Soft X-ray spectral variations of the narrow-line Seyfert 1 galaxy Markarian 766

• Published in: Monthly notices of the Royal Astronomical Society 1999-06, Vol.305 (4), p.775-794
• Main Authors: Page, M. J., Carrera, F. J., Mittaz, J. P. D., Mason, K. O.
• Format: Article
• Language: English
• Subjects: galaxies: active; galaxies: individual: mrk 766; galaxies: Seyfert; X-rays: galaxies
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1046/j.1365-8711.1999.02489.x

ABSTRACT The X‐ray variability of the narrow‐line Seyfert 1 galaxy Markarian 766 is studied using nine ROSAT PSPC data sets. The spectrum is well described by a power law combined with a blackbody (kT ∼ 70 eV) soft excess. Examination of flux ratio changes and variability amplitude in three X‐ray bands shows that the power‐law component varies continuously on time‐scales of ∼ 5000 s and is steeper when it is brighter. In contrast, variability of the soft excess is not detected. Spectral modelling of 31 spectra from different observations and at a range of count rates is also consistent with a picture in which the power law is steeper when it is brighter, and in which the soft‐excess component does not vary. The power‐law variability can be explained if the power law is produced by variable thermal or non‐thermal Comptonization of soft photons. This behaviour is similar to that of Galactic black hole candidates in the low state. The X‐ray and multiwavelength properties of Markarian 766 are shown to be very similar to those of other narrow‐line Seyfert 1s. This may mean that the rapid X‐ray variability seen in other narrow‐line Seyfert 1s may also not originate in their strong soft‐excess components.