arXiv subjects

Davide Lazzati

Publications and source records attributed to Davide Lazzati.

At least 127 records · Page 7Linked to original sources

arXiv2002

Time-Dependent Photoionization in a Dusty Medium I: Code Description and General Results

We present a time-dependent photoionization code that combines self-consistently metal evolution and dust destruction under an intense X-ray UV radiation field. Firstly, we extend the mathematical formulation of the time-dependent evolution of dust grains under an intense radiation flux with the inclusion of the process of ion field emission (IFE). We determine the relative importance of IFE with respect to X-ray and UV sublimation as a function of grain size, intensity and hardness of the incident spectrum. We then combine the processes of dust destruction with a photoionization code that follows the evolution of the ionization states of the metals and the relative radiative transitions. Our code treats, self-consistently, the gradual recycling of metals into gas as dust is sublimated away; it allows for any initial dust grain distribution and follows its evolution in space and time. In this first paper, we use our code to study the time-dependent behaviour of the X-ray and optical opacities in the nearby environment of a Gamma-ray Burst, and show how the time variability of the low-energy and high-energy opacities can yield powerful clues on the characteristics of the medium in which the bursts occur.

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arXiv2002

Soft X-ray emission lines in the early afterglow of gamma-ray bursts

We compute the luminosity of K_alpha emission lines produced by astrophysically abundant elements in the soft X-ray spectra of the early afterglow of gamma-ray bursts. We find that the detection of these lines can be a diagnostic for the geometrical set-up of the reprocessing material. In particular we can distinguish between a ``geometry dominated'' model, in which the line emission is coming from an extended region and its duration arises from light-travel-time effects and an ``engine dominated'' model, where the line emitting gas is in a smaller region, irradiated for a longer period. These lines therefore offer clues to the dynamics and time-scale of the explosion leading to a gamma-ray burst.

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arXiv2002

Precursors and electron-positron pair loading from erupting fireballs

Recent observations suggest that long-duration gamma-ray bursts and their afterglows are produced by highly relativistic jets emitted in core-collapse explosions. As the jet makes its way out of the stellar mantle, a bow shock runs ahead and a strong thermal precursor is produced as the shock breaks out. Such erupting fireballs produce a very bright gamma-ray precursor as they interact with the thermal break-out emission. The prompt gamma-ray emission propagates ahead of the fireball before it becomes optically thin, leading to electron-positron pair loading and radiative acceleration of the external medium. The detection of such precursors would offer the possibility of diagnosing not only the radius of the stellar progenitor and the initial Lorentz factor of the collimated fireball, but also the density of the external environment.

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arXiv2002

Hard X-ray afterglows of short GRBs

We report the discovery of a transient and fading hard X-ray emission in the BATSE lightcurves of a sample of short gamma-ray bursts. We have summed each of the four channel BATSE light curves of 76 short bursts to uncover the average overall temporal and spectral evolution of a possible transient signal following the prompt flux. We found an excess emission peaking ~30 s after the prompt one, detectable for ~100 s. The soft power-law spectrum and the time-evolution of this transient signal suggest that it is produced by the deceleration of a relativistic expanding source, as predicted by the afterglow model.

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arXiv2002

X-ray spectroscopy of gamma-ray bursts: the path to the progenitor

Despite great observational and theoretical effort, the burst progenitor is still a mysterious object. It is generally accepted that one of the best ways to unveil its nature is the study of the properties of the close environment in which the explosion takes place. We discuss the potentiality and feasibility of time resolved X-ray spectroscopy, focusing on the prompt gamma-ray phase. We show that the study of absorption features (or continuum absorption) can reveal the radial structure of the close environment, unaccessible with different techniques. We discuss the detection of absorption in the prompt and afterglow spectra of several bursts, showing how these are consistent with gamma-ray bursts taking place in dense regions. In particular, we show that the radius and density of the surrounding cloud can be measured through the evolution of the column density in the prompt burst phase. The derived cloud properties are similar to those of the star forming cocoons and globules within molecular clouds. We conclude that the burst are likely associated with the final evolutionary stages of massive stars.

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arXiv2001

The birth place of gamma-ray bursts: abundance gradients and constraints on progenitors

The physics of gamma-ray bursts (GRBs) and their offsets from the centers of their host galaxies are used to investigate the evolutionary state of their progenitors, motivated by the popular idea that GRBs are linked with the cataclysmic collapse of massive stars. We suggest that GRB progenitors in the inner and outer regions of hosts may be intrinsically different: outer bursts appear to have systematically greater isotropic equivalent energies (or narrower jets). This may provide an interesting clue to the nature of GRBs, and could reflect a relation between metallicity and the evolution of GRB progenitors. If true, then this offset-isotropic luminosity correlation is a strong argument for a collapsar origin of long-duration GRBs.

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arXiv2001

Determining the location of Gamma-Ray Bursts through the evolution of their soft X-ray absorption

We investigate through dedicated numerical simulations the evolution of the soft X-ray absorption properties of a cloud surrounding a gamma-ray burst source. We show that the absorption properties of the material are strongly modified by the ionization induced by the intense burst flux. We derive the temporal evolution of the measured column density as a function of the density and size of the absorbing medium. Even if their statistical significance is not extremely compelling, we find that the detection in several bursts of variable absorption during the gamma-ray phase can be accounted for if these bursts are associated to overdense regions in molecular clouds with properties similar to those of star formation globules. We fit our model variable column density to the data of GRB 980329 and GRB 780506, showing that with this method the size, density and density distribution of the material surrounding a burst can be measured.

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arXiv2001

Possible detection of hard X-ray afterglows of short gamma-ray bursts

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arXiv2001

Time dependent photoionization opacities in dense Gamma-Ray Burst environments

The recent detection of a transient absorption feature in the X-ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma-ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor.

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arXiv2001

Internal shocks in the jets of radio-loud quasars

The central engine causing the production of jets in radio sources may work intermittently, accelerating shells of plasma with different mass, energy and velocity. Faster but later shells can then catch up slower earlier ones. In the resulting collisions shocks develop, converting some of the ordered bulk kinetic energy into magnetic field and random energy of the electrons which then radiate. We propose that this internal shock scenario, which is the scenario generally thought to explain the observed gamma-ray burst radiation, can work also for radio sources in general, and for blazar in particular. We investigate in detail this idea, simulating the birth, propagation and collision of shells, calculating the spectrum produced in each collision, and summing the locally produced spectra from those regions of the jet which are simultaneously active in the observer's frame. We can thus construct snapshots of the overall spectral energy distribution as well as time dependent spectra and light curves. This allows us to characterize the predicted variability at any frequency, study correlations among the emission at different frequencies, specify the contribution of each region of the jet to the total emission, find correlations between flares at high energies and the birth of superluminal radio knots and/or radio flares. The model has been applied to qualitatively reproduce the observed properties of 3C 279. Global agreement in terms of both spectra and temporal evolution is found. In a forthcoming work, we explore the constraints which this scenario sets on the initial conditions of the plasma injected in the jet and the shock dissipation for different classes of blazars.

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arXiv2001

The BMW X-ray Cluster Survey

We describe the main features of the BMW survey of serendipitous X-ray clusters, based on the still unexploited ROSAT-HRI archival observations. The sky coverage, surface density and first deep optical CCD images of the candidates indicate that this sample can represent an excellent complement to the existing PSPC deep cluster surveys and will provide us with a fully independent probe of the evolution of the cluster abundance, in addition to significantly increasing the number of clusters known at z>0.6.

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arXiv2001

Early afterglows as probes for the reionization epoch

The nature of Gamma-Ray Burst progenitors is still a debated issue, but consensus is growing on the association of GRBs with massive stars. Furthermore, current models for the reionization of the universe consider massive Pop--III stars as the sources of the ionizing photons. There could then be a natural link between GRBs and reionization. The reionization epoch can be measured through prompt IR spectral observations of high redshift GRBs. For this, GRBs are better than quasars: they produce a smaller HII region even if they are much brighter than quasars (but for a much shorter time) and then, contrary to quasars, they do not modify the absorption properties of the surrounding IGM.

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arXiv2001

Illuminated, and enlightened, by GRB 991216

We consider models for the generation of the emission line recently discovered in the X-ray afterglow spectrum of several bursts, and especially of GRB 991216 observed by Chandra. These observations suggest the presence of 0.1-1 solar masses of iron in the vicinity of the bursts. We show that there are strong geometrical and kinematical constraints on the line emitting material. We discuss several classes of models, favoring one where the line photons are produced by reflection off the walls of a wide funnel of semi-opening angle 45 degrees, excavated in a young (a few months old) supernova remnant made of ~10 solar masses with radius 6x10^15 cm, and ~1 solar mass of iron, providing strong support for the SupraNova model.

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arXiv2000

Internal shocks in the jets of blazars

The development of instabilities leading to the formation of internal shocks is expected in the relativistic outflows of both gamma-ray bursts and blazars. The shocks heat the expanding ejecta, generate a tangled magnetic field and accelerate leptons to relativistic energies. While this scenario has been largely considered for the origin of the spectrum and the fast variability in gamma-ray bursts, here we consider it in the contest of relativistic jets of blazars. We calculate the expected spectra, light curves and time correlations between emission at different wavelengths. The dynamical evolution of the wind explains the minimum distance for dissipation (~10^{17} cm) to avoid $γ$--$γ$ collisions and the low radiative efficiency required to transport most of the kinetic energy to the extended radio structures. The internal shock model allows to follow the evolution of changes, both dynamical and radiative, along the entire jet, from the inner part, where the jet becomes radiative and emits at high energies ($γ$-jet), to the parsec scale, where the emission is mostly in the radio band (radio-jet). We have produced some animations that can be found at http://www.merate.mi.astro.it/~lazzati/3C279/, in which the temporal and spectral informations are shown together.

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arXiv1999

Constraints on the emission mechanisms of gamma-ray bursts

If the emission of gamma-ray bursts were due to the synchrotron process in the standard internal shock scenario, then the typical observed spectrum should have a slope F(nu) \propto nu^{-1/2}, which strongly conflicts with the much harder spectra observed. This directly follows from the cooling time being much shorter than the dynamical time. Particle re-acceleration, deviations from equipartition, fastly changing magnetic fields and adiabatic losses are found to be inadequate to account for this discrepancy. We also find that in the internal shock scenario the relativistic inverse Compton scattering is always as important as the synchrotron process, and faces the same problems. This indicates that the burst emission is not produced by relativistic electrons emitting synchrotron and inverse Compton radiation.

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arXiv1999

The detection of linear polarization in the afterglow of GRB 990510 and its theoretical implications

We present the recent discovery of linear polarization of the optical afterglow of GRB 990510. Effects that could introduce spurious polarization are discussed, showing that they do not apply to the case of GRB 990510, which is then intrinsically polarized. It will be shown that this observation constrains the emission mechanism of the afterglow radiation, the geometry of the fireball and degree of order of the magnetic field. We then present the theoretical interpretations of this observation with particular emphasis on the possibility of observing polarization in beamed fireballs.

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arXiv1999

Compton Dragged Gamma--Ray Bursts associated with Supernovae

It is proposed that the gamma-ray photons that characterize the prompt emission of Gamma-Ray Bursts are produced through the Compton drag process, caused by the interaction of a relativistic fireball with a very dense soft photon bath. If gamma-ray bursts are indeed associated with Supernovae, then the exploding star can provide enough soft photons for radiative drag to be effective. This model accounts for the basic properties of gamma-ray bursts, i.e. the overall energetics, the peak frequency of the spectrum and the fast variability, with an efficiency which can exceed 50%. In this scenario there is no need for particle acceleration in relativistic collisionless shocks. Furthermore, though Poynting flux may be important in accelerating the outflow, no magnetic field is required in the gamma-ray production. The drag also naturally limits the relativistic expansion of the fireball to Gamma < 10^4.

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arXiv1999

Constraints on the bulk Lorentz factor in the internal shock scenario for gamma-ray bursts

We investigate, independently of specific emission models, the constraints on the value of the bulk Lorentz factor Gamma of a fireball. We assume that the burst emission comes from internal shocks in a region transparent to Thomson scattering and before deceleration due to the swept up external matter is effective. We consider the role of Compton drag in decelerating fast moving shells before they interact with slower ones, thus limiting the possible differences in bulk Lorentz factor of shells. Tighter constraints on the possible range of Gamma are derived by requiring that the internal shocks transform more than a few per cent of the bulk energy into radiation. Efficient bursts may require a hierarchical scenario, where a shell undergoes multiple interactions with other shells. We conclude that fireballs with average Lorentz factors larger than 1000 are unlikely to give rise to the observed bursts.

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