The puzzling case of GRB 990123: prompt emission and broad-band afterglow modeling

¹ IASF-CNR, via Fosso del Cavaliere 100, 00133 Rome, Italy e-mail: Alessandra.Corsi@rm.iasf.cnr.it
² University “La Sapienza”, Piazzale Aldo Moro 5, 00185 Rome, Italy
³ ESA-ESTEC, Science Operations & Data Systems Division, SCI-SDG, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁴ IASF-CNR, via Gobetti 101, 40129 Bologna, Italy
⁵ Rome Astronomical Observatory, via di Frascati 33, 00044 Rome, Italy
⁶ Physics Department, University of Ferrara, via Paradiso 11, 00044 Rome, Italy
⁷ Space Research Organization Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁸ Astronomy Department, University of Bologna, via Ranzani1, 40126 Bologna, Italy
⁹ IASF-CNR, via Ugo la Malfa 153, 90146 Palermo, Italy
¹⁰ INAF, Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy

Received: 14 December 2004
Accepted: 21 April 2005

Abstract

We report on BeppoSAX simultaneous X- and γ-ray observations of the bright γ-ray burst (GRB) 990123. We present the broad-band spectrum of the prompt emission, including optical, X- and γ-rays, confirming the suggestion that the emission mechanisms at low and high frequencies must have different physical origins. In the framework of the standard fireball model, we discuss the X-ray afterglow observed by the Narrow Field Instruments (NFIs) on board BeppoSAX and its hard X-ray emission up to 60 keV several hours after the burst, detected for about 20 ks by the Phoswich Detection System (PDS). Considering the  keV and optical light curves, the  keV spectrum during the 20 ks in which the PDS signal was present and the 8.46 GHz upper limits, we find that the multi-wavelength observations cannot be readily accommodated by basic afterglow models. While the temporal and spectral behavior of the optical afterglow is possibly explained by a synchrotron cooling frequency between the optical and the X-ray energy band during the NFIs observations, in X-rays this assumption only accounts for the slope of the  keV light curve, but not for the flatness of the  keV spectrum. Including the contribution of Inverse Compton (IC) scattering, we solve the problem of the flat X-ray spectrum and justify the hard X-ray emission; we also suggest that the lack of a significant detection of  keV emission in the following 75 ks and last 70 ks spectra, should be related to poorer statistics rather than to an important suppression of IC contribution. However, considering also the radio band data, we find the 8.46 GHz upper limits violated. On the other hand, leaving unchanged the emission mechanism requires modifying the hydrodynamics by invoking an ambient medium whose density rises rapidly with radius and by having the shock losing energy. Thus we are left with an open puzzle which requires further inspection.