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 as cosmological tools

The use of Gamma Ray Bursts as ``standard candles'' has been made possible by the recent discovery of a very tight correlation between their rest frame intrinsic properties. This correlation relates the GRB prompt emission peak spectral energy E_peak to the energy E_gamma corrected for the collimation angle theta_jet of these sources. The possibility to use GRBs to constrain the cosmological parameters and to study the nature of Dark Energy are very promising.Comment: 6 pages, 3 figures, Proceedings of the workshop 'Astrophysical sources of high energy particles and radiation', Torun - Poland 20-24 June 2005, Ed. T. Bulik, B. Rudak, G. Madejsk

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

Black-body components in Gamma-Ray Bursts spectra?

We study 7 Gamma Ray Bursts (GRBs), detected both by the BATSE instrument, on-board the Compton Gamma Ray Observatory, and by the Wide Field Camera (WFC), on-board BeppoSAX. These bursts have measured spectroscopic redshifts and are a sizeable fraction of the bursts defining the correlation between the peak energy E_peak (i.e. the peak of the vFv spectrum) and the total prompt isotropic energy E_iso (the so called "Amati" relation). Recent theoretical interpretations of this correlation assume that black-body emission dominates the time resolved spectra of GRBs, even if, in the time integrated spectrum, its presence may be hidden by the change of its temperature and by the dilution of a possible non-thermal power law component. We perform a time resolved spectral analysis, and show that the sum of a power-law and a black-body gives acceptable fits to the time dependent spectra within the BATSE energy range, but overpredicts the flux in the WFC X-ray range. Moreover, a fit with a cutoff power-law plus a black-body is consistent with the WFC data, but the black-body component contributes a negligible fraction of the total flux. On the contrary, we find that fitting the spectra with a Band model or a simple cutoff power-law model yields an X-ray flux and spectral slope which well matches the WFC spectra.Comment: 14 pages, 13 figures, accepted for publication in MNRA

The peak luminosity - peak energy correlation in GRBs

We derive the peak luminosity - peak energy (L_iso - E_peak) correlation using 22 long Gamma-Ray Bursts (GRBs) with firm redshift measurements. We find that its slope is similar to the correlation between the time integrated isotropic emitted energy E_iso and E_peak (Amati et al. 2002). For the 15 GRBs in our sample with estimated jet opening angle we compute the collimation corrected peak luminosity L_gamma, and find that it correlates with E_peak. This has, however, a scatter larger than the correlation between E_peak and E_gamma (the time integrated emitted energy, corrected for collimation; Ghirlanda et al. 2004), which we ascribe to the fact that the opening angle is estimated through the global energetics. We have then selected a large sample of 442 GRBs with pseudo--redshifts, derived through the lag-luminosity relation, to test the existence of the L_iso-E_peak correlation. With this sample we also explore the possibility of a correlation between time resolved quantities, namely L_iso,p and the peak energy at the peak of emission E_peak,p.Comment: 5 pages, 5 figures, 2 tables - MNRAS Letters submitte

The Epeak-Eiso plane of long Gamma Ray Bursts and selection effects

We study the distribution of long Gamma Ray Bursts in the Ep-Eiso and in the Ep,obs-Fluence planes through an updated sample of 76 bursts, with measured redshift and spectral parameters, detected up to September 2007. We confirm the existence of a strong rest frame correlation Ep ~ Eiso^0.54+-0.01. Contrary to previous studies, no sign of evolution with redshift of the Ep-Eiso correlation (either its slope and normalisation) is found. The 76 bursts define a strong Ep,obs-Fluence correlation in the observer frame (Ep,obs ~ F^0.32+-0.05) with redshifts evenly distributed along this correlation. We study possible instrumental selection effects in the observer frame Ep,obs-Fluence plane. In particular, we concentrate on the minimum peak flux necessary to trigger a given GRB detector (trigger threshold) and the minimum fluence a burst must have to determine the value of Ep,obs (spectral analysis threshold). We find that the latter dominates in the Ep,obs-Fluence plane over the former. Our analysis shows, however, that these instrumental selection effects do not dominate for bursts detected before the launch of the Swift satellite, while the spectral analysis threshold is the dominant truncation effect of the Swift GRB sample (27 out of 76 events). This suggests that the Ep,obs-Fluence correlation defined by the pre--Swift sample could be affected by other, still not understood, selection effects. Besides we caution about the conclusions on the existence of the Ep,obs-Fluence correlation based on our Swift sample alone.Comment: To appear in MNRA

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&

Model-Independent Distance Measurements from Gamma-Ray Bursts and Constraints on Dark Energy

Gamma-Ray Bursts (GRB) are the most energetic events in the Universe, and provide a complementary probe of dark energy by allowing the measurement of cosmic expansion history that extends to redshifts greater than 6. Unlike Type Ia supernovae (SNe Ia), GRBs must be calibrated for each cosmological model considered, because of the lack of a nearby sample of GRBs for model-independent calibration. For a flat Universe with a cosmological constant, we find Omega_m=0.25^{+0.12}_{-0.11} from 69 GRBs alone. We show that the current GRB data can be summarized by a set of model-independent distance measurements, with negligible loss of information. We constrain a dark energy equation of state linear in the cosmic scale factor using these distance measurements from GRBs, together with the "Union" compilation of SNe Ia, WMAP five year observations, and the SDSS baryon acoustic oscillation scale measurement. We find that a cosmological constant is consistent with current data at 68% confidence level for a flat Universe. Our results provide a simple and robust method to incorporate GRB data in a joint analysis of cosmological data to constrain dark energy.Comment: 8 pages, 5 color figures. Version expanded and revised for clarification, and typo in Eqs.(3)(4)(12) corrected. PRD, in pres