SIFTO:AN EM PIRICAL M ETHOD FOR FITTING SN IA LIGHT CURVES 1

W epresentSiFTO,a new em piricalm ethod form odeling TypeIa supernovae(SNeIa)lightcurves by m anipulating a spectraltem plate.W em akeuseofhigh-redshiftSN data when trainingthem odel, allowing usto extend itbluerthan restfram e U. Thisincreasesthe utility ofourhigh-redshiftSN observationsby allowingustousem oreoftheavailabledata.W end thatwhen theshapeofthelight curveisdescribed using a stretch prescription,applying thesam estretch atallwavelengthsisnotan adequatedescription.SiFTO thereforeusesa generalization ofstretch which appliesdierentstretch factorsasa function ofboth thewavelength oftheobservedlterand thestretch in therest-fram e B band.W ecom pareSiFTO to otherpublished light-curvem odelsby applying them to thesam esetof SN photom etry,and dem onstratethatSiFTO and SALT2 perform betterthan thealternativeswhen judged by the scatteraround the besttlum inosity distance relationship. W e furtherdem onstrate thatwhen SiFTO and SALT2 aretrained on thesam edata setthecosm ologicalresultsagree. Subjectheadings: supernova:general