3.4.3. The Fe line
A further signature of X-ray reprocessing by cold material is the presence of a fluorescence emission line at 6.4keV. There is some evidence for the presence of a weak line at this energy in the X-ray spectrum of 3C 273. One of the GINGA observations (in July 1987) showed evidence for the line at the 99% confidence level. The line equivalent width was 50eV, the corresponding line flux was 4.5 ± 2.5 10-5photonscm-2 s-1 [Turner et al. 1990]. The other GINGA observations reported by [Turner et al. 1990] provided only upper limits to the line flux compatible with the one of July 1987. Since the continuum flux had varied significantly this means that the line equivalent width did vary. A further line detection is reported by [Grandi et al. 1997] in a SAX observation in July 1996. The equivalent width was 30 ± 12 eV. Other observations, by EXOSAT and ASCA provided only upper limits [Turner et al. 1990] and [Yaqoob et al. 1994]. This somewhat confused set of measurements probably means that there is a weak line present in the emission of the quasar but that the data available do not allow us to perform a convincing analysis of the line variability nor of its relationship with the continuum variations. This study will be of prime importance to understand where the cold matter emitting the fluorescence line is located with respect to the primary X-ray source.