Nearby Supernova Factory: Étalonnage des données de SNIFS et ...

tel-00372504, version 1 - 1 Apr 2009
CHAPTER 2. OBSERVATIONAL COSMOLOGY
SALT2 (Guy et al. 2007) is the evolution of SALT (Guy et al. 2005), used on the first
year SNLS cosmological analysis (Astier et al. 2006). It relies on an evolution model for the
spectral energy distribution (SED) of a supernova, and differs from the SALT approach in the
sense that spectra (contrary to photometric points only) and high-redshift supernovæ are now
used for model training. Each supernova is then treated as the departure from an “average” SN
spectral sequence, with possible variations of luminosity, color and spectral features.
The flux model for a supernova is of the type
FSN(p, λ) = x0 × [M0(p, λ) + x1M1(p, λ) + . . .] × exp [c CL(λ)] , (2.1)
where the flux at a certain phase p (relative to B maximum) and at rest-frame wavelength λ
depends on: Mi(p, λ), an average spectral sequence (similar in concept to the SN templates
by Nugent et al. (2002)) for i = 0, with additional components (i > 0) to account for SNe Ia
variability; CL(λ), an average color correction law 18 ; c, a color offset at maximum with respect
to the average, c = (B − V )max − 〈B − V 〉; x0, a normalization factor, allowing standardization
without knowledge of distances; and xi which are the intrinsic parameters for the SN, in practice
behaving equivalently to the stretch or ∆m15 (§ 2.3.2) parameters.
Flux
z = 0.47
z = 0.65
Rest Frame B−band
Rest Frame V−band
De−redshifted R−band
De−redshifted I−band
3000 4000 5000 6000 7000
Wavelength (Angstroms)
Figure 2.10: K-corrections for the same supernova SED at two different redshifts. The blue and red filled
lines correspond to both B and V rest-frame filters, and the dashed ones to de-redshifted R and I filters.
The agreement is good for the smaller redshift, but on higher z the need to use multiple observation filters
to describe the rest-frame ones arises. The requirement for a set of filters that extends well into the redder
part of the optical window, when observing very high redshift supernovæ, is obvious. From Nugent et al.
(2002).
This parametrization implicitly takes into account the so-called K-corrections (Fig. 2.10).
The need for this correction issues from the purely technical effect that occurs when we are
doing photometry, and trying to observe a continuous supernova SED redshifted through fixed
18 The fact that SALT[2] implicitly includes into a single term the supernovæ intrinsic color variation and the
color from the host galaxy, is criticized by Wood-Vasey et al. (2007), who separate both components in their
fitter. They however agree that the SALT2 approach seems to work rather well, and gives comparable results to
MLCS2k2.
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