1ES 1927+654: An AGN Caught Changing Look on a Timescale of Months

Abstract

We study the sudden optical and ultraviolet (UV) brightening of 1ES 1927+654, which until now was known as a narrow-line active galactic nucleus (AGN). 1ES 1927+654 was part of the small and peculiar class of "true Type-2" AGN, which lack broad emission lines and line-of-sight obscuration. Our highcadence spectroscopic monitoring captures the appearance of a blue, featureless continuum, followed several weeks later by the appearance of broad Balmer emission lines. This timescale is generally consistent with the expected light travel time between the central engine and the broad-line emission region in (persistent) broad-line AGN. Hubble Space Telescope spectroscopy reveals no evidence for broad UV emission lines (e.g., C iv λ1549, C iii] λ1909, Mg ii λ2798), probably owing to dust in the broad-line emission region. To the best of our knowledge, this is the first case where the lag between the change in continuum and in broad-line emission of a "changing-look" AGN has been temporally resolved. The nature and timescales of the photometric and spectral evolution disfavor both a change in line-of-sight obscuration and a change of the overall rate of gas inflow as driving the drastic spectral transformations seen in this AGN. Although the peak luminosity and timescales are consistent with 2 Trakhtenbrot et al.those of tidal disruption events seen in inactive galaxies, the spectral properties are not. The X-ray emission displays a markedly different behavior, with frequent flares on timescales of hours to days, and will be presented in a companion publication.

The Destruction and Recreation of the X-Ray Corona in a Changing-look Active Galactic Nucleus

“…The very soft X-ray emission observed after the event, and the fact that the optical/UV flux decreases following a trend consistent with the t −5/3 behavior (Trakhtenbrot et al 2019) are in agreement with the TDE scenario (e.g., Komossa 2015;Auchettl et al 2017), while the re-brightening observed in the X-rays is very different from what is typically observed in TDEs. Interestingly, the behavior seen in 1ES 1927+654 might be consistent with recent hydrodynamic simulations of a debris stream colliding with an accretion disk (Chan et al 2019).…”

“…1ES 1927+654 is a type-2 AGN that was recently found to develop broad optical emission lines after an optical outburst (23 December 2017; Trakhtenbrot et al 2019). Following the changing-look event, we started an intense 450-day X-ray monitoring campaign, which includes observations from NICER (265 pointings), Swift (14) and XMM-Newton/NuSTAR (3).…”

“…We were recently able to observe, for the first time, an AGN in the act of changing phase. 1ES 1927+654 (z = 0.019422, Trakhtenbrot et al 2019) was previously classified as a type 2 AGN (e.g., Gallo et al 2013 and references therein). In March 2018 the All-Sky Automated Survey for SuperNovae (Shappee et al 2014) showed that the source suddenly increased in flux by an order of magnitude in the optical V-band (ASASSN-18el/AT2018zf, Nicholls et al 2018).…”

“…Panel A: 0.5-10 keV light curve, including NICER (cyan circles), Swift/XRT (red squares) and XMM-Newton/NuSTAR (yellow diamonds) data. Our first X-ray observation (May 17 2018) was carried out ∼ 150 days after the detection of the outburst in the optical (December 23 2017; Trakhtenbrot et al 2019). The 0.5-10 keV luminosity before the event, inferred by the 2011 XMM-Newton observation (Gallo et al 2013), was 8.3 × 10 42 erg s −1 (horizontal green dot-dashed line).…”

“…Panel C: ratio between the power-law and blackbody flux in the 0.5-2 keV band. Panel D: optical/UV lightcurve (Trakhtenbrot et al 2019), showing a very different behavior with respect to the X-ray band. The time of the event (t 0 ) adopted for the t −5/3 curve (black dashed line) is December 23 2017.…”