Variability of the Seyfert galaxy MKN 766 in the ROSAT all sky survey.
Abstract
Results are presented from the ROSAT All Sky Survey (RASS) observation of the Seyfert galaxy Mkn 766. Using a technique specifically developed for the analysis of point sources in the RASS we show that Mkn 766 is highly variable. Comparison of the soft X-ray spectrum (0.1-2 keV) with EXOSAT Medium Energy archive data (2-10 keV) clearly shows that the spectrum of Mkn 766 is characterized by a strong soft- excess. The low energy resolution of the PSPC instrument on board the ROSAT spacecraft does not allow to distinguish between different spectral models for the soft-excess. Both a power-law or a thermal component (blackbody or bremsstrahlung) yield an acceptable fit. Of the two thermal models only the first can be accepted since the best-fitting parameters derived for latter are not consistent with bremsstrahlung emission. A comparison with theoretical models for the soft-excess is presented. The lack of a strong UV bump rules out models in which soft X-ray are produced by Comptonizing UV photons. The soft-excess in the spectrum of Mkn 766 can be fitted with the emission spectrum from an accretion disk. The accretion disk parameters derived from the fit, the mass and the accretion rate, are consistent with those found by comparing the two variability timescales observed in the lightcurve to two fundamental timescales operating in the disk.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- April 1993
- Bibcode:
- 1993A&A...271...18M