Davide Lazzati

The Collimation-corrected Gamma-Ray Burst Energies Correlate with the Peak Energy of Their νFν Spectrum

We consider all bursts with known redshift and νFν peak energy, E. For a good fraction of them an estimate of the jet opening angle is available from the achromatic break of their afterglow light curve. This allows the derivation of the collimation-corrected energy of the bursts, Eγ. The distribution of the values of Eγ is more spread out than in previous findings, covering about 2 orders of magnitude. We find a surprisingly tight correlation between Eγ and the source frame Epeak: E(1 + z) ∝ E. This correlation can shed light on the still uncertain radiation processes for the prompt GRB emission. More importantly, if the small scatter of this newly found correlation could be confirmed by forthcoming data, it would be possible to use it for cosmological purposes.

The Resolved Fraction of the Cosmic X-Ray Background

We present the X-ray source number counts in two energy bands (0.5-2 and 2-10 keV) from a very large source sample: we combine data of six different surveys, both shallow wide-field and deep pencil-beam, performed with three different satellites (ROSAT, Chandra, and XMM-Newton). The sample covers with good statistics the largest possible flux range so far: 2.4 × 10-17 to 10-11 ergs s-1 cm-2 in the soft band and 2.1 × 10-16 to 8 × 10-12 ergs s-1 cm-2 in the hard band. Integrating the flux distributions over this range and taking into account the (small) contribution of the brightest sources, we derive the flux density generated by discrete sources in both bands. After a critical review of the literature values of the total cosmic X-ray background (CXB) we conclude that, with the present data, 94.3 % and 88.8 % of the soft and hard CXB can be ascribed to discrete source emission. If we extrapolate the analytical form of the log N-log S distribution beyond the flux limit of our catalog in the soft band we find that the flux from discrete sources at ~3 × 10-18 ergs s-1 cm-2 is consistent with the entire CXB, whereas in the hard band it accounts for only 93% of the total CXB at most, hinting at a faint and obscured population to arise at even fainter fluxes.

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 also work for radio sources in general, and for blazars 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 observers frame. We can thus construct snapshots of the overall spectral energy distribution, time-dependent spectra and light curves. This allows us to characterize the predicted variability at any frequency, study correlations between the emission at different frequencies, specify the contribution of each region of the jet to the total emission, and find correlations between flares at high energies and the birth of superluminal radio knots and/or radio flares. The model has been applied to reproduce qualitatively the observed properties of 3C 279. Global agreement in terms of both spectra and temporal evolution is found. In a forthcoming work, we will explore the constraints that this scenario sets on the initial conditions of the plasma injected in the jet and the shock dissipation for different classes of blazars.

Afterglow light curves, viewing angle and the jet structure of γ-ray bursts

Gamma-ray bursts are often modelled as jet-like outflows directed towards the observer; the cone angle of the jet is then commonly inferred from the time at which there is a steepening in the power-law decay of the afterglow. We consider an alternative model in which the jet has a beam pattern where the luminosity per unit solid angle (and perhaps also the initial Lorentz factor) decreases smoothly away from the axis, rather than having a well-defined cone angle within which the flow is uniform. We show that the break in the afterglow light curve then occurs at a time that depends on the viewing angle. Instead of implying a range of intrinsically different jets – some very narrow, and others with a similar power spread over a wider cone – the data on afterglow breaks could be consistent with a standardized jet, viewed from different angles. We discuss the implication of this model for the luminosity function.

Temporal and Angular Properties of Gamma-Ray Burst Jets Emerging from Massive Stars

We study the long-term evolution of relativistic jets in collapsars and examine the effects of viewing angle on the subsequent gamma-ray bursts. We carry out a series of high-resolution simulations of a jet propagating through a stellar envelope in 2D cylindrical coordinates using the FLASH relativistic hydrodynamics module. For the first time, simulations are carried out using an adaptive mesh that allows for a large dynamic range inside the star while still being efficient enough to follow the evolution of the jet long after it breaks out from the star. We single out three phases in the jet evolution: a precursor phase in which relativistic material turbulently shed from the head of the jet first emerges from the star, a shocked-jet phase where a fully shocked jet of material is emerging, and an unshocked-jet phase where the jet consists of a free-streaming, unshocked core surrounded by a thin boundary layer of shocked-jet material. The appearance of these phases will be different to observers at different angles. The precursor has a wide opening angle, the shocked phase has a relatively narrow opening angle, and in the unshocked phase the opening angle increases logarithmically with time. As a consequence, some observers see prolonged dead times of emission even for constant properties of the jet injected in the stellar core. We also present an analytic model that is able to reproduce the overall properties of the jet and its evolution. In an appendix, we present 1D and 2D tests of the FLASH relativistic hydrodynamics module.

The Rates of Hypernovae and Gamma-Ray Bursts: Implications for Their Progenitors

A critical comparison of estimates for the rates of hypernovae (HNe) and gamma-ray bursts (GRBs) is presented. Within the substantial uncertainties, the estimates are shown to be quite comparable and give a galactic rate of 10-6 to 10-5 yr-1 for both events. These rates are several orders of magnitude lower than the rate of core-collapse supernovae, suggesting that the evolution leading to an HN/GRB requires special circumstances, very likely due to binary interactions. Various possible binary channels are discussed, and it is shown that these are generally compatible with the inferred rates.

The Giant X-Ray Flare of GRB 050502B: Evidence for Late-Time Internal Engine Activity

Until recently, X-ray flares during the afterglow of gamma-ray bursts (GRBs) were a rarely detected phenomenon; thus, their nature is unclear. During the afterglow of GRB 050502B, the largest X-ray flare ever recorded rose rapidly above the afterglow light curve detected by the Swift X-Ray Telescope. The peak flux of the flare was >500 times that of the underlying afterglow, and it occurred >12 minutes after the nominal prompt burst emission. The fluence of this X-ray flare, (1.0 ± 0.05) × 10-6 ergs cm-2 in the 0.2-10.0 keV energy band, exceeded the fluence of the nominal prompt burst. The spectra during the flare were significantly harder than those measured before and after the flare. Later in time, there were additional flux increases detected above the underlying afterglow, as well as a break in the afterglow light curve. All evidence presented below, including spectral and, particularly, timing information during and around the giant flare, suggests that this giant flare was the result of internal dissipation of energy due to late central engine activity, rather than an afterglow-related effect. We also find that the data are consistent with a second central engine activity episode, in which the ejecta is moving slower than that of the initial episode, causing the giant flare and then proceeding to overtake and refresh the afterglow shock, thus causing additional activity at even later times in the light curve.

The afterglow of GRB 021004: Surfing on density waves

We present a model for the early optical afterglow of GRB 021004. This burst had one of the earliest detected optical afterglows, allowing for a dense optical sampling. The lightcurve was peculiar, since bright bumps were superimposed to the regular power-law decay observed in many other events. We show that, given their time scale and shape, the bumps are likely due to the interaction of the fireball with moderate density enhancements in the ambient medium. The enhancements have a density contrast of order 10, modifying only slightly the dynamics of the fireball, which therefore surfs on them rather than colliding into them. A relativistic reverse shock does not develop. Since the interaction takes place when the fireball is still hyper-relativistic it is not possible to understand if the overdensities are localized in clumps or are spherically symmetric around the GRB progenitor. The monotonic decrease of the contrast of successive rebrightenings suggests however the presence of clumps embedded in a uniform environment. Such an interpretation, complemented by the detection of several high velocity absorption systems in the optical spectrum, strongly suggests that GRB 021004 exploded within the remnant of a previous explosion.

Constraints on the emission mechanisms of gamma-ray bursts

ABSTRACT If the emission of gamma–ray bursts were due to the synchrotron process in the stan-dard internal shock scenario, then the typical observed spectrum should have a slopeF ν ∝ν −1/2 , which strongly conflicts with the much harder spectra observed. Thisdirectly follows from the cooling time being much shorter than the dynamical time.Particle re–acceleration, deviations from equipartition, fastly changing magnetic fieldsand adiabatic losses are found to be inadequate to account for this discrepancy. Wealso find that in the internal shock scenario the relativistic inverse Compton scatteringis always as important as the synchrotron process, and faces the same problems. Thisindicates that the burst emission is not produced by relativistic electrons emittingsynchrotron and inverse Compton radiation.Key words: gamma rays: bursts — X–rays: general — radiation mechanisms: non–thermal 1 INTRODUCTIONSince the observational breakthrough by BeppoSAX (Costaet al. 1997; van Paradijs et al. 1997) the physics of gamma–ray bursts (GRB) has started to be disclosed. The huge en-ergy and power releases required by their cosmological dis-tances support the fireball scenario (Cavallo & Rees 1978;Rees & M´esz´aros 1992; M´esz´aros & Rees 1993), whose evo-lution and behavior is (unfortunately) largely independentof their origin.We do not know yet in any detail how the GRB event isrelated to the afterglow emission, but in the most acceptedpicture of formation of and emission from internal/externalshocks (Rees & M´esz´aros 1992; Rees & M´esz´aros 1994; Sari& Piran 1997), the former is due to collisions of pairs ofrelativistic shells (internal shocks), while the latter is gen-erated by the collisionless shocks produced by shells inter-acting with the interstellar medium (external shocks). Theshort spikes (t

PROBING THE ENVIRONMENT IN GAMMA-RAY BURSTS: THE CASE OF AN X-RAY PRECURSOR, AFTERGLOW LATE ONSET, AND WIND VERSUS CONSTANT DENSITY PROFILE IN GRB 011121 AND GRB 011211

In this paper we present BeppoSAX and XMM-Newton observations of two long gamma-ray bursts (GRBs), the X-ray-rich event of 2001 December 11 (GRB 011211) and the hard and very bright event of 2001 November 21 (GRB 011121). In both events we find evidence of a late X-ray burst taking place several minutes after the prompt emission. In the November burst the spectrum of the X-ray burst is much softer than that of the preceding prompt phase and consistent with the spectrum of the afterglow at 1 day. In addition, the tail of the X-ray burst and the light curve of the afterglow at 1 day are connected by a single power law ?(t - t0), when t0 corresponds with the onset of the X-ray burst. These evidences suggest that the late X-ray burst represents the onset of the afterglow. A similar conclusion is drawn for the December burst. The temporal and spectral behavior of the X-ray and optical afterglows indicate that the fireball evolution in the December burst takes place in an interstellar medium (ISM) environment. In contrast, in the November burst the wind case is revealed by an X-ray decay slower than that observed in the optical (?X = 1.29 ? 0.04 vs. ?O = 1.66 ? 0.06). The wind profile should change into a constant-density profile at large radii in order to reconcile late-time radio data with a jet. Two other results are obtained for this burst. An X-ray burst precedes the much harder GRB by about 30 s. Contrary to the prediction of simple models of precursor activity for collapsars, the precursors spectrum is not consistent with a blackbody. Finally, a substantial absorption column [NH = (7 ? 2) ? 1022?cm-2] is detected during the early part of the prompt emission. This is much greater than that of the wind, and it is thus likely associated with the region surrounding the burst.