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Title: Probing variability of GRBs on the finest time scales with Global Fit Analysis
Abstract: GRBs are known to manifest variability of flux on sub-millisecond time scales. The use of the Global Fit Analysis (GFA) allowed one to investigate spectral variability with almost the same time resolution thanks to special assumptions about intrinsic properties of the spectra. With the GFA we could also extend spectral analysis to far tails of GRBs where traditional spectroscopy fails due to low S/N ratio. In GFA, instead of parameterising individual spectra and analysing spectral evolution in terms of numerous individual spectral fit parameters we define a spectral evolution model with a set of constant global parameters and just a few (N=1-2) time dependent variables. Besides the time histories of physically meaningful spectral parameters GFA also allows us to obtain for a given GRB global cosmologically invariant parameters for further population studies.
Title: The mechanism of GRB afterglows
Abstract: The afterglow is believed to be emitted by a relativistic blast wave driven by the GRB ejecta into an ambient medium. I will describe recent advances in the theory of afterglows with an emphasis on the physical mechanism of the ejecta-medium interaction and the basic dynamics of the explosion.
Title: The Optical Light Curves of Gamma-Ray Bursts
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Title: X-ray lines, progenitor and beaming of GRBs
Abstract: The detection of X-ray lines in the afterglow of GRBs allow to probe the close GRB environment and then the nature of the progenitor. Moreover, being uncollimated, line emission can be used to constrain the energy budget of the bursts and therefore their beaming angle. I will review the present status of line detections and the implications they bear for the burst progenitor and global energetics.
Title: What can radio observations teach us about the progenitors of GRBs?
Abstract: Radio observations of GRB afterglows provide unique information in the framework of the afterglow model, ranging from a direct measurement of the size and expansion of the relativistic ejecta, to a determination of the star formation rate in the host galaxy. In this talk I will concentrate on recent observations which aim to extract information about the progenitors, by studying the relativistic nature of the ejecta, the circumburst environment, and the large scale environment. I will use GRB 011121, GRB 020405, and GRB 980703 as illustrative examples of interstellar scintillation, reverse shocks, broad-band modeling, and host galaxy observations, all of which point to a massive star origin for the long duration GRBs.
Title: UHE neutrinos and photons from jets in GRB
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Title: Aspects of the Amplification of Magnetic Fields
Abstract: Relatively strong magnetic fields are inferred from GRB afterglow spectra, and very strong fields are required to facilitate Poynting flux rotator/outflow models of GRB engines. Strong fields in the engine might help explain those in the external shock, but where do the engine magnetic fields come from in the first place? The associated physics of magnetic field amplification in magnetized turbulent rotators is a broadly relevant, but complex problem in astrophysics. It requires tackling magnetic dynamo theory and turbulent transport. Recent theoretical work and numerical simulations have led to progress. I will discuss the physical principles involved, distinguishing between "inverse" and "direct" dynamos. I will discuss where the magnetorotational instability fits into this picture, and address the role of magnetic helicity for outflow mediating fields. The importance of solving toy problems that facilitate a symbiosis between numerical experiment and semi-analytic theory will be emphasized.
Title: High frequencies in the power spectrum of Gamma-Ray Burst
Abstract: Power spectra of Gamma-Ray Bursts (GRB) were investigated in the frequency range of 0.01 - 500 Hz using BATSE data. For high frequency domain the time-tagged events (TTE) data type were used. The averaging of more than 200 Ksec of data allowed us to obtain statistically robust results on flux variability on various time scales. We also elaborated the model for dead time correction for large area detecors of BATSE instrument and tested it using the background data. We found that dead time significantly affects power density spectrum at high frequencies. Significant variability was detected in the averaged power spectrum up to 100 Hz. The upper limit for the variability beyond 100 Hz was also estimated. The power density spectrum shows a steepening at frequency 5-7 Hz with power law index changing from -1.6 to -2.2. We found that the averaged power spectrum of GRBs can be approximated by smoothed broken power low in the wide frequency range including high frequency part. Both break frequency and power low indices monotonously depend on burst intensity. Based on the obtained results a model of cosmological evolution of GRB source is discussed.
Title: Observable Consequences of Collimation in GRB Afterglows
Abstract: Because gamma ray bursts are emitted by ultrarelativistic ejecta, they are subject to strong relativistc beaming and it is effectively impossible to infer their collimation angles from gamma ray properties alone. Fortunately, afterglows do allow measurements of collimation angles precisely because they are emitted while the burst ejecta decelerate and so are subject to a lesser (and evolving) degree of relativistic beaming. Two tests of GRB collimation have reached the point of practical application: Light curve breaks (first proposed by Rhoads 1999), which have now been detected in many GRBs, and orphan afterglows (Rhoads 1997), which have been sought at many wavelengths but not yet convincingly detected. I will outline the physics and underlying both tests, with attention to both their strengths and possible pitfalls. One physical pitfall is that afterglow emission may be powered by different material from the GRB emission, since the minimum Lorentz factor to produce an afterglow is relatively low. I will discuss ways of addressing this ambiguity in the context of the orphan afterglow test. I will also summarize briefly the observational status and future prospects for both methods. If time permits, I will close with a brief discussion of other tests, notably including GRB remnant statistics and afterglo w polarization.
Title: Universal jet structure in GRBs
Abstract: Gamma ray bursts are often modelled as jet-like outflows directed towards the observer; the cone angle of the jet is then commonly inferred from the time at which there is a steepening in the power-law decay of the afterglow. We consider an alternative model in which the jet has a beam pattern where the luminosity per unit solid angle (and perhaps also the initial Lorentz factor) decreases smoothly away from the axis, rather than having a well-defined cone angle within which the flow is uniform. We show that the break in the afterglow light curve then occurs at a time that depends on the viewing angle. Instead of implying a range of intrinsically different jets - some very narrow, and others with similar power spread over a wider cone - the data on afterglow breaks could be consistent with a standardized jet, viewed from different angles.
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Title: \"The Collapsar Model for GRBs\" by S. E. Woosley and Weiqun Zhang
Abstract: A collapsar is a rotating massive star whose core collapses to a black hole and an accretion disk. Accretion energy is then converted, either by neutrino transport or MHD proceses, into jet-like outflows. Recent calculations relevant to gamma-ray bursts will be reviewed. We find the relativistic jet experiences a shock deep inside the star so that, when it emerges, it contains a large fraction of its energy in internal energy. The terminal Lorentz factor is developed only after the jet has escaped the star and expanded. The GRB light curve is thus insensitive to details of the central engine but influenced greatly by the Kelvin-Helmholtz instability between the jet and its coccoon as it passes through the star. A variety of different high energy transients will be observed depending upon the angle at which a standard collapsar model is observed. These range from ordinary GRBs to hard x-ray flashes and include events like GRB 980425 and SN 1998bw. Prospects for making short hard bursts from shock break out in the collapsar model will also be discussed.
Title: ion-dominated plasma and the origin of collimated jets
Abstract: Low cooling plasma associated with tremendous kinetic energies are likely to be the origin of the kpc-extended and well collimated jets. It is proposed that jets take-off from a layer, governed by highly diffusive, super-Keplerian rotating and thermally dominated by virial-hot ion-plasma. The runaway layer is located between the accretion disk and the corona surrounding the nucleus. The matter in the layer is causally connected to both the disk and to the central engine. If accretion flows are associated with large scale magnetic fields, we find that BH-corona are dynamically unstable to thermal conduction, and accretion disks become intrinsically advection-dominated. We confirm the capability of this multi-layer model by carrying 3D axi-symmetric quasi-stationary MHD calculations with high spatial resolution.
Title: MHD Jets, Flares, and Gamma Ray Bursts
Abstract: Using general relativistic MHD simulations, we study MHD disk-jets and Lebranc-Wilson type outflows as a model of the central engine of gamma ray bursts. We also discuss the analogy between solar flares/ coronal mass ejections and gamma ray bursts.
Title: Relativistic MHD jets and the GRB's
Abstract: The general properties of relativistic MHD jets are discussed -- the light cylinder, electric fields, high magnetization -- mainly using models of relativistic stationary MHD. These properties are then discussed within the context of GRB's. The basic difficulty with GRBs as generated by MHD jets is the point that in order to gain a high Lorentz factor the jet flow must be highly magnetized. On the other hand, the MHD condition breaks down for arbitrarily high magnetization.
Title: Magnetic fields in GRBs
Abstract: I will review the dynamical role of the magnetic fields in extracting the energy of a relativistic rotating progenitor (like black hole, super-massive neutron star or accretion disk), in collimating the outflow and in producing the observed high energy emission. In particular, I will concentrate on the dynamics of relativistic explosions in the wind zone assuming that most energy is released by the GRB progenitor in a form of toroidal magnetic fields (relativistic magnetic bubble). The expanding force-free and causally connected bubble is non-spherically symmetric with Lorentz factor of the containing contact discontinuity increasing towards the polar axes defined by the rotation of the progenitor. Such expanding magnetic bubble will produce a similarly anisotropic blast wave. Dissipative current instabilities developing at large distances near the axes of the flow result in particle acceleration and high energy emission.
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Title: MHD turbulence: implications for Gamma ray bursts
Abstract: Physics of the GRBs is intimately related to the processes of particle acceleration, transfer of energy from protons to electrons and ultimately to the interaction of particles with turbulent magnetic field. I shall discuss at length the properties of Alfven, slow and fast modes of compressible MHD turbulence and how those modes scatter and accelerate particles. Recent advances in the physics of those modes require substantial revisions of the known theories of cosmic ray dynamics and I shall compare newly obtained predictions with those in the literature. I shall also discuss how new understanding of MHD turbulence affects the energy transfer to high wavenumbers and how this affects the tranfer of energy from the proton beam to electrons.
Title: New picture of cosmic ray scattering and its implications for gamma ray burst
Abstract: Recent advances in understanding of magnetohydrodynamic (MHD) turbulence entail substantial revisions in our understanding of cosmic ray transport and acceleration including the acceleration of electrons within gamma ray fireball. We show that scattering efficiency by Alfvenic turbulence that most of the authors refer to in their models is marginal. On the other hand, we find that the gyroresonance with fast modes dominates cosmic rays scattering. This entails the dependence of scattering on plasma beta which is in agreement with the observed cosmic ray abundance constrains. We discuss the implications of the new picture of cosmic ray scattering for Gamma ray bursts.