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We compare the prompt X-ray (2-10 keV) emission of GRB990510 measured by theBeppoSAX Wide Field Cameras (WFC) during the burst to the X-ray afterglowdetected by the BeppoSAX Narrow Field Instruments. A single power-law model forthe afterglow, f(t) ~ t^{-1.42}, is ruled out. Provided the initial time of theafterglow is assumed to coincide with the last short pulse in the X-ray promptevent (i.e., 72 seconds after the GRB trigger time), the X-ray emission from\~80 to 10^5 seconds after the GRB trigger is well described by an externalshock expanding in a decelerating jet, in which synchrotron radiation takesplace. This model, represented by a double power-law of indices alpha_1 ~ 1 andalpha_2 ~ 2 before and after a jet collimation break time of ~0.5 days afterGRB, respectively, is consistent with the second and third upper limitsmeasured by the WFC, but not with the first. This may be related toinhomogeneities in the circumburst medium. Our finding indicates that thetemporal behavior of the GRB990510 X-ray afterglow is similar to that atoptical wavelengths, and thus strengthens the interpretation of themultiwavelength afterglow as synchrotron emission in a jet with decreasingLorentz factor. GRB990510 is thus the only burst in which evidence of aspreading jet has been found in X-rays.Comment: 7 pages, Latex, 3 PostScript figures, in press in the A&A. A  substantial addition has been made, wrt submitted version, due to evidence  that the GRB prompt event and its afterglow have different timing. Format  problem fixe

BeppoSAX confirmation of beamed afterglow emission from GRB 990510