Photospheric‐Phase Spectropolarimetry and Nebular‐Phase Spectroscopy of the Peculiar Type Ic Supernova 2002ap

Abstract

ABSTRACTThe early‐time optical spectrum of the Type Ic supernova (SN) 2002ap was characterized by unusually broad features, leading some authors to designate it a “hypernova.” We present optical spectropolarimetry of this object 16 and 37 days after the estimated date of explosion. After correcting for interstellar polarization, we find evidence for a high level of intrinsic continuum polarization at both epochs: p≳1.3% on day 16 and p≳1.0% on day 37. Prominent line polarization is also seen, especially in the trough of the Ca ii near‐infrared triplet during the second epoch. When interpreted in terms of the oblate, electron‐scattering model atmospheres of Höflich, our results imply an asymmetry of at least 20% (day 16) and 17% (day 37). The data suggest a fair degree of axisymmetry, although the polarization angle of the dominant polarization axes are different by about 55° between the two epochs, implying a complex morphology for the thinning ejecta. In particular, there exists some spectropolarimetric evidence for a different distribution of Ca relative to iron‐group elements. We also present flux spectra of SN 2002ap taken 131 and 140 days after the explosion. The spectra are characterized by a very weak continuum and broad emission lines, indicating that SN 2002ap has entered the nebular phase. The spectral features are substantially similar to those of “normal” SNe Ic in the nebular phase, and the emission lines are not significantly broader. However, some of the broad lines are very sharply peaked and may possess a narrow component (probably unresolved by our spectra, FWHM≲400 km s-1) that is redshifted by ∼580 km s-1 with respect to the systemic velocity of the host galaxy.