Gamma-ray Burst Outflows Research Articles

Neutron-loaded outflows in gamma-ray bursts

Relativistic neutron-loaded outflows in gamma-ray bursts are studied at their early stages, before deceleration by a surrounding medium. The outflow has four components: radiation, electrons, protons and neutrons. The components interact with each other and exchange energy as the outflow expands. The presence of neutrons significantly changes the outflow evolution. Before neutrons decouple from protons, friction between the two components increases their temperatures by many orders of magnitude. After the decoupling, the gradual neutron decay inside the outflow has a drag effect on the protons and reduces their final Lorentz factor. In inhomogenous outflows, this drag can reduce the Lorentz-factor contrast and affect the development and strength of internal shocks.

Peak Energy Clustering and Efficiency in Compact Objects

We study the properties of plasmas containing a low energy thermal photon component at comoving temperature \theta \equiv kT'/m_e c^2 \sim 10^{-5} - 10^{-2} interacting with an energetic electron component, characteristic of, e.g., the dissipation phase of relativistic outflows in gamma-ray bursts (GRB's), X-ray flashes, and blazars. We show that, for scattering optical depths larger than a few, balance between Compton and inverse-Compton scattering leads to the accumulation of electrons at values of $\gamma\beta ~ 0.15 - 0.3$. For optical depths larger than ~ 100, this leads to a peak in the comoving photon spectrum at 1-10 keV, very weakly dependent on the values of the free parameters. In particular, these results are applicable to the internal shock model of GRB, as well as to slow dissipation models, e.g. as might be expected from reconnection, if the dissipation occurs at a sub-photospheric radii. For GRB bulk Lorentz factors ~ 100, this results in observed spectral peaks clustering in the 0.1-1 MeV range, with conversion efficiencies of electron into photon energy in the BATSE range of ~ 30%.

Acoustic causality in relativistic shells and its implication for gamma-ray bursts

The motion of sound waves propagating in the perfect fluid with inhomogeneous background flow is effectively described as a massless scalar field on a curved space-time. This effective geometry is characterized by the acoustic metric, which depends on the background flow, and null geodesics on the geometry express the acoustic causal structure. Therefore by the effective geometry we can easily study the causality on the flows. In this paper, we consider a spherically symmetric, relativistic outflow and present the maximal causally connected region for a supersonic flow, which is a simple analog of gamma-ray bursts (GRBs). Causality in GRB outflows is important when we consider the coherency of the magnetic field. When the Lorentz factor of the radial velocity of the flow is constant or obeys the power law with respect to the radial coordinate r, we can solve it analytically. As a result we show that in the constant case the maximum angle is proportional to the inverse of the Lorentz factor and logarithmically increases with respect to r; in contrast, accelerative expansions in the power-law case make this angle bounded.

X-ray Flashes from Off-axis Nonuniform Jets

It is widely believed that outflows of gamma-ray bursts are jetted. Some people also suggest that the jets may have structures like $\epsilon(\theta)\propto\theta^{-k}$. We test the possibility of X-ray flashes coming from such jets in this paper. Both qualitative and quantitative analyses have shown that this model can reproduce most observational features for both X-ray flashes and gamma-ray bursts. Using common parameters of gamma-ray bursts, we have done both uniform and nonuniform jets' numerical calculations for their fluxes, spectra and peak energies. It seems that nonuniform jets are more appropriated to these observational properties than uniform ones. We also give a spectrum and flux fit to the most significant X-ray flash, XRF971019 by our model. We also have shown that in our model the observational ratio of gamma-ray bursts to X-ray flashes is about several.

High-Energy Cosmic Rays from Galactic and Extragalactic γ-Ray Bursts

A model for high-energy (≳ 10 14 eV) cosmic rays (HECRs) from galactic and extragalactic gamma-ray bursts (GRBs) is summarized. Relativistic outflows in GRBs are assumed to inject power-law distributions of CR protons and ions to the highest (≳ 10 20 eV) energies. A diffusive propagation model for HECRs from a single recent GRB within ≈ 1 kpc from Earth explains the CR spectrum near and above the knee. The CR spectrum at energies above ∼ 10 18 eV is fit with a component from extragalactic GRBs. By normalizing the energy injection rate to that required to produce the CR flux from extragalactic sources observed locally, we determine the amount of energy a typical GRB must release in the form of nonthermal hadrons. Our interpretation of the HECR spectrum requires that GRBs are hadronically dominated, which would be confirmed by the detection of HE neutrinos from GRBs.

Early afterglow emission from a reverse shock as a diagnostic tool for gamma-ray burst outflows

The gamma-ray burst‐afterglow transition is one of the most interesting and least studied gamma-ray burst 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 result from a reverse shock emission (in an interstellar medium 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 interstellar medium. We further show the following. (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. Ke yw ords: hydrodynamics ‐ shock waves ‐ gamma-rays: bursts.

Stability of an Ultrarelativistic Blast Wave in an External Medium with a Steep Power‐Law Density Profile

We examine the stability of self-similar solutions for an accelerating relativistic blast wave that is generated by a point explosion in an external medium with a steep radial density profile of a power-law index greater than 4.134. These accelerating solutions apply, for example, to the breakout of a gamma-ray burst outflow from the boundary of a massive star, as assumed in the popular collapsar model. We show that short wavelength perturbations may grow, but only by a modest factor 10.

Nucleosynthesis in $\gamma-$ray bursts outflows

It is shown that fusion of neutrons and protons to He-4 nuclei occurs in gamma-ray burst outflows in a process similar to big-bang nucleosynthesis in the early Universe. Only the surviving free neutrons can then decouple kinematically from the charged fluid so that the multi-GeV neutrino signal predicted from inelastic nuclear n-p collisions is significantly reduced. It is also argued that a sizeable fraction of ultra-high energy cosmic rays accelerated in gamma-ray bursts should be He-4 nuclei.

Magnetohydrodynamics of Gamma-Ray Burst Outflows

Using relativistic, axisymmetric, ideal MHD, we examine the outflow from a disk around a compact object, taking into account the baryonic matter, the electron-positron/photon fluid, and the large-scale electromagnetic field. Focussing on the parameter regime appropriate to gamma-ray burst outflows, we demonstrate, through exact self-similar solutions, that the thermal force (which dominates the initial acceleration) and the Lorentz force (which dominates further out and contributes most of the acceleration) can convert up to ~50% of the initial total energy into asymptotic baryon kinetic energy. We examine how baryon loading and magnetic collimation affect the structure of the flow, including the regime where emission due to internal shocks could take place.

Spectral Features from Ultrarelativistic Ions in Gamma-Ray Bursts?

Gamma ray burst outflows may entrain small blobs or filaments of dense, highly ionized metal rich material. Such inhomogeneities, accelerated by the flow to Lorentz factors in the range 10-100, could have a significant coverage factor, and give rise to broad features, especially due to Fe K-edges, which influence the spectrum below the MeV range, leading to a progressively decreasing hardness ratio.