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GRB 020410 is by far the longest gamma-ray burst (with a duration of about 1600 s) to have been followed up from the X-ray through the radio regime. Afterglow emission was detected in X-rays and at optical wavelengths while no emission was detected at 8 GHz brighter than 120 microJy. The decaying X-ray afterglow, back extrapolated to 11 hr after the burst, had a flux of 7.9 10^-12 cgs (2-10 keV); the brightest detected so far. No direct redshift determination is available yet for this GRB, but according to the empirical relationship between the peak energy in the \nu F_\nu spectrum and the isotropic energy output, z is constrained in the range 0.9-1.5. The reconstructed optical afterglow light curve implies at least two breaks in the simple power-law decay. This may be related to emergence of a SN, or refreshment of the external shock by a variation in the circumstellar medium. By comparing the backward extrapolation of the 2-10 keV afterglow decay it is shown that the long duration of the prompt emission is not related to an early onset of afterglow emission, but must be related to prolonged activity of the ``central engine''., Comment: 8 pages, 6 figures; submitted to A&A

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 $10^{-6}$ -- $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 a 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., Comment: Accepted by Astrophysical Journal Letters. 12 pages

We consider internal shocks as the main dissipation mechanism responsible for the emission in blazars and show that it can satisfactorily account for the properties of all blazars. In particular, we extend previous work (Spada et al. 2001) on powerful objects, to intermediate (BL Lac) and low power sources (Mkn 421), in order to reproduce the whole of the blazar sequence. The model self-consistently treats the dynamics, spectral emission and its variability. The key parameters driving the phenomenological sequence are the jet power and the properties of the broad line region, which regulate the cooling efficiency of the emitting particles and thus the shape of the spectral energy distribution. By assuming that the remaining parameters are similar for all objects it has been possible to reproduce the full range of the observed spectral "states". A more detailed comparison of the variability properties shows (for Mkn 421) a good agreement in the X-ray band, while in the optical the simulated flux appears to be too variable. For BL Lac lags (~10 days) are predicted between the gamma--rays and the infrared emission, Comment: 10 pages, 12 figures. Submitted to Astronomy and Astrophysics

The discovery of a supernova emerging at late times in the afterglow of GRB 030329 has apparently settled the issue on the nature of the progenitor of gamma-ray bursts. We now know that at least a fraction of cosmological GRBs are associated with the death of massive stars, and that the two explosions are most likely simultaneous. Even though the association was already suggested for GRB 980425, the peculiarity of that burst did not allow to extend the association to all GRBs. The issue is now to understand whether GRB 030329 is a "standard burst" or not. I will discuss some peculiarities of GRB 030329 and its afterglow lightcurve showing how, rather than a classical cosmological GRB, it looks more like a transition object linking weak events like GRB 980425 to the classical long duration GRBs. I will also discuss the problems faced by the Hypernova scenario to account for the X-ray features detected in several GRBs and their afterglows., Comment: 17 pages, 9 postscript figures. Invited review at the Xth Marcel Grossmann Meeting on General Relativity, Rio de Janeiro, Brazil, July 2003

The polarization curve of GRB 020813 is discussed and compared to different models for the structure, evolution and magnetisation properties of the jet and the interstellar medium onto which the fireball impacts. GRB 020813 is best suited for this kind of analysis for the smoothness of its afterglow light curve, ensuring the applicability of current models. The polarization dataset allows us to rule out the standard GRB jet, in which the energy and Lorentz factor have a well defined value inside the jet opening angle and the magnetic field is generated at the shock front. We explore alternative models finding that a structured jet or a jet with a toroidal component of the magnetic field can fit equally well the polarization curve. Stronger conclusions cannot be drawn due to the incomplete sampling of the polarization curve. A more dense sampling, especially at early times, is required to pin down the structure of the jet and the geometry of its magnetic field., Comment: 7 pages, 5 postscript figures, minor revisions according to the referee comments. A&A in press

We numerically compute light and polarisation curves of gamma-ray burst afterglows for various configurations of the jet luminosity structure and for different dynamical evolutions. We especially consider the standard homogeneous ``top hat'' jet and the ``universal structured jet'' with power-law wings. We also investigate a possible more physical variation of the ``top hat'' model: the ``Gaussian jet''. The polarisation curves for the last two jet types are shown here for the first time together with the computation of X-ray and radio polarised fluxes. We show that the lightcurves of the total flux from these configurations are very similar to each other, and therefore only very high quality data could allow us to pin down the underlying jet structure. We demonstrate instead that polarisation curves are a powerful means to solve the jet structure, since the predicted behaviour of polarisation and its position angle at times around the jet break are very different if not opposite. We conclude that the afterglow polarisation measurements provide clear footprints of any outflow energy distribution (unlike the lightcurves of the total flux) and the joint analysis of the total and polarised flux should reveal GRBs jet structure., Comment: 16 pages, 18 figures, MNRAS, temp, 321. Light curves and polarisation curves for a Gaussian jet added. Cartoon of the three jet structures added

The past year has witnessed a large increase in our knowledge of the polarization properties of Gamma-Ray Burst (GRB) radiation. In the prompt phase, the measurement (albeit highly debated) of a large degree of linear polarization in GRB 021206 has stimulated a deep theoretical study of polarization from GRB jets. The optical afterglow of GRB 030329, on the other hand, has been followed thoroughly in polarimetric mode, allowing for an unprecedented sampling of its polarization curve. I will review the present status of theories and observations of polarization in GRBs, focusing on how polarimetric observations and their modelling can give us informations on the structure and magnetisation o GRB jets which is not possible to obtain from their light curve., Comment: 6 pages, 3 postscript figures, invited review for the proceedings of 2003 Gamma-Ray Burst Symposium (Santa Fe, NM, September 8-12, 2003), to be published by AIP

We present spectroscopic and photometric observations of the afterglow of GRB 021211 and the discovery of its associated supernova, SN 2002lt. The spectrum shows a broad feature (FWHM = 150 A), around 3770 A (in the rest-frame of the GRB), which we interpret as Ca H+K blueshifted by 14400 km/s. Potential sources of contamination due to the host galaxy and/or residuals of telluric absorption have been analyzed and ruled out. Overall, the spectrum shows a suggestive resemblance with the one of the prototypical type-Ic SN 1994I. This might indicate that GRBs are produced also by standard type-Ic supernovae., Comment: 6 pages, 5 color figures. Proceedings of the 2003 GRB Conference (Santa Fe, NM, 2003 Sep 8-12). Needs aipprocs LaTeX class

We present the results of a VLT polarimetric monitoring campaign of the GRB 020813 optical afterglow carried out in three nights, from 0.88 to 4.20 days after the gamma-ray event. The mean values of the degree of linear polarization (P) and its position angle (Theta) resulting from our campaign are =1.18+/-0.10% and Theta = 148.7+/-2.3 deg after correcting for Galactic interstellar polarization. Our VLT data set is most naturally described by a constant degree of linear polarization and position angle, nonetheless a slow Theta evolution cannot be entirely ruled out by our data. The VLT monitoring campaign did not reveal either a significant Theta rotation respect to the Keck spectropolarimetric observations performed ~0.25 days after the GRB (Barth et al. 2003). However, is a factor of two lower than the polarization degree measured from Keck. Additionally, the VLT polarization data allowed us to construct an accurate V-band light curve. The V-band photometric data revealed a smooth light curve with a break located between the last Keck and our first VLT polarimetric measurement, 0.33 < t_(break, V) < 0.88 days after the GRB. The typical magnitude fluctuations of the VLT V-band lightcurve are 0.003 mag, 0.010 mag and 0.016 mag for our three observing nights, respectively. We speculate that the stability of Theta might be related to the smoothness of the light curve., Comment: 7 pages, 2 figures, accepted for publication in A&A. Minor changes included with respect to the previously posted version

The recent claim by Coburn & Boggs to have detected a very high degree of linear polarization in the prompt emission of GRB 021206 has stimulated interest in how much polarization could arise in gamma-ray bursts from synchrotron emission. Alternatively, as Shaviv & Dar have shown, GRB polarization could be produced by inverse Compton scattering in the point-source limit. We discuss polarization from a fireball that upscatters a soft radiation field. We show that, after the proper angular integration, the residual polarization can be large, in some cases approaching the point-source limit. We discuss the probability of realizing the geometrical conditions in which a large polarization is obtained showing that, for a particularly bright burst as GRB 021206, the detection of polarization at the first attempt in the Compton drag scenario is not unlikely., Comment: Accepted for publication in MNRAS

Polarization measurements for the optical counterpart to GRB 021004 are presented and discussed. Our observations were performed with the TNG and the VLT-UT3 (Melipal) during the first and fourth night after the gamma-ray burst discovery. We find robust evidence of temporal evolution of the polarization, which is therefore, at least partially, intrinsic to the optical transient. We do not find convincing evidence of wavelength dependence for the intrinsic polarization of the transient, in agreement with current polarization models for optical afterglows. We discuss the role of dust, both in our galaxy and in the host, in modifying the transmitted polarization vector, showing how a sizable fraction of the observed polarized flux is due to Galactic selective extinction, while it is not possible to single out any clear contribution from dust in the host galaxy. We discuss how our data compare to those obtained by different groups showing that a two-component model is required to describe the complete dataset. This is not surprising given the complex lightcurve of GRB 021004., Comment: 9 pages, 6 postscript figures, A&A in press

We present photometric and spectroscopic observations of the gamma-ray burst GRB 021211 obtained during the late stages of its afterglow. The light curve shows a rebrightening occurring ~25 days after the GRB. The analysis of a VLT spectrum obtained during the bump (27 days after the GRB) reveals a suggestive resemblance with the spectrum of the prototypical type-Ic SN 1994I, obtained about ~10 days past maximum light. Particularly we have measured a strong, broad absorption feature at 3770 A, which we have identified with Ca II blueshifted by ~14400 km/s, thus indicating that a supernova (SN) component is indeed powering the `bump' in the afterglow decay. Assuming SN 1994I as a template, the spectroscopic and photometric data together indicate that the SN and GRB explosions were at most separated by a few days. Our results suggest that GRBs might be associated also to standard type-Ic supernovae., Comment: 6 pages, 4 color figures. Accepted for publication in A&A Letters. Fig. 4 does not appair in the A&A version due to space restrictions. Includes aa.cls and txfonts.sty

We reanalyze the XMM--Newton data of GRB 011211 showing that the spectral features, interpreted by Reeves et al. (2002, 2003) as due thermal emission from a collisionally ionized plasma, can be also reproduced by a reflection model (with ionization parameter $\xi\sim 10^2$). We discuss the implications of this interpretation, estimating the total mass required in the simplified case of a funnel geometry. We conclude that a moderate clumping of the reprocessing material (corresponding to a filling factor of the order of $f\sim 10^{-3}$) is required. Finally we show that, if this interpretation is correct, a bright quasi--thermal component is expected in the optical--UV band (containing about 90% of the luminosity of the illuminating continuum), whose presence can be used to test the reflection model., Comment: revised version accepted for publication by A&A

We present optical and near-infrared (NIR) photometry of the bright afterglow of GRB 020813. Our data span from 3 hours to 4 days after the GRB event. A rather sharp achromatic break is present in the light curve, 14 hours after the trigger. In the framework of jetted fireballs, this break corresponds to a jet half opening angle of (1.9 +- 0.2) deg, the smallest value ever inferred for a GRB. We discuss our results in the framework of currently available models, and find that they have problems in explaining the joint temporal and spectral properties, and in particular the slow decay before the break., Comment: 5 pages, 4 (color) figures; minor changes introduced; published in A&A Letters; includes aa.cls and txfonts.sty

Gamma-Ray Burst afterglow polarization measurements, in spite of their intrinsic difficulties, have been carried out for a number of events that begins to be adequate to draw some general statistical conclusions. Although the presence of some degree of intrinsic polarization seems to be well established, there are still open problems regarding the polarization time evolution and the possible contribution of polarization induced by dust in the host galaxies., Comment: 6 pages, 4 figures. Solicited talk at the meeting: "Gamma Ray Bursts in the afterglow era - Third workshop.", Rome, September 2002

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 - 10^-11] cgs in the soft band and [2.1*10^-16 - 8*10^{-12}]cgs 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, the 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 cgs is consistent with the entire CXB, whereas in the hard band it accounts for only 93% of the total CXB at most, hinting for a faint and obscured population to arise at even fainter fluxes., Comment: Accepted for publication in ApJ

The properties of the ambient medium in which GRBs go off are an important piece of the puzzle, not only as an issue in itself, but because of their link with the nature of the progenitor. In this review, I describe and critically comment the various experiments proposed to study the burst environment. I discuss emission and absorption features techniques and compare their results to afterglow modelling. A consistent picture cannot be drawn, and I suggest that the evaporation of the soft X-ray absorbing column is the more promising of the tools, even though the fast reaction of Swift is required in order to obtain data of sufficiently high quality., Comment: 8 pages, 3 figures. Invited talk at the meeting: "Gamma Ray Bursts in the afterglow era - Third workshop. September 2002, Rome

We use the time-dependent photoionization and dust destruction code developed by Perna & Lazzati (2002) to study the time evolution of the medium in a dusty gaseous cloud illuminated by a bright central source that sets on at time zero. We study the case of a bright source, which lasts for a time scale much smaller than the recombination and dust creation time scales. For this reason an equilibrium is never reached. We show that the presence of dust and its properties, such as its composition, can have a big effect on the time scale for the evaporation of the soft X-ray absorption, in particular for ionizing sources with hard spectra. We discuss the profile of evaporation of the soft X-ray absorbing column, as well as how the apparent ionization state of the cloud evolves in time. We finally consider the apparent metallicity of the cloud that is left behind as a function of the cloud and ionizing source properties., Comment: Added missing information: 12 pages, 19 postscript figures, MNRAS in press

Polarization measurements for the optical counterpart to GRB 020405 are presented and discussed. Our observations were performed with the VLT-UT3 (Melipal) during the second and third night after the gamma-ray burst discovery. The polarization degree (and the position angle) appears to be constant between our two observations at a level around (1.5 - 2)%. The polarization can be intrinsic but it is not possible to unambiguously exclude that a substantial fraction of it is induced by dust in the host galaxy., Comment: 4 pages, 3 color figures, accepted for publication in A&A Letters. Contains aa.cls

The interaction of a radiation field with a dusty medium is a relevant issue in several astrophysical contexts. We use the time-dependent photoionization code in a dusty medium developed by Perna & Lazzati (2002), to study the modifications in the dust distribution and the relative optical opacities when a strong X-ray UV radiation flux propagates into a medium. We find that silicates are preferentially destroyed with respect to graphite, and the extinction curve becomes significantly flatter (hence implying less reddening), with the characteristic bump at lambda 2175 A highly suppressed, due to the destruction of the small graphite grains. This could explain the observational lack of such a feature in GRB afterglow and AGN spectra. For a very intense and highly variable source irradiating a compact and dense region, time variability in the optical opacity resulting from dust destruction can be observed on a relatively short timescale. We show that, under these circumstances, monitoring the time variability of the opacity can yield powerful clues on the properties of dust in the environment of the source. In particular, it allows to break the observational degeneracy as to whether a grey extinction is the result of a low dust-to-gas ratio or of a dust grain distribution that is skewed towards large grains., Comment: ApJ in press; corrected typo in Fig.2

We present the Brera Multi-scale Wavelet ROSAT HRI source catalogue (BMW-HRI) derived from all ROSAT HRI pointed observations with exposure time longer than 100 s available in the ROSAT public archives. The data were analyzed automatically using a wavelet detection algorithm suited to the detection and characterization of both point-like and extended sources. This algorithm is able to detect and disentangle sources in very crowded fields and/or in presence of extended or bright sources. Images have been also visually inspected after the analysis to ensure verification. The final catalogue, derived from 4,303 observations, consists of 29,089 sources detected with a detection probability of greater or equal 4.2 sigma. For each source, the primary catalogue entries provide name, position, count rate, flux and extension along with the relative errors. In addition, results of cross-correlations with existing catalogues at different wavelengths (FIRST, IRAS, 2MASS and GSC2) are also reported. All these information are available on the web via the DIANA Interface. As an external check, we compared our catalogue with the previously available ROSHRICAT catalogue (both in its short and long versions) and we were able to recover, for the short version, \~90% of the entries. We computed the sky coverage of the entire HRI data set by means of simulations. The complete BMW-HRI catalogue provides a sky coverage of 732 square degrees down to a limiting flux of ~1E-12 erg/s/cm2 and of 10 square degrees down to ~1E-14 erg/s/cm2. We were able to compute the cosmological log(N)-log(S) distribution down to a flux of about 1.2x1E-14 erg/s/cm2., Comment: 15 pages, 20 figures, accepted for publication in A&A

X-ray emission features have been detected in at least five GRB afterglows. Their detection and properties are of great importance in our understanding of the nature of the GRB progenitor. Moreover, they provide us with an un-collimated source of photons, allowing us to put firm constraints on the total energy produced in GRBs, and therefore on their degree of collimation. I will review the radiation mechanisms and geometries proposed as sources of the line emission, and discuss the energy budget of the lines and their implication for the overall energetics of the phenomenon., Comment: 12 pages, 2 postscript figures, Review talk at the NBSI workshop "Beaming and Jets in Gamma Ray Bursts", Copenhagen, August 12-30, 2002

It has been recently proposed that current burst and afterglow observations can be accounted for by a top hat jet structure as well as by a more universal structured outflow, in which the energy carried by the flow is maximum at the jet spine, decreasing outwards as a power-law with the off-axis angle. To pin down the GRBs jet structure is important in order to quantify the total energy released by the unknown engine and put constrains on its nature. In paper we will review the most popular GRBs jet structures, focusing on the predicted observed properties that could discriminate between them. In particular the behaviour of linearly polarised emission during the afterglow phase seems very promising, since the degree and temporal dependence are predicted to be very different from model to model. In addition, polarization measurements and detections are accumulating fast enouqgh to lead soon to a comprehension of magnetic field and GRBs jet configurations., Comment: 6 pages, 3 figures; proceeding of the workshop "Beaming and Jets in Gamma Ray Bursts", Copenhagen, August 12-30, 2002

We present a model for the early optical afterglow of GRB 021004. This burst had one of the earliest detected optical afterglows, allowing 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 due to the interaction of the fireball with moderate enhancements in the ambient medium density. 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 onto 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 in an average 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., Comment: 5 pages, 2 postscript figures, A&A reference added

The possible presence of thermal components in the early X-ray afterglows of gamma-ray bursts is investigated. We discuss both the presence of a thermal continuum and, in particular, of collisional X-ray emission lines. We compute the predicted luminosity by a thin plasma for a range of metallicities for the continuum and the K_alpha lines of the elements Mg, Si, S, Ar, Ca and Fe. We show that light travel effects are dominant in the determination of the thermal continuum and line luminosities, and derive the relevant equations. We conclude that thermal lines and continua are unlikely to dominate the early afterglow of GRBs, unless the explosion site is surrounded by a very massive and extremely clumped shell of material. Such conditions are difficult to envisage in the close environment of GRB progenitor, unless they are excited by some strong precursor activity, like in the Supranova scenario., Comment: Minor changes, A&A in press. Black and White printer friendly version available at http://www.ast.cam.ac.uk/~lazzati/thermal_bw.ps.gz

We consider the internal shock model as the dissipation mechanism responsible for the emission in blazars. It has been shown that this model is successful in reproducing the observed spectral energy distribution and the variability properties of a powerful blazar like 3C 279. However, the blazar family covers a wide range of spectral characteristics which appear to be correlated and the whole class can be seen as a sequence: the frequency and the intensity of the low energy versus high energy peak intensity increase with decreasing luminosity. We show that the internal shock model can satisfactorily account also for the properties of the low power blazars like BL Lac and Mkn 421 and it is successful in reproducing the blazar sequence., Comment: 7 pages, 1 figure. To be published in the Proceedings of the 5th AGN Italian Workshop, Como, 11-14 June 2002

We present and discuss polarimetric observations performed with the VLT-UT3 (Melipal) on the afterglow of GRB 011211, ~35 hours after the burst onset. The observations yielded a 3-sigma upper limit of P<2.7%. We discuss this result in combination with the lightcurve evolution, that may show a break approximately at the time of our observation. We show that our upper limit is consistent with the currently favored beamed fireball geometry, especially if the line of sight was not too close to the edge of the cone., Comment: 5 pages, 4 figures, 1 table; uses aa.cls (now included). Accepted for publication in A&A

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., Comment: 37 pages, 12 figures, final version accepted to ApJ

We analyze the power spectra of the lightcurves of long gamma-ray bursts, dividing the sample in bins of luminosity, using the recently discovered variability-luminosity correlation. We find that the value of the variability parameter strongly correlates with the frequency that contains most of the power in the burst comoving frame. We compute the average power spectra in luminosity bins. The average power spectrum is well described by a broken power-low and the break frequency is a function of the variability parameter, while the two slopes are roughly constant. This allow us to conclude that scattering processes do not play a relevant role in modelling the lightcurves. We finally discuss in which conditions scattering may still play a relevant role in shaping the spectra of GRBs., Comment: Minor changes according to referee comments. Accepted for publication in MNRAS

The recent report of iron line features in the afterglow of the gamma-ray burst (GRB) 991216 has important implications for the properties of the radiating material and hence the nature of the immediate burst environment. We argue that the putative strong Fe emission line can be attributed to the reflected emission from Thomson-thick matter which is illuminated by a power-law continuum. The ionization parameter of the material (i.e., the flux to density ratio) is around 10^3, resulting in a Fe K line from He-like iron. A supersolar abundance of iron is not required by the data. Interestingly, the ionizing continuum must be harder than the observed one. We interpret this as due to the fact that the observed continuum is dominated by the standard blast wave emission, while the line is produced by reprocessing material located at much smaller radii., Comment: 4 pages, 2 figures, accepted by A&A Letters

The emission features observed in the X-ray afterglow of Gamma Ray Bursts are extremely powerful. Since they last at least for several hours, they imply energies of the order of 1e49 ergs. This in turn implies that the energy contained in the illuminating continuum thought to be responsible of the line production must exceed 1e51 ergs. This is a strong lower limit to the energy reservoir of Gamma Ray Bursts, which is independent of collimation and beaming, and bears important consequences on the possible collimation of the fireball radiation and the density of the medium surrounding the burst., Comment: Accepted for publication in A&A (Letters)

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., Comment: 5 pages, 2 postscript figures, ApJL in press, minor changes due to space limitations

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., Comment: 3 pages, 3 postscript figures, to appear in "Gamma-Ray Burst and Afterglow Astronomy 2001", Woods Hole; 5-9 Nov, 2001

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., Comment: 4 pages, 6 postscript figures, to appear in "Gamma-Ray Burst and Afterglow Astronomy 2001", Woods Hole; 5-9 Nov, 2001

Chandra deep fields represent the deepest look at the X-ray sky. We analyzed the Chandra Deep Field South (CDFS) with the aid of a dedicated wavelet-based algorithm. Here we present a detailed description of the procedures used to analyze this field, tested and verified by means of extensive simulations. We show that we can safely reconstruct the LogN-Log S source distribution of the CDFS down to limiting fluxes of 2.4x10^-17 and 2.1x10^-16 erg s^-1 cm^-2 in the soft (0.5-2 keV) and hard (2-10 keV) bands, respectively, fainter by a factor ~ 2 than current estimates. At these levels we can account for ~ 90% of the 1-2 keV and 2-10 keV X-ray background., Comment: 27 pages 16 figures, accepted for publication in the Astrophysical Journal

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., Comment: 10 pages, 1 figure, accepted for publication in ApJ Letters

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 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., Comment: Corrected typo in Eq. 19

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., Comment: 12 pages, 5 figures, with final revisions, MNRAS in press

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., Comment: 8 pages, 8 postscript figures, accepted for publication in MNRAS

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., Comment: 6 pages, 4 postscript figures, accepted for publication in A&AL. This version of the manuscript contains one figure that will not appear in the journal due to space limitations

We present photometric and spectroscopic observations of the late afterglow of GRB 000911, starting ~1 day after the burst event and lasting ~8 weeks. We detect a moderately significant re-brightening in the R, I and J lightcurves, associated with a sizable reddening of the spectrum. This can be explained through the presence of an underlying supernova, outshining the afterglow ~30 days after the burst event. Alternative explanations are discussed., Comment: 7 pages, 3 postscript figures, A&A in press

The origin of the X-ray background, in particular at hard (2-10 keV) energies, has been a debated issue for more than 30 years. The Chandra deep fields provide the deepest look at the X-ray sky and are the best dataset to study the X-ray background. We searched the Chandra Deep Field South for X-ray sources with the aid of a dedicated wavelet-based algorithm. We are able to reconstruct the Log N-Log S source distribution in the soft (0.5-2 keV) and hard (2-10 keV) bands down to limiting fluxes of 2x10^{-17} erg s^{-1} cm^{-2} and 2x10^{-16} erg s^{-1} cm^{-2}, respectively. These are a factor ~5 deeper than previous investigations. We find that the soft relation continues along the extrapolation from higher fluxes, almost completely accounting for the soft X-ray background. On the contrary, the hard distribution shows a flattening below ~2x10^{-14} erg s^{-1} cm^{-2}. Nevertheless, we can account for >68% of the hard X-ray background, with the main uncertainty being the sky flux itself., Comment: Accepted for publication on ApJL. Two figures, requires emulateapj5 (included)

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., Comment: 5 pages, 4 postscript figures, MNRAS in press

Recent observations of X-ray afterglows reveal the presence of a redshifted Kalpha iron line in emission in four bursts. In GRB 991216, the line was detected by the low energy grating of Chandra, which showed the line to be broad, with a full width of ~15,000 km/s. These observations indicate the presence of a >1 solar mass of iron rich material in the close vicinity of the burst, most likely a supernova remnant. The fact that such strong lines are observed less than a day after the trigger strongly limits the size of the remnant, which must be very compact. If the remnant had the observed velocity since the supernova explosion, its age would be less than a month. In this case nickel and cobalt have not yet decayed into iron. We show how to solve this paradox., Comment: 3 pages, to appear in the proceedings of the the 2nd Workshop on Gamma-Ray Bursts in the Afterglow Era, Rome, Oct. 2000

The absorption feature detected in the prompt X-ray emission of GRB 990705 bears important consequences. We investigate different production mechanisms and we conclude that the absorbing material cannot be very close to the burster and is likely to be moderately clumped. These properties challenge any model in which the burst explodes in coincidence with the core-collapse of a massive rotating star. We show that the straightforward interpretation of the absorption feature as a photoionization K edge of neutral iron faces a severe problem in that it requires a huge amount of iron in the close vicinity of the burster. We then discuss an alternative scenario, in which iron ions are kept in a high ionization state by the burst flux, and the absorption feature is produced by resonant scattering from hydrogen-like iron, broadened by a range outflow velocities. In this case the physical conditions and geometry of the absorbing material are fully consistent with the presence of a young supernova remnant surrounding the burst site at a radius R ~ 10^{16} cm. We finally discuss how this remnant might affect the generation of afterglows with a standard power-law flux decay., Comment: 11 pages, 4 postscript figures, accepted for publication in ApJ

While all but one Gamma-Ray Bursts observed in the X-ray band showed an X-ray afterglow, about 60 per cent of them have not been detected in the optical band. We show that this is not due to adverse observing conditions. We then investigate the hypothesis that the failure of detecting the optical afterglow is due to absorption at the source location. We find that this is a marginally viable interpretation, but only if the X-ray burst and afterglow emission and the possible optical/UV flash do not destroy the dust responsible for absorption in the optical band. If dust is efficiently destroyed, we are led to conclude that bursts with no detected optical afterglow are intrinsically different., Comment: 3 pages, 1 fig., to appear in the proceedings of the the 2nd Workshop on Gamma-Ray Bursts in the Afterglow Era, Rome, Oct. 2000

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., Comment: 12 pages, 10 postscript figures, submitted to MNRAS

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., Comment: 6 pages,4 figures.To appear in the proceedings of the "X-Ray Astronomy 2000" conference (Sept. 2000, Palermo, Italy), PASP Conference Series, ed. R. Giacconi, L. Stella and S. Serio