GCN Circular 300

The U.S. Naval Observatory GRB team (F.J. Vrba, A.A. Henden, C. B.
Luginbuhl, B. Canzian, 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 a second night of calibrated photometric 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).
Deep Rc-band images formed from 12 stacked 10-minute exposures were
obtained on both nights.  No clearly variable object within the IPN
annulus was seen, down to a detection limit of roughly Rc=23.

One object 1.6 arcmin outside the revised IPN annulus (GCN 298) but
within the revised RXTE localization (GCN/RXTE_PCA Burst Position
Notice of 990507 02:31:42 UT) appears to be variable, fading from
approximately Rc=22.0 (+/- 0.2mag) on 990507 UT to below our detection
limit on 990508 UT.  The coordinates of this object are 11:54:57.80,
-26:43:03.8 (J2000, +/- 0.2 arcsec).  A 2.7 arcmin square finding chart
(NE = upper left) for this object can be found on
  ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb990506.gif

The BVRcIc photometric calibration file in this directory
(grb990506.dat) will be revised in the near future to reflect the
second night of all-sky photometry.

This GCN note can be cited.