Swift Monitoring of NGC 4151: Evidence for a Second X-Ray/UV Reprocessing

Abstract

Swift monitoring of NGC 4151 with an∼6hr sampling over a total of 69 days in early 2016 is used to construct light curves covering five bands in the X-rays (0.3-50keV) and six in the ultraviolet (UV)/optical (1900-5500Å). The three hardest X-ray bands (>2.5keV) are all strongly correlated with no measurable interband lag,while the two softer bands show lower variability and weaker correlations. The UV/optical bands are significantly correlated with the X-rays, lagging ∼3-4days behind the hard X-rays. The variability within the UV/optical bands is also strongly correlated, with the UV appearing to lead the optical by ∼0.5-1days. This combination of 3day lags between the X-rays and UV and 1day lags within the UV/optical appears to rule out the "lamp-post" reprocessing model in which a hot, X-ray emitting corona directly illuminates the accretion disk, which then reprocesses the energy in the UV/optical. Instead, these results appear consistent with the Gardner & Done picture in which two separate reprocessings occur: first, emission from the corona illuminates an extreme-UV-emitting toroidal component that shields the disk from the corona; this then heats the extreme-UV component,which illuminates the disk and drives its variability.

Continuum Reverberation Mapping of the Accretion Disks in Two Seyfert 1 Galaxies

“…Although the X-ray light curve would be expected to be smoothed if reprocessed at UV wavelengths, comparison of Figures 3 and the Appendix shows that the inferred driving light curve does not correlate very well with the observed X-ray variations (this is confirmed by the ICCF analysis from Section 3). This poor correlation was also seen in 2013 by Shappee et al (2014), and has been observed in other objects, including NGC 5548 (Uttley et al 2003;Edelson et al 2015), MR 2251-178 (Arévalo et al 2008), Mrk 79 (Breedt et al 2009), NGC 3783 ), and NGC 4151 (Edelson et al 2017). Several of these studies have been unable to represent the UV/optical light curves as a reprocessed (smoothed and shifted) version of the X-ray light curve (Arévalo et al 2008;Breedt et al 2009;Starkey et al 2017), which is problematic for a generic disk reprocessing model.…”

“…However, microlensing can only probe the disks in distant, high-luminosity quasars, while RM provides a means of probing accretion disks in local, low-luminosity AGN. It is therefore imperative to expand the sample of objects with secure interband continuum lags, and several such programs have been completed, with others still in progress (e.g., NGC 4151, Edelson et al 2017;NGC 4593, McHardy et al 2016).…”

Accretion Disk Reverberation with Hubble Space Telescope Observations of NGC 4593: Evidence for Diffuse Continuum Lags

“…However, the data have often suffered from poor temporal sampling (given the short lags), leading to large uncertainties in the lag. Several recent observing campaigns have changed this (McHardy et al 2014;Shappee et al 2014;Edelson et al 2015;Fausnaugh et al 2016;Edelson et al 2017). In particular, the AGN STORM campaign on NGC5548 used a combination of monitoring with the Hubble Space Telescope (HST) and the Neil Gehrels Swift Observatory (Swift hereafter), obtaining 282 observations over 125 days, with a mean sampling rate of less than 0.5 days (Edelson et al 2015).…”

“…In 2016, a high-cadence Swift monitoring campaign of the bright AGN NGC 4151 took place (Edelson et al 2017), with a sampling rate of approximately 6 hr over a 69day period. Here, the X-ray/UV/optical light curves are all well correlated.…”

“…This disconnect between the X-ray-to-UV and UV-tooptical lags is even more strongly in conflict with the standard model. One possible explanation could be that an additional component shields the disk from the corona and reprocesses the energy on a longer (e.g., dynamical) timescale, increasing the lag between the X-rays and optical/UV (Edelson et al 2017;Gardner & Done 2017).…”