Analysis of the Polarization and Flux Spectra of SN 1993J

Abstract

Synthetic polarization and flux spectra are presented for aspherical, electron scattering-dominated photospheres of Type II supernovae (SN II) in general and the specific case of SN 1993J. Monte Carlo calculations are based on the following assumptions: a) ellipsoidal envelopes with power law density profiles; b) occupation numbers given by local thermodynamical equilibrium (LTE); c) pure electron scattering for continuum opacities; d) lines treated in a Sobolev approximation with an assumed constant thermalization fraction; e) line transitions result in depolarization; and f) the temperature structure is given by a gray extended atmosphere.The observed luminosity of a Type II supernova depends on the unknown inclination angle i. Spectral analysis alone will fail to detect even strong deviations from spherical symmetry. Line scattering depolarizes incident polarized light, but the residual intrinsic polarization does not completely vanish because of electron scattering effects that depend on the electron density distribution. By combining results on the polarization and velocity structure of the emission lines, and the degree of polarization in the continuum, we place strong constraints on the degree of asphericity, the inclination of the system and the electron density distribution. In addition, since the problem is over constrained, we can independently test for the contribution to the polarization caused by aligned interstellar grains between the SN and earth.These modelling techniques have been applied to SN1993J. Both the flux spectra and the percentage of polarization as a function of wavelength can be reproduced by an aspherical model with an axis ratio of 0.6, radial electron densities proportional to r −5 and an effective temperature of 4800 K. In an oblate model, SN 1993J is seen almost equator on. The line forming region is still within the hydrogen rich part of the envelope 3 weeks after the explosion, although the continuum may form in deeper layers. Although the residual polarization across Hα is not zero, the interstellar component derived herein is consistent with that deduced by Trammell et al. (1993). The power of this technique for investigating the structure of other SN II, as a method for independently deriving the interstellar polarization, and the implications of these results on the use of SN II to determine distances through the Baade-Wesselink method are discussed.-2 -

Asphericity, Interaction, and Dust in the Type Ii-P/Ii-L Supernova 2013ej in Messier 74

“…The broad spectral features are indicative of SN ejecta energized by the passage of the reverse shock and show similarities with other well-studied corecollapse SNe that have exhibited evidence for persistent interaction, substantial polarization, and late-time thermal excess from dust, including SN 1993J (Höflich et al 1996;Tran et al 1997;Matheson et al 2000), SN 2004et (Kotak et al 2009; polarization noted by Leonard et al 2009), and SN 2007od (Andrews et al 2010Inserra et al 2011; polarization information from R. Chornock 2016, private communication). The late time UV-optical photometry of SN 2013ej from HST, out to day 1169, which is much brighter than what is expected from radioactive decay, is also indicative of ongoing CSM interaction.…”

“…We note that a similarly strong continuum and line polarization was observed in the Type IIb SN 1993J, which also showed signs of asymmetric CSM interaction during its recombination phase and at late times (see Figure 10). This SN was also modeled as a nearly edge-on oblate ellipsoidal photosphere (Höflich et al 1996), with potential contribution from circumstellar dust (Tran et al 1997). It appears that SN 2013ej and SN 1993J could have exploded into similar asymmetric circumstellar environments of tenuous, dusty CSM.…”

First multicolour polarimetry of TeV γ-ray binary HESS J0632+057 close to periastron passage

“…At this orbital phase, different events may occur, such as: the interaction of the stellar wind with the wind or jet of the compact object; a strong flow of matter from the Be star can be captured by the compact object resulting in the formation of an accretion disc or gaseous tail; the disruption of the circumstellar disc by the compact object etc. (see Hoeflich et al (1996); Moritani et al (2015); Zamanov et al (2016)). Therefore, some perturbation of circumstellar material at this orbital phase or the formation of an additional source of polarisation cannot be surprising.…”

A non-spherical core in the explosion of supernova SN 2004dj

“…The polarization data have been corrected for an interstellar polarization (ISP) contribution, which was derived by examining (1) the observed polarization in spectral regions dominated by the strongest emission lines (that is, Na I D, H ! " and the Ca II near-infrared triplet), which are believed to be intrinsically unpolarized 16 , and (2) the observed polarization at late times (that is, the nebular phase), when the electron-scattering optical depth of the atmosphere has dropped well below unity and the observed polarization at all wavelengths is largely due to ISP (see ref.…”