2006CBET..546....1D

K. Dawson, Physics Division, Lawrence Berkeley National Laboratory, Berkeley, reports on behalf of the Supernova Cosmology Project (G. Aldering, K. Barbary, K. Dawson, V. Fadeyev, G. Goldhaber, A. Gude, X. Huang, A. Kim, M. Kowalski, N. Kuznetsova, W. Lee, J. Meyers, P. Nugent, S. Perlmutter, D. Rubin, D. Schlegel, T. Spadafora, N. Suzuki, and L. Wang, Lawrence Berkeley National Laboratory; M. Doi, Y. Ihara, T. Morokuma, N. Takanashi, K. Tokita, and N. Yasuda, University of Tokyo; C. Lidman, European Southern Observatory; and R. Amanullah, A. Goobar, and V. Stanishev, Stockholm University) that an unusual transient located at R.A. = 14h32m27s.42, Decl. = +33o32'25".1 (equinox 2000.0), with a rise time of > 100 days, was discovered in images taken on Feb. 22 using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, and was observed at intervals of 15-25 days until just after peak magnitude on June 5. The new variable has increased by more than a factor of 200 in flux, with an estimated rise time of approximately 100 days in the observer frame; it has displayed little or no evolution in color, maintaining [F775W]-[F850LP] = -0.1 for more than 100 days with a scatter of 0.05 mag. Deep ACS reference images with exposure times of approximately 6400 s in F850LP and 1200 s in F775W show no indication of a host galaxy to an approximate limiting mag of 26.5 at 5-sigma confidence in both filters. Selected F850LP magnitudes for the new variable: 2006 Jan. 29 (and two earlier images, stacked), > 26.5 (not visible); Feb. 22, 23.7; May 22, 21.2; June 5, 21.3. Selected F775W magnitudes for the new variable: 2006 Jan. 29, > 26.5; Feb. 22, 23.5; May 22, 21.1; June 5, 21.2. From Feb. 22 to May 22, the object increased monotonically in flux.