F. Frontera, Istituto TESRE, CNR, Bologna and University of
Ferrara, Italy; M. Capalbi, M.R. Daniele, A. Malizia, M. Perri, BeppoSAX
SDC, Rome, Italy; E. Montanari, University of Ferrara, Italy; M. R.
Daniele, A. Paolino, R. Ricci, G. Tarei, A. Tesseri, BeppoSAX SOC,
Rome, Italy; C. Pastor, M. Stornelli, SAX-OCC, Rome, Italy; E. Pian,
Istituto TESRE, CNR, Bologna, Italy; G. Gandolfi and L. Piro, IAS,
CNR, Rome, Italy, on behalf of a larger collaboration, report:
"The BeppoSAX Wide Field Camera error box of GRB990806 BeppoSAX MAIL n.
99/22 and GCN #392) was observed with the BeppoSAX Narrow Field
Instruments (NFI) starting about 8 hrs after the burst in the period Aug.
06.9267-08.6563 UT. Preliminary analysis of MECS2+3 data shows a
previously unknown X-ray source, 1SAX J0310.6-6806, located at RA =
03h10m35s, Dec = -68o06'35'' (equinox 2000.0), with an error radius of 1'.
This position is very close to the centroid of the WFC error box. During
the observation, the 1.4-10 keV source flux decreases by a factor 4,
from (8.6 +- 2.4)x10E-3 cts/s, corresponding to (5.5 +- 1.5)x10E-13 erg
sE-1 cmE-2 to (2.3 +- 0.6)x10E-3 cts/s corresponding to
(1.5 +- 0.6)x10E-13 erg sE-1 cmE-2. For comparison a ROSAT source, 1RXS
J031250.0-680915, out of the WFC error box but visible in the same MECS
image, does not show any statistically significant variation during the
entire observation. We conclude that 1SAX J0310.6-6806 is likely the X-ray
afterglow of GRB990806."
This message can be cited