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We report on BeppoSAX simultaneous X- and gamma-ray observations of thebright GRB 990123. We present the broad-band spectrum of the prompt emission,including optical, X- and gamma-rays, confirming the suggestion that theemission mechanisms at low and high frequencies must have different physicalorigins. In the framework of the standard fireball model, we discuss the X-rayafterglow observed by the NFIs and its hard X-ray emission up to 60 keV severalhours after the burst, detected for about 20 ks by the PDS. Considering the2-10 keV and optical light curves, the 0.1-60 keV spectrum during the 20 ks inwhich the PDS signal was present and the 8.46 GHz upper limits, we find thatthe multi-wavelength observations cannot be readily accommodated by basicafterglow models. While the temporal and spectral behavior of the opticalafterglow is possibly explained by a synchrotron cooling frequency between theoptical and the X-ray energy band during the NFIs observations, in X-rays thisassumption only accounts for the slope of the 2-10 keV light curve, but not forthe flatness of the 0.1-60 keV spectrum. Including the contribution of InverseCompton (IC) scattering, we solve the problem of the flat X-ray spectrum andjustify the hard X-ray emission; we suggest that the lack of a significantdetection of 15-60 keV emission in the following 75 ks and last 70 ks spectra,should be related to poorer statistics rather than to an important suppressionof IC contribution. However, considering also the radio band data, we find the8.46 GHz upper limits violated. On the other hand, leaving unchanged theemission mechanism requires modifying the hydrodynamics by invoking an ambientmedium whose density rises rapidly with radius and by having the shock losingenergy. Thus we are left with an open puzzle which requires further inspection.Comment: Revised version, minor changes in Table 1 and Fig. 2; 21 pages  including 6 figures, 4 tables. Accepted for publication in A&A. The Appendix  will be available only on the electronic version of the journa

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