0.1–;20 keV Spectra of 3C 273 and E1821+643

Abstract

The high sensitivity and improved spectral resolution (ΔE/E = 0.4 at 1 keV) of ROSAT has enabled high quality spectra to be obtained in the soft X-ray (0.1−2 keV) band. When added to simultaneous Ginga data, spectra extending up to 20 keV are achieved. 3C273 was observed by ROSAT in a pointed observation in June 1990 and again in December 1990 during the survey, when it was also observed by Ginga . The December observation yielded a 3σ detection in the SI filter of the Wide Field Camera. E1821+643, benefits from being at a high galactic latitude and was observed for a total of 40 days during the ROSAT survey, with an effective exposure time of 9ks. Ginga observed the source during this period. It was also observed serendipitously during the pointed phase in February 1991. Both sources need a two component model to fit their combined 0.1−20 keV spectrum (see Tables 1 & 2). The spectrum of 3C273 has a significant soft excess which exists below 1.2 keV (source rest frame). This may be adequately parameterised by a 200eV Bremsstrahlung or a power law of α = 3.4, however, a single temperature Blackbody or Raymond-Smith model is not a good fit to the data, neither are elementary disc models. E1821+643 has a strong, steep soft excess which rises below 0.65 keV (source rest frame) in both ROSAT observations.