[en] The fully differential cross section for photon-electron pair production is integrated numerically over phase space. Results for the astrophysically interesting case are obtained, in which the interaction between an ultrarelativistic electron and a soft photon results in e^--e⁺ pair production. The positron spectrum is a function of the energies of both the photon and the electron, as well as the angle of interaction. An isotropic distribution of photons is assumed in integration over solid angle. The energy at which the positron distribution peaks is found to be inversely proportional to the photon energy and independent of the electron energy. The positron spectrum is integrated once more over initial electron energies for a power-law energy distribution of primary electrons. The same procedure is repeated for the recoil particle; it is shown that the peak of the recoil energy distribution depends linearly on the energy of the primary electron. Finally, semi-analytical expressions are obtained for the energy losses of the primary electrons. Triplet pair production turns out to be important in the magnetospheres of hot (T approx. 10⁷K), young (t less than or equal to 200 yrs) neutron stars. Interactions between thermal photons and highly relativistic electrons accelerated in the pulsar magnetosphere can initiate intense electromagnetic cascades. Finally, the possibility of a young pulsar (presumably arising from a recent supernova explosion) in the galactic center region is considered

[en] High-energy neutrons produced as a consequence of proton acceleration in active galactic nuclei (AGNs) can carry a substantial fraction of the initial luminosity outside of the central source. Part of this luminosity is expected to go into VHE and UHE γ-ray emission as a result of hadronic interactions of these relativistic particles with the accreting plasma. We find that the most efficient emitters will be AGNs with luminosities ∼0.01-0.1 of the Eddington luminosity. (author)

[en] A way is considered of positron production in pulsars: pair production from electron - photon collisions. It is shown that this process must be the dominant one for a young pulsar when its surface temperature is around 10⁷ K. This model is applied to the observed positron flux coming from the Galactic Center and it is shown that a pulsar with parameters close to those of the Crab at birth can explain, in principle, the flux. 10 references, 1 table

[en] A detailed model for positron production by a young pulsar is presented. It is shown that electromagnetic cascades can develop in a young pulsar's magnetosphere, and the model results are applied to the pulsar which is hypothesized to lie near the Galactic center. It is found that such a pulsar would be expected to produce relatively low energy electron-positron pairs with an efficiency rating high enough to explain the observed luminosity of the Galactic center annihilation line. Virtually all of the gamma ray continuum radiation produced in the cascades would be beamed along the magnetic poles of the neutron star, and therefore probably would not be observed from earth. Some observational predictions generated by the proposed model for the Galactic center positron source are given. 47 references

[en] The fully differential cross section for photon-electron pair production is integrated numerically over phase space. We obtain results for the astrophysically interesting case in which the interaction between an ultrarelativistic electron and a soft photon results in e^+- pair production. The positron spectrum is a function of the energies of both the photon and the electron, as well as the angle of interaction. We integrate over solid angle by assuming an isotropic distribution of photons. We find that the energy at which the positron distribution peaks is inversely proportional to the photon energy and independent of the electron energy. The positron spectrum is integrated once more over initial electron energies for a power-law energy distribution of primary electrons. The same procedure is repeated for the recoil particle; it is shown that the peak of the recoil energy distribution depends linearly on the energy of the primary electron. Finally, semianalytical expressions are obtained for the energy losses of the primary electrons

[en] We investigate the impact that the upper cutoff of the electron distribution has on the multiwavelength GRB afterglow spectra and on the corresponding X-ray light curves. We show under which conditions X-ray light curves with a plateau phase can be produced in this picture.

[en] The 'Supercritical Pile' is a very economical gamma-ray burst (GRB) model that provides for the efficient conversion of the energy stored in the protons of a relativistic blast wave (RBW) into radiation and at the same time produces-in the prompt GRB phase, even in the absence of any particle acceleration-a spectral peak at energy ∼1MeV. We extend this model to include the evolution of the RBW Lorentz factor Γ and thus follow its spectral and temporal features into the early GRB afterglow stage. One of the novel features of the present treatment is the inclusion of the feedback of the GRB produced radiation on the evolution of Γ with radius. This feedback and the presence of kinematic and dynamic thresholds in the model are sources of potentially very rich time evolution which we have began to explore. In particular, one can this way obtain afterglow light curves with steep decays followed by the more conventional flatter afterglow slopes, while at the same time preserving the desirable features of the model, i.e., the well-defined relativistic electron source and radiative processes that produce the proper peak in the νF _ν spectra. In this Letter, we present the results of a specific set of parameters of this model with emphasis on the multiwavelength prompt emission and transition to the early afterglow.

[en] We present a process that accounts for the steep decline and plateau phase of the Swift X-Ray Telescope (XRT) light curves, vexing features of gamma-ray burst (GRB) phenomenology. This process is an integral part of the 'supercritical pile' GRB model, proposed a few years ago to account for the conversion of the GRB kinetic energy into radiation with a spectral peak at E _pk ∼ m_ec ². We compute the evolution of the relativistic blast wave (RBW) Lorentz factor Γ to show that the radiation-reaction force due to the GRB emission can produce an abrupt, small (∼25%) decrease in Γ at a radius that is smaller (depending on conditions) than the deceleration radius R_D . Because of this reduction, the kinematic criticality criterion of the 'supercritical pile' is no longer fulfilled. Transfer of the proton energy into electrons ceases and the GRB enters abruptly the afterglow phase at a luminosity smaller by ∼m_p /m_e than that of the prompt emission. If the radius at which this slow-down occurs is significantly smaller than R_D , the RBW internal energy continues to drive the RBW expansion at a constant (new) Γ and its X-ray luminosity remains constant until R_D is reached, at which point it resumes its more conventional decay, thereby completing the 'unexpected' XRT light curve phase. If this transition occurs at R ≅ R_D , the steep decline is followed by a flux decrease instead of a 'plateau,' consistent with the conventional afterglow declines. Besides providing an account of these peculiarities, the model suggests that the afterglow phase may in fact begin before the RBW reaches R ≅ R_D , thus providing novel insights into GRB phenomenology.

[en] Supernova remnant blast shells can reach the flow speed v_s = 0.1c and shocks form at its front. Instabilities driven by shock-reflected ion beams heat the plasma in the foreshock, which may inject particles into diffusive acceleration. The ion beams can have the speed v_b ∼ v_s. For v_b << v_s the Buneman or upper-hybrid instabilities dominate, while for v_b >> v_s the filamentation and mixed modes grow faster. Here the relevant waves for v_b ∼ v_s are examined and how they interact nonlinearly with the particles. The collision of two plasma clouds at the speed v_s is modelled with particle-in-cell simulations, which convect with them magnetic fields oriented perpendicular to their flow velocity vector. One simulation models equally dense clouds and the other one uses a density ratio of 2. Both simulations show upper-hybrid waves that are planar over large spatial intervals and that accelerate electrons to ∼10 keV. The symmetric collision yields only short oscillatory wave pulses, while the asymmetric collision also produces large-scale electric fields, probably through a magnetic pressure gradient. The large-scale fields destroy the electron phase space holes and they accelerate the ions, which facilitates the formation of a precursor shock

[en] We present the statistics of the ratio, $\mathcal{R}$, between the prompt and afterglow “plateau” fluxes of gamma-ray bursts (GRBs). We define this as the ratio of the mean prompt energy flux in Swift BAT and the Swift XRT one, immediately following the steep transition between these two states and the beginning of the afterglow stage referred to as the “plateau”. Like the distribution of many other GRB observables, the histogram of $\mathcal{R}$ is log-normal with maximum at a value ${\mathcal{R}}_m\simeq <!--\; ???\; -->2000$, FWHM of about two decades, and with the entire distribution spanning about five decades in the value of $\mathcal{R}$. We note that the peak of the distribution is close to the proton-to-electron mass ratio $({\mathcal{R}}_m\simeq <!--\; ???\; -->m_p/m_e=1836)$, as proposed to be the case in an earlier publication, on the basis of a specific model of the GRB dissipation process. It therefore appears that, in addition to the values of the energy of peak luminosity $E_{\mathrm{p}\mathrm{k}}\sim <!--\; ???\; -->m_e{c}^2$, GRBs present us with one more quantity with an apparent characteristic value. The fact that the values of both these quantities ($E_{\mathrm{p}\mathrm{k}}$ and $\mathcal{R}$) are consistent with the same specific model invoked to account for the efficient conversion of their relativistic proton energies to electrons argues favorably for its underlying assumptions.