[Paper Review] The largest Swift AGN monitoring campaign: UV/optical variability in NGC 5548

Stephen Connolly, I. M. McHardy+8 more|arXiv (Cornell University)|Feb 26, 2015

Astrophysical Phenomena and Observations26 references3 citations

TL;DR

This study presents the largest Swift AGN monitoring campaign to date, analyzing 554 observations of NGC 5548 over 750 days to investigate UV/optical variability. It finds that UV/optical variations lag X-rays with a wavelength-dependent lag scaling as λ^1.23, consistent with X-ray reprocessing by the accretion disc, but the observed lags are ~3 times longer than predicted by the standard Shakura-Sunyaev disc model, suggesting a need for revised disc parameters or a clumpy, hotter, or larger disc structure.

ABSTRACT

We report on the largest Swift AGN monitoring program, concerning UV/optical variability in Seyferts. From 554 observations, over a 750d period, of the Seyfert galaxy NGC 5548, we see (McHardy et al. 2014) a good overall correlation between the X-ray and UV/optical bands,particularly on short timescales (tens of days). The UVOT bands are found to lag behind X-rays with a lag scaling as wavelength to the power 1.23 +/- 0.31, in excellent agreement with that expected (1.33) if UV/optical variability arises from reprocessing of X-rays by the accretion disc. However, the observed lags are ~3 times longer than expected from a standard Shakura-Sunyaev disc, raising real concerns about the detailed validity of this model. The results can be explained with a slightly larger mass and accretion rate, and a hotter disc, or if the disc is clumpy, thereby enhancing the emission from the outer regions.

Motivation & Objective

• To measure the X-ray to UV/optical variability lag in NGC 5548 with high temporal resolution using simultaneous Swift XRT and UVOT data.
• To test whether UV/optical variability arises from reprocessing of X-ray emission by the accretion disc, as predicted by reprocessing models.
• To determine whether the observed lags are consistent with the standard Shakura-Sunyaev disc model or require modifications such as higher mass, accretion rate, or disc clumpiness.
• To investigate the origin of long-timescale UV/optical variability not correlated with X-ray variations.

Proposed method

• Conducted 554 simultaneous X-ray (XRT) and UV/optical (UVOT) observations of NGC 5548 over 750 days, with a median cadence of ~2 days.
• Used discrete cross-correlation function (DCF) and interpolation cross-correlation function (ICCF) to measure time lags between X-ray and UVOT light curves across multiple bands.
• Applied a reprocessing model based on the lamppost geometry, assuming X-ray heating from a source above the disc, with parameters including black hole mass, accretion rate, X-ray luminosity, and source height H.
• Modelled the disc using X-ray and gravitational heating, incorporating disc albedo, inner radius R_in, and height H, to predict reprocessing lags.
• Compared observed lags with predictions from the standard disc model and explored parameter space (e.g., M_BH, ṁ_E, H, R_in) to reconcile observations with theory.
• Investigated alternative explanations such as disc clumpiness, larger disc size, or viscous propagation in the disc or corona for long-term variability.

Experimental results

Research questions

• RQ1 What is the time lag between X-ray and UV/optical variability in NGC 5548, and how does it scale with wavelength?
• RQ2 Is the observed UV/optical variability in NGC 5548 primarily driven by reprocessing of X-ray emission by the accretion disc?
• RQ3 Are the observed lags consistent with predictions from the standard Shakura-Sunyaev accretion disc model?
• RQ4 What physical mechanisms can explain the discrepancy between observed lags and standard model predictions?
• RQ5 What causes the long-term UV/optical variability that is not mirrored in X-ray light curves?

Key findings

• The X-ray to UVW2 lag is measured at 1.23 ± 0.31 days per decade of wavelength, consistent with a λ^1.23 scaling, strongly supporting X-ray reprocessing as the origin of short-timescale UV/optical variability.
• The observed lags are approximately three times longer than predicted by the standard Shakura-Sunyaev disc model, indicating a significant discrepancy with theoretical expectations.
• To reconcile observations with the standard model, the black hole mass must be increased to ~10^8 M⊙, the accretion rate to 0.06 of Eddington, and the X-ray source height to 20 Rg, or the disc must be clumpy and hotter.
• The long-term UV/optical rise from day 6470 to 6547, not mirrored in X-rays, cannot be explained by viscous propagation from large radii (timescales ~10 years), suggesting alternative mechanisms like a corona or inner disc perturbation.
• The dominance of X-ray leading UV/optical variability, despite potential seed photon fluctuations, is explained by the large solid angle of the disc and photon multiplication during reprocessing, which amplifies X-ray-driven variability.
• Microlensing observations supporting a larger disc size further suggest that the standard disc model is inadequate, reinforcing the need for modified or inhomogeneous disc structures.