Long-term optical, UV, and X-ray continuum variations in the changing-look AGN HE 1136-2304

¹ Institut für Astrophysik, Universität Göttingen, Friedrich-Hund Platz 1, 37077 Göttingen, Germany
e-mail: wkollat@astro.physik.uni-goettingen.de
² Department of Earth and Space Sciences, Morehead State University, Morehead, KY 40351, USA
³ Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstrasse 150, 44801 Bochum, Germany
⁴ Physics Department and the Haifa Research Center for Theoretical Physics and Astrophysics, University of Haifa, Haifa 3498838, Israel
⁵ School of Physics & Astronomy and the Wise Observatory, The Raymond and Beverly Sackler Faculty of Exact Sciences Tel-Aviv University, Tel-Aviv 69978, Israel
⁶ XMM-Newton Science Operations Centre, ESA, Villafranca del Casuntilo, Apartado 78, 28691 Villanueva de la Cañada, Spain

Received: 20 December 2017
Accepted: 6 April 2018

Abstract

Aims. A strong outburst in the X-ray continuum and a change of its Seyfert spectral type was detected in HE 1136-2304 in 2014. The spectral type changed from nearly Seyfert 2 type (1.95) to Seyfert 1.5 type in comparison to previous observations taken ten to twenty years before. In a subsequent variability campaign we wanted to investigate whether this outburst was a single event or whether the variability pattern following the outburst was similar to those seen in other variable Seyfert galaxies.

Methods. In addition to a SALT spectral variability campaign, we carried out optical continuum, as well as X-ray and UV (Swift) monitoring studies from 2014 to 2017.

Results. HE 1136-2304 strongly varied on timescales of days to months from 2014 to 2017. No systematic trends were found in the variability behavior following the outburst in 2014. A general decrease in flux would have been expected for a tidal disruption event. This could not be confirmed. More likely the flux variations are connected to irregular fluctuations in the accretion rate. The strongest variability amplitudes have been found in the X-ray regime: HE 1136-2304 varied by a factor of eight during 2015. The amplitudes of the continuum variability (from the UV to the optical) systematically decreased with wavelength following a power law F_var = a × λ^−c with c = 0.84. There is a trend that the B-band continuum shows a delay of three light days with respect to the variable X-ray flux. The Seyfert type 1.5 did not change despite the strong continuum variations for the period between 2014 and 2017.