The Host Galaxy and Rapidly Evolving Broad-line Region in the Changing-look Active Galactic Nucleus 1ES 1927+654

Abstract

Changing-look active galactic nuclei (AGNs) present an important laboratory to understand the origin and physical properties of the broad-line region (BLR). We investigate follow-up optical spectroscopy spanning ∼500 days after the outburst of the changing-look AGN 1ES 1927+654. The emission lines displayed dramatic, systematic variations in intensity, velocity width, velocity shift, and symmetry. Analysis of optical spectra and multiband images indicates that the host galaxy contains a pseudobulge and a total stellar mass of 3.56 − 0.35 + 0.38 × 10 9 M ⊙ . Enhanced continuum radiation from the outburst produced an accretion disk wind, which condensed into BLR clouds in the region above and below the temporary eccentric disk. Broad Balmer lines emerged ∼100 days after the outburst, together with an unexpected, additional component of narrow-line emission. The newly formed BLR clouds then traveled along a similar eccentric orbit (e ≈ 0.6). The Balmer decrement of the BLR increased by a factor of ∼4–5 as a result of secular changes in cloud density. The drop in density at late times allowed the production of He i and He ii emission. The mass of the black hole cannot be derived from the broad emission lines because the BLR is not virialized. Instead, we use the stellar properties of the host galaxy to estimate M BH = 1.38 − 0.66 + 1.25 × 10 6 M ⊙ . The nucleus reached near or above its Eddington limit during the peak of the outburst. We discuss the nature of the changing-look AGN 1ES 1927+654 in the context of other tidal disruption events.

Discovery of the luminous X-ray ignition eRASSt J234402.9−352640

“…We therefore conclude that this unusual flux ratio is intrinsic to the spectrum of J234402, although its origin remains unclear. Interestingly, Li et al (2022) find a low value of the Balmer decrement immediately following the flare in 1ES 1927+654, as low as ∼1, which increases to >3 on a timescale of hundreds of days. We will continue to monitor the evolution of this anomalous line ratio in J234402, to investigate whether it is related to the rapid changes in the BLR.…”

Changing-look Active Galactic Nuclei from the Dark Energy Spectroscopic Instrument. I. Sample from the Early Data

“…Additionally, the mechanisms responsible for these changes are yet to be fully understood. Three commonly proposed possible causes are: (1) changes in the central gas density due to the variation of the energy radiation intensity from the nucleus or accelerating outflows (Shapovalova et al 2010;LaMassa et al 2015;Ricci & Trakhtenbrot 2023); (2) a rapid increase or decrease of the gas density and accretion rate in the compact region originating from tidal disruption events (Blanchard et al 2017;Li et al 2022a); and (3) accretion rate changes caused by BLR evolution, accretion disk instability, or temporary accretion events (Esin et al 1997;Elitzur & Ho 2009;Dexter & Agol 2011;Elitzur et al 2014;MacLeod et al 2019;Śniegowska & Czerny 2020). CL AGNs may be attributed to various mechanisms or influenced by multiple physical processes.…”

Steep Balmer Decrement in Weak AGNs May Not Be Caused by Dust Extinction: Clues from Low-luminosity AGNs and Changing-look AGNs

“…The emission-line data of NGC 3516 are selected from Shapovalova et al (2019), where we exclude the data from 2014 to 2018 due to the Hβ line being too weak, and the measurements on the broad component are not accurate. The data of 1ES 1927+654 are selected from Li et al (2022). For CLAGNs with only two spectra in Jin et al (2022), we exclude the objects observed by the XingLong Telescope for bad spectral quality and several sources with very large differences of velocity even in active state (i.e., FWHM(Hβ)/FWHM (Hα) 1.5 or FWHM(Hα)/FWHM(Hβ) 1.5).…”