Abstracts of invited contributions
IceCube is a neutrino observatory located at the geographical South Pole. In two years the km^3 detector is expected to be complete. At the moment, data is being taken with 59 deployed strings, when completed it will be comprised of 80-strings plus 6 additional strings for the low energy array Deep Core. The strings are deployed in the deep ice between 1,450 and 2,450 meters depth, each string containing 60 optical sensors. During the year of 2007-2008 data were collected with 22 deployed strings. In this talk I will present some of the current scientific results of these data including the search reporting the measurement of 0.06% of large scale anisotropy. The data used in the large scale anisotropy analysis contains ~4.3 billion downward going muon events with a median energy per nucleon of ~14 TeV and a median angular resolution of 3 degrees. The energy dependence of this anisotropy at median energies of 12 TeV and 126 TeV is also presented in this work. The observed anisotropy has an unknown origin and we will discuss various possible explanations. Studies of the anisotropy could further enhance the understanding of the structure of the galactic magnetic field and possible cosmic ray sources.
Pulsars transmit their rotational energy to the outside would via relativistic MHD winds. I describe some recent developments in the physics of those winds, with particular attention to the decay of their magnetic fields in the regions between the light cylinders and the surrounding pulsar wind nebulae. I address both the acceleration of the flow and possible radiative signatures, as the magnetic energy decays. I suggest that part of the energy goes into accelerating a relativtsic beam in each wind's current sheet as a "runaway" phenomenon in the electric filed supported by the anomalous resistivity in the sheet. I also discuss some related issues in the dynamics of the current sheet separating the closed and open field regions within pulsars' magnetospheres, and relate these dissipative structures to pulsed gamma ray emission.
The High Energy Stereoscopic System (H.E.S.S.) is a southern hemisphere array of four Atmospheric Cherenkov telescopes observing the sky in the very high energy gamma-ray range (VHE; > 100 GeV). VHE observations are an invaluable tool to study the acceleration and propagation of energetic particles in many astrophysical systems where relativistic outflows are the main drivers of the emission, such as AGNs, GRBs and galactic binary systems. In this talk we will review the main results of H.E.S.S. observations of these objects, presenting the general picture that emerges from them. We will also comment on some prospects for future investigations with H.E.S.S.-II, to start operation next year.
The giant radio galaxy M 87 is located at a distance of ~16 Mpc and harbours a supermassive black hole in its center. The structure of its relativistic plasma jet is resolved at radio, optical and X-ray wavelengths. M 87 belongs to the class of active galactic nuclei (AGN) and is one of the few extragalactic TeV gamma-ray source not belonging to the class of blazars. This makes it a unique laboratory to study jet physics and the corresponding emission processes at very high energies (VHE; E>100 GeV). During the last 10 years M 87 has been regularly detected by several experiments at VHE. The current status as well as the prospects of future simultaneous multi-wavelength observations are discussed.
One of the fundamental properties of astrophysical magnetic fields is their ability to change their topology through magnetic reconnection and in doing so, to release magnetic energy, sometimes violently. In this talk, we will discuss the role of magnetic reconnection and associated heating and particle acceleration in jet/disk accretion systems, namely young stellar objects (YSOs), microquasars, and active galactic nuclei (AGNs). In the case of microquasars and AGNs, violent reconnection episodes between the magnetic field lines of the inner disk region and those that are anchored into the black hole are able to heat the coronal/disk gas and accelerate the plasma to relativistic velocities through a diffusive first-order Fermi-like process within the reconnection site that will produce intermittent relativistic ejections or plasmons. The resulting power-law electron distribution is compatible with the synchrotron radio spectrum observed during the outbursts of these sources. A diagram of the magnetic energy rate released by violent reconnection as a function of the black hole (BH) mass spanning 10^9 orders of magnitude shows that the magnetic reconnection power is more than sufficient to explain the observed radio luminosities of the outbursts, from microquasars to low luminous AGNs. In addition, the magnetic reconnection events cause the heating of the coronal gas which can be conducted back to the disk to enhance its thermal soft x-ray emission as observed during outbursts in microquasars. The decay of the hard x-ray emission after a radio flare could also be explained in this model due to the escape of relativistic electrons with the evolving jet outburst. In the case of YSOs, a similar magnetic configuration can be reached that can produce the observed x-ray flares in some sources and provide the heating at the jet launching base. Fully 3D MHD-particle-in-cell simulations of turbulent reconnection and particle acceleration will be also presented.
In this review I will survey the current state of particle acceleration theory as applied to astrophysical systems with relativistic outflows. Although certainly not the only mechanism for particle acceleration, discussion tends to be dominated by diffusive shock acceleration because this is the best understood process, at least for non-relativistic shocks. The exciting new development here is the growing appreciation and understanding of magnetic field amplification. In the relativistic regime shock acceleration is much less secure, but here also recent PIC simulations offer interesting insights and hope for progress in the near future.
We are in an exciting period of discovery for gamma-ray bursts. The Swift observatory is detecting 100 bursts per year, providing arcsecond localizations and sensitive observations of the prompt and afterglow emission. The Fermi observatory is observing 250 bursts per year with its medium-energy GRB instrument and about 10 bursts per year with its high-energy LAT instrument. In addition, rapid-response telescopes on the ground are providing new capabilities to study optical emission during the prompt phase and spectral signatures of the host galaxies. The combined data set is enabling great advances in our understanding of GRBs including afterglow physics, short burst origin, and high energy emission. This talk will highlight recent findings.
The Pierre Auger Observatory has measured that the flux of cosmic rays is strongly suppressed above 4x10**19 eV. This is consistent with the prediction that cosmic rays with larger energies can only arrive from nearby sources since they must surmount the losses caused by propagation through the cosmic microwave background. The Observatory has also found evidence that the inhomogeneous distribution of nearby extragalactic matter imprints its anisotropy upon the arrival directions of cosmic rays with energy above 6x10**19 eV. Correlations are observed with the positions of active galactic nuclei within 100 Mpc, and with other distributions of local extragalactic objects. An excess of events is observed from a region of the sky close to the location of the radio source Cen A. Current measurements do not identify neither individual sources nor a specific class of sites of origin. We review these results and discuss the perspectives that similar studies with future data, complemented with further measurements of composition of the cosmic rays, may identify their astrophysical sources.
One can imagine a number of mechanisms that could be the cause of brighter/fainter segments of jets. In a sense, jets might be easier to understand if they were featureless. However we observe a wide variety of structures which we call 'knots'. By considering the ramifications of the various scenarios for the creation of knots, we determine which ones or which classes are favored by the currently available multiwavelength data.
I will discuss properties of binary systems as Very High Energy (VHE) emitters. In particular, I will consider the implications of the most recent observations in gamma- and X-ray energy bands. Discussing different scenarios for gamma-ray production in such sources, I will the mostly focus on the formation of non-thermal emission in relativistic outflows, which are natural components of microquasars and binary pulsar systems.
In this talk I will summarise our recent results on the possible role of the Blandford-Znajek mechanism in the production of relativistic jets of Long Gamma Ray Bursts. In addition to the results of general relativistic MHD simulations, which have allowed to clarify particular aspects of the launch of black hole jets in the collapsar scenario, I will also discuss the possibility of explaining the "shallow decay phase" in the afterglows of LGRBs discovered by the Swift observatory. Finally, I will explore the intrinsic properties and the unusual observational signatures of the relativistic jets from the supercollapsars of the very first stars in the early Universe (z~20), which are expected to be very massive.
Relativistic jets are believed to be powered by the rotational energy of compact objects (black holes or neutron stars) or of their associated accretion disks, which is tapped through a magnetic field that threads the source. This talk will review our current understanding of the acceleration and collimation of such outflows -- with emphasis on recent developments - and will proceed to discuss the key open questions on this subject. The discussion will address both purely theoretical issues and the confrontation between theory and observations. Among the topics that will be considered are: the appropriate theoretical framework for modeling relativistic outflows (magnetodynamics or magnetohydrodynamics?), ideal-MHD vs. dissipative systems, the effect of the ambient radiation field, unique predictions of the magnetic acceleration model and their potential manifestations in GRB and AGN jets, dynamical stability properties, the origin of the high-energy emission and of the apparent variability of such jets, single vs. multi-component outflows, and common traits in relativistic jets from disparate astronomical objects (including their relation to the accretion flow).
I will present a comparison study of jets on all scales. The scaling of conditions in the central engine and of jet properties with the controlling parameters: black hole mass and spin, accretion rate and strength of magnetic field on the horizon, will be discussed. Environmental effects on jet propagation, collimation and dissipation in different systems will also be considered.
Thanks to its unprecedented sensitivity, large field of view and sky survey operating mode, the Fermi Gamma-ray Space Telescope has opened a new era in extragalactic gamma-ray astronomy. In the first year of science operations the Fermi/LAT has detected several hundreds blazars and a few radiogalaxies, which can be studied with unprecedented accuracy. In this talk, I will review the general properties of these sources and highlight the results of MW studies carried out on bright sources.
I will discuss how ultrarelativistic jets are produced in GRB and AGN systems as understood from jet theory, accretion disk theory, and general relativistic MHD simulations.
I will review the observational evidences for the connection between instabilities in accretion disks and the production of powerful relativistic jets in the three astrophysical manifestations of black holes in the universe: active galactic nuclei, stellar black hole binaries (microquasars), and gamma-ray-bursts.
We describe numerical simulations of magnetized relativistic jets, focusing in particular on the efficiency with which Poynting flux is converted to bulk kinetic energy of the outflowing gas. We show that efficient conversion occurs only for certain geometries. We also discuss the stability of jets to the kink mode.
The origin of dramatically different electron distributions responsible for Comptonization in black hole X-ray binaries (BHBs) in their various states is discussed. We solve the coupled kinetic equations for photons and electron-positron pairs without approximations on the relevant cross-sections accounting for Compton scattering, synchrotron radiation, pairp production and Coulomb collisions. In the absence of external soft photons, the pairs are efficiently thermalized by synchrotron self-absorption and Coulomb scattering even for pure nonthermal electron injection. The resulting quasi-thermal synchrotron self-Compton spectra have very stable slopes and electron temperatures similar to the hard states of BHBs. The hard spectral slopes observed in the X-rays, the cutoff at 100 keV, and the MeV tail together require low magnetic fields ruling out the magnetic dissipation mechanism. The motion of the accretion disk toward the black hole results in larger Compton cooling and lower equilibrium electron temperature. Our self-consistent simulations show that in this case the distributions of pairs and photons attain a power-law-dominated shape similar to what is observed in the soft state. The electron distribution in the Cyg X-1 soft state might require a strong magnetic field, being consistent with the magnetically dominated corona.
Review of cosmic gamma-ray burst will be presented focusing on phenomenological properties, models and unresolved puzzles of the exciting phenomenon. We discuss variability properties of light curves in both gamma-ray domain and optic, Supernovae connection and host galaxy statistics. Special attention is devoted to the prompt emission of GRB which is a clue to understanding central machine of gamma-bursts. Particular attention will be given to short duration gamma-ray bursts and possible new class of very short bursts. We also present most interesting results obtained with GRB-follow up network of CIS observatories covering 8 time zones.
All energetic phenomena associated with black holes, including large scale relativistic outflows, are fundamentally driven by processes occurring close to the event horizon. In recent years, new observational capabilities (especially sensitive X-ray spectroscopy) have given us unprecedented constraints on the physical conditions close to accreting black holes including (1) the physical properties of the inner accretion flow, (2) the physics of accretion disk winds, and (3) the role of gravitomagnetic forces (associated with black hole spin). I will summarize the current constraints provided by these observations. Drawing motivation from these observations, I will then proceed to discuss the behaviour of large-scale magnetic fields close to the black hole, and speculate upon the ingredients required for the formation of powerful relativistic jets.
MAGIC is a single-dish Cherenkov telescope located on La Palma (Spain), hence with an optimal view on the Northern sky. Sensitive to the 30 GeV - 30 TeV energy band, it is nowadays the only ground-based instrument being able to measure high-energy gamma-rays below 100 GeV. With the operation in coincidence with MACIC-II, starting in Fall 2009, the sensitivity will improve by a factor ~2. We review the experimental results obtained by MAGIC on the very-high-energy emission from astrophysical objects such as pulsars, pulsar wind nebulae, binary systems, active galactic nuclei and gamma-ray bursts.
I will review the most recent observational results regarding multiwavelength emission of extragalactic large-scale jets. I will attempt to identify the most relevant radiative processes involved. Next I will emphasize the resulting constrains on the energy dissipation and particle acceleration processes taking place thereby, as well as on the global structure of relativistic outflows in active galaxies. In particular, I will address a role of shocks and magnetic turbulence in energizing jet particles to ultrarelativistic energies. Brief comparison with the multiwavelength properties of small-scale (blazar) jets will be given.
I review the current knowledge of the high-energy emission from AGNs, with particular emphasis on the clues from the new observations of Fermi and Cherenkov telescopes of blazars and radiogalaxies. I discuss the main models advanced to account for the observed properties of the gamma-ray emission in these sources.
The origin of the X-ray emission from large-scale quasar jets has been the subject of active debate. Synchrotron and inverse-Compton (IC) models of the X-ray emission have very different implications to the jet physics on large scales and particle acceleration inside the jet. We present recent multiwavelength observations of quasar jets on large scales and discuss the radiation mechanism responsible for the quasar extended jets. Particular emphasis will be placed on new calculations of polarization properties of external IC emission. Using the same formalism, we also discuss how future X-ray polarization measurement of "small-scale" emission in high luminosity quasar jets can be used to infer physical conditions of the relativistic jets.
Relativistic jets and collimated outflows are ubiquitous phenomena in astrophysical settings, from young stellar objects up to Active Galactic Nuclei (AGNs). The observed emission from some of these jets can cover the whole electromagnetic spectrum, from radio to gamma rays. The relevant features of the spectral energy distributions depend on the nature of the source and on the characteristics of the surrounding environment. In this talk, I shall review the main physical processes that command the interactions between populations of relativistic particles locally accelerated in the jets, with matter, radiation and magnetic fields. Special attention will be given to the conditions that lead to the dominance of the different radiative mechanisms. Examples from various types of sources will be used to illustrate these effects.
The advent of advanced systems of atmospheric Cherenkov imaging telescope arrays has opened the extreme universe to observations with high sensitivity. Somewhat surprisingly, the ground-based techniques match, or exceed, the sensitivity of space telescopes at lower energies and hence provide complementary observations to the AGILE and Fermi missions. VERITAS (the Very Energetic Radiation Imaging Telescope Array System) in southern Arizona is one such system; it was commissioned in 2007. It consists of four telescopes of 12 m aperture and cameras with 499 pixels and is located in southern Arizona. The array was completed on-time and within budget and has satisfied or exceeded all technical specifications. VERITAS is now one of the most sensitive very high energy gamma-ray observatories in operation. Results of observations on known and newly discovered sources will be presented. It is now apparent that the emission of very high energy gamma rays is ubiquitous with the existence of nearly 100 sources, both galactic and extragalactic, established. TeV gamma-ray astronomy promises to be a fertile new discipline in high energy astrophysics.
The recent Fermi observation of GRB 080916C suggests that at least for this burst, the bright thermal emission associated with a hot fireball is missing, which suggests that the outflow has to be Poynting flux dominated. I will critically review the existing GRB prompt emission models (both the baryonic fireball internal shock model and the electromagnetic model), and discuss the recent progress of developing a new GRB prompt emission model in the Poynting-flux-dominated regime, namely, the internal collision-induced magnetic reconnection and turbulence (ICMRT) model.