Polarization lightcurves and position angle variation of beamed gamma-ray bursts

The recently detected linear polarization in the optical lightcurve of GRB 990510 renewed the interest on how polarization can be produced in gamma-ray burst fireballs. Here we present a model based on the assumption that we are seeing a collimated fireball, observed slightly off-axis. This introduces some degree of anisotropy, and makes it possible to observe a linearly polarized flux even if the magnetic field is completely tangled in the plane orthogonal to the line of sight. We construct the lightcurve of the polarization flux, showing that it is always characterized by two maxima, with the polarization position angle changing by 90 deg. between the first and the second maximum. The very same geometry here assumed implies that the total flux initially decays in time as a power law, but gradually steepens as the bulk Lorentz factor of the fireball decreases.Comment: 5 pages, 4 postscript figures, submitted to MNRAS letter

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.Comment: 5 pages, 3 figures, accepted for publication in MNRAS, pink page

Thermal components in the early X-ray afterglow of GRBs

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

The Brera Multi-scale Wavelet (BMW) ROSAT HRI source catalog. I: the algorithm

We present a new detection algorithm based on the wavelet transform for the analysis of high energy astronomical images. The wavelet transform, due to its multi-scale structure, is suited for the optimal detection of point-like as well as extended sources, regardless of any loss of resolution with the off-axis angle. Sources are detected as significant enhancements in the wavelet space, after the subtraction of the non-flat components of the background. Detection thresholds are computed through Monte Carlo simulations in order to establish the expected number of spurious sources per field. The source characterization is performed through a multi-source fitting in the wavelet space. The procedure is designed to correctly deal with very crowded fields, allowing for the simultaneous characterization of nearby sources. To obtain a fast and reliable estimate of the source parameters and related errors, we apply a novel decimation technique which, taking into account the correlation properties of the wavelet transform, extracts a subset of almost independent coefficients. We test the performance of this algorithm on synthetic fields, analyzing with particular care the characterization of sources in poor background situations, where the assumption of Gaussian statistics does not hold. For these cases, where standard wavelet algorithms generally provide underestimated errors, we infer errors through a procedure which relies on robust basic statistics. Our algorithm is well suited for the analysis of images taken with the new generation of X-ray instruments equipped with CCD technology which will produce images with very low background and/or high source density.Comment: 8 pages, 6 figures, ApJ in pres

The updated E_peak - E_gamma correlation in GRBs

The recently discovered correlation between the rest frame GRB peak spectral energy $E_{\rm peak}$ and the collimation corrected energy $E_\gamma$ in long GRBs is potentially very important, yet awaits confirmation from an independent sample. It may help to shed light on the radiation mechanism of the prompt GRB phase and on the way -- and in which form -- the energy is released from the central engine. We here present some additional evidence for the correlation (two new bursts) and re-derive the best-fit parameters. The tightness of the correlation is confirmed (sigma=0.1 dex). We show that this correlation allows us, for the first time, to use GRBs as cosmological probes to constrain the expansion history of the universe.Comment: 4 pages, 1 figure, submitted to Il Nuovo Cimento (4th Workshop Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 2004). Additional material at http://www.merate.mi.astro.it/~ghirla/deep/blink.ht

X-ray flares from propagation instabilities in long Gamma-Ray Burst jets

We present a numerical simulation of a gamma-ray burst jet from a long-lasting engine in the core of a 16 solar mass Wolf-Rayet star. The engine is kept active for 6000 s with a luminosity that decays in time as a power-law with index -5/3. Even though there is no short time-scale variability in the injected engine luminosity, we find that the jet's kinetic luminosity outside the progenitor star is characterized by fluctuations with relatively short time scale. We analyze the temporal characteristics of those fluctuations and we find that they are consistent with the properties of observed flares in X-ray afterglows. The peak to continuum flux ratio of the flares in the simulation is consistent with some, but not all, the observed flares. We propose that propagation instabilities, rather than variability in the engine luminosity, are responsible for the X-ray flares with moderate contrast. Strong flares such as the one detected in GRB 050502B, instead, cannot be reproduced by this model and require strong variability in the engine activity.Comment: 6 pages, MNRAS in pres

Gamma Ray Bursts: new rulers to measure the Universe

The best measure of the Universe should be done using a standard "ruler" at any redshift. Type Ia Supernovae (SN Ia) probe the universe up to z$\sim$1.5, while the Cosmic Microwave Background (CMB) primary anisotropies concern basically $z\sim$1000. Apparently, Gamma--Ray Bursts (GRBs) are all but standard candles. However, their emission is collimated and the collimation--corrected energy correlates tightly with the frequency at which most of the radiation of the prompt is emitted, as found by Ghirlanda et al. (2004). Through this correlation we can infer the burst energy accurately enough to probe the intermediate redshift ($z<10$) Universe. Using the best known 15 GRBs we find very encouraging results that emphasize the cosmological GRB role. A combined fit with SN Ia yields $\Omega_{\rm M}=0.37\pm0.10$ and $\Omega_{\Lambda}=0.87\pm 0.23$. Assuming in addition a flat Universe, the parameters are constrained to be $\Omega_{\rm M}=0.29\pm0.04$ and $\Omega_{\Lambda}=0.71\pm 0.05$. GRBs accomplish the role of "missing link" between SN Ia and CMB primary anisotropies. They can provide a new insight on the cosmic effects of dark energy, complementary to the one supplied by CMB secondary anisotropies through the Integrated Sachs Wolfe effect. The unexpected Standard Candle cosmological role of GRBs motivates the most optimistic hopes for what can be obtained when the GRB-dedicated satellite, Swift, will be launched.Comment: 11 pages, 4 color figures, ApJ Letters (vol. 613) in pres

Compton dragged gamma-ray bursts: the spectrum

We calculate the spectrum resulting from the interaction of a fireball with ambient soft photons. These photons are assumed to be produced by the walls of a funnel in a massive star. By parameterizing the radial dependence of the funnel temperature we calculate the deceleration of the fireball self-consistently, taking into account the absorption of high energy gamma-rays due to interaction with the softer ambient photons. The resulting spectrum is peaked at energies in agreement with observations, has a nu^2 slope in the X-ray band and a steep power-law high energy tail.Comment: 5 pages, 3 figures, accepted for publication in MNRAS, pink page

Short Gamma-Ray Bursts and Binary Mergers in Spiral and Elliptical Galaxies: Redshift Distribution and Hosts

To test whether the short GRB rates, redshift distribution and host galaxies are consistent with current theoretical predictions, we use avery large database of population synthesis calculations to examine BH-NS and NS-NS merger rates in the universe, factoring in (i) the star formation history of the universe, (ii) a heterogeneous population of star-forming galaxies, including spirals and ellipticals, and (iii) a simple flux-limited selection model for short GRB detection. When we require our models reproduce the known short GRB rates and redshift measurements (and, for NS-NS, the merger rates extrapolated from binary pulsars in the Galaxy), a small fraction of models reproduce all observations, both when we assume a NS-NS and a BH-NS origin for bursts. Most commonly models produce mergers preferentially in spiral galaxies if short GRBs arise from NS-NS mergers alone. Model universes where present-day binary mergers occur preferentially in elliptical galaxies necessarily include a significant fraction of binaries with long delay times between birth and merger (often $O(10{\rm Gyr})$). Though long delays occur, almost all of our models predict that a higher proportion of short GRBs should occur at moderate to high redshift (e.g., $z>1$) than has presently been observed, in agreement with recent observations which suggest a selection bias towards successful follow-up of low-redshift short GRBs. Finally, if only a fraction of BH-NS mergers have the right combination of masses and spins to make GRBs, then at best only a small fraction of BH-NS models could be consistent with all {\em current} available data. (Abridged)Comment: 14 figures, using bitmapped fonts (via eps2eps) to fit in archive space restrictions; better resolution figures are available from the author. Accepted for publication in ApJ. v3 updates reference