Supernovae Shedding Light on Gamma-Ray Bursts

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.Comment: 20 pages, 13 figures, invited review at the 4th Workshop Gamma-Ray Bursts in the Afterglow Era, Rome,18-22 October 2004. Editors: L. Piro, L. Amati, S. Covino, and B. Gendre. Il Nuovo Cimento, in pres

Cutoff-effects in the spectrum of dynamical Wilson fermions

We investigate the low-lying eigenvalues of the improved Wilson-Dirac operator in the Schroedinger functional with two dynamical quark flavors. At a lattice spacing of approximately 0.1 fm we find more very small eigenvalues than in the quenched case. These cause problems with HMC-type algorithms and in the evaluation of fermionic correlation functions. Through a simulation at a finer lattice spacing we are able to establish their nature as cutoff-effectsComment: Lattice2004(machines), 3 pages, 3 figures, talk by R.

Non-perturbative renormalization of the axial current with dynamical Wilson fermions

We present a new normalization condition for the axial current, derived from the PCAC relation with non-vanishing quark mass. This condition is expected to reduce mass effects in the chiral extrapolation of the results for the normalization factor Z_A. The application to the two-flavor theory with improved Wilson fermions shows that this expectation is indeed fulfilled. Using the Schroedinger functional setup we calculate Z_A(g_0^2) as well as the vector current normalization factor Z_V(g_0^2) for beta = 6/g_0^2 >= 5.2.Comment: 15 pages, 4 figures, 2 tables, JHEP styl

On Core Collapse Supernovae in Normal and in Seyfert Galaxies

This paper estimates the relative frequency of different types of core-collapse supernovae, in terms of the ratio f between the number of type Ib--Ic and of type II supernovae. We estimate f independently for all normal and Seyfert galaxies whose radial velocity is <=14000 km/s, and which had at least one supernova event recorded in the Asiago catalogue from January 1986 to August 2000. We find that the ratio f is approx. 0.23+/-0.05 in normal galaxies. This value is consistent with constant star formation rate and with a Salpeter Initial Mass Function and average binary rate approx. 50 %. On the contrary, Seyfert galaxies exceed the ratio f in normal galaxies by a factor approx. 4 at a confidence level >= 2 sigma. A caveat is that the numbers for Seyferts are still small (6 type Ib-Ic and 6 type II supernovae discovered as yet). Assumed real, this excess of type Ib and Ic with respect to type II supernovae, may indicate a burst of star formation of young age (<= 20 Myr), a high incidence of binary systems in the inner regions (r <= 0.4 R25) of Seyfert galaxies, or a top-loaded mass function.Comment: Accepted for Publication in MNRA

Two populations of progenitors for type Ia SNe?

We use recent observations of type Ia Supernova (SN Ia) rates to derive, on robust empirical grounds, the distribution of the delay time (DTD) between the formation of the progenitor star and its explosion as a SN. Our analysis finds: i) delay times as long as 3-4 Gyr, derived from observations of SNe Ia at high redshift, cannot reproduce the dependence of the SN Ia rate on the colors and on the radio-luminosity of the parent galaxies, as observed in the local Universe; ii) the comparison between observed SN rates and a grid of theoretical "single-population" DTDs shows that only a few of them are possibly consistent with observations. The most successful models are all predicting a peak of SN explosions soon after star formation and an extended tail in the DTD, and can reproduce the data but only at a modest statistical confidence level; iii) present data are best matched by a bimodal DTD, in which about 50% of type Ia SNe (dubbed "prompt" SN Ia) explode soon after their stellar birth, in a time of the order of 10^8 years, while the remaining 50% ("tardy" SN Ia) have a much wider distribution, well described by an exponential function with a decay time of about 3 Gyr. This fact, coupled with the well established bimodal distribution of the decay rate, suggests the existence of two classes of progenitors. We discuss the cosmological implications of this result and make simple predictions. [Abridged]Comment: 11 pages, MNRAS, in press, modified after referee's comment

On the Evolution of the Cosmic Supernova Rates

Ongoing searches for supernovae (SNe) at cosmological distances have recently started to provide a link between SN Ia statistics and galaxy evolution. We use recent estimates of the global history of star formation to compute the theoretical Type Ia and Type II SN rates as a function of cosmic time from the present epoch to high redshifts. We show that accurate measurements of the frequency of SN events in the range 0<z<1 will be valuable probes of the nature of Type Ia progenitors and the evolution of the stellar birthrate in the universe. The Next Generation Space Telescope should detect of order 20 Type II SNe per 4'x 4' field per year in the interval 1<z<4.Comment: LaTeX, 19 pages, 3 figures, to be published in the MNRA