X-ray, optical and radio wavelengths from gamma-ray bursts (GRBs), called the afterglow, is likely produced by the external forward shock model. In this model, the GRB jet interacts with the circum-stellar medium and drives a shock that heats the medium, which radiates via synchrotron emission. In this work, we carried out a detailed analysis of the late time afterglow data of GRB 090902B using a very careful accounting of the Inverse Compton losses. We find that in the context of the external forward shock model, the only viable option to explain the X-ray and optical data of GRB 090920B is to have the electron energy distribution deviate from a power-law shape and exhibit some slight curvature immediately downstream of the shock front (we explored other models that rely on a single power-law assumption, but they all fail to explain the observations). We find the fraction of the energy of shocked plasma in magnetic field to be ∼10 −6 using late time afterglow data, which is consistent with the value obtained using early gamma-ray data. Studies like the present one might be able to provide a link between GRB afterglow modelling and numerical simulations of particle acceleration in collisionless shocks. We also provide detailed calculations for the early (10 3 s) high-energy (>100 MeV) emission and confirm that it is consistent with origin in the external forward shock. We investigated the possibility that the ∼10 keV excess observed in the spectrum during the prompt phase also has its origin in the external shock and found the answer to be negative.
53709333
Supernovae-induced accretion and star formation in the inner kiloparsec of a gaseous disc
We consider the effects of supernovae (SNe) on accretion and star formation in a massive gaseous disc in a large primeval galaxy. The gaseous disc we envisage, roughly 1 kpc in size with ≳108 M⊙ of gas, could have formed as a result of galaxy mergers where tidal interactions removed angular momentum from gas at larger radius and thereby concentrated it within the central ∼1 kpc region. We find that SNe lead to accretion in the disc at a rate of roughly 0.1–1 M⊙ yr−1 and induce star formation at a rate of ∼10–100 M⊙ per year which contributes to the formation of a bulge; a part of the stellar velocity dispersion is due to SN shell speed from which stars are formed and a part due to the repeated action of stochastic gravitational field of SNe remnant network on stars. The rate of SN in the inner kpc is shown to be self-regulating, and it cycles through phases of low and high activity. The SN-assisted accretion transports gas from about 1 kpc to within a few pc of the centre. If this accretion were to continue down to the central black hole then the resulting ratio of black hole mass to the stellar mass in the bulge would be of the order of ∼ 10−2–10−3, in line with the observed Magorrian relation.
53321571
COLLAPSAR ACCRETION AND THE GAMMA-RAY BURST X-RAY LIGHT CURVE
We present axisymmetric hydrodynamical simulations of the long-term accretion of a rotating gamma-ray burst (GRB) progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive-mesh-refinement code FLASH in two spatial dimensions and with an explicit shear viscosity. The evolution of the central accretion rate exhibits phases reminiscent of the long GRB γ-ray and X-ray light curve, which lends support to the proposal by Kumar et al. that the luminosity is modulated by the central accretion rate. In the first "prompt" phase, the black hole acquires most of its final mass through supersonic quasiradial accretion occurring at a steady rate of ~0.2 M ☉ s–1. After a few tens of seconds, an accretion shock sweeps outward through the star. The formation and outward expansion of the accretion shock is accompanied with a sudden and rapid power-law decline in the central accretion rate , which resembles the L X ∝ t –3 decline observed in the X-ray light curves. The collapsed, shock-heated stellar envelope settles into a thick, low-mass equatorial disk embedded within a massive, pressure-supported atmosphere. After a few hundred seconds, the inflow of low angular momentum material in the axial funnel reverses into an outflow from the thick disk. Meanwhile, the rapid decline of the accretion rate slows down, which is potentially suggestive of the "plateau" phase in the X-ray light curve. We complement our adiabatic simulations with an analytical model that takes into account the cooling by neutrino emission and estimate that the duration of the prompt phase can be ~20 s. The model suggests that the steep decline in GRB X-ray light curves is triggered by the circularization of the infalling stellar envelope at radii where the virial temperature is below 1010 K, such that neutrino cooling is inefficient and an outward expansion of the accretion shock becomes imminent; GRBs with longer prompt γ-ray emission should have more slowly rotating envelopes.
53385261
Supernovae Powered by Collapsar Accretion in Gamma-Ray Burst Sources
The association of long-duration gamma-ray bursts (LGRBs) with Type Ic supernovae presents a challenge to supernova explosion models. In the collapsar model for LGRBs, gamma rays are produced in an ultrarelativistic jet launching from the magnetosphere of the black hole that forms in the aftermath of the collapse of a rotating progenitor star. The jet is collimated along the star's rotation axis, but the concomitant luminous supernova should be relatively?though certainly not entirely?spherical, and should synthesize a substantial mass of 56Ni. Our goal is to provide a qualitative assessment of the possibility that accretion of the progenitor envelope onto the black hole, which powers the LGRB, could also deposit sufficient energy and nickel mass in the envelope to produce a luminous supernova. For this, the energy dissipated near the black hole during accretion must be transported outward, where it can drive a supernova-like shock wave. Here we suggest that the energy is transported by convection and develop an analytical toy model, relying on global mass and energy conservation, for the dynamics of stellar collapse. The model suggests that a ~10 000 km s?1 shock can be driven into the envelope and that ~1051 erg explosions are possible. The efficiency with which the accretion energy is being transferred to the envelope is governed by the competition of advection and convection at distances ~100-1000 km from the black hole and is sensitive to the values of the convective mixing length, the magnitude of the effective viscous stress, and the specific angular momentum of the infalling envelope. Substantial masses of 56Ni may be synthesized in the convective accretion flow over the course of tens of seconds from the initial circularization of the infalling envelope around the black hole. The synthesized nickel is convectively mixed with a much larger mass of unburned ejecta.
120167461
Anomalous behavior of the level density parameter in neutron and charged particle evaporation
The compound nucleus $^{76}\mathrm{Kr}*$ was populated at the excitation energy of $75\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ and angular momentum of $39\ensuremath{\hbar}$ in fusion reactions with two complementary, mass-symmetric $(^{31}\mathrm{P}+^{45}\mathrm{Sc})$ and mass-asymmetric $(^{12}\mathrm{C}+^{64}\mathrm{Zn})$ entrance channels. The neutron evaporation spectra were measured and compared with the predictions of statistical model calculations using the transmission coefficients for the spherical nuclei in the inverse absorption channel, the rotating liquid drop model moment of inertia, and the normal systematic value of $A∕8$ for the level density parameter $a$. The results for the mass-asymmetric reaction are found to be consistent with the predictions of the statistical model calculations. However, for the mass-symmetric reaction $(^{31}\mathrm{P}+^{45}\mathrm{Sc})$, the experimental spectra are found to be harder than the theoretical neutron spectra and the statistical model calculations require a lower value of $A∕10$ for the parameter $a$ to reproduce the shape of the experimental spectra, indicating the neutron to be evaporated at higher temperature for the same excitation energy and angular momentum in symmetric system. According to the dynamical model, the formation time $(37\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}\phantom{\rule{0.3em}{0ex}}\mathrm{sec})$ of the compound nucleus for the symmetric $^{31}\mathrm{P}+^{45}\mathrm{Sc}$ system is significantly higher than that $(29\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}\phantom{\rule{0.3em}{0ex}}\mathrm{sec})$ for the asymmetric $^{12}\mathrm{C}+^{64}\mathrm{Zn}$ system. This may probably lead to the formation of a temperature-equilibrated dinuclear complex that may be responsible for neutron emission at higher temperature in the case of the symmetric system.
120052850
Search for entrance channel effects in heavy ion induced fusion reactions through the compound system79Rb
124715654
Search for an effect of shell closure on nuclear dissipation via a neutron-multiplicity measurement
118602066
On the smoothness of multi center coplanar black hole and membrane horizons
We study the differentiability of the metric and other fields at any of the horizons of multi center Reissner-Nordstrom black hole solutions in $d \ge 5$ and of multi center $M2$ brane solutions. The centers are distributed in a plane in transverse space, hence termed coplanar. We construct the Gaussian null co-ordinate system for the neighborhood of a horizon by solving the geodesic equations in expansions of (appropriate powers of) the affine parameter. Organizing the harmonic functions that appear in the solution in terms of what can be called generalized Gegenbauer polynomials is key to obtaining the solution to the geodesic equations in a compact and manageable form. We then compute the metric and other fields in the Gaussian null co-ordinate system and find that the differentiability of the coplanar solution is \emph{identical to} the differentiability of the collinear solution (centers distributed on a line in transverse space). The results of this paper thus run counter to a suggestion in the literature that posits reduction in the degree of smoothness to accompany reduction in symmetries. We end the paper with a conjecture on the degree of smoothness of the most general multi center solution, the one with centers distributed arbitrarily and hence possessing no transverse spatial isometries.
120551696
alpha-particle emission as a probe of dynamical deformations
We have measured alpha-particle spectra at different laboratory angles from the fusion reaction Si-28+V-51 at 140 MeV. These spectra deviate at higher energies from the statistical model calculations using rotating liquid drop model values of the moment of inertia. In order to explain the experimental spectra, changes were required in the moments of inertia corresponding to deformation, which suggests dynamical effects on the deexcitation process for charged-particle emission. The comparison of decay times with formation times implies that light charged particles are preferentially emitted prior to the full relaxation of the compound nucleus. Statistical models and dynamical calculations have been employed in an attempt to interpret the experimental data. The results were compared with the Si-28+Al-27 system studied earlier in order to understand the symmetric and asymmetric entrance channel effects in the formation of the compound system.