A new method to calibrate the magnitudes of type Ia supernovae at maximum light

We present a new empirical method for fitting multicolor light curves of Type Ia supernovae (SNe). Our method combines elements from two widely used techniques in the literature: the Delta m(15) template fitting method (Phillips et al.) and the Multicolor Light-Curve Shape method (MLCS; Riess et al.). An advantage of our technique is the ease of adding new colors, templates, or parameters to the fitting procedure. We use a large sample of published light curves to calibrate the relations between the absolute magnitudes at maximum and the postmaximum decline rate Delta m(15) in BVRI filters. If we perform a cut in reddening and compare an unreddened with a reddened sample, we find that the two samples produce relations that are marginally consistent with each other. We find that individual subsamples from a given survey or publication have significantly tighter relationships between light-curve shape and luminosity than the relationship derived from the sum of all the samples, pointing to uncorrected systematic errors in the photometry, mainly in BV filters. Using our method, we calculate luminosity distances and host galaxy reddening to 89 SNe in the Hubble flow and construct a low-z Hubble diagram. The dispersion of the SNe in the Hubble diagram is sigma = 0.20 mag, or an error of similar to 9% in distance to a single SN. Our technique produces similar or smaller dispersion in the low-z Hubble diagram than other techniques in the literature.