X-ray variability of Seyfert 1.8/1.9 galaxies

¹ Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía, s/n, 18008 Granada, Spain
e-mail: lorena.hernandez@iaps.inaf.it
² INAF–Istituto di Astrofisica e Planetologia Spaziali di Roma (IAPS-INAF), via del Fosso del Cavaliere 100, 00133 Roma, Italy
³ Instituto de radioastronomía y Astrofísica (IRyA-UNAM), 3–72 (Xangari), 8701 Morelia, Mexico
⁴ European Space Research and Technology Centre (ESA/ESTEC), Kepleriaan 1, 2201 AZ Noordwijk, The Netherlands

Received: 20 January 2017
Accepted: 13 March 2017

Abstract

Context. Seyfert 1.8/1.9 are sources showing weak broad H_α components in their optical spectra. According to unification schemes, they are seen with an edge-on inclination, similar to type 2 Seyfert galaxies, but with slightly lower inclination angles.

Aims. We aim to test whether Seyfert 1.8/1.9 have similar properties at UV and X-ray wavelengths.

Methods. We used the 15 Seyfert 1.8/1.9 in the Véron Cetty and Véron catalog with public data available from the Chandra and/or XMM–Newton archives at different dates, with timescales between observations ranging from days to years. All the spectra of the same source were simultaneously fit with the same model and different parameters were left free to vary in order to select the variable parameter(s). Whenever possible, short-term variations from the analysis of the X-ray light curves and long-term UV variations from the optical monitor onboard XMM–Newton were studied. Our results are homogeneously compared with a previous work using the same methodology applied to a sample of Seyfert 2.

Results. X-ray variability is found in all 15 nuclei over the aforementioned ranges of timescales. The main variability pattern is related to intrinsic changes in the sources, which are observed in ten nuclei. Changes in the column density are also frequent, as they are observed in six nuclei, and variations at soft energies, possibly related to scattered nuclear emission, are detected in six sources. X-ray intra-day variations are detected in six out of the eight studied sources. Variations at UV frequencies are detected in seven out of nine sources.

Conclusions. A comparison between the samples of Seyfert 1.8/1.9 and 2 shows that, even if the main variability pattern is due to intrinsic changes of the sources in the two families, these nuclei exhibit different variability properties in the UV and X-ray domains. In particular, variations in the broad X-ray band on short timescales (days to weeks), and variations in the soft X-rays and UV on long timescales (months to years) are detected in Seyfert 1.8/1.9 but not in Seyfert 2. Overall, we suggest that optically classified Seyfert 1.8/1.9 should be kept separated from Seyfert 2 galaxies in UV/X-ray studies of the obscured AGN population because their intrinsic properties might be different.