The U.S. Naval Observatory GRB team (F.J. Vrba, A.A. Henden, B. Canzian,
C.B. Luginbuhl, S.E. Levine, H.H. Guetter, J.A. Munn), D.H. Hartmann
(Clemson Univ.), M.C. Jennings (IGPP, UCR visitor), and A.J. Castro-Tirado
(LAEFF-INTA, IAA-CSIC, Spain), M.R. Zapatero-Osorio, R. Casas,
V. Motta (IAC, Spain), J. Gorosabel (LAEFF-INTA), and J. Greiner (AIP, Germany)
report observations of the GRB 990506 field, obtained at the U.S. Naval Observatory,
Flagstaff Station with the 1.0-m reflector. Observations in B, V, R(Cousins),
and I(Cousins) using a CCD with an 11x11 arcmin field were centered approximately
at the center of the intersection of the IPN3 (GCN 291) and the revised RXTE
x-ray localization (of UT 1999 May 07 02:31:42) and thus cover an area several
times the size of this intersection. A series of local standards covering this
field and based on all-sky photometry was established and preliminary photometric
values, with +/- 0.05 mag uncertainty to V = 18.5, are available at
ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb990506.dat, along with astrometry
accurate to approximately 0.1 arcsec. The RXTE/IPN3 locus was inspected for objects
which had visually different brightness than on the Digital Sky Survey (DSS).
One object, located near the center of this intersection at R.A. = 11:54:52.92,
DEC = -26:43:39.9 (+/- 0.2 arcsec in both coordinates)(J2000), was notably absent
from the DSS image. However, at V = 21.0, V-R = 1.6 (+/- 0.2) its apparently red
color (if not due to variability) might be why it is not visible on the DSS.
As of the time of this writing, observations are continuuing in order to determine
if this object is variable.
This GCN note can be cited.