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Photospheric-Phase Spectropolarimetry and Nebular-Phase Spectroscopy of the Peculiar Type Ic Supernova 2002ap
The early-time optical spectrum of the Type Ic supernova (SN) 2002ap wascharacterized by unusually broad features, leading some authors todesignate it a ``hypernova.'' We present optical spectropolarimetry ofthis object 16 and 37 days after the estimated date of explosion. Aftercorrecting for interstellar polarization, we find evidence for a highlevel of intrinsic continuum polarization at both epochs: p>~1.3% onday 16 and p>~1.0% on day 37. Prominent line polarization is alsoseen, especially in the trough of the Ca II near-infrared triplet duringthe second epoch. When interpreted in terms of the oblate,electron-scattering model atmospheres of Höflich, our results implyan asymmetry of at least 20% (day 16) and 17% (day 37). The data suggesta fair degree of axisymmetry, although the polarization angle of thedominant polarization axes are different by about 55° between thetwo epochs, implying a complex morphology for the thinning ejecta. Inparticular, there exists some spectropolarimetric evidence for adifferent distribution of Ca relative to iron-group elements. We alsopresent flux spectra of SN 2002ap taken 131 and 140 days after theexplosion. The spectra are characterized by a very weak continuum andbroad emission lines, indicating that SN 2002ap has entered the nebularphase. The spectral features are substantially similar to those of``normal'' SNe Ic in the nebular phase, and the emission lines are notsignificantly broader. However, some of the broad lines are very sharplypeaked and may possess a narrow component (probably unresolved by ourspectra, FWHM<~400 km s-1) that is redshifted by ~580 kms-1 with respect to the systemic velocity of the hostgalaxy.
| The Metamorphosis of SN 1998bw
We present and discuss the photometric and spectroscopic evolution ofthe peculiar SN 1998bw, associated with GRB 980425, through an analysisof optical and near-IR data collected at ESO-La Silla. The spectroscopicdata, spanning the period from day -9 to day +376 (relative to Bmaximum), have shown that this supernova (SN) was unprecedented,although somewhat similar to SN 1997ef. Maximum expansion velocities ashigh as 3×104 km s-1 to some extent mask itsresemblance to other Type Ic SNe. At intermediate phases, betweenphotospheric and fully nebular, the expansion velocities(~104 km s-1) remained exceptionally high comparedto those of other recorded core-collapse SNe at a similar phase. Themild linear polarization detected at early epochs suggests the presenceof asymmetry in the emitting material. The degree of asymmetry, however,cannot be decoded from these measurements alone. The He I 1.083 and2.058 μm lines are identified, and He is suggested to lie in an outerregion of the envelope. The temporal behavior of the fluxes and profilesof emission lines of Mg I] λ4571, [O I] λλ6300,6364, and a feature ascribed to Fe are traced to stimulate futuremodeling work. The uniqueness of SN 1998bw became less obvious once itentered the fully nebular phase (after 1 yr), when it was very similarto other Type Ib/c-IIb objects, such as the Type Ib SN 1996N and theType IIb SN 1993J, even though SN 1998bw was 1.4 mag brighter than SN1993J and 3 mag brighter than SN 1996N at a comparable phase. Thelate-phase optical photometry, which extends up to 403 days after Bmaximum, shows that the SN luminosity declined exponentially butsubstantially faster than the decay rate of 56Co. Theultraviolet-optical-infrared bolometric light curve, constructed usingall available optical data and the early JHK photometry presented inthis work, shows a slight flattening starting on about day +300. Sinceno clear evidence of ejecta-wind interaction was found in the late-timespectroscopy (see also the work of Sollerman and coworkers), this may bedue to the contribution of the positrons since most γ-rays escapethermalization at this phase. A contribution from the superposed H IIregion cannot, however, be excluded. Based on observations collected atESO-La Silla.
| Supernova 1998bw in ESO 184-G82
IAUC 6969 available at Central Bureau for Astronomical Telegrams.
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