33,491 research outputs found
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 yr that may
imply the ratio GRB/Hypernovae to be in the range . In the same
framework we find the ratio GRB/SNe-Ibc to be . 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
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