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At a redshift of 8.3 GRB 090423 marks the highest known redshift object in the Universe. By combining our radio measurements with existing X-ray and infrared observations, we estimated the kinetic energy of the afterglow, the geometry of the outflow and the density of the circumburst medium. Our best fit model is a quasi-spherical, high-energy explosion in a low, constant-density medium. We compare the properties of GRB 090423 with a sample of GRBs at moderate redshifts. We find that the high energy and afterglow properties of GRB 090423 are not sufficiently different from other GRBs to suggest a different kind of progenitor, such as a Population III star. However, we argue that it is not clear that the afterglow properties alone can provide convincing identification of Population III progenitors. We suggest that the millimeter and centimeter radio detections of GRB 090423 at early times contained emission from a reverse shock component. This has important implications for the detection of high redshift GRBs by the next generation of radio facilities. [ABSTRACT FROM AUTHOR]

The epochs of origin of the first stars and galaxies, and subsequent growth of the first supermassive black holes, are among the most fundamental questions. Observations of the highest redshift Gamma-Ray Bursts (GRBs) will be the most compelling in situ probe of the history of initial star formation and consequent epoch of reionization if their prompt and precise detection can be followed immediately by sensitive near-IR imaging and spectroscopy. Blazars are the persistent analogs of GRBs and for the same reason (beaming) can be observed at highest redshifts where they might best trace the high accretion rate-driven jets and growth of supermassive black holes in galaxies. The proposed EXIST mission can uniquely probe these questions, and many others, given its unparalled combination of sensitivity and spatial-spectral-temporal coverage and resolution. Here we provide a brief summary of the mission design, key science objectives, mission plan and readiness for EXIST, as proposed to Astro2010. [ABSTRACT FROM AUTHOR]

The observation of the very early stages of the Universe represents one of the main challenges of modern cosmology. 200–300 million years after the Big Bang stars began to form, thus providing the Universe with the first sources of light and heat after the Big Bang. This event marks the transition between the epoch when the Universe was dark and neutral and the time when the Universe became fully ionized. The direct investigation of the early Universe has been usually accomplished by observing distant quasars, but a new fundamental tool is now at hand thanks to Gamma-ray Bursts (GRBs). GRBs are gamma-ray flashes, detected from space, produced by rare types of massive stellar explosions. Their rapidly fading afterglows are often bright at optical wavelengths, such that GRBs are detectable up to cosmological distances. Here we report on the Swift observation of GRB 090423 and the near-infrared spectroscopic measurement of its redshift z = 8.1-0.3+0.1. This redshift is substantially higher than that measured any other cosmological object. This GRB was produced in a cosmic explosion when the Universe was only ∼4% of its current age. Unexpectedly, this primordial object exhibits properties similar to those of GRBs observed at low/intermediate redshifts, suggesting that the mechanisms and progenitors which gave rise to GRBs about 600 million years after the Big Bang are not markedly different from those producing GRBs ∼10 billion years later. [ABSTRACT FROM AUTHOR]

The brightest events in a time series of cosmological transients obey an observation time dependence which is often overlooked. This dependence can be exploited to probe the global properties of electromagnetic and gravitational wave transients [1, 2]. We describe a new relation based on a peak flux-observation time distribution and show that it is invariant to the luminosity distribution of the sources [3]. Applying this relation, in combination with a new data analysis filter, to Swift gamma-ray burst data, we demonstrate that it can constrain their rate density. [ABSTRACT FROM AUTHOR]

The JEM-EUSO mission explores the origin of the extreme energy comic-rays (EECRs) above 1020 eV and challenges to the limit of the basic physics, through the observations, of their arrival directions and energies. It is designed to observe more than 1,000 events of EECRs above 7×1019 eV in its five-year operation with an exposure larger than 1 million km2·sr·year. The super-wide-field (60 degrees) telescope with a diameter of about 2.5 m looks down the atmosphere of the night-side of the earth to detect near UV photons (330–400 nm, both fluorescent and Cherenkov photons) emitted from the giant air-shower produced by an EECR. The arrival direction map with 1,000 events naturally tells us the origin of the EECRs and allows us to identify the EECR sources to known astronomical objects. The comparison among the energy spectra of the spatially resolved individual sources will clarify the acceleration/emission mechanism, and also finally confirm the Greisen-Zatse′pin-Kuzmin process for the validation of Lorentz invariance up to γ∼1011. Neutral components (neutrinos and gamma rays) can also be detected as well, if their fluxes are high enough. The JEM-EUSO mission is planned to be launched by a H2B rocket about 2015 and transferred to ISS by H2 Transfer Vehicle (HTV). It will be attached to the external experiment platform of “KIBO” which completed July 2009 by STS-127 mission of the space shuttle. The first flight of HTV by H2B rocket was successfully done in September and October 2009. [ABSTRACT FROM AUTHOR]

The Extreme Universe Space Observatory onboard the Japanese Experiment Module is an international mission designed to identify the astrophysical origin and physical nature of ultra high-energy (UHE) cosmic particles with energies E>7×1019 eV. It uses a near-UV 2.5 m diameter telescope with a field of view of 60 degrees to detect the fluorescence and Cherenkov light emitted along the linear track generated by UHE particle traversing the earth’s atmosphere. The JEM-EUSO mission is designed to observe at least 103 particles. The energy and arrival direction of each particle will be accurately measured while all-sky is covered and monitored. The primary science goal of the mission is particle astronomy. The high statistics of JEM-EUSO will be essential to identify the sources of the highest energy particles, and to measure their spectrum. This will allow a deeper understanding of the production, acceleration, and in situ propagation mechanisms of UHE cosmic rays. Through the study of the sky distribution of these extreme events, we can probe the galactic and local extragalactic magnetic field. The spectrum of scientific goals of the JEM-EUSO mission includes as exploratory objectives the detection of UHE γ-rays and neutrinos, and tests of relativity and quantum gravity effects at extreme energies. Other aspects of fundamental physics that will be studied include predictions of top-down models and the behavior of neutrino cross sections at extreme energies. Eventually, JEM-EUSO will also systematically survey atmospheric phenomena over the Earth surface. In this paper we summarize the science case and objectives of this challenging but fascinating mission. [ABSTRACT FROM AUTHOR]

Gamma ray bursts (GRBs) are high energy photons detected during a short interval of time. They are signs of the most energetic events in the universe. Photons detected show a non thermal spectrum, very well described by the peak energy and two smoothly connected power laws with different slopes or indices. This spectrum is named Band spectrum and serves actually as the most important source of information on their physical origin. It characterizes not only the overall emission, but also the instantaneous one, found in small successive intervals of time during the event. The time-resolved spectra show an obvious evolution in energy, which is generally accepted and supported by several arguments. There is also some evidence in measuring a kind of evolution in slope parameters, which would be considered as a serious source of information for the physical mechanism. In this work we try to examine the limit in observation bands, which we suspect as a reason for the apparent variation in time of spectral indices. Our work is based on a numeric and certified code. We simulate GRBs assuming synchrotron emission in the internal shocks model. For each time-resolved spectrum we find spectral parameters based on the Band function. [ABSTRACT FROM AUTHOR]

In this paper we want to discuss the status of Astroparticle Physics, which is now in its golden age. Astroparticle physics—the new branch of physics born roughly at the end of the 1980-ies—takes into account results obtained from the observations of cosmic sources via photonic experiments and particle experiments, and try to detect their global behaviour in order to understand the physical processes giving rise to them. All this for a better knowledge of the physics governing the whole Universe. Because of a limited length of the paper we will discuss only a selection of arguments, which are biased by our knowledge, as follows: 1. Introduction 2. Few Remarkable Notes 3. Background in the Universe 4. A Round-the-Universe Cruise 5. Prospects Conclusions [ABSTRACT FROM AUTHOR]

Diffusive shock acceleration (DSA) at relativistic shocks is expected to be an important acceleration mechanism in a variety of astrophysical objects including extragalactic jets in active galactic nuclei and gamma ray bursts. These sources remain strong and interesting candidate sites for the generation of ultra-high energy cosmic rays. In this paper, key predictions of DSA at relativistic shocks that are salient to the issue of cosmic ray ion and electron production are outlined. Results from a Monte Carlo simulation of such diffusive acceleration in test-particle, relativistic, oblique, MHD shocks are presented. Simulation output is described for both large angle and small angle scattering scenarios, and a variety of shock obliquities including superluminal regimes when the de Hoffman-Teller frame does not exist. The distribution function power-law indices compare favorably with results from other techniques. They are found to depend sensitively on the mean magnetic field orientation in the shock, and the nature of MHD turbulence that propagates along fields in shock environs. An interesting regime of flat spectrum generation is addressed, providing evidence for its origin being due to shock drift acceleration. The impact of these theoretical results on gamma-ray burst and blazar science is outlined. Specifically, Fermi gamma-ray observations of these cosmic sources are already providing significant constraints on important environmental quantities for relativistic shocks, namely the frequency of scattering and the level of field turbulence. [ABSTRACT FROM AUTHOR]

Highly relativistic jets are a key element of current gamma-ray burst (GRB) models, where the jet kinetic energy is converted to radiation energy at optically thin shocks. Mildly relativistic jets with smaller Lorentz factors are typically optically thick to gamma rays, and do not produce the spectacular GRB phenomenon. Jets which stall inside the progenitor similarly do not produce a GRB. However, various studies suggest that these jets are more common than GRB-producing jets. Here we report on our study of high-energy neutrino emission from these hidden jets. We describe the detection prospects with near-future neutrino detectors, and discuss how the presence of jets can be studied with neutrinos. The neutrino horizon for hidden jets is of order 10 Mpc, a volume which contains at least a few supernova per year. [ABSTRACT FROM AUTHOR]

Despite the pre-Swift expectation that bright optical flashes from reverse shocks would be prevalent in early-time afterglow emission, rapid response observations show this not to be the case. Although very bright at early times, some GRBs such as GRB 061007 and GRB 060418, lack the short-lived optical flash from the reverse shock within minutes after the GRB. In contrast, other optical afterglows, such as those of GRB 990123, GRB 021211, GRB 060111B, GRB 060117, GRB 061126, and recently GRB 080319B, show a steep-to-flat transition within first 103 s typical of a rapidly evolving reverse+forward shock combination. We review the presence and absence of the reverse shock components in optical afterglows and discuss the implications for the standard model and the magnetization of the fireball. We show that the previously predicted optical flashes are likely to occur at lower wavelengths, perhaps as low as radio wavelengths and, by using the case of GRB 061126 we show that the magnetic energy density in the ejecta, expressed as a fraction of the equipartion value, is a key physical parameter. [ABSTRACT FROM AUTHOR]

Thanks to its great capability Swift succeeds detecting an afterglow more than 93% of the time BAT triggers a new event. In a few cases, mainly Short GRBs or late observations, XRT did not detect any emission. However, for at least seven Long GRBs XRT could not measure any X-afterglow either, even as early as 100 s after the BAT trigger. The detection of this new class of GRBs without any X-ray emission has raised many questions. Are these extreme naked GRB? Is this a completely new class of objects with an uncommon emission release? In this contribution we present a study on these unusual events. We analyse their peculiar behaviour in the Gamma and X-ray bands and discuss their singular origin. [ABSTRACT FROM AUTHOR]

The Energetic X-ray Imaging Survey Telescope (EXIST) mission concept was selected for further study under the Astrophysics Strategic Mission Concept Study (ASMCS) program. The mission design is optimized for study of high-z GRBs as probes of the early Universe. With a 4.5 m2 imaging 5–600 keV (CZT) detector imaging a 90°×70° (>10% coding fraction) field of view with 2’ resolution and <20” (90% conf.) positions for >5σ sources, EXIST will perform rapid (<200 sec) slews onto GRBs and obtain prompt images and spectra with a co-aligned soft X-ray telescope 0.1–10 keV) and with a 1.1 m optical-IR telescope (IRT) simultaneously in 4 bands (0.3–0.52 μm, 0.52–0.9 μm, 0.9–1.38 μm, and 1.38–2.3 μm) for prompt identification (by dropouts or variability) of the GRB within a 3.75×4.25’ FoV. An autonomous spacecraft re-point (<2’) will then (at ∼300 sec after GRB trigger) put the GRB on a 0.2×10” slit for either R = 3000 (for AB <21) or R = 30 (for AB ∼21–25) prompt spectra over the 0.3–0.9 μm and 0.9–2.3 μm bands. This will provide onboard redshifts within ∼500–2000 sec for most GRBs, reaching z ∼20 (for Lyman-α breaks) if such GRBs exist, and spectra for studies of the host galaxy and local re-ionization bubble as well as intervening cosmic structure. With ∼600 GRBs/yr expected, EXIST will open a new era in studies of the early Universe as well as carry out a rich program of AGN and transient-source science. An overview of the GRB science objectives and a brief discussion of the overall mission design and operations is given, and example high-z GRB spectra are shown. EXIST is being proposed to the Astro2010 Decadal Survey as a Medium Class mission that could be launched as early as 2017. [ABSTRACT FROM AUTHOR]

Although many X-ray afterglows follow a “canonical” steep-shallow-standard decay pattern, some break the mould. The start of the Swift X-ray light-curve of GRB 080307 showed an unusual smooth rise, at the beginning of which the emission softened. After this brightening, the emission followed a simple power-law decay, with no requirement for breaks. It is conjectured that the early softening is related to the tail of the prompt emission, which then fades rapidly away, allowing the rise of the afterglow to be seen. The optical afterglow was briefly detected by Gemini, Faulkes Telescope South and UKIRT, and the host galaxy by WHT, though no redshift was determined. [ABSTRACT FROM AUTHOR]

We present spectroscopy of the host of GRB 051022 with GMOS nod and shuffle on Gemini South and NIRSPEC on Keck II. We determine a metallicity for the host of log(O/H)+12 = 8.77 using the R23 method (Kobulnicky & Kewley 2004 scale) making this the highest metallicity long burst host yet observed. The galaxy itself is unusually luminous for a LGRB host with a rest frame B band absolute magnitude -21.5 and has the spectrum of a rapidly star-forming galaxy. Our work raises the question of whether other dark burst hosts will show high metallicities. [ABSTRACT FROM AUTHOR]

I discuss some recent developments concerning the prompt emission of gamma-ray bursts, in particular the jet properties and radiation mechanisms, as exemplified by the naked-eye burst GRB 080319b, and the prompt X-ray emission of XRB080109/SN2008d, where the progenitor has, for the first time, been shown to contribute to the prompt emission. I discuss then some recent theoretical calculations of the GeV/TeV spectrum of GRB in the context of both leptonic SSC models and hadronic models. The recent observations by the Fermi satellite of GRB 080916C are then reviewed, and their implications for such models are discussed, together with its interesting determination of a bulk Lorentz factor, and the highest lower limit on the quantum gravity energy scale so far. [ABSTRACT FROM AUTHOR]

The principal paradigm for gamma-ray bursts (GRBs) suggests that the prompt transient gamma-ray signal arises from multiple shocks internal to the relativistic expansion. This paper explores how GRB prompt emission spectra can constrain electron (or ion) acceleration properties at the relativistic shocks that pertain to GRB models. The array of possible high-energy power-law indices in accelerated populations is highlighted, focusing on how spectra above 1 MeV can probe the field obliquity in GRB internal shocks, and the character of hydromagnetic turbulence in their environs. When encompassing the MeV-band spectral break, fits to BATSE/EGRET burst data indicate that the preponderance of electrons responsible for the prompt emission reside in an intrinsically non-thermal population. This differs markedly from typical populations generated in acceleration simulations; potential resolutions of this conflict such as the action of self-absorption are mentioned. Spectral modeling also suggests that the synchrotron mechanism is favored over synchrotron self-Compton scenarios due to the latter’s typically broad curvature near the peak. Such diagnostics will be enhanced by the broadband spectral coverage of bursts by the Fermi Gamma-Ray Space Telescope; the GBM will provide key information on the lower energy portions of the non-thermal particle population, while the LAT will constrain the power-law regime of particle acceleration. [ABSTRACT FROM AUTHOR]

Radiation from GRBs in the prompt phase, flares and an afterglow is thought to be produced by accelerated electrons in magnetic fields. Such emission may be produced at collisionless shocks of baryonic outflows or at reconnection sites (at least for the prompt and flares) of the magnetically dominated (Poynting flux driven) outflows, where no shocks presumably form at all. An astonishing recent discovery is that during reconnection strong small-scale magnetic fields are produced via the Weibel instability, very much like they are produced at relativistic shocks. The relevant physics has been successfully and extensively studied with the PIC simulations in 2D and, to some extent, in 3D for the past few years. We discuss how these simulations predict the existence of MeV-range synchrotron/jitter emission in some GRBs, which can be observed with Fermi. Recent results on modeling of the spectral variability and spectral correlations of the GRB prompt emission in the Weibel-jitter paradigm applicable to both baryonic and magnetic-dominated outflows is reviewed with the emphasis on observational predictions. [ABSTRACT FROM AUTHOR]

We present the observations of the long-duration Gamma-Ray Burst GRB 080916C by the Fermi Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT). This event was observed from 8 keV to a photon with an energy of 13.2 GeV. It develops over a 1400 s interval during which the highest number of photons with energy above 100 MeV are detected from a burst. The onset of the high-energy (>100 MeV) emission is delayed by ∼4.5 s with respect to the low-energy (<1 MeV) emission, which is not detected past 200 s. The broad-band spectrum of the burst is consistent with a single spectral form. [ABSTRACT FROM AUTHOR]

Continued study of the BATSE catalog verifies previously-identified correlations between pulse lag and pulse duration and corresponding anti-correlations between both properties and pulse peak flux for a large sample of Long GRB pulses; the study also finds correlations between pulse peak lags, pulse asymmetry, and pulse hardness. These correlations apparently can be used to delineate Long GRBs from Short ones. Correlated pulse properties represent constraints that can be used to guide theoretical modeling, whereas bulk prompt emission properties appear to be constructed by combining and smearing out pulse characteristics in ways that potentially lose valuable information. [ABSTRACT FROM AUTHOR]

Correlations among pulse properties in the prompt emission of long GRBs can potentially be used as cosmological distance indicators to estimate redshifts of GRBs to which these pulses belong. We demonstrate application of this technique to a sample of GRBs for which redshifts are not known. We also study the scatter of predicted redshifts of pulses found within individual bursts. We explore the characteristics of this scatter in hopes of identifying systematic corrections and/or pulse subsets that can be used to increase the technique’s reliability. [ABSTRACT FROM AUTHOR]

We present multiwavelength observations of the optically flaring source SWFIT J195509+261406 (initially mimicking a GRB), and discuss its nature considering several possibilities, amongst them an ultracompact low-mass X-ray binary. Finally we suggest a likely magnetar nature implying a missing link to the dim isolated neutron stars. [ABSTRACT FROM AUTHOR]

The Fermi Gamma Ray Burst Monitor (GBM) enhances the science return from the Fermi mission by providing observations of GRBs in the energy range 8 keV to 40 MeV, thereby extending GRB spectra three decades below the Large Area Telescope (LAT) threshold of ∼20 MeV. GBM employs twelve sodium iodide scintillation detectors and two bismuth germinate scintillation detectors to observe the full unocculted sky. The flight software localizes GRBs and can generate a request to autonomously repoint the spacecraft to place a burst within the LAT field of view. GBM has a trigger threshold of 0.74 photons-cm-2-s-1 and triggers on ∼260 bursts per year, as well as a variety of other gamma-ray transients. [ABSTRACT FROM AUTHOR]

Supernovae and Gamma-Ray bursts display many similarities, both in their observational qualities and in the engines behind these powerful explosions. Although not identical, there is a strong synergy in the study of these two objects. There is much the gamma-ray burst field can learn from the lessons of the more-developed supernova field, but the supernova field can also learn from new techniques developed for gamma-ray burst studies. Here I review some of the “lessons learned” from these fields to help foster this synergy. [ABSTRACT FROM AUTHOR]

The full catalogue of gamma-ray bursts detected by the HETE-2 spacecraft experiment is almost complete. It contains a unique set of more than 250 events. About one third of them has a coverage of prompt emission down to 2 keV (thanks to wide-field X-ray detectors), providing an unprecedented insight into the classes of X-ray rich bursts and X-ray flashes. Also the coverage of the optical afterglow of these bursts (detected in 28 cases) seems to be richer than in the case of Swift GRBs, probably because, with less frequent burst localization, follow-up observations were more available. We present statistical properties of the sample, with some emphasis on the analysis of the prompt emission in multiple spectral bands. [ABSTRACT FROM AUTHOR]

We review the observational status of the Supernova/Gamma-Ray Burst connection. Several lines of evidence suggest that long duration Gamma-ray Bursts are associated with bright SNe-Ic. However recent observations of GRB 060614 puzzle this scenario, pointing out the existence of long-duration Gamma-ray Burst not accompanied by a bright supernova. An analysis of the association GRB 060218/SN 2006aj and X081009/SN 2008D finds that SNe and GRBs are coeval events within ∼0.1 days. Current estimates of the SN and GRB rates yield a ratio GRB/SNe-Ibc in the range ∼0.4%–3%. [ABSTRACT FROM AUTHOR]

Optical emission from supernovae (SNe) reflects physical properties of SNe, such as mass and kinetic energy of the ejecta and the ejected 56Ni mass, which are closely related to the central remnants, explosion scenarios and progenitor stars of SNe. Modelling of the optical emission has been done mostly under the assumption of spherical symmetry. In this paper, we present multidimensional modelling of optical emission using a multi-dimensional Monte-Carlo radiative transfer code, SAMURAI (SupernovA MUlti-dimensional RAdIative transfer code). We show that all the optical observations of SN 1998 bw/GRB 980425, including the light curve and optical spectra at early and late phases, are consistent with polar-viewed radiation of the aspherical explosion model. The kinetic energy, which has been estimated to be 30–50×1051 ergs by spherical models, can be reduced to 20×1051 ergs. Such reduction of the kinetic energy is less effective for off-axis or less aspherical cases. As an observational test of SN asphericity, we also show our recent spectropolarimetric observations of Type Ic SN 2007 gr with Subaru telescope. The data clearly indicate the difference in the distribution of the newly synthesized elements and the elements in the pre-explosion star. [ABSTRACT FROM AUTHOR]

The launch of the Swift mission has opened up new windows in the study of the Gamma-Ray Burst. The optical follow-up of the early afterglow is one among the scientific fields that have been revolutionized. Thanks to the fast broadcast of the GRB coordinates and to the rapid response mode systems built up in 8-meter class telescopes, these afterglows can be observed from hours down to a few minutes after the GRB explosion. The high signal-to noise achievable allows us to acquire high resolution spectra, which carry a lot of information both on the GRB environment and the Inter-Stellar Medium of high redshift galaxies. In this paper, I will give a short review of what we have learned through the high resolution spectroscopy of GRB afterglows; I will focus in particular on the case of GRB080319B, the first naked-eye GRB observed to date, for which high resolution observations reach an unprecedented level of the signal to noise ratio. [ABSTRACT FROM AUTHOR]

The Swift Gamma-Ray Burst Explorer has revolutionized the study of these remarkable high-energy explosions. We summarize the technical developments which lead to the creation of the Swift mission, and outline the highlights of the first three years, and the prospects ahead. [ABSTRACT FROM AUTHOR]

The Milagro detector surveyed the sky continuously in the Very High Energy regime from January 2000 through March 2008. During that time, over 130 GRBs were detected and well localized by satellites within its 2 sr field of view. We have used Milagro data to search for >1 GeV emission from these bursts. Milagro is a water Cerenkov detector designed primarily for observations in the 0.1–100 TeV energy range. In the standard mode of operation, Milagro data is used to reconstruct the direction of an incoming high energy particle by analyzing the timing information of a large number of photomultiplier tubes that are triggered in coincidence by the air shower generated when such a particle interacts with the Earth’s atmosphere. Milagro data, however, can also be analyzed in “scaler mode,” where the rates of individual photomultiplier tubes can be used to detect emission above 1 GeV (albeit with no directional information). Here we present results from both techniques for all known GRBs detected by BATSE, BeppoSax, HETE-2, INTEGRAL, Swift, and the IPN, within the field of view of Milagro in its 8 years of operation. [ABSTRACT FROM AUTHOR]

Some models for quantum gravity (QG) violate Lorentz invariance and predict an energy dependence of the speed of light, leading to a dispersion of high-energy gamma-ray signals that travel over cosmological distances. Limits on the dispersion from short-duration substructures observed in gamma-rays emitted by gamma-ray bursts (GRBs) at cosmological distances have provided interesting bounds on Lorentz invariance violation (LIV). Recent observations of unprecedentedly fast flares in the very-high energy gamma-ray emission of the active galactic nuclei (AGNs) Mkn 501 in 2005 and PKS 2155–304 in 2006 resulted in the most constraining limits on LIV from light-travel observations, approaching the Planck mass scale, at which QG effects are assumed to become important. I review the current status of LIV searches using GRBs and AGN flare events, and discuss limitations of light-travel time analyses and prospects for future instruments in the gamma-ray domain. [ABSTRACT FROM AUTHOR]

The Large Area Telescope (LAT) instrument on the Fermi mission is revealing the rich spectral and temporal gamma-ray burst phenomena in the >100 MeV band. The synergy with Fermi’s GBM detectors links these observations to those in the well-explored 10–1000 keV range; the addition of the >100 MeV band observations brings new hint and new information about burst emission in both the prompt and afterglow phases. In this contribution we describe the prospects for the Fermi Large Area Telescope (LAT) in observing Gamma-Ray Bursts (GRBs), and some preliminary results. Here we focus our attention on the importance of multi frequencies approach to GRB science during the just began Fermi era. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts are observed to emit at least up to GeV energies, and their photon spectrum at the source is expected to extend up to TeV, due to either or both leptonic and hadronic mechanisms. I review some recent developments in GRB phenomenology in the light of Swift and other sub-MeV measurement, as well as recent theoretical work. I discuss then the acceleration of cosmic rays in GRB, which can extend to GZK energies, and the possibility of acceleration in GRB-related or other hypernovae. In both, synchrotron and inverse Compton, as well as hadronic processes, can lead to GeV-TeV gamma-rays measurable by GLAST, AGILE, or ACTs, providing useful probes of the burst physics and model parameters. Photo-meson and pp interactions also produce neutrinos at energies ranging from sub-TeV to EeV, which are targets for experiments such as IceCube, ANITA and KM3NeT. [ABSTRACT FROM AUTHOR]

Swift observations have revealed interesting but puzzling data that demand a rethink of the origins of gamma-ray bursts (GRBs) and their afterglows. The chromatic breaks in X-ray/optical afterglow lightcurves stimulated several innovative suggestions, most invoking a non-forward-shock origin of the X-ray afterglows. The status of both the observational facts and the theoretical models is critically reviewed. Besides the late “internal” emission from a long-live central engine, most observed X-ray afterglows likely still include the contribution of the traditional forward shock component. The physical nature (e.g. energy dissipation mechanism, emission site, and radiation mechanism) of the GRB prompt emission is currently not identified. The motivations and issues of three proposed prompt emission sites are reviewed. Several independent methods, invoking prompt gamma-ray, X-ray, optical and GeV emission information, respectively, have been applied to constrain the unknown emission site. Tentative evidence suggests a large prompt emission radius. Finally, the implications of the broad band high quality data of the “naked eye” GRB 080319B for our understanding of the afterglow and prompt emission mechanisms are discussed. [ABSTRACT FROM AUTHOR]

We summarise the X-ray temporal and spectral variability properties of GRBs as observed using the Swift satellite. Despite much individual complexity, the flux and spectral variability can be reasonably well described by a combination of two components—which we denote as the prompt and the afterglow. The first, prompt component consists of the burst and its initial decay while the second, afterglow component fits the X-ray plateau phase and subsequent decline observed in the majority of GRBs. When strong spectral variability occurs it is associated with the prompt component while the X-ray plateau and later emission shows little if any spectral variability. We briefly compare the observations with some of the proposed models. Any model for the early or late emission must explain the differences in both temporal and spectral behaviour. [ABSTRACT FROM AUTHOR]

When discussing the afterglows of gamma-ray bursts analytically, it is usually assumed that the external shock is always ultra-relativisitc, with the bulk Lorentz factor much larger than 1. However, we show that the deceleration of the external shock is actually very quick. The afterglow may typically enter the nonrelativistic phase in several days to teens of days, and may even enter the deep Newtonian phase in tens of days to several months. One thus should be careful in using those familiar analytical expressions that are derived only under the ultra-relativistic assumption. To explain the observed afterglows that typically last for a few weeks to several months, we need to consider the dynamics and radiation in the nonrelativisitic phase. [ABSTRACT FROM AUTHOR]

GRB 980703 has extensive available multiband afterglow data. Especially, its radio afterglows were very bright and monitored until more than 1000 days after the trigger time. This makes GRB 980703 a precious sample in GRB research. We calculate the radio and optical/NIR afterglows of GRB 980703 numerically, by using a set of generic dynamical equations. It is found that the observations consist with the theoretical expectation of the standard fireball model well. Our result suggests that the jet opening angle is ∼13°, the number density of the surrounding medium is ∼30 cm-3, and the isotropic equivalent kinetic energy of the explosion is ∼3.8×1052 ergs. [ABSTRACT FROM AUTHOR]

GRB080319B reached 5th optical mag during the burst. Thanks to the VLT/UVES rapid response mode, we observed its afterglow just 8m:30s after the GRB onset when R was ∼12. This allowed us to obtain the best signal-to-noise, high resolution spectrum of a GRB afterglow ever (S/N ∼50). The spectrum is rich of absorption features belonging to the main system at z = 0.937, divided in at least six components spanning a velocity range of 120 km/s. The observations caught the absorbing gas in a highly excited state, producing the strongest FeII fine structure lines ever observed in a GRB. A few hours later the optical depth of these lines was reduced by a factor of 4–20, and the optical/UV flux by a factor of ∼60. This proves that the excitation of the observed fine structure lines is due to “pumping” by the GRB UV photons. A comparison of the observed ratio between the number of photons absorbed by the excited and ground state of the FeII suggests that the six absorbers are ∼18–34 kpc from the GRB site, with component I ∼2 times closer to the GRB site than components III to VI. This suggests a structured ISM in this galaxy complex. [ABSTRACT FROM AUTHOR]

A comprehensive analysis of the X-ray afterglows of GRBs from the early shallow decay to late jet like decay phases observed with Swift/XRT is presented. We find that no significant spectral evolution is observed from the shallow to normal decay segments in the XRT lightcurves, and the normal decay segment is roughly consistent with external-shock models, favoring the idea that the shallow decay segment may be due to long-lasted refreshed shocks, but the observed chromatic breaks in the X-ray and optical bands of some bursts raise concerns for this scenario. GRBs 060413, 060522, 060607A, and 070110 are four significant outliers of this scenario in our sample, and they likely have an internal origin, which demands continuous operation of a long-term central engine. In order to explore the late jet-like breaks in the afterglow light curves we analyze the Swift XRT data for 179 GRBs and the optical afterglow data for 57 pre- and post-Swift GRBs. We find that no textbook version of jet break is found, in which the data fully satisfy jet break criteria. By relaxing one or more requirements, candidates to various degrees are identified. The break time in the X-ray band is earlier than that in the optical bands, and among thirteen bursts having both the optical and X-ray light curves, only seven have an achromatic break. Even in these cases only one band data satisfies the closure relations. These are great issues to interpret these candidates as jet breaks and further inferring GRB energetics. [ABSTRACT FROM AUTHOR]

The Swift Ultra-Violet/Optical Telescope has observed ∼200 gamma-ray bursts (GRBs) in its first two-and-a-half years of operation. From this collection of observations we have obtained ∼40 well sampled light curves. Using this dataset we present general optical/UV properties of GRBs, including filter dependent temporal slopes and color-color relationships. We also show that correlations exist between both the 11 hour X-ray flux (0.3–10 keV) and the prompt gamma-ray fluence (15–150 keV) and the v-band magnitude at 2000 s. [ABSTRACT FROM AUTHOR]

During the gravitational core collapse of a massive progenitor star which may give rise to at least a class of gamma-ray bursts (GRBs) associated with supernovae, a stellar core rapidly passes through a short yet important phase of neutronization, producing a huge amount of energetic neutrinos and photons which contribute to the total pressure within the progenitor core. The collection of neutrinos, photons and gas materials together may be approximated as a fluid with a polytropic index γ = 4/3 under the action of self-gravity. With a substantial generalization and using analytical and numerical methods (Lou & Cao 2008), we recently constructed and examined various self-similar solutions to describe collapses, rebound shocks and flows systematically in a γ = 4/3 polytropic gas mixture with spherical symmetry, and compare our results with those of Goldreich & Weber (1980). It is also possible to construct central void solutions without or with shocks. Various features and characteristics of this nonlinear relativistically hot gas dynamics, including asymptotic and exact solutions, are presented. This more general polytropic model analysis provides the dynamic basis of understanding the evolution of rebound shocks in supernovae (SNs) and the results may be also utilized to benchmark hydrodynamic simulations. [ABSTRACT FROM AUTHOR]

We review the models of Gamma-Ray Bursts (GRBs) with relativistic baryon-dominated ejecta: the Weibel-jitter model and the vorticity model. The former may also be relevant to the magnetic-dominated outflows. We discuss the origin of magnetic fields, mechanisms of electron heating/acceleration, nature and properties of the emitted radiation and the relevance of computer modeling to realistic GRBs. We discuss observational effects and predictions that, hopefully, will allow us to discriminate between the models. [ABSTRACT FROM AUTHOR]

POET (Polarimeters for Energetic Transients) is a Small Explorer mission concept proposed to NASA in January 2008. The principal scientific goal of POET is to measure GRB polarization between 2 and 500 keV. The payload consists of two wide FoV instruments: a Low Energy Polarimeter (LEP) capable of polarization measurements in the energy range from 2–15 keV and a high energy polarimeter (Gamma-Ray Polarimeter Experiment—GRAPE) that will measure polarization in the 60–500 keV energy range. Spectra will be measured from 2 keV up to 1 MeV. The POET spacecraft provides a zenith-pointed platform for maximizing the exposure to deep space. Spacecraft rotation will provide a means of effectively dealing with systematics in the polarization response. POET will provide sufficient sensitivity and sky coverage to measure statistically significant polarization for up to 100 GRBs in a two-year mission. Polarization data will also be obtained for solar flares, pulsars and other sources of astronomical interest. [ABSTRACT FROM AUTHOR]

The X and Gamma-ray telescope ECLAIRs is foreseen to be launched on a low Earth orbit (h = 630 km, i = 30°) aboard the SVOM satellite (Space-based multi-band astronomical Variable Objects Monitor), a French-Chinese mission with Italian contribution. Observations are expected to start in 2013 (Wei, J., these proceedings). It has been designed to detect and localize Gamma-Ray Bursts (GRBs) or persistent sources of the sky, thanks to its wide field of view (≈2 sr) and its remarkable sensitivity in the 4–250 keV energy range, with enhanced imaging sensitivity in the 4–70 keV energy band. These characteristics are well suited to detect highly redshifted GRBs, and consequently to provide fast and accurate triggers to other onboard or ground-based instruments able to follow-up the detected events in a very short time from the optical wavelength bands up to the few MeV Gamma-Ray domain. [ABSTRACT FROM AUTHOR]

The ARGO-YBJ experiment is a full coverage array made of Resistive Plate Chambers (RPCs) operated in streamer mode to detect the extensive array showers (EASs). With large field of view (>2sr) and high duty cycle (>90%), the ARGO-YBJ detector is the right ground-based apparatus to detect VHE GRBs, while the high altitude (4300 m a.s.l) and scaler working mode enable it to detect GRBs in the GeV region as high energy follow-up observation of satellites such as SWIFT. If a GRB signal is detected, the four scaler channels, which correspond to different energy thresholds, can provide further information on the energy spectrum and its cut-off. More than 40 SWIFT GRBs were in ARGO-YBJ field of view, no significant excess was detected till now. [ABSTRACT FROM AUTHOR]

It was widely shown that the cosmological parameters and dark energy can be constrained by using type-Ia supernovae (SNe Ia). Recently gamma-ray bursts (GRBs) were also argued to be promising standard candles for cosmography. In this paper, We calibrate the luminosity relations of GRBs and present constraints on cosmographic parameters by combining a recent GRB sample with SNe Ia. We find the ΛCDM model is consistent with the current data using the model independent constraints on cosmographic parameters. Furthermore, we find that the XCDM model and generalized Chaplygin gas(GCG) model could be distinguished by snap parameters. [ABSTRACT FROM AUTHOR]

Preliminary results of our analysis on the extended emission of short/medium duration GRBs observed with Swift/BAT are presented. The Bayesian blocks algorithm is used to analyze the burst durations and the temporal structure of the lightcurves in different energy bands. We show here the results of three bursts (GRBs 050724, 061006 and 070714B) that have a prominent soft extended emission component in our sample. The extended emission of these bursts is a continuous, flickering-liked component, lasting ∼100 seconds post the GRB trigger at 15–25 keV bands. Without considering this component, the three bursts are classified as short GRBs, with T90 = 2∼3 seconds. GRB 060614 has an emission component similar to the extended emission, but this component has pulse-liked structure, possibly indicating that this emission component is different from that observed in GRBs 050724, 061006, and 070714B. Further analysis on the spectral evolution behavior of the extended emission component is on going. [ABSTRACT FROM AUTHOR]

A sample of 286 gamma-ray bursts (GRBs) detected by the Swift satellite and 358 GRBs detected by the RHESSI satellite are studied statistically. Previously published articles, based on the BATSE GRB Catalog, claimed the existence of an intermediate subgroup of GRBs with respect to duration. We use the statistical χ2 test and the F-test to compare the number of GRB subgroups in our databases with the earlier BATSE results. Similarly to the BATSE database, the short and long subgroups are well detected in the Swift and RHESSI data. However, contrary to the BATSE data, we have not found a statistically significant intermediate subgroup in either Swift or RHESSI data. [ABSTRACT FROM AUTHOR]

In this work we analyse statistical properties of 82 long-GRBs with confirmed redshift and well-sampled light curves observed in R-band. The data gathered here show a cosmological evolution trend on various intrinsic GRB features. In particular, we find that the optical burst duration, the isotropic optical luminosity at the observed maximum and the time-integrated isotropic energy are all redshift dependent. The lack of correlations between the redshift and the observational GRB quantities (i.e. optical fluence and observed peak flux) points out that the detected trends are not affect by significant selection effects. This fact suggests that the intrinsic optical afterglow luminosity follows the cosmological evolution of a circumburst environment which determines the optical afterglow luminosity rate. It is interesting to note that the similar analysis performed for the main parameters of the gamma-ray emission show no evidence for a hypothetical redshift-dependent effect according to which the characteristics of a GRB-event depend on its location in the universe. Furthermore, there are correlations between the luminosity, the total energy and the duration of the gamma-ray and optical emission separately, which can arise from universal features of the observed lightcurves. [ABSTRACT FROM AUTHOR]

Using the 2.4m MDM and 8.4m Large Binocular Telescope, we observed nine GRB afterglows to systematically probe the late time behaviors of afterglows including jet breaks, flares, and supernova bumps. In particular, the LBT observations have typical flux limits of 25–26 mag in the Sloan r′ band, which allows us to extend the temporal baseline for measuring jet breaks by another decade in time scale. We detected four jet breaks (including a “textbook” jet break in GRB070125) and a fifth candidate, all of which are not detectable without deep, late time optical observations. In the other four cases, we do not detect the jet breaks either because of contamination from the host galaxy light, the presence of a supernova bump, or the intrinsic faintness of the optical afterglow. This suggests that the basic picture that GRBs are collimated is still valid and that the apparent lack of Swift jet breaks is due to poorly sampled afterglow light curves, particularly at late times. Besides the jet breaks, we also detected late time flares, which could attribute to late central engine activities, and two supernova bumps. [ABSTRACT FROM AUTHOR]