Implications of the Measured Image Size for the Radio Afterglow of GRB 030329

We use data on the image size of the radio afterglow of GRB 030329 (Taylor et al. 2004) to constrain the physical parameters of this explosion. Together with the observed broad band spectrum, this data over-constrains the physical parameters, thus enabling to test different GRB jet models for consistency. We consider two extreme models for the lateral spreading of the jet: model 1 with relativistic expansion in the local rest frame, and model 2 with little lateral expansion as long as the jet is highly relativistic. We find that both models are consistent with the data for a uniform external medium, while for a stellar wind environment model 1 is consistent with the data but model 2 is disfavored by the data. Our derivations can be used to place tighter constraints on the dynamics and structure of GRB jets in future afterglows, following a denser monitoring campaign for the temporal evolution of their image size.Comment: 12 pages, 6 figues; submitted to Ap

GRB990123: Evidence that the Gamma Rays Come from a Central Engine

GRB990123 was a long complex gamma-ray burst with an optical transient that started early within the gamma-ray phase. The peak and power law decay of the early optical emission strongly indicates the presence of a decelerating relativistic shell during that phase. Prior to this burst, it was not known if the shell decelerated during the burst, so an external shock origin for the gamma rays was still possible. If the gamma-rays are produced in the external shock, then the pulse widths should reflect the observed deceleration of the shell and increase by about 2.3. We analyze the fine time structure observed in the gamma-ray data from BATSE and determine that the width of the peaks do not increase as expected for a decelerating shell; the later pulses are, at most, a factor of 1.15 longer than the earlier pulses. We also analyze the variability to determine what fraction of the shell's surface could be involved in the production of the gamma rays, the so-called surface filling factor. For GRB990123 we find a filling factor of 0.008. The lack of pulse width evolution eliminates the only remaining kinematically acceptable external shock explanation for the gamma-ray phase and, thus, the gamma rays must originate at a central engine.Comment: 14 pages, 3 embedded figues, Latex, Submitted to ApJ

Tunable crystal structure and proton conductivity of lanthanide nitrilotrismethylphosphonates

Metal phosphonates are multifunctional solids with remarkable stability and proton conducting properties owing to their structure is usually composed of extended hydrogen-bond networks that favor proton transfer pathways [1]. Moreover, these properties can be enhanced by appropriate modification of the synthesis conditions [2, 3]. In this communication, a new family of isostructural 2D layered compounds based on lanthanide nitrilotris-methylphosphonates is reported. These compounds have been isolated at room temperature and have the general formula Ln[N(CH2)3(PO3H2)2(PO3H)(H2O)]SO4·2H2O (Ln= Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb). The coordination environment of Ln3+ is composed by eight oxygen atoms from three different ligands and two oxygens from bound waters. This connectivity creates positive charged layers connected to sulfate ions through hydrogen-bonds. These compounds show promising proton conductivity with values ranging between 7.6·10-2 and 3.8·10-2 S·cm-1 at 80 °C and 95% RH and low activation energy corresponding to Grotthuss-type proton transfer mechanism. In addition, a structural transformation occurs at T > 70 °C accompanied by a remarkable enhanced conductivity. Studies on the structure-properties relationships will be discussed.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. MINECO: MAT2016-77648-R Junta Andalucía: P12-FQM-1656 y FQM-11

e+/- Pair Loading and the Origin of the Upstream Field in GRB Shocks

We investigate here the effects of plasma instabilities driven by rapid e(sup plus or minus) pair cascades, which arise in the environment of GRB sources as a result of back-scattering of a seed fraction of their original spectrum. The injection of e(sup plus or minus) pairs induces strong streaming motions in the ambient medium. One therefore expects the pair-enriched medium ahead of the forward shock to be strongly sheared on length scales comparable to the radiation front thickness. Using three-dimensional particle-in-cell simulations, we show that plasma instabilities driven by these streaming e(sup plus or minus) pairs are responsible for the excitation of near-equipartition, turbulent magnetic fields. Our results reveal the importance of the electromagnetic filamentation instability in ensuring an effective coupling between e(sup plus or minus) pairs and ions, and may help explain the origin of large upstream fields in GRB shocks

Accretion modes in collapsars - prospects for GRB production

We explore low angular momentum accretion flows onto black holes formed after the collapse of massive stellar cores. In particular, we consider the state of the gas falling quasi-spherically onto stellar-mass black holes in the hypercritical regime, where the accretion rates are in the range 0.001 - 0.5 solar masses per second and neutrinos dominate the cooling. Previous studies have assumed that in order to have a black hole switch to a luminous state, the condition l >> r_g c, where l is the specific orbital angular momentum of the infalling gas and r_g is the Schwarszchild radius, needs to be fulfilled. We argue that flows in hyperaccreting, stellar mass disks around black holes are likely to transition to a highly radiative state when their angular momentum is just above the threshold for disk formation, l ~ 2 r_g c. In a range where l lies between r_g c and 2 r_g c, a dwarf disk forms in which gas spirals rapidly into the black hole due to general relativistic effects, without any help from horizontal viscous stresses. For high rotation rates with l greater than 2 r_g c, the luminosity is supplied by large, hot equatorial bubbles around the black hole. The highest neutrino luminosities are obtained for l ~ 2 r_g c, and this value of angular momentum also produces the most energetic neutrinos, and thus also the highest energy deposition rates. Given the range of l explored in this work, we argue that, as long as l is greater than 2 r_g c, low angular momentum cores may in fact be better suited for producing neutrino--driven explosions following core collapse in supernovae and gamma ray bursts.Comment: Revised version following referee's comments. References added. Accepted for publication in Ap

Tidally-induced thermonuclear Supernovae

We discuss the results of 3D simulations of tidal disruptions of white dwarfs by moderate-mass black holes as they may exist in the cores of globular clusters or dwarf galaxies. Our simulations follow self-consistently the hydrodynamic and nuclear evolution from the initial parabolic orbit over the disruption to the build-up of an accretion disk around the black hole. For strong enough encounters (pericentre distances smaller than about 1/3 of the tidal radius) the tidal compression is reversed by a shock and finally results in a thermonuclear explosion. These explosions are not restricted to progenitor masses close to the Chandrasekhar limit, we find exploding examples throughout the whole white dwarf mass range. There is, however, a restriction on the masses of the involved black holes: black holes more massive than $2\times 10^5$ M$_\odot$ swallow a typical 0.6 M$_\odot$ dwarf before their tidal forces can overwhelm the star's self-gravity. Therefore, this mechanism is characteristic for black holes of moderate masses. The material that remains bound to the black hole settles into an accretion disk and produces an X-ray flare close to the Eddington limit of $L_{\rm Edd} \simeq 10^{41} {\rm erg/s} M_{\rm bh}/1000 M$_\odot$), typically lasting for a few months. The combination of a peculiar thermonuclear supernova together with an X-ray flare thus whistle-blows the existence of such moderate-mass black holes. The next generation of wide field space-based instruments should be able to detect such events.Comment: 8 pages, 2 figures, EuroWD0

Simulation Study of Magnetic Fields Generated by the Electromagnetic Filamentation Instability

We have investigated the effects of plasma instabilities driven by rapid e(sup plus or minus) pair cascades, which arise in the environment of GRB sources as a result of back-scattering of a seed fraction of the original spectrum. The injection of e(sup plus or minus) pairs induces strong streaming motions in the ambient medium. One therefore expects the pair-enriched medium ahead of the forward shock to be strongly sheared on length scales comparable to the radiation front thickness. Using three-dimensional particle-in-cell simulations, we show that plasma instabilities driven by these streaming e(sup plus or minus) pairs are responsible for the excitation of near-equipartition, turbulent magnetic fields. Our results reveal the importance of the electromagnetic filamentation instability in ensuring an effective coupling between e(sup plus or minus) pairs and ions, and may help explain the origin of large upstream fields in GRB shocks

Weibel instability and associated strong fields in a fully 3D simulation of a relativistic shock

Plasma instabilities (e.g., Buneman, Weibel and other two-stream instabilities) excited in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a new 3-D relativistic particle-in-cell code, we have investigated the particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. The simulation has been performed using a long simulation system in order to study the nonlinear stages of the Weibel instability, the particle acceleration mechanism, and the shock structure. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic (HD) like shock structure. In the leading shock, electron density increases by a factor of 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. We discuss the possible implication of our simulation results within the AGN and GRB context.Comment: 4 pages, 3 figures, ApJ Letters, in pres