The Relative Wavelength Independence of IR Time Lags in NGC 4151 during the Years 2010–2015

Abstract

We present results of a study of the correlation between the infrared (JHKL) and optical (B) fluxes of the nucleus of the Seyfert galaxy NGC 4151 for the years 2010 -2015 using our own data (partially published) in combination with published data of Roberts and Rumstey (2012), Guo et al. (2014) and Schnülle et al. (2013Schnülle et al. ( , 2015.We find similar lags for each of the HKL passbands relative to the optical of 37 ± 3 days. The lags are the same to within the accuracy of measurement. We do not confirm a significant decrease in the lag for HKL in 2013-2014 previously reported by Schnülle et al. (2015), but we find that the lag of the short-lag component of J increased. We discuss our results within the framework of the standard model, where the variable infrared radiation is mainly due to the thermal reemission of short-wave radiation by dust clouds close to a variable central source. There is also some contribution to the IR emission from the accretion disk, and this contribution increases with decreasing wavelength. The variability in J and K is not entirely simultaneous, which may be due to the differing contributions of the radiation from the accretion disk in these bands. The absence of strong wavelength-dependent changes in infrared lag across the HKL passbands can be explained by having the dust clouds during 2010-2015 be located beyond the sublimation radius.The relative wavelength independence of the infrared lags is also consistent with the hollow bi-conical outflow model of Oknyansky et al. (2015).

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

“…These two leading possibilities -intrinsic variability and variable obscuration -do not need to be mutually exclusive. For example, a dramatic change in the energy generation could cause the sublimation of dust in clouds near the central source energy source (Oknyansky et al, 2017;Oknyansky, Shenavrin, Metlova, & Gaskell, 2019). If such sublimation occurs in some clouds along the line of sight, then the rise in the UV can be explained in part with the change in obscuration.…”

“…These investigations of NGC 1566 have long history (see e.g. references at da Silva, Steiner, & Menezes (2017); Oknyansky, Winkler, et al (2019); Oknyansky et al (2020); Parker et al (2019)). The NGC1566 is one of the brightest and nearest galaxies with AGN in the South Hemisphere.…”

“…The NGC1566 is one of the brightest and nearest galaxies with AGN in the South Hemisphere. This object is also nearest Changing Look (CL) † Changing looks of the nucleus of the Seyfert galaxy NGC 1566 AGN (Oknyansky et al, 2018;Oknyansky, Winkler, et al, 2019). It is one of two (also NGC3516) firstly discovered (soon after discovery of variability of AGNs in continuum by Fitch, Pacholczyk, & Weymann (1967)) CL AGNs (Andrillat & Souffrin, 1968;Pastoriza & Gerola, 1970), however this designation came into common use only in the past decade.…”

“…NGC 1566 is a galaxy with a very well-studied variable active nucleus, however the spectral and photometric data have big gaps. The most intensive multiwave photometic observations were done during past years after discovery of a new reawakening (Cutri, Mainzer, Dyk, & Jiang, 2018;Ferrigno et al, 2018;Grupe, Komossa, & Schartel, 2018;Kuin et al, 2018;Parker et al, 2019) and new CL phase (see Oknyansky et al (2018); Oknyansky, Winkler, et al (2019); Oknyansky et al (2020) and references there). During these few years 2 CLs were discovered in the object: first happened in 2018 when object changed from Sy1.9 to Sy1.2 (Oknyansky, Winkler, et al (2019) = Paper I) and second one in 2019 when object changed back to the low state (Oknyansky et al (2020) = Paper II).…”

“…The most intensive multiwave photometic observations were done during past years after discovery of a new reawakening (Cutri, Mainzer, Dyk, & Jiang, 2018;Ferrigno et al, 2018;Grupe, Komossa, & Schartel, 2018;Kuin et al, 2018;Parker et al, 2019) and new CL phase (see Oknyansky et al (2018); Oknyansky, Winkler, et al (2019); Oknyansky et al (2020) and references there). During these few years 2 CLs were discovered in the object: first happened in 2018 when object changed from Sy1.9 to Sy1.2 (Oknyansky, Winkler, et al (2019) = Paper I) and second one in 2019 when object changed back to the low state (Oknyansky et al (2020) = Paper II). This was happened about 60 years after previous CLs (Pastoriza & Gerola, 1970;Quintana et al, 1975a), but, taking into account too long gaps in the spectral coverage, some other possible CLs could well have been missed.…”