X-ray spectrum of NGC 3783 - Detection of soft excess and an emission line

Abstract

Results of the X-ray (0.1-10 keV) spectral analysis of the Seyfert I galaxy, NGC 3783, which was observed with EXOSAT on three epochs between 1983 and 1985, are presented here. No low energy absorption, intrinsic to the source, was found in this galaxy and the spectral index of the power law fit did not vary during the EXOSAT observations. Soft excess was detected in this source which has displayed correlated variabilities with the soft spectral index (of the broken power law fits) and the low energy count rates (or fluxes) of the galaxy. Soft excess was maximum when the source was in brightest state in 1984. Also, strong flux (both soft and hard) variations of NGC 3783 have been found on the time scales of 6 months. For the first time, we report the presence of a highly significant emission line around 6.0 keV in the spectrum of this galaxy. Measured equivalent width of the line is best estimated to be about 146 +/- 77 eV which indicates that the origin of the line may be due to the fluorescence of the cold iron around the central continuum source.