Discovery of a type II supernova ROTSE3 J122836.3+153449

ATel #1389; F. Yuan (U. Mich.), R. Quimby (Caltech), E. Rykoff (UCSB), M. D. Sisson, D. Chamarro, C. Akerlof, T. Mckay, J. M. Miller (U. Mich.) and J. C. Wheeler (U. Texas) on behalf of the ROTSE collaboration; J. S. Bloom, R. J. Foley, M. Modjaz, and A. A. Miller (UC Berkeley); P. J. Brown (Penn State)
on 16 Feb 2008; 14:44 UT
Credential Certification: Fang Yuan (yuanfang@umich.edu)

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We report the discovery of a type IIn supernova in unfiltered CCD images taken with the 0.45-m ROTSE-IIIb telescope at McDonald Observatory. The supernova, located at RA: 12:28:36.25 Dec:+15:34:49.1 (J2000), is 1.0" west and 0.6" south from the center of an anonymous galaxy seen in SDSS. It was discovered on Jan. 10.41 UT at 18.7+/-0.2 mag. The SN reached 18.2+/-0.2 mag on Jan. 15.36 UT and then decayed. The most recent detection showed it at 18.6+/-0.1 mag on Feb 12.27 UT. Magnitudes quoted above are unfiltered calibrated to SDSS r band. A finding chart of the SN can be found at: http://www.rotse.net/rsvp/j122836.3+153449/j122836.3+153449.jpg

Optical spectra (420-890 nm) of ROTSE3 J122836.3+153449 obtained on Jan. 29.35 UT under poor conditions and again on Jan. 30.35 UT with the 9.2m 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 + broad). Overall, the spectra are similar to the Type IIn supernova, 1996L at early times (Benetti et al. 1999, MNRAS 305,811). The Jan 30 data also clearly show narrow emission features from OIII 495.9,500.7-nm and OII 372.7-nm. All narrow features are consistent with a systematic redshift of z=0.234.

A spectrum (330-980 nm) obtained on Jan. 31.50 UT by E. Ofek (Caltech) with the Palomar 200" telescope (+ Double Beam Spectrograph) and one obtained on Feb 08.12 UT by J. S. Bloom, R. J. Foley, M. Modjaz, and A. A. Miller (UC Berkeley) with the 10-m Keck I telescope (+LRIS) confirm the behavior described above and also exhibits narrow He I emission lines and weak, narrow absorption features corresponding to the MgII 279.6,280.4-nm doublet. The He I lines are further confirmation that the object is a SN and not an AGN. The Mg II absorption sets the minimum redshift for the supernova at z=0.234. We also note that the narrow [O II] 372.7-nm emission is more extended along the slit in the 2-D spectrogram than is the SN continuum, and it is offset as well. This indicates that ROTSE3 J122836.3+153449 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, the second brightest supernova observed and a possible pair-instability SN (Ofek et al. 2007, ApJ, 659, 13; Smith et al., 2007, ApJ, 666, 1116).

ToO observations of ROTSE3 J122836.3+153449 were obtained with Swift XRT and UVOT on Feb. 6th, 7th and 13th UT. No X-ray emission is detected at the position of the supernova. The 1.1ks cleaned observation on Jan 6th yields a 3-sigma upper limit of 2e-13 erg/cm2/s in the 0.3-10keV energy range, assuming a power-law spectrum with photon index of 2. ROTSE3 J122836.3+153449 is detected by UVOT in all filters on Feb. 6th. The magnitudes are reported below. Note that the aperture used to get the UVOT photometry includes the center of the host, which introduced large uncertainty in the magnitudes.

  
Filter      Tstart      Tstop      T_exp(s)      Magnitude 
  
v          16:48:10    19:59:55     211.9       18.8+/-0.2 
  
b          16:41:35    19:54:35     211.9       19.2+/-0.2 
  
u          16:40:13    19:53:28     211.9       18.2+/-0.2 
  
uvw1       16:37:35    19:52:22     424.5       18.6+/-0.2 
  
uvm2       16:49:30    18:26:59     270.3       18.5+/-0.2 
  
uvw2       16:42:57    19:58:48     849.7       18.9+/-0.3