BeppoSAX confirmation of beamed afterglow emission from GRB990510
Abstract
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. The X-ray emission from ~100 to 10^5 seconds after the GRB 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 alpha_2 ~2 before and after a jet collimation break time of ~1 day after GRB, respectively, is consistent with the tail of the burst temporal profile and upper limits measured by the WFC. A single power-law f(t) ~t^{-1.42} is instead ruled out. 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 Lorenz factor. GRB990510 is thus the only burst in which evidence of a spreading jet has been found in X-rays. The synchrotron cooling frequency appears to be evolving consistently with the above picture.Keywords
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