Reverberation mapping of active galactic nuclei: from X-ray corona to dusty torus

Abstract

The central engines of active galactic nuclei (AGNs) are powered by accreting supermassive black holes, and while AGNs are known to play an important role in galaxy evolution, the key physical processes occur on scales that are too small to be resolved spatially (aside from a few exceptional cases). Reverberation mapping is a powerful technique that overcomes this limitation by using echoes of light to determine the geometry and kinematics of the central regions. Variable ionizing radiation from close to the black hole drives correlated variability in surrounding gas/dust but with a time delay due to the light travel time between the regions, allowing reverberation mapping to effectively replace spatial resolution with time resolution. Reverberation mapping is used to measure black hole masses and to probe the innermost X-ray emitting region, the UV/optical accretion disk, the broad emission line region, and the dusty torus. In this article, we provide an overview of the technique and its varied applications.

Keywords: Astrophysics methods; Extragalactic astrophysics; Observational astronomy.

Graphical abstract

Figure 1

A cross-sectional schematic of an AGN The schematic highlights the main components and the four types of reverberation discussed in this review: X-ray reverberation (Section 2), optical/UV continuum reverberation (Section 3), broad line region reverberation (Section 4), and dust reverberation (Section 5) with general radial scales from the black hole indicated by labels.

Figure 2

A demonstration of X-ray lags at different frequencies Left: A portion of a simulated (noiseless) hard band (1–4 keV; red) and soft band (0.3–1 keV; black) light curve based on the properties of 1H0707-495, displaying variability on a range of timescales. On long timescales (low temporal frequencies), the hard band lags the soft, but on short timescales (high temporal frequencies), the soft band lags the soft. This demonstrates the benefit of studying X-ray reverberation (soft lags) using frequency-resolved timing analysis. Right: The observed lag-frequency spectrum of Seyfert galaxy 1H0707-495 (Zoghbi et al., 2010), showing the hard lags (positive lags) at low frequencies and the soft lags (negative lags) at high frequencies. Figure courtesy of Abdu Zoghbi.

Figure 3

Reflection features in the energy spectrum and the lag spectrum Left: the NuSTAR spectrum of SWIFT J2127.4 + 5654, with a power law model of the continuum divided out. The ratio plot shows the broad Fe K line and Compton hump (adapted from Marinucci et al., 2014). Right: The observed high-frequency time lags from PG 1244 + 026 (Kara et al., 2014) fitted with an iron K reverberation model generated from a general relativistic ray-tracing simulation of a lamppost corona irradiating a razor-thin disk (Cackett et al., 2014).

Figure 4

Iron K lags in a sample of Seyfert AGNs (left), and the geometry and corresponding response functions for a corona with different heights (right) Left: The iron K lag amplitude vs. black hole mass for a sample of Seyfert AGNs (updated from Kara et al. (2016)). The spread is likely in part due to uncertainties in black hole mass (not plotted) and also due to intrinsic differences in corona/disk geometry between different AGN Right: Alston et al. (2020) showed that reverberation can measure variable corona/disk geometries in a single AGN. The top image shows a simple schematic of the geometry of the model, and the bottom image shows the corresponding impulse response functions for the two different height coronae.

Figure 5

Three of the continuum light curves of NGC 5548 from the AGN STORM campaign The light curves are (A) 1367Å from HST, (B) UVW1 (2600Å) from Swift, and (C) i (7648Å) from ground-based telescopes. The light curves are all highly correlated. Vertical dotted lines are plotted to guide the eye. The right-hand image shows the cross-correlation function with respect to the 1367Å light curve for the UVW1 (blue, solid line) and i (orange, dashed line) bands, with the correlation peaking at a lag of around 0.9 days for UVW1 and 4 days for i. Data from Fausnaugh et al. (2016).

Figure 6

Wavelength-dependent continuum lags in NGC 5548 (Fausnaugh et al., 2016), NGC 4593 (Cackett et al., 2018), and Mrk 142 (Cackett et al., 2020) Data from Swift (black circles), HST (red squares), and ground-based observatories (blue triangles). The solid lines show the best-fitting $\tau \propto {\lambda }^{4/3}$ relation (excluding the X-rays and $u/U$ bands), while the dashed lines show the predicted lags based on reasonable estimates of the black hole mass and accretion rate (Equation 12 in Fausnaugh et al., 2016)). The difference between the observed (solid line) and predicted (dashed line) relations is the “accretion disk size problem”. The lags in the $u/U$ bands (around 3500Å) show a clear excess, and in NGC 4593, the lag spectrum reveals a broad excess around the Balmer jump. Finally, note also how there is no consistent relationship between the X-rays and the best-fitting ${\lambda }^{4/3}$ relation.

Figure 7

Model wavelength-dependent lags from the BLR diffuse continuum The lags are measured relative to three different lag-less driving continuum light curves. Note lags are expected at all wavelengths but especially prominent around the Balmer and Paschen jumps. Taken from (Korista and Goad, 2019).

Figure 8

A recent example of BLR reverberation showing spectra (left) and light curves (right) Left: Mean and root-mean-square (rms) optical spectra of NGC 3783 collected over the course of 4 months in early 2020. The rms spectrum shows the spectral components that were variable during the observing campaign. Right: Light curves and cross-correlation functions for the continuum and the strong broad emission lines. From Bentz et al. (2021).

Figure 9

The $R_{\text{BLR}}-L_{\text{AGN}}$ relationship for all AGNs with Hβ reverberation lags The black points show local Seyferts, while the blue point shows a sample of rapidly accreting AGNs and the red points show a sample of AGNs at higher redshift. From Fonseca Alvarez et al., 2020.

Figure 10

A comparison of dust and BLR radii from Koshida et al. (2014) Red circles indicate K-band dust reverberation radii from Koshida et al. (2014), purple open squares indicate K-band interferometric radii (Kishimoto et al., 2011; Weigelt et al., 2012), green dots at dust radii from SED fitting (Mor and Netzer, 2012), and blue error bars indicate BLR reverberation radii from Bentz et al. (2009). Dust radii are approximately a factor of 4 larger than the BLR radii.