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