A&A 500, L17-L20 (2009)
DOI: 10.1051/0004-6361/200911973

Letter

Using spectral flux ratios to standardize SN Ia luminosities

The Nearby Supernova Factory: S. Bailey¹, G. Aldering², P. Antilogus¹, C. Aragon², C. Baltay³, S. Bongard¹, C. Buton⁴, M. Childress^{2, 5}, N. Chotard⁴, Y. Copin⁴, E. Gangler⁴, S. Loken², P. Nugent², R. Pain¹, E. Pecontal⁶, R. Pereira⁴, S. Perlmutter^{2, 5}, D. Rabinowitz³, G. Rigaudier⁶, K. Runge², R. Scalzo³, G. Smadja⁴, H. Swift^{2, 5}, C. Tao⁷, R. C. Thomas², and C. Wu<sup>1

1</sup>  Laboratoire de Physique Nucléaire et des Hautes Énergies, CNRS/IN2P3; Universités Paris VI et Paris VII, 4 place Jussieu, Tour 43, 75252 Paris Cedex 05, France
    e-mail: stephen.bailey@in2p3.fr
²  Physics Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
³  Department of Physics, Yale University, New Haven, CT 06250-8121, USA
⁴  Université de Lyon, 69622, France; Université de Lyon 1, France; CNRS/IN2P3, Institut de Physique Nucléaire de Lyon, France
⁵  Department of Physics, University of California Berkeley, Berkeley, CA 94720, USA
⁶  Université de Lyon, 69622, France; Université de Lyon 1, France; CRAL, Observatoire de Lyon, 69230 Saint Genis Laval, France; CNRS, UMR 5574; ENS de Lyon, France
⁷  CPPM, 163 Av. Luminy, 13288 Marseille Cedex 09, France

Received 2 March 2009 / Accepted 4 Mai 2009

Abstract
We present a new method to standardize type Ia supernova (SN Ia) luminosities to 0.13 mag using flux ratios from a single flux-calibrated spectrum per SN. Using Nearby Supernova Factory spectrophotomery of 58 SNe Ia, we performed an unbiased search for flux ratios that correlate with SN Ia luminosity. After developing the method and selecting the best ratios from a training sample, we verified the results on a separate validation sample and with data from the literature. We identified multiple flux ratios whose correlations with luminosity are stronger than those of light curve shape and color, previously identified spectral feature ratios, or equivalent width measurements. In particular, the flux ratio has a correlation of 0.95 with SN Ia absolute magnitudes. Using this single ratio as a correction factor produces a Hubble diagram with a residual scatter standard deviation of  mag, compared with  mag when fit with the SALT2 light curve shape and color parameters x₁ and c. The ratio is an effective correction factor for both extrinsic dust reddening and instrinsic variations such as those of SN 1991T-like and SN 1999aa-like SNe. When combined with broad-band color measurements, spectral flux ratios can standardize SN Ia magnitudes to ~0.12 mag. These are the first spectral metrics that give robust improvements over the standard normalization methods based upon light curve shape and color, and they provide among the lowest scatter Hubble diagrams ever published.

Key words: stars: supernovae: general -- cosmology: observations

© ESO 2009

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