Circumnuclear Material Research Articles

The Fe Kα Compton shoulder in accreting supermassive black holes

ABSTRACT Reprocessed X-ray radiation from active galactic nuclei (AGN) carries important information about the properties of the circumnuclear material around the black hole. The X-ray photons travel from the very centre of the system and interact with that material often producing strong emission lines. The $\rm {Fe\, K\alpha }$ Compton shoulder (CS) is formed by fluorescent $\rm {Fe\, K\alpha }$ photons that perform Compton scatterings with the intercepting material and lose energy to form the distinct shoulder shape. In this work, we use the ray-tracing code RefleX to explore how the physical properties of the medium, as well as its geometry, affect the shape of the CS. We start by running simulations using a simple toroidal reflector, to test the effect of the metal composition, metallicity, column density, dust presence and velocity on the $\rm {Fe\, K\alpha }$ line, and its CS. We confirm that the shape of the CS is sensitive to the optical depth of the intercepting medium, which can be regulated by either changing the metal composition or the line-of-sight column density of the circumnuclear material. Next, we create a series of models, which feature different geometrical configurations of dust and gas, and explore how the CS is affected by such configurations finding that components that can regulate the line-of-sight column density affect the $\rm {Fe\, K\alpha }$ and its CS. Finally, we test whether observatories such as the recently launched XRISM and future Athena will make the CS a useful spectral feature of nearby AGN, by applying specific models on simulated spectra of the Circinus galaxy. The CS has the potential to be used to help constrain properties of the circumnuclear material yet with some limitations.

Multi-epoch X-ray spectral analysis of Centaurus A: Revealing new constraints on iron emission line origins

Abstract We conduct X-ray reverberation mapping and spectral analysis of the radio galaxy Centaurus A to uncover its central structure. We compare the light curve of the hard X-ray continuum from Swift Burst Alert Telescope observations with that of the Fe K$\alpha$ fluorescence line, derived from the Nuclear Spectroscopic Telescope Array (NuSTAR), Suzaku, XMM–Newton, and Swift X-ray Telescope observations. The analysis of the light curves suggests that a top-hat transfer function, commonly employed in reverberation mapping studies, is improbable. Instead, the relation between these light curves can be described by a transfer function featuring two components: one with a lag of $0.19_{- 0.02}^{+ 0.10} \,\, \mathrm{pc}/c$, and another originating at $r \\gt 1.7 \,\, \mathrm{pc}$ that produces an almost constant light curve. Further, we analyze the four-epoch NuSTAR and six-epoch Suzaku spectra, considering the time lag of the reflection component relative to the primary continuum. This spectral analysis supports that the reflecting material is Compton-thin, with $N_{\mathrm{H}} = 3.14_{-0.74}^{+0.44} \times 10^{23} \,\, \mathrm{cm}^{-2}$. These results suggest that the Fe K$\alpha$ emission may originate from Compton-thin circumnuclear material located at a sub-parsec scale, likely a dust torus, and materials at a greater distance.

O iv]- and [Ne v]-weak Active Galactic Nuclei Hidden by Compton-thick Material in Late Mergers

We study “buried” active galactic nuclei (AGNs) almost fully covered by circumnuclear material in ultra/luminous infrared galaxies (U/LIRGs), which show weak ionized lines from narrow-line regions. Employing an indicator of a [O iv] 25.89 or [Ne v] 14.32 μm line to 12 μm AGN luminosity ratio, we find 17 buried AGN candidates that are [O iv]-weak (L [O IV]/L 12,AGN ≤ −3.0) or [Ne v]-weak (L [Ne V]/L 12,AGN ≤ −3.4) among 30 AGNs in local U/LIRGs. For the [O iv]-weak AGNs, we estimate their covering fractions of Compton-thick (CT; N H ≥ 1024 cm−2) material with an X-ray clumpy torus model to be fCT(spec)=0.55±0.19 on average. This value is consistent with the fraction of CT AGNs ( fCT(stat)=53%±12% ) among the [O iv]-weak AGNs in U/LIRGs and much larger than that in Swift/Burst Alert Telescope (BAT) AGNs (23% ± 6%). The fraction of [O iv]-weak AGNs increases from 27−10+13% (early) to 66−12+10% (late mergers). Similar results are obtained with the [Ne v] line. The [O iv]- or [Ne v]-weak AGNs in late mergers show larger N H and Eddington ratios (λ Edd) than those of the Swift/BAT AGNs, and the largest N H is ≳1025 cm−2 at logλEdd∼−1 , close to the effective Eddington limit for CT material. These suggest that (1) the circumnuclear material in buried AGNs is regulated by the radiation force from high-λ Edd AGNs on the CT obscurers, and (2) their dense material with large fCT(spec) (∼0.5 ± 0.1) in U/LIRGs is a likely cause of a unique structure of buried AGNs, whose amount of material may be maintained through merger-induced supply from their host galaxies.

BASS. XLII. The Relation between the Covering Factor of Dusty Gas and the Eddington Ratio in Nearby Active Galactic Nuclei

Accreting supermassive black holes (SMBHs) located at the centers of galaxies are typically surrounded by large quantities of gas and dust. The structure and evolution of this circumnuclear material can be studied at different wavelengths, from the submillimeter to the X-ray. Recent X-ray studies have shown that the covering factor of the obscuring material tends to decrease with increasing Eddington ratio, likely due to radiative feedback on dusty gas. Here we study a sample of 549 nearby (z ≲ 0.1) hard X-ray (14–195 keV) selected nonblazar active galactic nuclei (AGN) and use the ratio between the AGN infrared and bolometric luminosity as a proxy of the covering factor. We find that, in agreement with what has been found by X-ray studies of the same sample, the covering factor decreases with increasing Eddington ratio. We also confirm previous findings that showed that obscured AGN typically have larger covering factors than unobscured sources. Finally, we find that the median covering factors of AGN located in different regions of the column density–Eddington ratio diagram are in good agreement with what would be expected from a radiation-regulated growth of SMBHs.

Torus skin outflow in a near-Eddington quasar revealed by spectropolarimetry

ABSTRACT Even when the direct view towards the active nucleus is obscured, nuclear emission propagating along other directions can scatter off surrounding material, become polarized and reach the observer. Spectropolarimetry can thus be an important tool in investigating the circumnuclear geometry and kinematics of quasars on scales that cannot yet be probed via direct observations. Here, we discuss an intriguing class of quasars where the polarization position angle swings by large amounts ($\sim 90\deg$) within an emission line. We investigate a kinematic model in which the scattering dust or electrons are in an axisymmetric outflow. We propagate Stokes parameters in a variety of geometries of emitter, scatterer, and observer. We use these models to predict polarization fraction, line profiles, and polarization position angles and compare them to observations. We demonstrate that the swinging polarization angle can be a result of the geometry of the outflow and the orientation of the observer. Polarization properties of a near-Eddington extremely red quasar SDSS J1652 can be successfully explained by a model in which the quasar is surrounded by a geometrically thick disc, whose ‘skin’ is outflowing at ∼1000 km s−1 and acts as the scatterer on scales of a few tens of pc. The line of sight to the observer in this source is within or close to the skin of the torus, in agreement with multiwavelength data. Spectropolarimetric data and models presented here strongly support the thick-disc geometry of circumnuclear material suggested by recent numerical simulations of high-rate accretion flows on to black holes.

Multi-epoch hard X-ray view of Compton-thick AGN Circinus Galaxy

ABSTRACT The circumnuclear material around active galactic nuclei (AGNs) is one of the essential components of the obscuration-based unification model. However, our understanding of the circumnuclear material in terms of its geometrical shape, structure, and its dependence on accretion rate is still debated. In this paper, we present the multi-epoch broad-band X-ray spectral modelling of a nearby Compton-thick AGN in Circinus galaxy. We utilize all the available hard X-ray (>10 keV) observations taken from different telescopes, i.e. BeppoSAX, Suzaku, NuSTAR, and AstroSat, at 10 different epochs across 22 yr from 1998 to 2020. The 3.0–79 keV broad-band X-ray spectral modelling using physically motivated models, namely MYTorus, borus02, and uxclumpy, infers the presence of a torus with a low covering factor of 0.28, an inclination angle of 77°–81° and Compton-thick line-of-sight column densities (NH,LOS = 4.13–9.26 × 1024 cm−2) in all the epochs. The joint multi-epoch spectral modelling suggests that the overall structure of the torus is likely to remain unchanged. However, we find tentative evidence for the variable line-of-sight column density on time-scales ranging from 1 d to 1 week to a few years, suggesting a clumpy circumnuclear material located at subparsec to tens of parsec scales.

Host Dark Matter Halos of Wide-field Infrared Survey Explorer-selected Obscured and Unobscured Quasars: Evidence for Evolution

Obscuration in quasars may arise from steep viewing angles along the dusty torus, or instead may represent a distinct phase of supermassive black hole growth. We test these scenarios by probing the host dark matter halo environments of ∼1.4 million Wide-field Infrared Survey Explorer-selected obscured and unobscured quasars at 〈z〉 = 1.4 using angular clustering measurements as well as cross-correlation measurements of quasar positions with the gravitational lensing of the cosmic microwave background. We interpret these signals within a halo occupation distribution framework to conclude that obscured systems reside in more massive effective halos (∼1012.9 h −1 M ⊙) than their unobscured counterparts (∼1012.6 h −1 M ⊙), though we do not detect a difference in the satellite fraction. We find excellent agreement between the clustering and lensing analyses and show that this implies the observed difference is robust to uncertainties in the obscured quasar redshift distribution, highlighting the power of combining angular clustering and weak lensing measurements. This finding appears in tension with models that ascribe obscuration exclusively to orientation of the dusty torus along the line of sight, and instead may be consistent with the notion that some obscured quasars are attenuated by galaxy-scale or circumnuclear material during an evolutionary phase.

Probing the Structure and Evolution of BASS Active Galactic Nuclei through Eddington Ratios

We constrain the intrinsic Eddington ratio (λ Edd) distribution function for local active galactic nuclei (AGN) in bins of low and high obscuration and ], using the Swift Burst Alert Telescope 70 month/BASS DR2 survey. We interpret the fraction of obscured AGN in terms of circumnuclear geometry and temporal evolution. Specifically, at low Eddington ratios (−2), obscured AGN outnumber unobscured ones by a factor of ∼4, reflecting the covering factor of the circumnuclear material (0.8, or a torus opening angle of ∼34°). At high Eddington ratios (−1), the trend is reversed, with <30% of AGN having , which we suggest is mainly due to the small fraction of time spent in a highly obscured state. Considering the Eddington ratio distribution function of narrow-line and broad-line AGN from our prior work, we see a qualitatively similar picture. To disentangle temporal and geometric effects at high λ Edd, we explore plausible clearing scenarios such that the time-weighted covering factors agree with the observed population ratio. We find that the low fraction of obscured AGN at high λ Edd is primarily due to the fact that the covering factor drops very rapidly, with more than half the time spent with <10% covering factor. We also find that nearly all obscured AGN at high-λ Edd exhibit some broad lines. We suggest that this is because the height of the depleted torus falls below the height of the broad-line region, making the latter visible from all lines of sight.

Supermassive black holes at high redshift are expected to be obscured by their massive host galaxies’ interstellar medium

We combine results from deep ALMA observations of massive (M* &gt; 1010 M⊙) galaxies at different redshifts to show that the column density of their interstellar medium (ISM) rapidly increases toward early cosmic epochs. Our analysis includes objects from the ASPECS and ALPINE large programs, as well as individual observations ofz ∼ 6 quasar hosts. When accounting for non-detections and correcting for selection effects, we find that the median surface density of the ISM of the massive galaxy population evolves as ∼(1 + z)3.3. This means that the ISM column density toward the nucleus of az &gt; 3 galaxy is typically &gt; 100 times larger than locally, and it may reach values as high as Compton-thick atz ≳ 6. Remarkably, the median ISM column density is on the same order of what is measured from X-ray observations of large active galactic nucleus (AGN) samples already atz ≳ 2. We have developed a simple analytic model for the spatial distribution of ISM clouds within galaxies, and estimate the total covering factor toward active nuclei when obscuration by ISM clouds on the host scale is added to that of parsec-scale circumnuclear material (the so-called torus). The model includes clouds with a distribution of sizes, masses, and surface densities, and also allows for an evolution of the characteristic cloud surface density with redshift, Σc, * ∝ (1 + z)γ. We show that, forγ = 2, such a model successfully reproduces the increase in the obscured AGN fraction with redshift that is commonly observed in deep X-ray surveys, both when different absorption thresholds and AGN luminosities are considered. Our results suggest that 80–90% of supermassive black holes in the early Universe (z &gt; 6 − 8) are hidden to our view, primarily by the ISM in their hosts. We finally discuss the implications of our results and how they can be tested observationally with current and forthcoming facilities (e.g., VLT, E-ELT, ALMA, and JWST) and with next-generation X-ray imaging satellites. By extrapolating the observed X-ray nebulae around local AGN to the environments of supermassive black holes at high redshifts, we find ≲1″ nebulae impose stringent design constraints on the spatial resolution of any future X-ray imaging Great Observatory in the coming decades.

Infrared Spectral Energy Distribution and Variability of Active Galactic Nuclei: Clues to the Structure of Circumnuclear Material

The active galactic nucleus (AGN) phenomenon results from a supermassive black hole accreting its surrounding gaseous and dusty material. The infrared (IR) regime provides most of the information to characterize the dusty structures that bridge from the galaxy to the black hole, providing clues to the black hole growth and host galaxy evolution. Over the past several decades, with the commissioning of various ground, airborne and space IR observing facilities, our interpretations of the AGN circumnuclear structures have advanced significantly through an improved understanding of how their dust emission changes as a function of wavelength and how the heating of the dusty structures responds to variations in the energy released from the central engine. In this review, we summarize the current observational knowledge of the AGN IR broad-band spectral energy distributions (SEDs) and the IR time variability behavior covering large ranges of AGN luminosity and redshift, and discuss some first-order insights into the obscuring structures and host galaxy IR properties that can be obtained by integrating the relevant observations into a coherent picture.

Are Delayed Radio Flares Common in Tidal Disruption Events? The Case of the TDE iPTF 16fnl

Abstract Radio emission from tidal disruption events (TDEs) originates from an interaction of an outflow with the super-massive black hole (SMBH) circumnuclear material (CNM). In turn, this radio emission can be used to probe properties of both the outflow launched at the event and the CNM. Until recently, radio emission was detected only for a relatively small number of events. While the observed radio emission pointed to either relativistic or sub-relativistic outflows of different nature, it also indicated that the outflow has been launched shortly after stellar disruption. Recently, however, delayed radio flares, several months and years after stellar disruption, were reported in the case of the TDE ASASSN-15oi. These delayed flares suggest a delay in the launching of outflows and thus may provide new insights into SMBH accretion physics. Here, we present a new radio data set of another TDE, iPTF 16fnl, and discuss the possibility that a delayed radio flare also has been observed in this case, ∼5 months after optical discovery, suggesting that this phenomenon may be common in TDEs. Unlike ASASSN-15oi, the data for iPTF 16fnl is sparse and the delayed radio flare can be explained by several alternative models: among them are a complex varying CNM density structure and a delayed outflow ejection.

Properties of the Obscuring Torus in NGC 1052 from Multiepoch Broadband X-Ray Spectroscopy

Abstract Obscuration of the innermost parts of active galactic nuclei (AGN) is observed in the majority of the population both in the nearby universe and at high redshift. However, the nature of the structures causing obscuration, especially in low-luminosity AGN, is poorly understood at present. We present a novel approach to multiepoch broadband X-ray spectroscopy, anchored in the long-term average spectrum in the hard X-ray band, applied to the nearby, X-ray-bright AGN in the galaxy NGC 1052. From spectral features due to X-ray reprocessing in the circumnuclear material, based on a simple, uniform-density torus X-ray reprocessing model, we find a covering factor of 80%–100% and a globally averaged column density in the range (1−2) × 1023 cm−2. This closely matches the independently measured variable line-of-sight column density range, leading to a straightforward and self-consistent picture of the obscuring torus in NGC 1052, similar to several other AGN in recent literature. Comparing this X-ray-constrained torus model with measurements of spatially resolved subparsec absorption from radio observations, we find that it may be possible to account for both X-ray and radio data with a torus model featuring a steep density gradient along the axis of the relativistic jets. This provides a valuable direction for the development of improved physical models for the circumnuclear environment in NGC 1052 and potentially in a wider class of AGN.

Caltech–NRAO Stripe 82 Survey (CNSS). III. The First Radio-discovered Tidal Disruption Event, CNSS J0019+00

Abstract We present the discovery of a nuclear transient with the Caltech–NRAO Stripe 82 Survey (CNSS), a dedicated radio transient survey carried out with the Karl G. Jansky Very Large Array (VLA). This transient, CNSS J001947.3+003527, exhibited a turn-on over a timescale of ≲1 yr, increasing in flux density at 3 GHz from &lt;0.14 mJy in 2014 February to 4.4 ± 0.1 mJy in 2015 March, reaching a peak luminosity of around 2015 October. The association of CNSS J0019+00 with the nucleus (Gaia and our very-long baseline interferometry positions are consistent to within 1 pc) of a nearby S0 Seyfert galaxy at 77 Mpc, together with the radio spectral evolution, implies that this transient is most likely a tidal disruption event (TDE). Our equipartition analysis indicates the presence of a ∼15,000 km s−1 outflow, having energy ∼1049 erg. We derive the radial density profile for the circumnuclear material in the host galaxy to be proportional to R −2.5. All of these properties suggest resemblance with radio-detected thermal TDEs like ASASSN-14li and XMMSL1 J0740-85. No significant X-ray or optical emission is detected from CNSS J0019+00, although this may simply be due to the thermal emission being weak during our late-time follow-up observations. From the CNSS survey we have obtained the first unbiased measurement of the rate of radio TDEs, R(&gt;500μJy) of about 2 × 10−3 deg−2, or equivalently a volumetric rate of about 10 Gpc−3 yr−1. This rate implies that all-sky radio surveys such as the VLA Sky Survey and those planned with ASKAP, will find many tens of radio TDEs over the next few years.

A Hard Look at Local, Optically Selected, Obscured Seyfert Galaxies*

Abstract We study the X-ray spectra of a sample of 19 obscured, optically selected Seyfert galaxies (Sy 1.8, 1.9, and 2) in the local universe (d ≤ 175 Mpc), drawn from the CfA Seyfert sample. Our analysis is driven by the high sensitivity of NuSTAR in the hard X-rays, coupled with soft X-ray spectra using XMM-Newton, Chandra, Suzaku, and Swift/XRT. We also analyze the optical spectra of these sources in order to obtain accurate mass estimates and Eddington fractions. We employ four different models to analyze the X-ray spectra of these sources, which all provide consistent results. We find that 79%–90% of the sources are heavily obscured with line-of-sight column density N H &gt; 1023 cm−2. We also find a Compton-thick (N H &gt; 1024 cm−2) fraction of 37%–53%. These results are consistent with previous estimates based on multiwavelength analyses. We find that the fraction of reprocessed to intrinsic emission is positively correlated with N H and negatively correlated with the intrinsic, unabsorbed X-ray luminosity (in agreement with the Iwasawa–Taniguchi effect). Our results support the hypothesis that radiation pressure regulates the distribution of the circumnuclear material.

Modelling broad emission lines in active galactic nuclei

Abstract Broad Emission Lines are the most characteristic features of Active Galaxies, but the mechanism of creating a medium able to emit these intense lines is not quite clear. Observations clearly indicate that the motion of the material is predominantly Keplerian, with traces of inflow and clear signatures of outflow, but this still does not point out whether the lines partially come from the disk surface, or exclusively from the circumnuclear material, and whether this material originates from the disk as a wind, or comes, at least partially, from outside. I review the basic scenarios for the formation of the Broad Line Region (BLR), and the recent progress in modelling the physical conditions in he emitting medium. The current state is the outer radius of the BLR is fixed by the dust sublimation temperature in the medium exposed to the irradiation from the central source, the inner radius is likely fixed by the dust sublimation temperature in the atmosphere of the non-illuminated accretion disk, and the local cloud density is a universal number fixed by the radiation pressure confinement. The time-dependent aspects of the BLR formation, however, still wait for serious modelling effort matching the quality of the observational data.

New Constraints on the Geometry and Kinematics of Matter Surrounding the Accretion Flow in X-Ray Binaries from Chandra High-energy Transmission Grating X-Ray Spectroscopy

Abstract The narrow, neutral Fe Kα fluorescence emission line in X-ray binaries (XRBs) is a powerful probe of the geometry, kinematics, and Fe abundance of matter around the accretion flow. In a recent study it has been claimed, using Chandra High-Energy Transmission Grating (HETG) spectra for a sample of XRBs, that the circumnuclear material is consistent with a solar-abundance, uniform, spherical distribution. It was also claimed that the Fe Kα line was unresolved in all cases by the HETG. However, these conclusions were based on ad hoc models that did not attempt to relate the global column density to the Fe Kα line emission. We revisit the sample and test a self-consistent model of a uniform, spherical X-ray reprocessor against HETG spectra from 56 observations of 14 Galactic XRBs. We find that the model is ruled out in 13/14 sources because a variable Fe abundance is required. In two sources a spherical distribution is viable, but with nonsolar Fe abundance. We also applied a solar-abundance Compton-thick reflection model, which can account for the spectra that are inconsistent with a spherical model, but spectra with a broader bandpass are required to better constrain model parameters. We also robustly measured the velocity width of the Fe Kα line and found FWHM values of up to ∼5000 km s−1. Only in some spectra was the Fe Kα line unresolved by the HETG.

Nuclear obscuration in active galactic nuclei

The material surrounding accreting supermassive black holes connects the active galactic nucleus with its host galaxy and, besides being responsible for feeding the black hole, provides important information on the feedback that nuclear activity produces on the galaxy. In this Review, we summarize our current understanding of the close environment of accreting supermassive black holes obtained from studies of local active galactic nuclei carried out in the infrared and X-ray regimes. The structure of this circumnuclear material is complex, clumpy and dynamic, and its covering factor depends on the accretion properties of the active galactic nucleus. In the infrared, this obscuring material is a transition zone between the broad- and narrow-line regions, and, at least in some galaxies, it consists of two structures: an equatorial disk/torus and a polar component. In the X-ray regime, the obscuration is produced by multiple absorbers across various spatial scales, mostly associated with the torus and the broad-line region. In the coming decade, the new generation of infrared and X-ray facilities will greatly contribute to our understanding of the structure and physical properties of nuclear obscuration in active galactic nuclei. The material surrounding accreting supermassive black holes connects them with their hosts. From studies in the infrared and X-rays, the structure of this material is found to be complex, clumpy and dynamic.

The close environments of accreting massive black holes are shaped by radiative feedback.

The majority of the accreting supermassive black holes in the Universe are obscured by large columns of gas and dust. The location and evolution of this obscuring material have been the subject of intense research in the past decades, and are still debated. A decrease in the covering factor of the circumnuclear material with increasing accretion rates has been found by studies across the electromagnetic spectrum. The origin of this trend may be driven by the increase in the inner radius of the obscuring material with incident luminosity, which arises from the sublimation of dust; by the gravitational potential of the black hole; by radiative feedback; or by the interplay between outflows and inflows. However, the lack of a large, unbiased and complete sample of accreting black holes, with reliable information on gas column density, luminosity and mass, has left the main physical mechanism that regulates obscuration unclear. Here we report a systematic multi-wavelength survey of hard-X-ray-selected black holes that reveals that radiative feedback on dusty gas is the main physical mechanism that regulates the distribution of the circumnuclear material. Our results imply that the bulk of the obscuring dust and gas is located within a few to tens of parsecs of the accreting supermassive black hole (within the sphere of influence of the black hole), and that it can be swept away even at low radiative output rates. The main physical driver of the differences between obscured and unobscured accreting black holes is therefore their mass-normalized accretion rate.

The NuSTAR View of the Seyfert 2 Galaxy NGC 4388

Abstract We present an analysis of NuSTAR X-ray observations in the 3–79 keV energy band of the Seyfert 2 galaxy NGC 4388, taken in 2013. The broadband sensitivity of NuSTAR, covering the Fe Kα line and Compton reflection hump, enables tight constraints to be placed on reflection features in AGN X-ray spectra, thereby providing insight into the geometry of the circumnuclear material. In this observation, we found the X-ray spectrum of NGC 4388 to be well described by a moderately absorbed power law with non-relativistic reflection. We fit the spectrum with phenomenological reflection models and a physical torus model and find the source to be absorbed by Compton-thin material (N H = (6.5 ± 0.8) × 1023 cm−2) with a very weak Compton reflection hump (R &lt; 0.09) and an exceptionally large Fe Kα line eV) for a source with weak or no reflection. Calculations using a thin-shell approximation for the expected Fe Kα EW indicate that an Fe Kα line originating from Compton-thin material presents a possible explanation.

Measuring light echoes in NGC 4051

Five archived X-ray observations of NGC 4051, taken using the NuSTAR observatory, have been analysed, revealing lags between flux variations in bands covering a wide range of X-ray photon energy. In all pairs of bands compared, the harder band consistently lags the softer band by at least 1000s, at temporal frequencies ~5E-5 Hz. In addition, soft-band lags up to 400s are measured at frequencies ~2E-4 Hz. Light echos from an excess of soft band emission in the inner accretion disk cannot explain the lags in these data, as they are seen in cross-correlations with energy bands where the softer band is expected to have no contribution from reflection. The basic properties of the time delays have been parameterised by fitting a top hat response function that varies with photon energy, taking fully into account the covariance between measured time lag values. The low-frequency hard-band lags and the transition to soft-band lags are consistent with time lags arising as reverberation delays from circumnuclear scattering of X-rays, although greater model complexity is required to explain the entire spectrum of lags. The scattered fraction increases with increasing photon energy as expected, and the scattered fraction is high, indicating the reprocessor to have a global covering fraction ~50% around the continuum source. Circumnuclear material, possibly associated with a disk wind at a few hundred gravitational radii from the primary X-ray source, may provide suitable reprocessing.