N. Masetti, E. Palazzi, E. Pian, F. Frontera (ITESRE, CNR, Bologna),
P.M. Vreeswijk, E. Rol, T.J. Galama (U. of Amsterdam), M. Della Valle
(U. of Padua), E. Costa, M. Feroci, L. Piro (IAS, CNR, Rome),
J. van Paradijs (U. of Amsterdam and U. of Alabama in Huntsville),
C. Kouveliotou (NASA-MSFC/USRA), O. Hainaut, A. Pizzella (ESO-La Silla),
and D. Koester (Institut fuer Astronomie und Astrophysik, Kiel),
on behalf of the BeppoSAX and Amsterdam/Huntsville GRB optical
follow-up teams, report:
"Images of the GRB 990506 refined error box (Hurley et al., GCN #298)
were acquired with start times 1999 May 7.09, 7.26, 8.04, 9.98 and 10.00
UT (exposure times are reported below) at ESO-La Silla with the 2.2m
telescope plus WFI.
Zero-point R calibration was determined using the photometry by Vrba et
al. (GCN #305) with an accuracy of 0.07 mag.
Comparison between the 15-min images of May 7.09 and 8.04 shows no object
varying by more than 0.3 magnitudes (3-sigma) down to R~22.6 within the
GRB error box.
In order to reach a deeper magnitude limit, we summed the images of May 7
(total limiting magnitude: R~24.0) and compared them with that of May 9
and with that of May 10. In this case no object varying by more than 0.3
magnitudes down to R~23.0 is present inside the error box.
In all cases, no clearly variable object was seen within the IPN error box
down to the detection limits of the frames.
Limiting magnitudes for the five epochs cited above are reported in the
Start time (UT) Exp. time R mag
1999 May 07.09 15 min. 23.8
1999 May 07.26 15 min. 23.0
1999 May 08.04 15 min. 23.6
1999 May 09.98 30 min. 24.3
1999 May 10.00 30 min. 24.3
This message can be cited."