Z. Tsvetanov, J. Blakeslee, H. Ford, Johns Hopkins University; D. Magee, G. Illingworth, University of California Santa Cruz; A. Riess, Space Telescope Science Institute; and the ACS Science Team report the discovery of a second apparent supernova in observations of the Hubble Deep Field North (HDFN) taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). The object is located at R.A. = 12h36m55s.35, Decl. = +62d12'46".1 and was detected in exposures taken in the F775W filter on May 11.39 UT, the G800L grism (May 11.52 UT), and the F850LP filter (May 21.17 UT). The AB magnitudes of the object on these dates are F775W = 24.00 +/- 0.12 and F850LP = 24.15 +/- 0.12. The on-orbit photometric calibration of the ACS in these bandpasses has not yet been completed, and the quoted errors are dominated by the zero-point uncertainty, estimated at 0.1 mag. The object was not present in WFPC2 F814W images of the HDFN taken by HST in Dec. 1995. The spectrum extracted from the ACS G800L grism exposure appears to be that of a Type Ia supernova at a redshift z = 1.06 observed near maximum. The spectrum shows pronounced absorption features due to Ca II H & K and the iron-peak elements (Fe, Co). The object is -1.57"E, 0.65"N with respect to the nearest bright reference galaxy, object 3-486.0 in the HDF catalog (Williams et al. 1996, A.J. 112, 1335); however, this galaxy cannot be the host, as it has a measured redshift of z = 0.79 (Cohen et al. 1996, ApJ, 471, L5). No clear host galaxy can be identified at present.
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