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

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

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

Reconsidering the origin of the X-ray emission lines in GRB 011211

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&

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

The deepest X-ray look at the Universe

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

Iron line emission in X-ray afterglows

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. 200