The detection and X-ray view of the changing look AGN HE 1136-2304

TL;DR

This paper reports the detection of a significant X-ray flare in the changing look AGN HE 1136-2304, accompanied by optical spectral changes indicating a transition from Seyfert 2 to Seyfert 1.5, likely caused by increased accretion rate.

Contribution

It presents multi-wavelength observations of a changing look AGN and discusses the physical mechanisms behind its variability, highlighting the role of accretion rate changes.

Findings

X-ray flux increased by a factor of 30

Optical broad lines and blue continuum flux increased significantly

X-ray spectra show moderate absorption and a 100 keV cutoff

Abstract

We report the detection of high-amplitude X-ray flaring of the AGN HE 1136-2304, which is accompanied by a strong increase in the flux of the broad Balmer lines, changing its Seyfert type from almost type 2 in 1993 down to 1.5 in 2014. HE 1136-2304 was detected by the XMM-Newton slew survey at >10 times the flux it had in the ROSAT all-sky survey, and confirmed with Swift follow-up after increasing in X-ray flux by a factor of 30. Optical spectroscopy with SALT shows that the AGN has changed from a Seyfert 1.95 to a Seyfert 1.5 galaxy, with greatly increased broad line emission and an increase in blue continuum AGN flux by a factor of > 4. The X-ray spectra from XMM-Newton and NuSTAR reveal moderate intrinsic absorption and a high energy cutoff at 100 keV. We consider several different physical scenarios for a flare, such as changes in obscuring material, tidal disruption events, and an increase in the accretion rate. We find that the most likely cause of the increased flux is an increase in the accretion rate, although it could also be due to a change in obscuration.