publications.htm

Authors: G. Ghirlanda, G. Ghisellini, D. Lazzati, C. Firmani
Title: Gamma Ray Bursts: New Rulers to measure the Universe
ref: 2004, ApJ, v 613, L13
Abstract: 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~1.5, while the cosmic microwave background (CMB) primary anisotropies concern basically z~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. 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 Ω_M=0.37+/-0.10 and Ω_Λ=0.87+/-0.23. Assuming in addition a flat universe, the parameters are constrained to be Ω_M=0.29+/-0.04 and Ω_Λ=0.71+/-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 us with the most optimistic hopes for what can be obtained when the GRB-dedicated satellite, Swift, is launched.

Authors: C. Firmani, G. Ghisellini, G. Ghirlanda, V. Avila-Reese
Title: A new method optimized to use Gamma Ray Bursts as cosmic rulers
ref: 2005, MNRAS, 360, L1
Abstract: We present a new method aimed to handle long Gamma-Ray Burst (GRBs) as cosmic rulers. The recent discovery of a tight correlation between the collimation corrected GRB energy and the peak of the gamma-ray spectrum has opened the possibility to use GRBs as a new category of standard candles. Unfortunately, because of the lack of low-z GRBs, up to now this correlation is obtained from high-z GRBs with the consequence that it depends on the cosmological parameters we pretend to constrain. Hopefully this circularity problem will be solved when, in a few years, the low-z GRB sample will be increased enough. In the meanwhile we present here a new Bayesian method that eases the aforesaid circularity problem, and allows to introduce new constrains on the cosmological (Om,OL) diagram as well as to explore the universe kinematics up to z~3. The method we propose offers the further advantage to make handy the problem of the (Om,OL) loitering line singularity which inevitably appears when standard candles with z>2 are used. The combination of GRB with SN Ia data makes the popular LambdaCDM cosmology more consistent with the Hubble diagram at a 68% confidence level. For a flat cosmology we find Om=0.28\pm0.03 for the combined GRB+SN Ia data set. Correspondingly, the transition redshift between cosmic deceleration and acceleration is z_T=0.73\pm0.09, slightly larger than the value found by considering SNe Ia alone. We briefly discuss our results also in terms of non--LambdaCDM dark energy models.

Authors: G. Ghirlanda, G. Ghisellini, C. Firmani, A. Celotti, Z. Bosnjak
Title: The peak luminosity - peak energy correlation in GRBs
ref: 22005, MNRAS, 360, L45
Abstract: 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.

Authors: G. Ghirlanda, G. Ghisellini, C. Firmani
Title: Probing the existence of the Epeak - Eiso correlation in long Gamma Ray Bursts
ref: 22005, MNRAS, 361, L10
Abstract: We probe the existence of the E_peak-E_iso correlation in long GRBs using a sample of 442 BATSE bursts with known E_peak and with redshift estimated through the lag-luminosity correlation. This sample confirms that the rest frame peak energy is correlated with the isotropic equivalent energy. The distribution of the scatter of the points around the best fitting line is similar to that obtained with the 27 bursts with spectroscopic redshifts. We interpret the scatter in the E_peak-E_iso plane as due to the opening angle distribution of GRB jets. By assuming that the collimation corrected energy correlates with E_peak we can derive the observed distribution of the jet opening angles, which turns out to be log-normal with a peak value of ~6.5 degrees.

Authors: G. Ghirlanda, G. Ghisellini, D. Lazzati and C. Firmani
Title: The updated Epeak - Egamma correlation
ref: 22005, Il Nuovo Saggiatore, Ref. Proc. 4th Rome Workshop (2004)
Abstract: 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.

Authors: G. Ghisellini, G. Ghirlanda, D. Lazzati, C. Firmani, V. Avila - Reese
Title: Cosmology with Gamma Ray Bursts
ref: 22005, Il Nuovo Saggiatore, Ref. Proc. 4th Rome Workshop (2004)
Abstract: Apparently, Gamma--Ray Bursts (GRBs) are all but standard candles. Their emission is collimated into a cone and the received flux depends on the cone aperture angle. Fortunately we can derive the aperture angle through an achromatic
steepening of the lightcurve of the afterglow, and thus we can measure the ``true" energetics of the prompt emission.
Ghirlanda et al. (2004) found that this collimation-corrected energy correlates tightly with the frequency at which most of the radiation of the prompt is emitted. Through this correlation we can infer the burst energy accurately enough for a cosmological use. Using the best known 15 GRBs we find very encouraging results that emphasize the cosmological GRB role. Probing the universe with high accuracy up to high redshifts, GRBs establish a new insight on the cosmic expanding acceleration history and accomplish the role of ``missing link" between the Cosmic Microwave Background and type Ia supernovae, motivating the most optimistic hopes for what can be obtained from the bursts detected by SWIFT.

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