Spectral optical monitoring of a double-peaked emission line AGN Arp 102B

Variability of spectral lines and continuum^⋆

¹ Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz, Karachaevo-Cherkesia 369167, Russia
e-mail: ashap@sao.ru
² Astronomical Observatory, Volgina 7, 11160 Belgrade 74, Serbia
³ Isaac Newton Institute of Chile, Yugoslavia Branch, 11060 Belgrade, Serbia
⁴ Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), Apartado Postal 51 y 216, CP 72000, Puebla, Mexico
⁵ Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia
⁶ Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁷ Sternberg Astronomical Institute, 119992 Moscow, Russia
⁸ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁹ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
¹⁰ Instituto de Astronomía, Universidad Nacional Autónoma de Mexico, Apartado Postal 70-264, Mexico, D.F. 04510, Mexico
¹¹ Universidad Politécnica de Baja California, Av. de la Industrial 291, 21010 Mexicali, B.C., Mexico

Received: 26 April 2013
Accepted: 10 August 2013

Abstract

Context. We present results of long-term (1987–2010) optical spectral monitoring of the broad-line radio galaxy Arp 102B, a prototype of an active galactic nucleus with double-peaked broad emission lines that are commonly assumed to be emitted from an accretion disk.

Aims. To explore the structure of the broad-line region (BLR), we analyze the light-curves of the broad Hα and Hβ lines and the continuum flux. We aim to estimate the dimensions of the broad-line emitting regions and the mass of the central black hole.

Methods. We used the cross correlation function to find lags between the lines and continuum variations. We investigated the correlation between line and continuum fluxes in more detail and explored periodical variations of the red-to-blue line flux ratio using Lomb-Scargle periodograms.

Results. The line and continuum light-curves show several flare-like events. The fluxes in lines and in the continuum show no significant change (around 20%) during the monitored period. We found a weak correlation between the line and continuum flux variation that may indicate that the line variation is weakly connected with the variation of the central photoionization source. In spite of this weak line-continuum correlation, we estimated a time lag for Hβ of about 20 days using various methods. The correlation between the Hβ and Hα flux variation is significantly higher than that between the lines and continuum. During the monitored period, the Hβ and Hα lines show double-peaked profiles, and we found an indication for a periodical oscillation in the red-to-blue flux ratio of the Hα line. The estimated mass of the central black hole is ~1.1 × 10⁸  M_⊙, which agrees with the mass estimated from the M-σ ∗ relation.