[IAUC] CBET 1462: 20080806 : SUPERNOVA 2008es

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                                                  Electronic Telegram No. 1462
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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SUPERNOVA 2008es
     F. Yuan, University of Michigan; R. Quimby, California Institute of
Technology; T. McKay, D. Chamarro, M. D. Sisson, and C. Akerlof, University
of Michigan; and J. C. Wheeler, University of Texas, on behalf of the ROTSE
collaboration, report the discovery of a supernova (mag about 18.1) in
unfiltered CCD images taken on Apr. 26.23 UT with the 0.45-m ROTSE-IIIb
telescope at McDonald Observatory; the object was at mag 17.8 on images
taken during Apr. 28.21-May 6.23.  The new object is located at R.A. =
11h56m49s.13, Decl. = +54o27'25".7 (equinox 2000.0; uncertainty < 1").  The
transient is near a group of galaxies at z = 0.062 in Sloan Digital Sky
Survey (SDSS) images, but no counterpart is visible at the position of
2008es; the closest SDSS source is a galaxy (magnitude r = 23.1) some 8"
away.  A finding chart of the object can be found at the following website
URL:  http:/www.rotse.net/rsvp/j115649.1+542726/j115649.1+542726.jpg.
There are no known x-ray or radio sources catalogued via NED or SIMBAD at
the location of the optical transient.  Additional magnitudes for 2008es
were obtained with the Palomar 1.5-m reflector beginning on May 2,
summarized by the following data, which indicate a slow brightening trend
in bands g, r, and i:  May 2.27, g = 18.1, r = 18.1, i = 18.2; May 8.24,
g = 17.7, r = 17.8, i = 17.8.  Spectra of 2008es were obtained with the
9.2-m Hobby-Eberly Telescope (+ Marcario Low-Resolution Spectrograph;
range 420-890 nm) -- on May 1.24 by S. Odewahn and V. Rileyand, and on
May 8.25 by S. Rostopchin and A. Westfall -- and on May 2.29 by B. Cenko
with the Palomar 5-m telescope (+ Double Beam Spectrograph; range 320-900
nm).  The spectroscopic data from the three nights all show a mostly
featureless, blue continuum (f_nu approximately nu^0.35, where f_nu is
flux density and nu is the frequency); there is a broad emission feature
at 565.0 nm, which is seen at all three epochs.  This transient was
initially reported as a possible background active galactic nucleus
(http://www.astronomerstelegram.org/?read=1515) and was observed by
several groups (ibid., /?read=1524, 1576, 1578, 1593).
     R. Chornock, A. A. Miller, D. A. Perley, and J. S. Bloom, University of
California, Berkeley, report on further spectroscopic observations of the
2008es, observing the object for 840 s using the Low-Resolution Imaging
Spectrometer (Oke et al. 1995, PASP 107, 375) on the Keck I 10-m telescope
on Aug. 3.25 UT.  The initial observations of this object (reported at the
following website URL:  http://www.astronomerstelegram.org/?read=1576)
showed a blue and largely featureless spectrum, but at later epochs several
unidentified weak spectral features appeared (ibid., /?read=1576, 1593).
The object now has developed a prominent broad (FWHM about 10000 km/s)
emission feature centered near 790 nm, which is identified as H-alpha near
redshift 0.2.  The spectrum shows several other P-Cyg absorption features
consistent with higher-order Balmer lines and Fe II at a similar redshift,
leading to the conclusion that the object is a type-II supernova.
Application of the "SuperNova IDentification" code (Blondin and Tonry 2007,
Ap.J. 666, 1024) to this spectrum confirms the identification as a type-II
supernova and gives a best-fit redshift of z = 0.206 +/- 0.005.  At this
redshift, the peak apparent optical magnitude of around 17.8 (see text by
Yuan et al., above) corresponds to an absolute magnitude of M_V less-than-
or-equal-to -22.2, making this object one of the most luminous supernovae
ever observed, comparable to, if not brighter than, the extreme supernova
2006gy (Smith et al. 2007, Ap.J. 666, 1116; Ofek et al. 2007, Ap.J. 659, L13).
Chornock et al. thank M. Malkan (UCLA) for the exchange of observing time.


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                         (C) Copyright 2008 CBAT
2008 August 6                    (CBET 1462)              Daniel W. E. Green