ASAS-SN Discovery of Long-term fading in Mrk 926 and SCAT Classification as an Optical Changing-Look AGN

ATel #16140; J. T. Hinkle, M. E. Huber, A. V. Payne, B. J. Shappee (IfA, Hawai'i), M. A. Tucker (Ohio State), K. Auchettl (University of Melbourne), A. Do, W. B. Hoogendam (IfA, Hawai'i), K. Z. Stanek, C. S. Kochanek, T. A. Thompson, J. M. M. Neustadt (Ohio State), T. W.-S. Holoien (Carnegie Observatories), J. L. Prieto (Diego Portales; MAS), S. Dong (KIAA-PKU)
on 18 Jul 2023; 23:34 UT
Credential Certification: Jason Hinkle (jhinkle6@hawaii.edu)

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During normal operations of the All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al. 2014, Kochanek et al. 2017) a large (~0.5 mag) decrease in brightness was observed for the AGN Mrk 926. The long-term ASAS-SN and Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018) light curves show that Mrk 926 has been steadily fading in brightness for ~5.5 years. This flux decrease is accompanied by a decrease in the optical variability amplitude. Mrk 926 was first identified as a Seyfert galaxy by Ward et al. (1978), with subsequent spectroscopic follow-up suggesting a Seyfert 1/1.5 classification (Durret & Bergeron 1988, Véron-Cetty & Véron 2006, Kollatschny & Zetzl 2010).

Because of the large flux decrease, we obtained a spectrum with the Supernova Integral Field Spectrograph (SNIFS; Lantz et al. 2004) on the University of Hawaii 88-inch telescope on 2023-07-16 as part of the Spectroscopic Classification of Astronomical Transients (SCAT) program (ATel #11444, Tucker et al. 2022). Compared to an archival SDSS spectrum taken in December 2001, the spectrum is redder and shows significantly weaker broad H-beta and other higher order Balmer lines. The broad H-alpha line flux has also decreased and it appears that Mrk 926 has changed type and is now a Type 1.8/1.9 Seyfert. A comparison of the two spectra can be found below.

We have requested Swift ToO observations and are obtaining follow-up spectra. We encourage further follow-up of this source, particularly with high S/N spectroscopy.

Comparison of SNIFS and SDSS Spectra