2006CBET..546....2G

A Keck-telescope LRIS spectrum obtained on May 29 shows no obvious Fe, He, Si, or H features. Redward of 600 nm, the spectrum is blue and relatively featureless, with only one shallow absorption feature at about 635 nm. There is significant absorption at wavelengths below about 600 nm, with several additional broad (FWHM approximately 20 nm) absorption features at approximately 535, 485, and 430 nm. Earlier spectra obtained with Subaru on April 23 and the Very Large Telescope on May 19 indicate that these features are not evolving rapidly. If absorption at about 600 nm can be attributed to Ca H and K, the corresponding redshift would be z = 0.54, with peak magnitude approximately one mag brighter than a typical type-Ia supernova at that redshift. The lower limit on the absolute magnitude of the host at this redshift would be M > -14.5. Although the latest photometric point suggests that the target is now past maximum, it is still bright enough to be followed spectroscopically with 8- to 10-m-class telescopes. A finding chart can be found at http://supernova.lbl.gov/ACSclustersearch/ .