Electronic Telegram No. 1262 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions) CBAT@CFA.HARVARD.EDU (science) URL http://www.cfa.harvard.edu/iau/cbat.html SUPERNOVA 2008am F. Yuan, University of Michigan; R. Quimby, California Institute of Technology (CIT); E. Rykoff, University of California at Santa Barbara; M. D. Sisson, D. Chamarro, C. Akerlof, T. McKay, and J. M. Miller, University of Michigan; and J. C. Wheeler, University of Texas -- on behalf of the ROTSE collaboration -- and J. S. Bloom, R. J. Foley, M. Modjaz, and A. A. Miller, University of California at Berkeley (UCB); and P. J. Brown, Pennsylvania State University, report the discovery of a type-IIn supernova in unfiltered CCD images taken with the 0.45-m ROTSE-IIIb telescope at McDonald Observatory: SN 2008 UT R.A. (2000.0) Decl. Mag. Offset 2008am Jan. 10.41 12 28 36.25 +15 34 49.1 18.7 1".0 W, 0".6 S SN 2008am reached mag 18.2 +/- 0.2 on Jan. 15.36 UT and then decayed; the most recent detection showed it at mag 18.6 +/- 0.1 on Feb. 12.27 (the magnitudes given above are unfiltered but calibrated to the SDSS r band). A finding chart of 2008am can be found at the following website URL: http://www.rotse.net/rsvp/j122836.3+153449/j122836.3+153449.jpg. Optical spectra (range 420-890 nm) of 2008am, obtained on Jan. 29.35 under poor conditions and again on Jan. 30.35 with the 9.2-m Hobby-Eberly Telescope (+ Marcario Low-Resolution Spectrograph) by S. Rostopchin and V. Riley, show it to be a type-II supernova. The spectra consist of blue continua superposed by hydrogen Balmer emission lines (narrow and broad). Overall, the spectra are similar to those of the type-IIn supernova 1996L (cf. IAUC 6346) at early times (Benetti et al. 1999, MNRAS 305, 811). The Jan. 30 data also clearly show narrow emission features from O III 495.9- and 500.7-nm and O II 372.7-nm. All narrow features are consistent with a systematic redshift of z = 0.234. A spectrogram (range 330-980 nm), obtained on Jan. 31.50 UT by E. Ofek (CIT) with the Palomar 5-m telescope (+ Double Beam Spectrograph), and another one obtained on Feb. 8.12 by J. S. Bloom, R. J. Foley, M. Modjaz, and A. A. Miller (UCB) with the 10-m Keck I telescope (+ LRIS), confirm the behavior described above; these spectra also exhibit narrow He I emission lines and weak, narrow absorption features corresponding to the Mg II 279.6- and 280.4-nm doublet. The He I lines are further confirmation that the object is a supernova and not an active galactic nucleus. The Mg II absorption sets the minimum redshift for the supernova at z = 0.234. The narrow [O II] 372.7-nm emission is more extended along the slit in the two-dimensional spectrogram (i.e., the extension is seen in the direction along the slit orthogonal to the dispersion) than is the supernova continuum, and it is offset as well; this indicates that 2008am is offset from its host galaxy and that the host light may contribute to the narrow emission features. Note that the redshift derived above implies a peak absolute magnitude of -22.1, which is comparable to that of 2006gy (cf. CBETs 644, 647, 648, 695), the second brightest supernova observed and a possible "pair-instability" supernova (Ofek et al. 2007, Ap.J. 659, 13; Smith et al. 2007, Ap.J. 666, 1116). Target-of-opportunity observations of 2008am were obtained with the Swift XRT satellite (+ UVOT) on Feb. 6, 7, and 13 UT. No x-ray emission is detected at the position of the supernova. The 1100-s cleaned observation on Jan. 6 yields a 3-sigma upper limit of 2 x 10^(13) erg/cm2/s in the energy range 0.3-10 keV, assuming a power-law spectrum with photon index of 2. SN 2008am was detected by UVOT in all filters on Feb. 6, and the magnitudes through the various filters, all for the general period Feb. 6.69-6.83, are as follows: v = 18.8 +/- 0.2; b = 19.2 +/- 0.2; u = 18.2 +/- 0.2; [uvw1] = 18.6 +/- 0.2; [uvm2] = 18.5 +/- 0.2; [uvw2] = 18.9 +/- 0.3. Note that the aperture used to get the UVOT photometry includes the center of the host, which introduced large uncertainty in the magnitudes. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2008 CBAT 2008 February 16 (CBET 1262) Daniel W. E. Green