Discovery of a probably sub-luminous type Ia supernova ROTSE3 J032048.6+412145

ATel #1690; F. Yuan (U. Mich.), R. Quimby (Caltech), D. Chamarro, A. Uecker, M. D. Sisson, C. Akerlof, (U. Mich.) and J. C. Wheeler (U. Texas) on behalf of the ROTSE collaboration
on 2 Sep 2008; 16:05 UT
Credential Certification: Fang Yuan (yuanfang@umich.edu)

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We report the discovery of a supernova, located at RA: 03:20:18.59 Dec: +41:21:45.1(J2000), in unfiltered CCD images taken with the 0.45-m ROTSE-IIIb telescope at McDonald Observatory. The supernova is detected at ~18.3 mag on Jul. 30.36 UT and ~18.0 mag on Aug. 1.34 UT and observed to have decayed to ~18.8 on Aug 9.33 UT. It was not detected to ~18.7 mag before discovery on Jul. 24.38 UT. There is no apparent host seen in the DSS image, but the location is in the middle of a group of galaxies in the Perseus Cluster. The object was recently identified by KAIT as a potential supernova or transient (W. Li, private communication). A ROTSE finding chart of the SN can be found at http://www.rotse.net/rsvp/j032018.6+412145/j032018.6+412145.jpg

Spectra (420-890 nm) of the target obtained under poor conditions on Aug. 21.37 UT by M. Shetrone and A. Westfall, and under improved conditions on Aug. 26.37 by J. Caldwell and V. Riley with the 9.2m Hobby-Eberly Telescope (+ Marcario Low-Resolution Spectrograph), show broad (FWHM ~ 10000 km/s) emission features similar to spectra of Type Ia supernovae in the nebular phase. The best fit match from SNID (Blondin & Tonry 2007, ApJ 666, 1024), is to SN 1991bg around 30 days past maximum light; however, the fit is imperfect (rlap ~ 6.5) and results in a formal redshift estimate of z=0.008 ±0.008. By eye, the data longward of 6300 A are similar to 1991bg around 50 days post maximum, but with a much stronger emission peak at about 8700 A (observed). From this fit we estimate a redshift of approximately z=0.01.

If the object is in the Perseus Galaxy Cluster (Abell 426), then correcting for Galactic Extinction the peak magnitude would correspond to about -16.7 absolute, which would be consistent with a subluminous, 1991bg-like supernova at this distance (z=0.0179, Struble & Rood, 1999, ApJS 125,35).