Chryssa Kouveliotou

GRB 130427A occurred in a relatively nearby galaxy; its prompt emission had the largest GRB fluence ever recorded. The afterglow of GRB 130427A was bright enough for the Nuclear Spectroscopic Telescope ARray (NuSTAR) to observe it in the 3-79 keV energy range long after its prompt emission (~1.5 and 5 days). This range, where afterglow observations were previously not possible, bridges an important spectral gap. Combined with Swift, Fermi, and ground-based optical data, NuSTAR observations unambiguously establish a single afterglow spectral component from optical to multi-GeV energies a day after the event, which is almost certainly synchrotron radiation. Such an origin of the late-time Fermi/Large Area Telescope >10 GeV photons requires revisions in our understanding of collisionless relativistic shock physics.

GRB 130427A occurred in a relatively nearby galaxy; its prompt emission had the largest GRB fluence ever recorded. The afterglow of GRB 130427A was bright enough for the Nuclear Spectroscopic Telescope ARray (NuSTAR) to observe it in the 3-79 keV energy range long after its prompt emission (~1.5 and 5 days). This range, where afterglow observations were previously not possible, bridges an important spectral gap. Combined with Swift, Fermi and ground-based optical data, NuSTAR observations unambiguously establish a single afterglow spectral component from optical to multi-GeV energies a day after the event, which is almost certainly synchrotron radiation. Such an origin of the late-time Fermi/LAT > 10 GeV photons requires revisions in our understanding of collisionless relativistic shock physics.

We present the results of our broadband spectral analysis of 42 SGR J1550–5418 bursts simultaneously detected with the Swift/X-ray Telescope (XRT) and the Fermi/Gamma-ray Burst Monitor (GBM), during the 2009 January active episode of the source. The unique spectral and temporal capabilities of the XRT windowed timing mode have allowed us to extend the GBM spectral coverage for these events down to the X-ray domain (0.5-10 keV). Our earlier analysis of the GBM data found that the SGR J1550–5418 burst spectra were described equally well with either a Comptonized model or with two blackbody functions; the two models were statistically indistinguishable. Our new broadband (0.5-200 keV) spectral fits show that, on average, the burst spectra are better described with two blackbody functions than with the Comptonized model. Thus, our joint XRT-GBM analysis clearly shows for the first time that the SGR J1550–5418 burst spectra might naturally be expected to exhibit a more truly thermalized character, such as a two-blackbody or even a multi-blackbody signal. Using the Swift and RXTE timing ephemeris for SGR J1550–5418 we construct the distribution of the XRT burst counts with spin phase and find that it is not correlated with the persistent X-ray emission pulse phase from SGR J1550–5418. These results indicate that the burst emitting sites on the neutron star need not to be co-located with hot spots emitting the bulk of the persistent X-ray emission. Finally, we show that there is a significant pulse phase dependence of the XRT burst counts, likely demonstrating that the surface magnetic field of SGR J1550–5418 is not uniform over the emission zones, since it is anticipated that regions with stronger surface magnetic field could trigger bursts more efficiently.

Using our new 3‐D relativistic electromagnetic particle (REMP) code parallelized with MPI, we have investigated long‐term particle acceleration associated with an relativistic electron‐positron jet propagating in an unmagnetized ambient electron‐positron plasma. The simulations have been performed using a much longer simulation system than our previous simulations in order to investigate the full nonlinear stage of the Weibel instability and its particle acceleration mechanism. Cold jet electrons are thermalized and ambient electrons are accelerated in the resulting shocks. The acceleration of ambient electrons leads to a maximum ambient electron density three times larger than the original value. Behind the bow shock in the jet shock strong electromagnetic fields are generated. These fields may lead to the afterglow emission. We have calculated the time evolution of the spectrum from two electrons propagating in a uniform parallel magnetic field to verify the technique.

The TAROT telescopes (Télescopes à Action Rapide pour les Objets Transitoires) are two robotic observatories designed to observe the prompt optical emission counterpart and the early afterglow of gamma ray bursts (GRBs). We present the multi‐wavelength study of those gamma‐ray bursts observed by TAROT. These events are characterized by the presence at early time of a rising in their optical light curves lasting a few hundred of seconds. In one case (GRB 060904B), a flare occurs at similar time in the X‐ray band, while in the other cases the X‐ray light curves appear smooth during the optical rise. We investigate the possible nature of this behavior and conclude that a multi‐component emission is mandatory to explain the optical‐to‐X‐ray afterglow.

We present here a concept for a light and low‐power wide field monitor working in the X‐ray range, suitable for simultaneous imaging of large portions of the sky and GRB localizations. Our concept evolves from and improves on the design of the 5‐kg SuperAGILE experiment, flying on the AGILE mission and currently delivering arcmin‐localizations of GRBs at a rate of about 1/month. Similar to SuperAGILE, our concept is based on position sensitive silicon detectors equipped with one‐dimensional coded masks. Different options are available for the detector, whose properties, combined with the scientific requirements, drive the design of the experiment. Our approach is based on a modular detector. The experiment design can then be tailored to specific scientific goals of the experiment or the mission (e.g., to cite GRBs only: the brightest GRBs/XRFs on a large field of view—FoV, or many low‐fluence GRBs/XRFs on a smaller FoV, or the low energy spectrum of the prompt event, or the detection of high‐z GRBs). In this paper we describe the concept, the main detector properties and outline some possible experiment configurations, with examples of their expected performance. Different experiment configurations in terms of area, FoV, angular resolution may be designed starting from the same detectors. Instead, the band‐pass is mostly related to the detector properties. A key point of our project is the high degree of readiness of the detectors that are at production level and may be immediately proposed for a future experiment onboard missions with high readiness requirements.