GRB 981226

F. Frontera and E. Montanari, Universita, Ferrara; L. A. Antonelli, Osservatorio Astronomico, Rome; P. Giommi, Agenzia Spaziale Italiana, Rome; L. Piro, IAS, CNR, Rome; M. R. Daniele, A. Coletta, and C. De Libero, BeppoSAX, Rome, on behalf of a large collaboration, report: "The BeppoSAX Wide Field Camera error box of GRB 981226 (IAUC 7074) was observed with the BeppoSAX Narrow Field Instruments (NFI) starting about 11 hr after the burst in the period Dec. 26.878-29.148 UT. Preliminary analysis of MECS2+3 data shows a previously unknown x-ray source, 1SAX J2329.6-2356, located at R.A. = 23h29m36s.0, Decl. = -23o55'58" (equinox 2000.0), with an error radius of 1'. During the first 33.3 hr of observation, the 2-10-keV source flux decreases by a factor 3, from 0.0043 +/- 0.0010 counts/s, corresponding to (3.0 +/- 0.7) x 10E-13 ergE-1 cmE-2, down to 0.00167 +/- 0.00046 counts/s, corresponding to (0.98 +/- 0.30) x 10E-13 ergE-1 cmE-2. The measurements during the subsequent 18.36 hr show that the flux of 1SAX J2329.6-2356 does not vary significantly from the last value. Very likely this source is the x-ray afterglow of GRB 981226."