Electronic Telegram No. 2228 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION CBAT Director: Daniel W. E. Green; Room 209; Dept. of Earth and Planetary Sciences; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbat@iau.org; cbatiau@eps.harvard.edu URL http://www.cfa.harvard.edu/iau/cbat.html SUPERNOVA 2010bh R. Chornock, A. M. Soderberg, R. J. Foley, E. Berger, A. Frebel, and P. Challis, Harvard-Smithsonian Center for Astrophysics; J. D. Simon and S. Sheppard, Carnegie Observatories, report on the spectroscopic discovery of a supernova associated with GRB 100316D (Stamatikos et al., GCN 10496; visible at http://gcn.gsfc.nasa.gov/gcn3/10496.gcn3); the designation 2010bh was announced on CBET 2227: "Nightly observations were obtained with the MagE, LDSS3, and IMACS spectrographs on the twin 6.5-m Magellan telescopes, starting on Mar. 18.0 UT, approximately 1.5 days after the Burst Alert Telescope trigger. All spectra included the variable point source found by Levan et al. (GCN 10523) and Wiersema et al. (GCN 10525) within the slit aperture. Our spectra show superposed high-equivalent-width nebular emission lines at z = 0.059, in agreement with Vergani et al. (GCN 10512). Our earliest spectra show a very blue continuum with a few weak stellar features, indicative of some starlight from a young stellar population falling within our spectroscopic aperture, but no other obvious features. By the time of our most recent MagE spectrum (range 350-1000 nm), from Mar. 22.0 UT (5.5 days after the GRB), a few broad undulations in the continuum have developed, although contamination from the galaxy light, and possibly an afterglow, remains significant. The strongest feature has a flux peak near 785 nm (in the rest frame) and a minimum near 728 nm. An additional local minimum in the continuum is located near 570 nm, with a broad maximum located to the red of that. The appearance of these undulations in the continuum is broadly similar to those seen in peculiar broad-lined type-Ic supernovae such as SN 1998bw." NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2010 CBAT 2010 March 24 (CBET 2228) Daniel W. E. Green