AAS Meeting #193 - Austin, Texas, January 1999
Session 47. Supernovae
Display, Thursday, January 7, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[47.05] Spectropolarimetry of the Peculiar Type Ic Supernovae 1998bw and 1997ef

L. E. Kay (Barnard College, Columbia Univ.), J. P. Halpern, K. M. Leighly (Columbia Univ.), A. V. Filippenko, A. Barth, E. C. Moran, D. C. Leonard (UC Berkeley), A. M. Magalh\~aes (IAGUSP), S. Heathcote (CTIO)

We report on spectropolarimetry of SN 1998bw, possibly associated with GRB 980425, obtained with the CTIO 4-m telescope on June 20-23 1998. Over the wavelength range 390-750 nm, we measure intrinsic linear polarization of 0.53 +/- 0.08 percent at position angle 49 +/- 3 deg, after correcting for Galactic interstellar polarization using the star HD 184100, which has polarization of 0.75 +/- 0.01 percent at p.a. 176.5 +/- 2.5 deg. This measured interstellar polarization is consistent with the Galactic extinction in this direction, estimated to be E(B-V) = 0.059 from IRAS maps, or E(B-V) = 0.079 from 21-cm H I. Interstellar polarization in the host galaxy ESO 184-G82 is expected to be negligible based on the relative absence of Na I D absorption at z = 0.00841 +/- 0.00005, the redshift of the environment of the supernova from narrrow H II region emission lines in its spectrum. Polarization appears highest in between emission features in the total flux spectrum, which strengthens the interpretation of the polarization as intrinsic to the supernova. This modest polarization is less than that of some type-II supernovae, but greater than that of type-Ia supernovae, which are generally unpolarized. This supports the interpretation of SN 1998bw, a peculiar type-Ic supernova, as a core-collapse event in which the observed polarization is due to moderate asymmetry in either the photosphere of the ejecta or an overlying scattering envelope. However, this result does not strongly constrain arguments about whether some supernovae emit gamma-ray bursts, since such emission may come from a mildly relativistic shock associated with the radio emission and above the optical photosphere, without any requirements on beaming or orientation.

The spectrum most closely resembles that of the peculiar SN 1997ef. This was observed on three occasions over 2.5 months with the spectropolarimeter on Keck and is polarized less than 0.8 percent (with an estimated maximum Galactic interstellar polarization of 0.38 percent, using an E(B-V) of 0.042 from IRAS maps).

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