BeppoSAX confirmation of beamed afterglow emission from GRB 990510

¹ Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34131 Trieste, Italy
² Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, Via Gobetti 101, 40129 Bologna, Italy
³ Istituto di Astrofisica Spaziale, CNR, via Fosso del Cavaliere, Area di Ricerca Tor Vergata, 00133 Rome, Italy
⁴ Dipartimento di Astronomia, Università di Bologna, Via Ranzani 1, 40127 Bologna, Italy
⁵ Dipartimento di Fisica, Università di Ferrara, Via Paradiso 11, 44100 Ferrara, Italy
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ Department of Astrophysical Sciences, Princeton University, NJ 08544, USA
⁸ Institute for Advanced Study, Olden Lane, Princeton, NJ 08540, USA

Corresponding author: E. Pian, pian@tesre.bo.cnr.it

Received: 6 December 2000
Accepted: 26 March 2001

Abstract

We compare the prompt X-ray (2-10 keV) emission of GRB 990510 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, , 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 s after the GRB trigger time), the X-ray emission from ~80 to 10⁵ s 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 and 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 GRB 990510 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. GRB 990510 is thus the only burst in which evidence of a spreading jet has been found in X-rays.