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Gamma Ray Bursts (GRB) prompt energetics, though widely dispersed, correlate with their spectral peak energies. The dispersion of this color—energy correlation is reduced if GRB energetics are corrected for the jet opening angle leading to the possibility to use GRBs to constrain the cosmological parameters. One of the issues presently debated is the possibility that these correlations are due to instrumental selection effects. Through the time resolved spectral analysis of Fermi bursts, it is shown that a color—luminosity correlation exists within individual GRBs and that it is similar to that observed among different bursts. This result reassures against the possibility that the color—luminosity correlation is due to selection effects and calls for a robust physical interpretation. [ABSTRACT FROM AUTHOR]

We compute the emission from internal shocks using a new approach, where a detailed model for the dynamics of shock waves is coupled to a radiative code that solves the time evolution of shock-accelerated electrons and emitted photons in the comoving frame of the shocked material, taking into account all relevant radiative processes. Using this model, we discuss which process (synchrotron vs inverse Compton) is most likely dominating the observed low gamma-ray emission and consider a possible mechanism to reconcile the synchrotron process with the observed spectral slope (α∼-1) in this energy range. We then focus on the high-energy emission (Fermi-LKT range) and show that very different spectral shapes can be expected. We describe physical diagnostics to constrain the properties of the relativistic outflow, the physical conditions in the shocked medium and the dominant radiative process. [ABSTRACT FROM AUTHOR]

The apparent spectral evolution observed in the steep decay phase of many GRB earlyX-ray afterglows raises a great concern of the high-latitude “curvature effect” interpretation of this phase. However, previous curvature effect models only invoked a simple power law spectrum upon the cessation of the prompt internal emission. We investigate the “curvature effect” model of a more general non-power-law spectrum and and test this model with the Swift/XRT observations By comparing the simulated lightcurves/spectra with the observed ones, we show that one can reproduce both the observed lightcurve and the apparent spectral evolution of GRB 050814 using a model invoking a cut-off power-law spectrum. This suggests that at least for some GRBs, the fast spectral evolution in the steep-decay phase can be explained by the curvature effect of a non-power-law spectrum. [ABSTRACT FROM AUTHOR]

After the launch and successful beginning of operations of the FERMI satellite, the topics related to high-energy observations of gamma-ray bursts have obtained a considerable attention by the scientific community. Undoubtedly, the diagnostic power of high-energy observations in constraining the emission processes and the physical conditions of gamma-ray burst is relevant. We briefly discuss how gamma-ray burst observations with ground-based imaging array Cerenkov telescopes, in the GeV-TeV range, can compete and cooperate with FERMI observations, in the MeV-GeV range, to allow researchers to obtain a more detailed and complete picture of the prompt and afterglow phases of gamma-ray bursts. [ABSTRACT FROM AUTHOR]

H.E.S.S. is currently the most sensitive instrument in the very-high-energy (VHE; >100 GeV) gamma-ray domain and has revealed many new sources along the Galactic Plane. After the very successful first VHE Galactic survey of 2004/2005, H.E.S.S. continued and extended that survey in 2006/2007, discovering a number of new sources, many of which are unidentified. Some of the unidentified H.E.S.S. sources have no counterparts at any other wavelength; some of them have X-ray counterparts, but most of them have no counterparts at GeV energies, i.e. among unidentified EGRET sources. Hence the Fermi GST observations could be a key point in order to identify them and to explain the physical processes that rule these sources. [ABSTRACT FROM AUTHOR]

H.E.S.S. is currently the most sensitive instrument in the very-high-energy (VHE; >100 GeV) gamma-ray domain and has revealed many new sources along the Galactic Plane. After the very successful first VHE Galactic survey of 2004/2005, H.E.S.S. continued and extended that survey in 2006/2007, discovering a number of new sources, many of which are unidentified. In this context, we will present the results on a new source which is located near the Galatic Center, extended and very faint (∼1% of the Crab luminosity), almost at the lower end of H.E.S.S. sensitivity: HESS J1741-302. [ABSTRACT FROM AUTHOR]

H.E.S.S. is currently the most sensitive instrument in the very-high-energy (VHE; >100 GeV) gamma-ray domain and has revealed many new sources along the Galactic Plane. After the very successful first VHE Galactic survey of 2004/2005, H.E.S.S. continued and extended that survey in 2006/2007, discovering a number of new sources, many of which are unidentified. In this context, we will present the results on a new source which is located near the Galatic Center, extended and very faint (∼1% of the Crab luminosity), almost at the lower end of H.E.S.S. sensitivity: HESS J1741-302. [ABSTRACT FROM AUTHOR]

We present a model for high-energy emission in microquasars where the energy content of the jets is dominated by relativistic protons. We also include a primary leptonic component. Particles are accelerated up to relativistic energies in a compact region located near the base of the jet, where most of the emission is produced. We calculate the production spectrum due to proton and electron synchrotron radiation and photohadronic interactions. The target field for proton-photon collisions is provided by the synchrotron radiation in the acceleration region. In models with a significant leptonic component, strong internal photon-photon absorption can attenuate the emission spectrum at high energies. Depending on the values of the parameters, our model predicts luminosities in the range 1034–1037 erg s-1 up to GeV energies, with a high-energy tail that can extend up to 1016 eV. In some cases, however, absorption effects can completely suppress the emission above 10 GeV, giving rise to different spectral shapes. These results can be tested in the near future by observations with instruments like GLAST-Fermi, HESS II and MAGIC II. [ABSTRACT FROM AUTHOR]

Quasars remain one of the most powerful probes of the early Universe, providing measurements and constraints for re-ionization, early structure formation, and early chemical enrichment epochs [1], [2], [3]. The obscured phase of QSOs evolution, as well as their accretion history, can be best followed by observing QSOs bright in MeV gamma-rays [4]. By analysing resonant absorption troughs in spectral energy distribution of flaring QSOs one can measure the (baryonic) absorbing column, e.g. baryonic content, and metallicity of the QSO host galaxy [5], while the flare strength will give information on the accretion rate of the QSO powering supermassive black hole. By measuring the baryonic absorbing column for QSOs at different redshifts one can follow the early obscured evolution of AGN, as well as the evolution of the IGM transmission at the end of the cosmic re-ionization epoch, while absorption by metals constrain the early chemical evolution of the IGM. These possibilities can be explored in the near future by observations with instruments like, already launched, Fermi-GLAST. Even earlier phases of the Universe evolution, at redshifts z>6, can be studied by applying the gamma-ray resonance absorption method to the GRBs prompt emission anaysis. This type of a study can be done in a foreseeable future with a MeV-range covering space instrument like GRIPS. [ABSTRACT FROM AUTHOR]

The measured redshifts of gamma-ray bursts (GRBs), which were first detected by the Swift satellite, seem to be bigger on average than the redshifts of GRBs detected by other satellites. We analyzed the redshift distribution of GRBs triggered and observed by different satellites (Swift[1], HETE2[2], BeppoSax, Ulyssses). After considering the possible biases significant difference was found at the p = 95.70% level in the redshift distributions of GRBs measured by HETE and the Swift. [ABSTRACT FROM AUTHOR]

The apparent spectral evolution observed in the steep decay phase of many GRB early X-ray afterglows raises a great concern of the high-latitude “curvature effect” interpretation of this phase. However, previous curvature effect models only invoked a simple power law spectrum upon the cessation of the prompt internal emission. We investigate the “curvature effect” model of a more general non-power-law spectrum and and test this model with the Swift/XRT observations By comparing the simulated lightcurves/spectra with the observed ones, we show that one can reproduce both the observed lightcurve and the apparent spectral evolution of GRB 050814 using a model invoking a cut-off power-law spectrum. This suggests that at least for some GRBs, the fast spectral evolution in the steep-decay phase can be explained by the curvature effect of a non-power-law spectrum. [ABSTRACT FROM AUTHOR]

In spite of extensive efforts of the spectral and lightcurve observations, the emission mechanism of γ-ray bursts (GRBs) is still unclear. Recently the observational techniques of X-ray and γ-ray polarizations have been developed rapidly, and several satellites with high sensitivity for such high energy polarizations have been planned. They will provide us the first definitive detections of GRB polarizations and enable us to discuss their statistical properties. We calculate the distribution of GRB polarizations in the synchrotron models (with globally ordered magnetic field and with small-scale random magnetic field) and Compton drag model. The results show that the distribution of the polarization degrees of ∼100 bursts detected by a realistic polarimeter satellite, such as POET, may distinguish the three models, and provide direct diagnostics of the emission mechanism, the magnetic field configuration, and the geometry of GRB outflows. [ABSTRACT FROM AUTHOR]

The flare activity that is observed in GRBs soon after the prompt emission with the XRT (0.3–10 KeV) instrument on Board of the Swift satellite is leading to important clues in relation to the physical characteristics of the mechanism generating the emission of energy in Gamma Ray Bursts. We will briefly refer to the results obtained with the recent analysis [1] and [2] and discuss the preliminary results we obtained with a new larger sample of GRBs [limited to early flares] based on fitting of the flares using the Norris 2005 profile. We find, in agreement with previous results, that XRT flares follow the main characteristics observed in [3] for the prompt emission spikes. The estimate of the flare energy for the subsample with redshift is rather robust and an attempt is made, using the redshisft sample, to estimate how the energy emitted in flares depends on time. We used a H0 = 70 km/s/Mpc, ΩΛ = 0.7, Ωm = 0.3 cosmology. [ABSTRACT FROM AUTHOR]

Emission from an expanding fireball surface must be influenced by the delay of time of different areas of the fireball surface, the variation of the intensity due to the growing of the fireball radius, the variation of the time contracted factor and the shifting of the intrinsic spectrum associated with the angle relative to the line of sight. Besides these, the emission is confined by the fireball surface itself, which together with the intrinsic emission limits will play a role in constraining photons that will reach the observer expected at any particular observation time. Taking all these factors into account comes to a full knowledge of the so-called curvature effect. I present a summary of our group’s work carried on in the past few years. Our work covers the derivation of basic formulas of the curvature effect, its application to the prompt emission of GRBs, and the application to the early X-ray afterglow emission of the events. In particular, an interpretation based on the curvature effect, for the new puzzling finding of the softening process detected in the early X-ray emission of Swift bursts is presented. [ABSTRACT FROM AUTHOR]

We performed the first systematic search for the minimum variability time scale between 0.3 and 10 keV studying the 28 brightest early (<3000 s) afterglows detected by Swift-XRT up to March 2008. We adopt the power spectrum analysis in the time domain: unlike the Fourier spectrum, this is suitable to study the rms variations at different time-scales. We find that early XRT afterglows show variability in excess of the Poissonian noise level on time-scales as short as ∼1 s (rest frame value), with the shortest tmin associated with the highest energy band. The γ—ray prompt emission of GRB 080319B shows a characteristic average variability time-scale ∼1 s; this parameter undergoes a remarkable evolution during the prompt emission (BAT observation). [ABSTRACT FROM AUTHOR]

Long-duration gamma-ray bursts (GRBs) are believed to be produced by the core collapse of massive stars and hence to be connected with supernovae (SNe). Indeed, for four pairs of GRBs and SNe, spectroscopically confirmed connection has been firmly established. For more than a dozen of GRBs the SN signature (the ‘red bump’) has been detected in the afterglow lightcurves. Based on the four pairs of GRBs and SNe with spectroscopically confirmed connection a tight correlation was found between the peak spectral energy of GRBs and the peak bolometric luminosity of the underlying SNe. The recent discovery of X-ray flash 080109 associated with a normal core-collapse SN 2008D confirmed this relation and extended the GRB-SN connection. Progress on the GRB-SN connection is briefly reviewed. [ABSTRACT FROM AUTHOR]

Two classes of gamma-ray bursts have been identified in the BATSE catalogs characterized by durations shorter and longer than about 2 seconds. There are, however, some indications for the existence of a third type of burst. Swift satellite detectors have different spectral sensitivity than pre-Swift ones for gamma-ray bursts. Therefore it is worth to reanalyze the durations and their distribution and also the classification of GRBs. Using The First BAT Catalog the maximum likelihood estimation was used to analyzed the duration distribution of GRBs. The three log-normal fit is significantly (99.54% probability) better than the two for the duration distribution. Monte-Carlo simulations also confirm this probability (99.2%). [ABSTRACT FROM AUTHOR]

One of the most interesting discoveries of the Swift mission is the wide variety of X-ray behaviors observed in the early GRB afterglows. We selected a large sample of bright X-ray flares observed by XRT up to March 2008. The Norris et al. (2005) profile was adopted to characterize the morphological and timing properties of the flares in different energy bands, such as the peak, rise and decay times, the width and the asymmetry of each flare. These parameters were used to estimate other properties in a homogeneous way, such as the temporal lag between the same pulse seen at different energy bands, or the width of the flare as a function of energy, thus allowing us to derive important clues on the underlying physical mechanism and the connections with the central engine. [ABSTRACT FROM AUTHOR]

In this paper we make an attempt to combine the two kinds of data from the Swift-XRT instrument (windowed timing and photon counting modes) and the from BAT. A thorough desription of the applied procedure will be given. We apply various binning techniques to the different data: Bayes blocks, exponential binning and signal-to-noise type of binning. We present a handful of lightcurves and some possible applications. [ABSTRACT FROM AUTHOR]

The precise localization of short/hard (Type I) gamma-ray bursts (GRBs) in recent years has answered many questions but raised even more. I present some results of a systematic study of the optical afterglows of long/soft (Type II) and short/hard (Type I) GRBs, focusing on the optical luminosity as another puzzle piece in the classification of GRBs. [ABSTRACT FROM AUTHOR]

We briefly review the main results of the observations carried out during the prompt emission of the exceptionally bright GRB 080319B. [ABSTRACT FROM AUTHOR]

Swift has revealed features in GRB early light curves, such as steep decays and X-ray flares, whose properties are consistent with an internal origin though they are far from understood. The steep X-ray decay is often explained using the curvature effect; however a significant number of GRBs display strong spectral evolution during this phase, and a new mechanism must be invoked to explain this. Of particular interest are the longest duration GRBs in which the early emission can be studied in most detail. Here we present data for GRB 070616, in which the prompt emission shows a complex multipeaked structure, leading to one of the longest prompt emission durations ever recorded. We take advantage of extensive coverage of such a long burst by all Swift instruments. Combining data from Swift and Suzaku we study the evolution of the prompt emission spectrum, following the temporal variability of the peak energy and spectral slope. [ABSTRACT FROM AUTHOR]

We define a new characteristic photon energy for the Band function, Ewz, and describe a new way of estimating the peak luminosity of the prompt emission from GRBs, Liso, which is not dependent on the time binning. We show that Ewz and Liso are correlated for all bursts, long and short. The new photon energy-luminosity relationship gives us a strong indication that the underlying process responsible for the prompt GRB emission is thermal. [ABSTRACT FROM AUTHOR]

Using the discriminant analysis of the multivariate statistical analysis we compared the distribution of the physical quantities of the optically dark and bright GRBs, detected by the BAT and XRT on board of the Swift Satellite. We found that the GRBs having detected optical transients (OT) have systematically higher peak fluxes and lower HI column densities than those without OT. [ABSTRACT FROM AUTHOR]

The late-time optical/radio afterglows of γ-ray bursts (GRBs) are believed to be synchrotron emission of electrons accelerated in relativistic collisionless shocks propagating in the ambient medium of the sources. However, the fraction f of electrons that are coupled to protons and accelerated remains unclear and a large number of thermal electrons that are not coupled to protons may be left behind. If f<1, the true explosion energies of GRBs are f-1 times larger than those commonly estimated with f = 1. Thus the value of f gives an important constraint on the nature of the central engine of GRBs and the physics of collisionless shocks. Although early-time radio observations can probe the thermal electrons, they are difficult at present. We show that the Faraday rotation effects of the thermal electrons may suppress the linear polarization of the afterglow at frequencies higher than the absorption frequency in the late time, if the magnetic field is ordered at least in parts, and that f can be constrained through the observation of the effects. We find that those effects may be detected with late-time, >=1 day, polarimetry with ALMA for a burst occurring within 1 Gpc (i.e., z≃0.2), if f∼10-1. [ABSTRACT FROM AUTHOR]

Forty years after the discovery of rotation-powered pulsars, we still do not understand many aspects of their pulsed emission. In the last few years there have been some fundamental developments in acceleration and emission models. I will review both the basic physics of the models as well as the latest developments in understanding the high-energy emission of rotation-powered pulsars. The prospects for a revolution of the field is about to take place with the launch of AGILE and GLAST. Prospects for using this large increase in γ-ray data for the study of pulsar physics will be discussed. [ABSTRACT FROM AUTHOR]

We present a detailed study of the spectral and temporal properties of the X-ray flares emission of several GRBs. We select a sample of GRBs which X-ray light curve exhibits large amplitude variations with several rebrightenings superposed on the underlying three-segment broken powerlaw that is often seen in Swift GRBs. We try to understand the origin of these fluctuations giving some diagnostic in order to discriminate between refreshed shocks and late internal shocks. For some bursts our time-resolved spectral analysis supports the interpretation of a long-lived central engine, with rebrightenings consistent with energy injection in refreshed shocks as slower shells generated in the central engine prompt phase catch up with the afterglow shock at later times. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Rotation-powered pulsars are excellent laboratories for study of particle acceleration as well as fundamental physics of strong gravity, strong magnetic fields, high densities and relativity. I will review some outstanding questions in pulsar physics and the prospects for finding answers with GLAST LAT observations. LAT observations should significantly increase the number of detected radio-loud and radio-quiet gamma-ray pulsars, including millisecond pulsars, giving much better statistics for elucidating population characteristics, will measure the high-energy spectrum and the shape of spectral cutoffs and determine pulse profiles for a variety of pulsars of different age. Further, measurement of phase-resolved spectra and energy dependent pulse profiles of the brighter pulsars should allow detailed tests of magnetospheric particle acceleration and radiation mechanisms, by comparing data with theoretical models that have been developed. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Almost all the X-ray afterglows of gamma-ray bursts (GRBs) observed by the Swift satellite have a shallow decay phase in the first thousands of seconds. We show that in an inhomogeneous jet (multiple-subjet or patchy-shell) model the superposition of the afterglows of off-axis subjets (patchy shells) can have the shallow decay phase. The necessary condition for obtaining the shallow decay phase is that γ-ray bright subjets (patchy shells) should have γ-ray efficiency higher than previously estimated, and should be surrounded by γ-ray dim subjets (patchy shells) with low γ-ray efficiency. Our model predicts that events with dim prompt emission have the conventional afterglow light curve without the shallow decay phase like GRB 050416A. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]