The puzzling case of GRB 990123: multiwavelength afterglow study

¹ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy (formerly IASF/CNR, Bologna) e-mail: maiorano@bo.iasf.cnr.it
² INAF - Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40126 Bologna, Italy
³ Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
⁴ INAF - Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34100 Trieste, Italy
⁵ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Palermo, via La Malfa 153, 90146 Palermo, Italy (formerly IASF/CNR, Palermo)
⁶ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy (formerly IASF/CNR, Roma)
⁷ Astrophysics Research Institute – Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK
⁸ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
⁹ SRON, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
¹⁰ ESTEC/ESA, SCI-SDG, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands

Received: 14 December 2004
Accepted: 15 April 2005

Abstract

We report on the BeppoSAX data analysis of the afterglow of Gamma-Ray Burst (GRB) 990123, one of the brightest GRBs detected by BeppoSAX. Mainly due to its exceptional brightness, this is the only source for which the Wide Field Cameras have allowed an early detection of the X-ray afterglow between ~20 and 60 min after the GRB trigger. Besides, again for the first time, high-energy emission from the afterglow was detected up to 60 keV. For the X-ray afterglow we found a power-law decay with index ; the spectrum has a power-law shape with photon index . The backward extrapolation of the afterglow decay smoothly reconnects with the late GRB emission, thus suggesting that both emissions are produced by the same phenomenon. An extensive set of multiwavelength observations for the GRB 990123 afterglow made during the BeppoSAX pointing was collected from the literature. The hard X-ray to radio range coverage allowed to construct a spectral flux distribution and to perform an analysis of the GRB afterglow in the context of the “fireball” model. We also report the results of temporal and spectral analysis of an X-ray source serendipitously observed about 22' north of the GRB afterglow, along with the optical spectroscopy of its possible counterpart to establish the nature of this source.