  We compare the prompt X-ray (2-10 keV) emission of GRB990510 measured by the
BeppoSAX Wide Field Cameras (WFC) during the burst to the X-ray afterglow
detected by the BeppoSAX Narrow Field Instruments. A single power-law model for
the afterglow, f(t) ~ t^{-1.42}, is ruled out. Provided the initial time of the
afterglow is assumed to coincide with the last short pulse in the X-ray prompt
event (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 external
shock expanding in a decelerating jet, in which synchrotron radiation takes
place. This model, represented by a double power-law of indices alpha_1 ~ 1 and
alpha_2 ~ 2 before and after a jet collimation break time of ~0.5 days after
GRB, respectively, is consistent with the second and third upper limits
measured by the WFC, but not with the first. This may be related to
inhomogeneities in the circumburst medium. Our finding indicates that the
temporal behavior of the GRB990510 X-ray afterglow is similar to that at
optical wavelengths, and thus strengthens the interpretation of the
multiwavelength afterglow as synchrotron emission in a jet with decreasing
Lorentz factor. GRB990510 is thus the only burst in which evidence of a
spreading jet has been found in X-rays.
