Showing papers in "Nuovo Cimento Della Societa Italiana Di Fisica A-nuclei Particles and Fields in 2005"

High redshift supernovae: Cosmological implications

Abstract: We review the findings for the values of the cosmological parameters as derived from high-redshift SNIa measurements. The most recent results confirm the picture of a non-empty inflationary Universe that is consistent with a cosmological constant Ω A ≃ 0.7. This implies that the expansion of the Universe is currently accelerated by the action of some mysterious dark energy. We also discuss the possibility and the consequences of the fact that SNIa may not be perfect standard candles, in the sense of having properties in the early Universe that are systematically different from those they have at the present times.

Supernovae shedding light on gamma-ray bursts

Abstract: We review the observational status of the Supernova (SN)/Gamma-Ray Burst (GRB) connection. In section 2 we provide a short summary of the observational properties of core-collapse SNe. In sections 3-6 we review the circumstantial evidences and the direct observations that support the existence of a deep connection between the death of massive stars and GRBs. Present data suggest that SNe associated with GRBs form a heterogeneous class of objects including both bright and faint Hypernovae and perhaps also `standard' Ib/c events. In section 7, we provide an empirical estimate of the rate of Hypernovae, for a ``MilkyWay-like'' galaxy, of about $\sim 2.6\times 10^{-4}$ yr$^{-1}$ that may imply the ratio GRB/Hypernovae to be in the range $\sim 0.03-0.7$. In the same framework we find the ratio GRB/SNe-Ibc to be $\sim 0.008\div 0.05$. In section 8 we discuss the possible existence of a lag between the SN explosion and the associated gamma-ray event. In the few SN/GRB associations so far discovered the SN explosions and GRB events appear to go off simultaneously. In section 9 we present the conclusions and highlight the open problems that Swift hopefully will allow us to solve.

The Conspicuous gamma-ray burst of 30 May 1996

Abstract: The spectra of the majority of bursts exhibit a low-energy power law index, a, that is either a constant or becomes softer with time. However, in the burst of 30 May 1996 a becomes harder. Here we show that this behavior can be explained by a hybrid model consisting of a thermal and a non-thermal component. In this burst the power law index of the non-thermal component changes drastically from s similar to -1.5 to s similar to -0.67 at approximately 5 seconds after the trigger, thereby revealing, at low energies, the thermal component with its hard Rayleigh-Jeans tail. This leads to the large alpha-values that are found if the Band function is fitted to the spectra. We suggest that the change in s could be due to a transition from fast to slow cooling of the electrons emitting in the BATSE range. This could be due to the fact that the magnetic field strength becomes weaker.

RHESSI as Gamma Ray Burst Polarimeter

Abstract: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was designed to measure hard X-rays and γ-rays from solar flares. With its big detection area and thin side shielding it also proved to be well suited for studying Gamma Ray Bursts (GRB). Polarization analysis is feasible as well, due to a big modulation factor (MF), though serious constraints on the minimum detectable polarization (MDP) come from detection efficiency of double scattered photons. More constraints are given by background of accidental and real coincidences.

Cosmology with gamma ray bursts

Abstract: Thanks to their large luminosity, GRBs are detectable up to very high redshift (z=8.2 the present record holder). This makes GRBs very appealing for cosmological purposes as, for example, the possibility to put some independent constraints on the cosmological parameters. Similarly to Supernovae type Ia, GRBs are not characterised by a unique value of their luminosity/energetics. The use of several empirical correlations between the energy/power of GRBs and their peak energy has been proposed to overcome this problem. This solution, however, faces several problems, such as the lack of low redshift calibrators for all the proposed correlations, the large dispersion of several of these correlations, the lack of their theoretical interpretation and the still small number of objects. We review the methods proposed to use GRBs as standard candles. We discuss advantages and limitations of the correlations commonly used and the present status of constraining the cosmological parameters through GRBs.

GRB afterglows: Deep Newtonian phase and its application

Abstract: Gamma-ray burst afterglows have been observed for months or even years in a few cases. It is worth noting that at such late stages, the remnants should have entered the deep Newtonian phase, during which the majority of shock-accelerated electrons will no longer be highly relativistic. To calculate the afterglows, we must assume that the electrons obey a power law distribution according to their kinetic energy, not simply the Lorentz factor.