Effects of the explosion asymmetry and viewing angle on the Type Ia supernova colour and luminosity calibration★

Abstract

Phenomenological relations exist between the peak luminosity and other observables of type Ia supernovae (SNe Ia) that allow one to standardize their peak luminosities. However, several issues are yet to be clarified: SNe Ia show colour variations after the standardization. Also, individual SNe Ia can show residuals in their standardized peak absolute magnitude at the level of ∼0.15 mag. In this paper, we explore how the colour and luminosity residual are related to the wavelength shift of nebular emission lines observed at ≳150 d after the maximum light. A sample of 11 SNe Ia which likely suffer from little host extinction indicates a correlation (3.3σ) between the peak B−V colour and the late‐time emission‐line shift. Furthermore, a nearly identical relation applies for a larger sample in which only three SNe with B−V≳ 0.2 mag are excluded. Following the interpretation that the late‐time emission‐line shift is a tracer of the viewing direction from which an off‐centre explosion is observed, we suggest that the viewing direction is a dominant factor controlling the SN colour and that a large part of the colour variations is intrinsic, rather than due to the host extinction. We also investigate a relation between the peak luminosity residuals and the wavelength shift in nebular emission lines in a sample of 20 SNe. We thereby found a hint of a correlation (at ∼1.6σ level). The confirmation of this will require a future sample of SNe with more accurate distance estimates. Radiation transfer simulations for a toy explosion model where different viewing angles cause the late‐time emission‐line shift are presented, predicting a strong correlation between the colour and shift, and a weaker one for the luminosity residual.

VLT Spectropolarimetry of the Type Ia SN 2005ke

“…56 Ni) in Type Ia SNe, these have been inferred from the NIR line profile at 1.65 μm (Maeda et al 2011), and from observations of the remnant of S-Andromedae, which has been attributed to the class of SN 1986G-like objects (Fesen et al 2007). The amount of "offcenterness" is less than about 2000 km s −1 .…”

“…P as a function of the inclination for a source located off-center, at a distance D = R ph /R of 3 (solid blue) and 1.5 (dotted red) (adopted from Höflich et al 1995). Note that D = 3 corresponds to SN 2005ke at day −8, when the photosphere R ph is formed at about 10 000 km s −1 , and assuming the maximum off-center 56 Ni distribution suggested by Maeda et al (2011) for core-normal SNe Ia.…”

“…We note that the deflagration-detonation transition may occur both during the initial expansion phase or after a pulsation Höflich 1995;Höflich et al 2002). We choose a scenario with M WD = M Ch because only these models burn under sufficiently high densities to undergo electron capture as indicated by observations Motohara et al 2006;Fesen et al 2007;Maeda et al 2011). We note that the evidence for electron capture has been found in core-normal and moderately subluminous SNe Ia, SN 1986G-like events, but there is a lack of observations of 91bg-like events.…”

Early ultraviolet emission in the Type Ia supernova LSQ12gdj: No evidence for ongoing shock interaction

“…In a tamped detonation, the ejecta interact with an external envelope; in a pulsating delayed detonation, an initial pulsation fails to unbind the white dwarf, and a shock is formed when the outer layers fall back on to the inner layers before the final explosion. The asymmetric models of Maeda et al (2010aMaeda et al ( , 2011) also produce low-velocity gradients, although in this case the density enhancement occurs only along the line of sight. Alternatively, the plateau may trace not the density but the composition of the ejecta, i.e.…”

Photometric properties of intermediate-redshift Type Ia supernovae observed by the Sloan Digital Sky Survey-II Supernova Survey

“…SNe viewed from the side closer to the ignition center will appear bluer, and those viewed from the other side will appear redder. Maeda et al (2011) shows a difference of the peak luminosity changes ∼ 30% due to the viewing angle. The colour of the SN also seems to be correlated with the ejecta velocity (Foley & Kasen 2011;Foley 2012), with highvelocity SNe tend to be redder than the low-velocity ones and the offset is ∼ 0.04 mag.…”