Central Bureau for Astronomical Telegrams
IAUC 6969: 1998bw
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Circular No. 6969
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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SUPERNOVA 1998bw IN ESO 184-G82
L. E. Kay, J. P. Halpern, and K. M. Leighly, Columbia
University; S. Heathcote, Cerro Tololo Interamerican Observatory
(CTIO); and A. M. Magalhaes, Astronomico e Geofisico, Universidade
de Sao Paulo, report on spectropolarimetry of SN 1998bw, possibly
associated with GRB 980425 (cf. IAUC 6901, 6903), obtained with the
CTIO 4-m telescope on June 20, 21, and 23 UT: "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."
A. V. Filippenko, University of California at Berkeley,
comments on the total flux spectrum obtained above: "The spectrum
most closely resembles those of the peculiar SN 1997ef, but perhaps
evolving more slowly. It is not typical of type-Ic supernovae;
indeed, the spectrum does not match any of the known spectral
classes, but perhaps 'peculiar type Ic' is the best choice at this
time. The object appears to be starting a transition to the
nebular phase, but identifications of the broad emission features
are uncertain. The feature at 650 nm is probably [Fe III], as is
the one at 455 nm (perhaps with some contribution from Mg I]).
That at 590 nm may be a mixture of [Co III] and Na D. The
strongest feature, at 545 nm, may consist of Fe, Co, and Ti lines."
(C) Copyright 1998 CBAT
1998 July 10 (6969) Daniel W. E. Green
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