An X-ray variable absorber within the broad line region in Fairall 51

¹ Astronomical Institute, Academy of Sciences, Boční II 1401, 14100 Prague, Czech Republic
e-mail: jiri.svoboda@asu.cas.cz
² Dr. Karl Remeis Sternwarte, Sternwartstrasse 7, 96049 Bamberg, Germany
³ European Space Astronomy Centre of ESA, PO Box 78, Villanueva de la Cañada, 28691 Madrid, Spain
⁴ Departamento de Astrofisica Extragalactica y Cosmologia, Instituto de Astronomia, Universidad Nacional Autonoma de Mexico (UNAM), Apartado Postal 70-264, 04510 México, Mexico
⁵ Osservatorio Astronomico di Roma (INAF), via Frascati 33, 00040 Monteporzio Catone Roma, Italy

Received: 3 December 2014
Accepted: 13 April 2015

Abstract

Context. Fairall 51 is a polar-scattered Seyfert 1 galaxy, a type of active galaxy believed to represent a bridge between unobscured type-1 and obscured type-2 objects. Fairall 51 has shown complex and variable X-ray absorption, but little is known about its origin.

Aims. In our research, we observed Fairall 51 with the X-ray satellite Suzaku in order to constrain a characteristic time scale of its variability.

Methods. We performed timing and spectral analysis of four observations separated by 1.5, 2, and 5.5 day intervals.

Results. We found that the 0.5–50 keV broadband X-ray spectra are dominated by a primary power-law emission (with the photon index ~2). This emission is affected by at least three absorbers with different ionisations (log ξ ≈ 1–4). The spectrum is shaped further by a reprocessed emission, possibly coming from two regions, the accretion disc and a more distant scattering region. The accretion disc emission is smeared by the relativistic effects, from which we measured the spin of the black hole as a ≈ 0.8 ± 0.2. We found that most of the spectral variability can be attributed to the least ionised absorber whose column density changed by a factor of two between the first (highest-flux) and the last (lowest-flux) observation.

Conclusions. A week-long scale of the variability indicates that the absorber is located at the distance ≈0.05 pc from the centre, i.e., in the broad line region.