X‐Ray Lines from Gamma‐Ray Bursts

Monthly Notices of the Royal Astronomical Society, 2003

The Gamma-Ray Burst (GRB) 021211 detected by the High Energy Transient Explorer (HETE) II had a simple light-curve in the x-ray and gamma-ray energy bands containing one peak and little temporal fluctuation other than the expected Poisson variation. Such a burst offers the best chance for a unified understanding of the gamma-ray burst and afterglow emissions. We provide a detailed modeling of the observed radiation from GRB 021211 both during the burst and the afterglow phase. The consistency between early optical emission (prior to 11 minutes), which presumably comes from reverse shock heating of the ejecta, and late afterglow emission from forward shock (later than 11 minutes) requires the energy density in the magnetic field in the ejecta, expressed as fraction of the equipartition value or ǫ B , to be larger than the forward shock at 11 minutes by a factor of about 10 3 . We find that the only consistent model for the gamma-ray emission in GRB 021211 is the synchrotron radiation in the forward shock; to explain the peak flux during the GRB requires ǫ B in forward shock at deceleration to be larger than the value at 11 minutes by a factor of about 10 2 .

Nature, 2005

NOTE: this paper has been accepted for publication in Nature, but it is embargoed for discussion in the popular press until formal publication in Nature. Long gamma-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which gives rise to a highly relativistic jet. Internal inhomogeneities in the expanding flow give rise to internal shock waves that are believed to produce the gamma-rays we see 1,2. As the jet travels further outward into the surrounding circumstellar medium 'external' shocks give rise to the afterglow emission seen in the X-ray, optical and radio bands 1,2. Here we report on the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours. This steep decline, together with detailed spectral properties of two particular bursts, shows that violent shock interactions take place in the early jet outflows. The GRB prompt-ray emission usually goes through a strong spectral evolution with the peak of the emission rising to higher energies in the early phases and then moving to lower energies 3 , while the subsequent afterglow phase has an X-ray spectrum

Monthly Notices of the Royal Astronomical Society: Letters, 2006

Data from the Swift satellite has enabled us for the first time to provide a complete picture of the gamma-ray burst emission mechanism and its relationship with the early afterglow emissions. We show that gamma-ray photons for two bursts, 050126 & 050219A, for which we have carried out detailed analysis were produced as a result of the synchrotron self-Compton process in the material ejected in the explosion when it was heated to a mildly relativistic temperature at a distance from the center of explosion of order the deceleration radius. Both of these bursts exhibit rapidly declining early X-ray afterglow lightcurves; this emission is from the same source that produced the gamma-ray burst. The technique we exploit to determine this is very general and makes no assumption about any particular model for gamma-ray generation except that the basic radiation mechanism is some combination of synchrotron and inverse-Compton processes in a relativistic outflow. For GRB 050219A we can rule out the possibility that energy from the explosion is carried outward by magnetic fields, and that the dissipation of this field produced the γ-ray burst.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2004

Observational evidence of iron absorption and emission lines in X-ray spectra of Gamma-Ray Bursts is quite compelling. I will briefly review the results, summarize different models and describe the connection with massive progenitors in star-forming regions implied by these results. This link is also supported by measurements of the X-ray absorbing gas in several GRB's, with column density consistent with that of Giant Molecular Clouds harbouring star-formation in our Galaxy, as well as by evidences gathered in other wavelengths. However, the volume density inferred by the fireball-jet model is much lower than typical of a GMC, and I will confront this with the alternative explanation of fireball expansion in a high dense medium, outlining the problems that both models have at present. Finally I will briefly summarize some results on dark GRB's, and describe the prospects of high resolution X-ray spectroscopy in getting closer to the central environment of GRB, and far in the Early Universe by using GRB as beacons to probe star and galaxy formation.

Arxiv preprint astro-ph/ …, 2000

Gamma-ray burst astronomy has undergone a revolution in the last three years, spurred by the discovery of fading long-wavelength counterparts. We now know that at least the long duration GRBs lie at cosmological distances with estimated electromagnetic energy release of 10 51 -10 53 erg, making these the brightest explosions in the Universe. In this article we review the current observational state, beginning with the statistics of X-ray, optical, and radio afterglow detections. We then discuss the insights these observations have given to the progenitor population, the energetics of the GRB events, and the physics of the afterglow emission. We focus particular attention on the evidence linking GRBs to the explosion of massive stars. Throughout, we identify remaining puzzles and uncertainties, and emphasize promising observational tools for addressing them. The imminent launch of HETE-2 and the increasingly sophisticated and coordinated ground-based and space-based observations have primed this field for fantastic growth.

Monthly Notices of the Royal Astronomical Society, 1999

The discovery of a powerful and transient iron line feature in the X-ray afterglow spectra of gamma-ray bursts would be a major breakthrough for understanding the nature of their progenitors. and report such a detection in the afterglow of GRB 970508 and GRB 970828, respectively. We discuss how such a strong line could be produced in the various scenarios proposed for the event progenitor. We show that the observed line intensity requires a large iron mass, concentrated in the vicinity of the burst. The previous explosion of a supernova, predicted in the Supranova scenario, is the most straightforward way to account for such a large amount of matter. We discuss three different physical processes that could account for the line: recombination, reflection and thermal emission. Among these, reflection and thermal emission may explain the observed line features: reflection should be important if the remnant is optically thick, while thermal lines can be produced only in a thin plasma. The recombination process requires extremely high densities to efficiently reprocess the burst photons, whereas this process could work during the Xray afterglow. Future key observations for discriminating the actual radiating process are discussed.

The Astrophysical Journal, 2002

Using GRB 991216 as a prototype, it is shown that the intensity substructures observed in what is generally called the "prompt emission" in gamma ray bursts (GRBs) do originate in the collision between the accelerated baryonic matter (ABM) pulse with inhomogeneities in the interstellar medium (ISM). The initial phase of such process occurs at a Lorentz factor γ ∼ 310. The crossing of ISM inhomogeneities of sizes ∆R ∼ 10 15 cm occurs in a detector arrival time interval of ∼ 0.4 s implying an apparent superluminal behavior of ∼ 10 5 c. The long lasting debate between the validity of the external shock model vs. the internal shock model for GRBs is solved in favor of the first.

International Journal of Modern Physics D, 2003

ical predictions and the observations are documented. The afterglow power-law indexes in the EMBH theory are compared and contrasted with the ones in the literature, and no beaming process is found for GRB 991216. Finally, some preliminary results relating the observed time variability of the E-APE to the inhomogeneities in the interstellar medium are presented, as well as some general considerations on the EMBH formation. The issue of the GSTS paradigm will be the object of a forthcoming publication and the relevance of the iron-lines observed in GRB 991216 is shortly reviewed. The general conclusions are then presented based on the three fundamental parameters of the EMBH theory: the dyadosphere energy, the baryonic mass of the remnant, the interstellar medium density. An in depth discussion and comparison of the EMBH theory with alternative theories is presented as well as indications of further developments beyond the radial approximation, which will be the subject of paper II in this series. 4 Future needs for specific GRB observations are outlined.

The Astrophysical Journal, 2012

The Swift burst GRB 110205A was a very bright burst visible in the Northern hemisphere. GRB 110205A was intrinsically long and very energetic and it occurred in a low-density interstellar medium environment, leading to delayed afterglow emission and a clear temporal separation of the main emitting components: prompt emission, reverse shock, and forward shock. Our observations show several remarkable features of GRB 110205A : the detection of prompt optical emission strongly correlated with the BAT light curve, with no temporal lag between the two ; the absence of correlation of the X-ray emission compared to the optical and high energy gamma-ray ones during the prompt phase ; and a large optical re-brightening after the end of the prompt phase, that we interpret as a signature of the reverse shock. Beyond the pedagogical value offered by the excellent multi-wavelength coverage of a GRB with temporally separated radiating components, we discuss several questions raised by our observations: the nature of the prompt optical emission and the spectral evolution of the prompt emission at high-energies (from 0.5 keV to 150 keV) ; the origin of an X-ray flare at the beginning of the forward shock; and the modeling of the afterglow, including the reverse shock, in the framework of the classical fireball model.

The Astrophysical Journal, 2005