91 research outputs found
Linearized iterative least-squares (LIL): A parameter fitting algorithm for component separation in multifrequency CMB experiments such as Planck
We present an efficient algorithm for the least squares parameter fitting
optimized for component separation in multi-frequency CMB experiments. We
sidestep some of the problems associated with non-linear optimization by taking
advantage of the quasi-linear nature of the foreground model. We demonstrate
our algorithm, linearized iterative least-squares (LIL), on the publicly
available Planck sky model FFP6 simulations and compare our result with the
other algorithms. We work at full Planck resolution and show that degrading the
resolution of all channels to that of the lowest frequency channel is not
necessary. Finally we present results for the publicly available Planck data.
Our algorithm is extremely fast, fitting 6 parameters to 7 lowest Planck
channels at full resolution (50 million pixels) in less than 160 CPU-minutes
(or few minutes running in parallel on few tens of cores). LIL is therefore
easily scalable to future experiments which may have even higher resolution and
more frequency channels. We also naturally propagate the uncertainties in
different parameters due to noise in the maps as well as degeneracies between
the parameters to the final errors on the parameters using Fisher matrix. One
indirect application of LIL could be a front-end for Bayesian parameter fitting
to find the maximum of the likelihood to be used as the starting point for the
Gibbs sampling. We show for rare components, such as the carbon-monoxide
emission, present in small fraction of sky, the optimal approach should combine
parameter fitting with model selection. LIL may also be useful in other
astrophysical applications which satisfy the quasi-linearity criteria.Comment: Accepted versio
Time of primordial Be-7 conversion into Li-7, energy release and doublet of narrow cosmological neutrino lines
One of the important light elements created during the big bang
nucleosynthesis is Be-7 which then decays to Li-7 by electron capture when
recombination becomes effective but well before the Saha equilibrium
recombination is reached. This means that Be-7 should wait until its
recombination epoch even though the half-life of the hydrogenic beryllium atom
is only 106.4 days. We calculate when the conversion from primordial Be-7 to
Li-7 occurs taking into account the population of the hyperfine structure
sublevels and solving the kinetic equations for recombination, photoionization
and conversion rate. We also calculate the energies and the spectrum of narrow
neutrino doublet lines resulting from Be-7 decay.Comment: Minor typos correcte
- …