GRB990510: on the possibility of a beamed X-ray afterglow

We discuss the prompt emission of the γ-ray burst (GRB) 990510 and its subsequent X-ray afterglow from 8.0 to 44.3 hrs after the prompt emission, using observations with the Gamma-ray Burst Monitor and Narrow Field Instruments on BeppoSAX. In the 40–700 keV band, GRB990510 had a fluence of ∼1.9×10−5 erg cm−2, whereas it reached a peak flux of ∼2.4× 10−6 erg cm−2 s−1. The X-ray afterglow decay light curve can be satisfactorily described by a single power law with index of −1.42 ± 0.07. Both the X-ray and optical behaviour of the afterglow can be explained by γ-ray burst debris expanding as a jet; we find that the cooling frequency is (fixed) between the optical and X-ray wavelength bands. Subject headings: gamma rays: bursts — X-rays: general