Electronic Telegram No. 546
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
CBAT@CFA.HARVARD.EDU (science)
URL http://cfa-www.harvard.edu/iau/cbat.html


VARIABLE OBJECT IN BOOTES
     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.
     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/ .


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2006 CBAT
2006 June 15                     (CBET 546)               Daniel W. E. Green