2005CBET..340....1M

G. R. Meurer, Johns Hopkins University; N. Pirzkal and A. Riess, Space Telescope Science Institute; N. P. Hathi, S. Malhotra, and J. Rhoads, Arizona State University, and the "Probing Evolution And Reionization Spectroscopically" (PEARS) Science Team report the discovery of an apparent supernova in the GOODS-North survey field, found in spectroscopic data obtained with the Hubble Space Telescope's Advanced Camera for Surveys (ACS) Wide Field Camera (WFC) using the G800L grism. The new object appears in two images obtained with the F606W filter (approximating a wide V band), used to align the data, at R.A. = 12h36m32s.253, Decl. = +62o15'14".48 (equinox 2000.0), which is 1".82 to the northwest of the galaxy GOODS J123632.48+621513.6 (whose spectroscopic redshift is z = 0.6827 ± 0.0003 according to Wirth et al. 2004, A.J. 127, 3121). Nothing is visible at this location on ACS/WFC GOODS F606W (approximating a broad V band) images taken between Nov. 2002 and May 2003 (limiting 10-sigma AB magnitude 28.33). The supernova appears in all of 12 grism images of the same field obtained from Dec. 13.130 to 18.729 UT. The combined grism spectrum is consistent with that of a type-I supernova (and most likely a type-Ia event) at z = 0.68 near maximum light. The mean flux density of the supernova spectra from 600 to 850 nm corresponds to an AB magnitude of about 24.2 ± 0.2 at the middle of the observations, fading by about 0.04 mag per day over the interval of observation. Measurement of two images obtained with ACS/WFC and the F606W filter (exposure times of 399 s) yields the following magnitudes: Dec. 15.849, 25.282 ± 0.042; Dec. 18.712, 25.333 ± 0.044. The difference between the grism flux density and the direct-image flux density is consistent with the spectrum of a type-Ia supernova near maximum light at z = 0.68, which should have an AB magnitude color of V-I approximately 1.3.