Nonlinear Decline‐Rate Dependence and Intrinsic Variation of Type Ia Supernova Luminosities

Abstract

Published B and V fluxes from nearby Type Ia supernovae are fitted to light-curve templates with 4-6 adjustable parameters. Separately, B magnitudes from the same sample are fitted to a linear dependence on B −V color within a post-maximum time window prescribed by the cmagic method. These fits yield two independent SN magnitude estimates B max and B BV . Their difference varies systematically with decline rate ∆m 15 in a form that is compatible with a bilinear but not a linear dependence; a nonlinear form likely describes the decline-rate dependence of B max itself. A Hubble fit to the average of B max and B BV requires a systematic correction for observed B −V color that can be described by a linear coefficient R = 2.59 ± 0.24, well below the coefficient R B ≈ 4.1 commonly used to characterize the effects of Milky Way dust. At 99.9% confidence the data reject a simple model in which no color correction is required for SNe that are clustered at the blue end of their observed color distribution. After systematic corrections are performed, B max and B BV exhibit mutual rms intrinsic variation equal to 0.074 ± 0.019 mag, of which at least an equal share likely belongs to B BV . SN magnitudes measured using maximum-luminosity or cmagic methods show comparable rms deviations of order ≈0.14 mag from the Hubble line. The same fit also establishes a 95% confidence upper limit of 486 km s −1 on the rms peculiar velocity of nearby SNe relative to the Hubble flow.

Determination of the Hubble Constant, the Intrinsic Scatter of Luminosities of Type Ia Supernovae, and Evidence for Nonstandard Dust in Other Galaxies

“…Note that they also derived their E(B À V ) host values from the peak V À I colors, which usually have larger scatter (e.g., 0.08-0.09 mag) and may not be as reliable as a reddening indicator. Very recently, Wang et al (2006) determined a lower value of R B ¼ 2:59 AE 0:24 from a combined analysis of the maximum luminosity and the luminosity in the CMAGIC region. This value may parameterize well the effects of both extinction by the host galaxy and intrinsic SN color dependence on luminosity, but not necessarily the true R-value for distant galaxies since they did not try to disentangle these two effects in the analysis.…”

“…These methods are fundamentally identical by utilizing the relationship between SN Ia light-curve shape and peak luminosity. The color-magnitude intercept method (CMAGIC) of Wang et al (2003Wang et al ( , 2006 shows some variance, which replaces the magnitude at maximum with the more uniform magnitude at a given value of the color index. The above methods can yield distances to SN Ia host galaxies with a relative precision approaching 8%-11%, which demonstrates the power of SNe Ia as cosmological lighthouses for extragalactic distance scales.…”

Gamma-ray burst theory after Swift

“…Regardless of the question whether the flare photons are produced by the central engine or within the blast wave itself, this can produce a detectable flare in the GeV-TeV (Wang et al 2006;Galli & Piro in press) or sub-GeV region (Fan & Piran 2006b).…”

Nearby Supernova Factory Observations of SN 2005gj: Another Type Ia Supernova in a Massive Circumstellar Envelope

“…Instead of the value R B $ 4:1 expected for dust extinction in the Galaxy and LMC, values R B $ 2Y3 are often found (Tripp & Branch 1999;Phillips et al 1999;Wang et al 2003Wang et al , 2006Knop et al 2003). In this hypothesis, dust in the immediate vicinity of the SN or at the highest velocities of its ejecta scatters SN light into the observer line of sight, thus adding back light and effectively reducing derived values of R k in the optical.…”