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The Mysterious Transient ROTSE3 J115649.1+542726 is an Extremely Luminous Type II SN at z = 0.21

ATEL # 1644; R. Chornock, A. A. Miller, D. A. Perley, and J. S. Bloom (University of California, Berkeley)
on 3 Aug 2008; 12:55 UT
Password Certification: J. S. Bloom (jbloom@cfa.harvard.edu)

Subjects: Optical, Request for Observations, Nova, Supernova, Transients

We report on further spectroscopic observations of the transient ROTSE3 J115649.1+542726 discovered by Yuan et al. (ATEL #1515) and followed up by several groups (ATEL #1524, ATEL #1576, ATEL #1578, and ATEL #1593). We observed the object for 840 s using the Low-Resolution Imaging Spectrometer (LRIS; Oke et al. 1995) on the Keck I 10-m telescope on 2008 Aug 3.25 UT. Our initial observations of this object (ATEL #1576) showed a blue and largely featureless spectrum, but at later epochs several unidentified weak spectral features appeared (ATEL #1576 and ATEL #1593). The object now has developed a prominent broad (FWHM ≅ 10,000 km/s) emission feature centered near 7900 , which we identify as Hα near redshift 0.2. The spectrum shows several other P-Cygni absorption features consistent with higher-order Balmer lines and Fe II at a similar redshift, leading us to conclude that the object is a type-II supernova. We applied the "SuperNova IDentification" code (SNID; Blondin and Tonry 2007, ApJ, 666, 1024) to this spectrum. SNID confirms the identification as a SN II and gives a best-fit redshift of z = 0.206 0.005.

At this redshift, the peak apparent optical magnitude of ≤ 17.8 (ATEL #1515) corresponds to an absolute brightness of MV ≤ -22.2 mag, making this object one of the most luminous supernovae ever observed, comparable to, if not brighter than, the extreme SN 2006gy (Smith et al. 2007, ApJ, 666, 1116; Ofek et al. 2007, ApJL, 659, 13).

We thank M. Malkan (UCLA) for the exchange of observing time.


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