A second example of a Ia supernova associated with a super-Chandrasekhar mass white dwarf

ATel #1212; ROTSE: F. Yuan, C. Akerlof, J. Miller (University of Michigan), R. Quimby (University of Texas) on behalf of a larger collaboration; Dartmouth: C. Peters and J. Thorstensen; The Nearby Supernova Factory: C. Baltay, A. Bauer, D. Rabinowitz, R. Scalzo (Yale University, New Haven, CT), G. Rigaudier, E. Pecontal (Centre de Recherche Astronomique de Lyon), C. Buton, Y. Copin, E. Gangler, G. Smadja, C. Tao (Institut de Physique Nucleaire de Lyon), P. Antilogus, S. Bailey, R. Pain, R. Pereira, C. Wu (Laboratoire de Physique Nucleaire et de Haute Energies de Paris), G. Aldering, C. Aragon, S. Bongard, M. Childress, S. Loken, P. Nugent, S. Perlmutter, K. Runge, R. C. Thomas, B. A. Weaver (Lawrence Berkeley National Laboratory, Berkeley, CA); and D. Birchall (Institute for Astronomy, University of Hawaii, Hilo, HI); and J. Cough, J. Holtzman (New Mexico State University, Las Cruces, NM); Cal Tech: A. Rau, M. Kasliwal (Caltech), A. Gal-Yam (Weizman Institute of Science, Rehovot, Israel)
on 11 Sep 2007; 17:50 UT
Credential Certification: Fang Yuan (yuanfang@umich.edu)

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We report the discovery of a supernova spectroscopically similar to the possibly super-Chandrasekhar mass Type Ia SNLS-03D3bb (Howell et al. 2006, Nature 443, 308).

The supernova, designated as ROTSE3 J011051+152740, is located at RA: 01:10:51.37 Dec: 15:27:39.9 (J2000, uncertainty < 1"). It was discovered at 18.3 ± 0.1 mag in unfiltered CCD images taken on Aug 19.28 UTC by the 0.45m ROTSE-IIIb telescope at McDonald Observatory, Texas. The field has been monitored by ROTSE-III telescopes on a daily basis before and after the discovery, weather permitting. The supernova was undetected to a limiting magnitude of 20.1 on Aug 14.30 UTC and initially detected by ROTSE-IIIb on Aug 16.29 UTC at magnitude 19.5 ± 0.1. It rose to 17.2 ± 0.1 mag on Aug 26.38 UTC and remained ~17 mag until most recently observed on Sep 9.98 UTC by ROTSE-IIIc (at the H.E.S.S. site at Mt. Gamsberg, Namibia). All magnitudes quoted above are unfiltered and calibrated relative to USNO-B1.0 R. Spectra taken with the MDM 2.4m telescope on Aug 22.4 UTC and the HET telescope on Aug 29 failed to show significant features that could identify the nature of the transient. Further information on the ROTSE observations including a finding chart and a light curve can be found at: http://www.rotse.net/transients/j0110+1527/index.html.

The Nearby Supernova Factory (SNfactory) reports its independent discovery of this supernova (SNfactory target designation SNF20070825-001) subsequent to the ROTSE3 detection, in images obtained Aug 25.4 UTC using the QUEST-II camera on the Palomar Oschin 48-inch telescope, operated by the Palomar-QUEST Consortium. On Aug 25 the supernova was at magnitude 18.4, and on Sep 03 it had brightened to 17.7 (both calibrated to R-band as derived from comparison with the USNO-B1.0 catalog). The supernova was not detected in images obtained Aug 09.5 UTC. A spectrum (range 3200-10000 AA), obtained by the SNfactory on Aug 26.5 UTC with the SuperNova Integral Field Spectrograph (SNIFS) on the University of Hawaii 2.2-meter telescope, consists of a blue continuum with scattered small-amplitude absorption features superimposed. Lacking additional constraints (such as host galaxy redshift), the target was not positively identified as a Type Ia supernova.

Additional spectroscopy of the object was obtained by Kasliwal, Rau and Gal-Yam using the Double Spectrograph mounted on the Hale 5m telescope at Palomar observatory, on Sep 6.45 UTC. Inspection of a quick reduction of this spectrum suggested it was a type Ia supernova, and further analysis using the Superfit SN spectral identification code (Howell et al. 2005, ApJ, 634, 1190) shows the best matches are obtained with type Ia SNe well before peak, indicating the object must have been detected shortly after explosion.

An additional SNIFS spectrum, obtained Sep 10.5 UTC, is an unambiguous match to that of the Canada-France-Hawaii Supernova Legacy Survey supernova SNLS-03D3bb (SN 2003fg). A comparison plot (corrected to rest-frame, assuming z = 0.07 for this object) is linked at: http://snfactory.lbl.gov/snf/data/SNF20070825-001.png. The C II identification around 4000 AA reported by Howell et al. can also be made for this target: after a small correction for the B-band telluric absorption, C II 6580 AA is clearly visible, as predicted by Howell et al.

We note that, assuming z = 0.07 and the magnitudes reported above (as bright as 17 mag), the object is clearly overluminous (absolute magnitude M < -20) for a type Ia SN, agreeing well with the spectroscopic results suggesting a super-Chandra event.