Type Ia supernovae from violent mergers of carbon–oxygen white dwarfs: polarization signatures

Abstract

The violent merger of two carbon-oxygen white dwarfs has been proposed as a viable progenitor for some Type Ia supernovae. However, it has been argued that the strong ejecta asymmetries produced by this model might be inconsistent with the low degree of polarisation typically observed in Type Ia supernova explosions. Here, we test this claim by carrying out a spectropolarimetric analysis for the model proposed by Pakmor et al. (2012) for an explosion triggered during the merger of a 1.1 M and 0.9 M carbon-oxygen white dwarf binary system. Owing to the asymmetries of the ejecta, the polarisation signal varies significantly with viewing angle. We find that polarisation levels for observers in the equatorial plane are modest ( 1 per cent) and show clear evidence for a dominant axis, as a consequence of the ejecta symmetry about the orbital plane. In contrast, orientations out of the plane are associated with higher degrees of polarisation and departures from a dominant axis. While the particular model studied here gives a good match to highly-polarised events such as SN 2004dt, it has difficulties in reproducing the low polarisation levels commonly observed in normal Type Ia supernovae. Specifically, we find that significant asymmetries in the element distribution result in a wealth of strong polarisation features that are not observed in the majority of currently available spectropolarimetric data of Type Ia supernovae. Future studies will map out the parameter space of the merger scenario to investigate if alternative models can provide better agreement with observations.

Linear spectropolarimetry of 35 Type Ia supernovae with VLT/FORS: an analysis of the Si ii line polarization

“…SN 2004dt has an exceptionally high peak polarization of the Si ii line (Wang et al 2006). Predictions of polarization in the violent merger scenario are in good agreement with such observations (Bulla et al 2016a), however, the explosion scenario is still debated (see also Altavilla et al 2007). In addition, SN 2003eh also shows high peak polarization of the Si ii line (∼ 0.8 per cent), however, it is not shown in Figure 12, because the lack of photometric data prevented us from deriving the decline rate ∆m 15 .…”

“…There are two clear outliers: SN 2004dt, which has been already studied by Wang et al (2006) and may be the result of a violent merger (Bulla et al 2016a), and SN 2003eh. There is no photometry available in the literature for this object; therefore the ∆m 15 value for this supernova is not known.…”

Light and Color Curve Properties of Type Ia Supernovae: Theory Versus Observations

“…The explosions will produce a wide variety of envelope masses ranging from well below M Ch to super-M Ch and very asymmetric density distributions in their ejecta. This class of objects will show a significant continuum polarization that has not be observed, and we must expect a directional dependence of the luminosity, which would result in a wide spread of observed ∆m 15 and ∆m 15,s (Höflich et al 2006;Patat et al 2012;Bulla et al 2016).…”

Evolution of the electric field along null rays for arbitrary observers and spacetimes

“…The results derived here might be relevant whenever we want to infer or estimate intrinsic properties of an object with unknown motion by observing its light intensity and polarization. That is the case for the many probes of state-of-the-art cosmology and astrophysics, such as: the polarization field of the cosmic microwave background (CMB), whose B modes carry information on the existence of primordial gravitational waves generated by inflation [15,16], that may be affected by gravitational lensing [17][18][19]; the polarization of light coming from supernovae, giving clues of a possible anisotropy in the explosion events [20][21][22]; the accurate determination of black hole masses by means of signatures in the polarization of light emitted by their accretion discs [23]; the study of standard sirens as detected by laser interferometry [24] and their optical counterpart [25]; the description of light traveling through Sagnac interferometers [26][27][28].…”