Multiwavelength monitoring and reverberation mapping of a changing look event in the Seyfert galaxy NGC 3516

Abstract

We present the results of photometric and spectroscopic monitoring campaigns of the changing look AGN NGC 3516 carried out in 2018 to 2020 covering the wavelength range from the X-ray to the optical. The facilities included the telescopes of the CMO SAI MSU, the 2.3-m WIRO telescope, and the XRT and UVOT of Swift. We found that NGC 3516 brightened to a high state and could be classified as Sy1.5 during the late spring of 2020. We have measured time delays in the responses of the Balmer and He ii λ4686 lines to continuum variations. In the case of the best-characterized broad H β line, the delay to continuum variability is about 17 d in the blue wing and is clearly shorter, 9 d, in the red, which is suggestive of inflow. As the broad lines strengthened, the blue side came to dominate the Balmer lines, resulting in very asymmetric profiles with blueshifted peaks during this high state. During the outburst the X-ray flux reached its maximum on 2020 April 1 and it was the highest value ever observed for NGC 3516 by the Swift observatory. The X-ray hard photon index became softer, ∼1.8 in the maximum on 2020 April 21 compared to the mean ∼0.7 during earlier epochs before 2020. We have found that the UV and optical variations correlated well (with a small time delay of 1–2 d) with the X-ray until the beginning of 2020 April, but later, until the end of 2020 June, these variations were not correlated. We suggest that this fact may be a consequence of partial obscuration by Compton-thick clouds crossing the line of sight.

Coevolution of Broad Emission Lines and X-Ray Spectrum in Changing-look AGNs

“…A change in the broad Hβ line profile was found in NGC 3516 (Shapovalova et al 2019). Oknyansky et al (2021) suggest that inflowing BLR clouds exist based on a study of the double-peaked Hβ lines with the reverberation mapping method. In the future, monitoring the broad lines during the changing-look phase of AGNs will further shed light on the physics of broad lines in AGNs.…”

“…Before 2007, the source is observed with BELs. Then, the source transited to the faint state with only NELs during 2014-2019, and returned to the bright state in 2020 and BELs appear again (Oknyansky et al 2021). As shown in the top right panel of Figure 1, seven observations with BELs follow a tight positive correlation of the X-ray photon index and Eddington-scaled X-ray luminosity of Γ = (1.21 ± 0.08) log (L 2-10 keV /L Edd ) + (5.22 ± 0.21) (r = 0.96 with p < 10 −3 ).…”

Searching for changing-state AGNs in massive datasets -- I: applying deep learning and anomaly detection techniques to find AGNs with anomalous variability behaviours

“…Finally, there are nine candidates with atypical spectra (ZTF DR5 IDs: 427114100001870, 474105300001176, 526115100009333, 578114100000128, 616105200002737, 626104200000260, 664111300002498, 789103200005093, and 818114300000932), with two of them (ZTF DR5 IDs: 626104200000260, and 664111300002498) showing very asymmetric broad line profiles, similar to those observed in NGC 3516, a known CSAGN (Oknyansky et al 2021).…”

Changing looks of the nucleus of the Seyfert galaxy NGC 1566 compared with other changing‐look AGNs

“…That can explain variations of obscuration. The H α /H β is higher in low states (see e.g. Shapovalova et al 2010 and Paper II). Variability of coronal lines might be common property for CL AGNs. CL AGNs have low accretion rate ≪1% Edd in minimum and few % in maximum. The significant strengthening of the UV Balmer continuum might be common for CL AGNs since they were noted in NGC 2617 (Shappee et al 2014), NGC 3516 (Oknyansky et al 2021) and some other CL AGNs.…”

“…However, such a strong correlation of the X‐ray and UV/Opt is not common among AGNs (see e.g., Buisson et al 2017; G. S. Edelson et al 2000), there are exceptions for some other CL AGNs, viz. NGC 2617, NGC 4151, NGC 3516, and NGC 5548 (see e.g., R. Edelson et al 2017; McHardy et al 2014; Oknyansky et al 2021, 2017; Shappee et al 2014). The UV/Optical variations are typically delayed relative to the X‐ray fluctuations by about a few days.…”