A Possible ∼20 yr Periodicity in Long-term Optical Photometric and Spectral Variations of the Nearby Radio-quiet Active Galactic Nucleus Ark 120

Abstract

We study the long-term variability in the optical monitoring database of Ark 120, a nearby radio-quiet active galactic nucleus (AGN) at a distance of 143 Mpc (z = 0.03271). We compiled the historical archival photometric and spectroscopic data since 1974 and conducted a new two-year monitoring campaign in 2015-2017, resulting in a total temporal baseline over four decades. The long-term variations in the optical continuum exhibit a wave-like pattern and the Hβ integrated flux series varies with a similar behaviour. The broad Hβ profiles have asymmetric double peaks, which change strongly with time and tend to merge into a single peak during some epochs. The period in the optical continuum determined from various period-search methods is about 20 years and the estimated false alarm probability with null hypothesis simulations is about 1 × 10 −3 . The overall variations of the broad Hβ profiles also follow the same period. However, the present database only covers two cycles of the suggested period, which strongly encourages continued monitoring to track more cycles and confirm the periodicity. Nevertheless, in light of the possible periodicity and the complicated Hβ profile, Ark 120 is one candidate of the nearest radio-quiet AGNs with possible periodic variability and it is thereby a potential candidate host for a sub-parsec supermassive black hole binary. (15) the All-Sky Automated Survey for Supernovae database (Shappee et al. 2014; Kochanek et al. 2017). a The flux of [O III] λ5007 used for absolute calibration of spectra in the dataset. b The photometric data of all the datasets are at V -band, except for dataset M79 at B-band.c This dataset has no temporal overlap with the other datasets so that it is impossible to do intercalibration. We simply adjust the fluxes to make the mean and standard deviation to be at the same scales as those of the other datasets. We do not use this dataset for periodicity analysis.d This dataset includes 15 additional observations (50,863-52,282) that have not been public.e There is a large flux difference with respect to the other datasets. We offset the fluxes by −20.0 × 10 −15 erg s −1 cm −2 Å −1 before performing intercalibration.f This dataset has a poor temporal overlap with the other datasets and we simply fix ϕ = 1.

On the Time Scales of Optical Variability of AGN and the Shape of Their Optical Emission Line Profiles

“…Possible interpretations of periodicities are discussed in many works [7,[9][10][11]30,31,35,36,50,51,54,70]. Assuming circular orbits in the disk as we did here, we suggest that possible source of periodicity should be in the AD, amplifying an emission contribution at that radius.…”

“…Unfortunately, it is hard to find cases of AGN light curves with several repeating patterns [4,5,9,10,29,30,36], needed for the clear detection of periodicity, above the red noise level, since the length of current AGN monitoring campaigns are of the order of orbiting time scales. Therefore, as an indicator of orbiting effects we could be tracing the broad emission line shifts, and if the periodicity in radial velocity curves is the same, it could indicate that the mechanism which drives both curves could be linked to the orbiting within the broad line region [7,[9][10][11]31,37]. Radial velocity curves are harder to obtain due to even shorter records of spectral observations, and therefore only a few candidates are detected, such as NGC 4151 with a 15.9 year periodicity [6,7], NGC 5548 with a ≈15 year periodicity [9,10], and Ark 120 with a ≈20 year periodicity [11].…”

“…Therefore, as an indicator of orbiting effects we could be tracing the broad emission line shifts, and if the periodicity in radial velocity curves is the same, it could indicate that the mechanism which drives both curves could be linked to the orbiting within the broad line region [7,[9][10][11]31,37]. Radial velocity curves are harder to obtain due to even shorter records of spectral observations, and therefore only a few candidates are detected, such as NGC 4151 with a 15.9 year periodicity [6,7], NGC 5548 with a ≈15 year periodicity [9,10], and Ark 120 with a ≈20 year periodicity [11]. It is interesting that current results of all these cases show that the radial velocity curves of the red side of broad emission lines show larger amplitude shift with more significant indication of periodicity, as in case of NGC 3516 [38], where the same effect is observed in red wing of Fe Kα line.…”

“…Unfortunately, most AGN monitoring campaign time intervals are not sufficiently long for detecting periodicity in optical domain of spectra, since expected orbiting time scales in AGN ADs that would affect the optical part of the spectrum are of the order of years and decades. Luckily, significant periodicities are detected in several AGN which are extensively monitored for sufficiently long time, such as OJ287 [4,5], NGC 4151 [6][7][8], NGC 5548 [9,10], Ark 120 [11], 3c273 [12,13].…”

Spectral energy distributions of candidate periodically variable quasars: testing the binary black hole hypothesis

“…The optical flux periodicity may be caused by accretion rate changes due to the intrinsic binary orbital tidal torque modulation (e.g., MacFadyen & Milosavljević 2008;Shi et al 2012;Roedig et al 2012;D'Orazio et al 2013;Farris et al 2014;Tang et al 2018), and/or the apparent Doppler boost modulation from the highly relativistic motion of gas in the mini accretion disk around the secondary BH (e.g., D'Orazio et al 2015b;Charisi et al 2018). While 100 quasars with candidate periodicity have been proposed as evidence for BS-BHs (e.g., Valtonen et al 2008;Graham et al 2015a,b;Liu et al 2015Liu et al , 2016Bon et al 2016;Charisi et al 2016;Zheng et al 2016;Li et al 2019), even the strongest candidate periodicity has been shown to be subject to false positives due to stochastic quasar variability given the uneven sampling, limited time baseline, and/or relatively low sensitivity. For example, the blazar OJ 287 has been suggested to host a BSBH based on the evidence for a 12-year periodicity in the optical and radio light curves (e.g., Sillanpaa et al 1996;Valtaoja et al 2000;Valtonen et al 2016), where the double-peaked flares have been interpreted as the result of a secondary BH punching through the accretion disk of the primary (e.g., Takalo 1994;Valtonen et al 2008) or accretion disk precession driven by the gravitational torque of a companion BH (e.g., Katz 1997).…”