6,236 research outputs found
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
A polarisation modulation scheme for measuring vacuum magnetic birefringence with static fields
A novel polarisation modulation scheme for polarimeters based on Fabry-Perot
cavities is presented. The application to the proposed HERA-X experiment aiming
to measuring the magnetic birefringence of vacuum with the HERA superconducting
magnets is discussed
Frequency locking to a high-finesse Fabry-Perot cavity of a Frequency doubled Nd:YAG laser used as the optical phase modulator
We report on the frequency locking of a frequency doubled Nd:YAG laser to a
45 000 finesse, 87-cm-long, Fabry-Perot cavity using a modified form of the
Pound-Drever-Hall technique. Necessary signals, such as light phase modulation
and frequency correction feedback, are fed direcly to the infrared pump laser.
This is sufficient to achieve a stable locking of the 532 nm visible beam to
the cavity, also showing that the doubling process does not degrade laser
performances.Comment: submitted to Review of Scientific Instrument
Modelling the nova rate in galaxies
We compute theoretical nova rates as well as type Ia SN rates in galaxies of
different morphological type (Milky Way, ellipticals and irregulars) by means
of detailed chemical evolution models, and compare them with the most recent
observations. The main difference among the different galaxies is the assumed
history of star formation. In particular, we predict that the nova rates in
giant ellipticals such as M87 are 100-300 nova/yr, about a factor of ten larger
than in our Galaxy (25 nova/yr), in agreement with very recent estimates from
HST data. The best agreement with the observed rates is obtained if the
recurrence time of novae in ellipticals is assumed to be longer than in the
Milky Way. This result indicates that the star formation rate in ellipticals,
and in particular in M87, must have been very efficient at early cosmic epochs.
We predict a nova rate for the LMC of 1.7 nova/yr, again in agreement with
observations. We compute also the K- and B-band luminosities for ellipticals of
different luminous mass and conclude that there is not a clear trend for the
luminosity specific nova rate with luminosity among these galaxies. However,
firm conclusions about ellipticals cannot be drawn because of possible
observational biases in observing these objects. The comparison between the
specific nova rates in the Milky Way and the LMC indicates a trend of
increasing nova rate passing from the Galaxy towards late-type spirals and
Magellanic irregulars.Comment: 9 pages, 5 figures, Astronomy and Astrophysics accepte
cPNN: Continuous Progressive Neural Networks for Evolving Streaming Time Series
Dealing with an unbounded data stream involves overcoming the assumption that data is identically distributed and independent. A data stream can, in fact, exhibit temporal dependencies (i.e., be a time series), and data can change distribution over time (concept drift). The two problems are deeply discussed, and existing solutions address them separately: a joint solution is absent. In addition, learning multiple concepts implies remembering the past (a.k.a. avoiding catastrophic forgetting in Neural Networks’ terminology). This work proposes Continuous Progressive Neural Networks (cPNN), a solution that tames concept drifts, handles temporal dependencies, and bypasses catastrophic forgetting. cPNN is a continuous version of Progressive Neural Networks, a methodology for remembering old concepts and transferring past knowledge to fit the new concepts quickly. We base our method on Recurrent Neural Networks and exploit the Stochastic Gradient Descent applied to data streams with temporal dependencies. Results of an ablation study show a quick adaptation of cPNN to new concepts and robustness to drifts
SN/GRB connection: a statistical approach with BATSE and Asiago Catalogues
Recent observations suggest that some types of GRB are physically connected
with SNe of type Ib/c. However, it has been pointed out by several authors that
some GRBs could be associated also with other types of core-collapse SNe (type
IIdw/IIn). On the basis of a comphrensive statistical study, which has made use
of the BATSE and Asiago catalogues, we have found that: i) the temporal and
spacial distribution of SNe-Ib/c is marginally correlated with that of the
BATSE GRBs; ii) we do not confirm the existence of an association between GRBs
and SNe-IIdw/IIn.Comment: Proceeding of the 4th workshop on Gamma Ray Bursts in the Afterglow
Era, Rome, 2004; 4 page
How many supernovae are we missing at high redshift?
Near-infrared and radio searches for core-collapse supernovae (CC SNe) in the
local universe have shown that the vast majority of the events occurring in
massive starburst are missed by the current optical searches as they explode in
very dusty environments. Recent infrared observations have shown that the
fraction of star-formation activity that takes place in very luminous dusty
starbursts sharply increases with redshift and becomes the dominant star
formation component at z>0.5. As a consequence, an increasing fraction of SNe
are expected to be missed by high-redshift optical searches. We estimate that
5-10% of the local CC SNe are out of reach of the optical searches. The
fraction of missing events rises sharply toward z=1, when about 30% of the CC
SNe will be undetected. At z=2 the missing fraction will be about 60%.
Correspondingly, for type Ia SNe, our computations provide missing fractions of
15% at z=1 and 35% at z=2. Such large corrections are crucially important to
compare the observed SN rate with the expectations from the evolution of the
cosmic star formation history, and to design the future SN searches at high
redshifts.Comment: 9 pages, MNRAS, in press, a few typos correcte
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