GRB 060218 and the outliers with respect to the Ep-Eiso correlation

GRB 031203 and GRB 980425 are the two outliers with respect to the Ep-Eiso correlation of long GRBs. Recently Swift discovered a nearby extremely long GRB 060218 associated with a SN event. The spectral properties of this bursts are striking: on the one hand its broad band SED presents both thermal and non-thermal components which can be interpreted as due to the emission from the hot cocoon surrounding the GRB jet and as standard synchrotron self absorbed emission in the GRB prompt phase, respectively; on the other hand it is its long duration and its hard--to--soft spectral evolution which make this underluminous burst consistent with the Ep-Eiso correlation of long GRBs. By comparing the available spectral informations on the two major outliers we suggests that they might be twins of 060218 and, therefore, only apparent outliers with respect to the Ep_Eiso correlation. This interpretation also suggests that it is of primary importance the study the broad band spectra of GRBs in order to monitor their spectral evolution throughout their complete duration.Comment: To appear in the conference proceeding of the IV workshop on "Science with the new generation of high energy Gamma-Ray Experiment", 20-22 June 2006, Isola d'Elb

Did we observe the supernova shock breakout in GRB 060218?

If the early optical data of GRB 060218 up to 1e5 s are interpreted as the black-body flux associated with the supernova shock breakout, we can derive lower limits to the bolometric luminosity and energetics of this black-body component. These limits are more severe for the very early data that imply energetics of order of 1e51 erg. These values, puzzlingly large, are rather independent of the assumed time profile of the emitting surface, provided that the corresponding radius does not increase superluminally. Another concern is the luminosity of the black-body component observed in the X-rays, that is large and appears to be produced by an approximately constant temperature and a surface area increasing only slowly in time. Although it has been suggested that the long X-ray black-body duration is consistent with the supernova shock breakout if anisotropy is assumed, the nearly constant emitting surface requires some fine tuning, allowing and suggesting an alternative interpretation, i.e. emission from late dissipation of the fireball bulk kinetic energy. This in turn requires a small value of the bulk Lorentz factor.Comment: 5 pages, 2 figures, revised version, MNRAS Letters, in pres

Cosmological constraints with GRBs: homogeneous medium vs wind density profile

We present the constraints on the cosmological parameters obtained with the $E_{\rm peak}$--$E_{\gamma}$ correlation found with the most recent sample of 19 GRBs with spectroscopically measured redshift and well determined prompt emission spectral and afterglow parameters. We compare our results obtained in the two possible uniform jet scenarios, i.e. assuming a homogeneous density profile (HM) or a wind density profile (WM) for the circumburst medium. Better constraints on $\Omega_{M}$ and $\Omega_{\Lambda}$ are obtained with the (tighter) $E_{\rm peak}$--$E_{\gamma}$ correlation derived in the wind density scenario. We explore the improvements to the constraints of the cosmological parameters that could be reached with a large sample, $\sim$ 150 GRBs, in the future. We study the possibility to calibrate the slope of these correlations. Our optimization analysis suggests that $\sim 12$ GRBs with redshift $z\in(0.9,1.1)$ can be used to calibrate the $E_{\rm peak}$--$E_{\gamma}$ with a precision better than 1%. The same precision is expected for the same number of bursts with $z\in(0.45,0.75)$. This result suggests that we do not necessarily need a large sample of low z GRBs for calibrating the slope of these correlations.Comment: 7 pages, 7 figures, submitted to A&

Constraining the location of the emitting region in Fermi blazars through rapid gamma-ray variability

We consider the 1.5 years Fermi Large Area Telescope light curves (E > 100 MeV) of the flat spectrum radio quasars 3C 454.3 and PKS 1510-089, which show high activity in this period of time. We characterise the duty cycle of the source by comparing the time spent by the sources at different flux levels. We consider in detail the light curves covering periods of extreme flux. The large number of high-energy photons collected by LAT in these events allows us to find evidence of variability on timescales of few hours. We discuss the implications of significant variability on such short timescales, that challenge the scenario recently advanced in which the bulk of the gamma-ray luminosity is produced in regions of the jet at large distances (tens of parsec) from the black hole.Comment: 5 pages, 5 figures, accpted for publication in MNRAS Letters

Evidence for anisotropy in the distribution of short-lived gamma-ray bursts

Measurements of the two-point angular correlation function w(\theta) for 407 short gamma-ray bursts collected in the Current BATSE Catalogue reveal a ~2 \sigma deviation from isotropy on angular scales \theta ~ 2-4 degrees. Such an anisotropy is not observed in the distribution of long gamma-ray bursts and hints to the presence of repeated bursts for up to ~13% of the sources under exam. However, the available data cannot exclude the signal as due to the presence of large-scale structure. Under this assumption, the amplitude of the observed w(\theta) is compatible with those derived for different populations of galaxies up to redshifts ~0.5, result that suggests short gamma-ray bursts to be relatively local sources.Comment: 5 pages, 4 figures, submitted to MNRA

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