RHESSI Hard X-ray imaging spectroscopy of the 2002 July 23 Solar Gamma-ray Flare

The GOES X4.8 flare of 23 July 2002 is the first gamma-ray flare seen by the recently launched Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). In this paper, first results of X-ray (>3~keV) imaging spectroscopy of this flare are discussed and related to images seen in EUV (TRACE, EIT), Hα (BBSO), and radio waves (Nobeyama). In particular, solar flare bremsstrahlung emission above 300 keV is imaged for the first time. During the first minutes of the impulsive phase of the flare, three main hard X-ray (>30~keV) sources are seen. Two of the sources show similar spectra with a break around 60~keV and exponents around -1.7 below the break and -3.0 above. The spectrum of the third source (that is observed in-between the two other sources, but slightly displaced to the side) is best fitted with a single power law with an exponent of around -3.0. Later in the impulsive phase, only the two sources with similar spectra are seen. The spatial separation of these sources is linearly increasing with a velocity of ~36 km/s over more than 5 minutes. This separation is comparable to the footpoint separation reported by Sakao et al. (1994). More detailed analysis on the temporal evolution of the spectra and a comparison with imaging observations at other wavelengths will be presented.