Relativistic Disk Research Articles

A Library of Synthetic X-Ray Spectra for Fitting Tidal Disruption Events

We present a tabulated version of our slim-disk model for fitting tidal disruption events (TDEs). We create a synthetic X-ray spectral library by ray-tracing stationary general relativistic slim disks and including gravitational redshift, Doppler, and lensing effects self-consistently. We introduce the library to reduce computational expense and increase access for fitting future events. Fitting requires interpolation between the library spectra; the interpolation error in the synthetic flux is generally <10% (it can rise to 40% when the disk is nearly edge-on). We fit the X-ray spectra of the TDEs ASASSN-14li and ASASSN-15oi, successfully reproducing our earlier constraints on black hole mass M • and spin a • from full on-the-fly ray-tracing. We use the library to fit mock observational data to explore the degeneracies among parameters, finding that (1) spectra from a hotter thermal disk and edge-on inclination angle offer tighter constraints on M • and a •; (2) the constraining power of spectra on M • and a • increases as a power law with the number of X-ray counts, and the index of the power law is higher for hotter thermal disk spectra; (3) multiepoch X-ray spectra partially break the degeneracy between M • and a •; (4) the time-dependent level of X-ray absorption can be constrained from spectral fitting. The tabulated model and slim-disk model are available at https://doi.org/10.25739/hfhz-xn60.

Estimating the Spin of the Black Hole Candidate MAXI J1659-152 with the X-Ray Continuum-fitting Method

Abstract As a transient X-ray binary, MAXI J1659-152 contains a black hole candidate as its compact star. MAXI J1659-152 was discovered on 2010 September 25 during its only known outburst. Previously published studies of this outburst indicate that MAXI J1659-152 may have an extreme retrograde spin, which, if confirmed, would provide an important clue as to the origin of black hole spin. In this paper, utilizing updated dynamical binary system parameters (i.e., the black hole mass, the orbital inclination, and the source distance) provided by Torres et al., we analyze 65 spectra of MAXI J1659-152 from RXTE/PCA, in order to assess the spin parameter. With a final selection of nine spectra matching our f sc ≲ 25%, soft state criteria, we apply a relativistic thin disk spectroscopic model kerrbb2 over 3.0–45.0 keV. We find that inclination angle correlates inversely with spin, and, considering the possible values for inclination angle, we constrain spin to be −1 &lt; a * ≲ 0.44 at a 90% confidence interval via X-ray continuum fitting. We can only rule out an extreme prograde (positive) spin. We confirm that an extreme retrograde solution is possible and is not ruled out by considering accretion torques given the young age of the system.

Time-resolved spectroscopy on the heartbeat state of GRS 1915+105 using AstroSat

ABSTRACT AstroSat spectra of the black hole system GRS 1915+105 during the heartbeat state (with a varying oscillation period from 150 to 100 s) were analysed using a truncated relativistic disc model along with a Comptonization component. Spectra were fitted for segments of length ∼24 s. The oscillation can be described as coordinated variations of the accretion rate, Comptonized flux, and the inner disc radius, with the latter ranging from 1.235 to 5 gravitational radii. Comparison with results from the χ and intermediate states shows that while the accretion rate and the high-energy photon index were similar, the inner disc radius and the fraction of Comptonized photons were larger for these states than for the heartbeat one. The coronal efficiency $\eta \equiv L_{ac}/\dot{M} c^2$, where Lac is the radiative luminosity generated in the corona, is found to be approximately ${\propto}\dot{M}^{-2/3}$ for all the observations. The efficiency decreases with inner radii for the heartbeat state but has similar values for the χ and intermediate states where the inner radii are larger. The implications of these results are discussed.

Broad-band spectral study of LMXB black hole candidate 4U 1957+11 with NuSTAR

We present here the results of broadband spectral analysis of a low-mass X-ray binary and a black hole candidate 4U 1957+11. The source was observed nine times with the Nuclear Spectroscopic Telescope Array (NuSTAR) between September 2018 and November 2019. During these observations, the spectral state of 4U 1957+11 evolved marginally. The disc dominant spectra are described well with a hot, multicolor disc blackbody with disc temperature varying in the range kT in ∼ 1.35–1.86 keV and a non-thermal component having a steep slope (Γ = 2–3). A broad Fe emission line feature (5–8 keV) was observed in the spectra of all the observations. The relativistic disc model was used to study the effect of distance, inclination and the black hole mass on its spin. Simulations indicate a higher spin for smaller distances and lower black hole masses. At smaller distances and higher masses, spin is maximum and almost independent of the distance. An inverse correlation exists between the spin and the spectral hardening factor for all the cases. The system prefers a moderate spin of about 0.85 for black hole masses between 4–6 M ⊙ for a 7 kpc distance.

Magnetised tori in the background of a deformed compact object

The aim of this work is to study the relativistic accretion thick disc model raised by a deformed compact object that slightly deviated from spherical up to the quadrupole moment by utilising the q-metric. This metric is the simplest asymptotically flat solution of Einstein’s equation with quadrupole moment. We studied the effects of quadrupole moments in combination with the parameters of the thick magnetised disc model via studying the properties of these equilibrium sequences of magnetised, non-self-gravitating discs in this space-time. We employed different angular momentum distributions and discussed the procedure of building this toroidal disc model based on a combination of approaches previously considered in the literature. We show the properties of this relativistic accretion disc model and its dependence on the initial parameters. In addition, this theoretical model can serve as the initial data for numerical simulations.

The first simultaneous X-ray broadband view of Mrk 110 with XMM-Newton and NuSTAR

Context. Soft and hard X-ray excesses, compared to the continuum power-law shape between ∼2−10 keV, are common features observed in the spectra of active galactic nuclei (AGN) and are associated with the accretion disc-corona system around the supermassive black hole. However, the dominant process at work is still highly debated and has been proposed to be either relativistic reflection or Comptonisation. Such an investigation can be problematic for AGN that have significant intrinsic absorption, either cold or warm, which can severely distort the observed continuum. Therefore, AGN with no (or very weak) intrinsic absorption along the line-of-sight, called bare AGN, are the best targets for directly probing disc-corona systems. Aims. We aim to characterise the main X-ray spectral physical components from the bright bare broad-line Seyfert 1 AGN Mrk 110, as well as the physical process(es) at work in its disc-corona system viewed almost face-on. Methods. We perform the X-ray broadband spectral analysis thanks to two simultaneous XMM-Newton and NuSTAR observations performed on November 16−17, 2019, and April 5−6, 2020. We also use a deep NuSTAR observation obtained in January 2017 for the spectral analysis above 3 keV. Results. The broadband X-ray spectra of Mrk 110 are characterised by the presence of a prominent and absorption-free smooth soft X-ray excess, moderately broad O VII and Fe Kα emission lines, and a lack of a strong Compton hump. The continuum above ∼3 keV is very similar at both epochs, while some variability (stronger when brighter) is present for the soft X-ray excess. A combination of soft and hard Comptonisation by a warm and hot corona, respectively, plus mildly relativistic disc reflection reproduce the broadband X-ray continuum very well. The inferred warm corona temperature, kTwarm ∼ 0.3 keV, is similar to the values found in other sub-Eddington AGN, whereas the hot corona temperature, kThot ∼ 21−31 keV (depending mainly on the assumed hot corona geometry), is found to be in the lower range of the values measured in AGN.

Revealing the nature of the transient source MAXI J0637-430 through spectro-temporal analysis

ABSTRACT We study the spectral and temporal properties of MAXI J0637-430 during its 2019–2020 outburst using Neutron Star Interior Composition Explorer (NICER), AstroSat , and Swift–XRT data. The source was in a disc dominant state within a day of its detection and traces out a ‘c’ shaped profile in the HID, similar to the ‘mini’-outbursts of the recurrent BHB 4U 1630-472. Energy spectrum is obtained in the 0.5−10 keV band with NICER and Swift–XRT, and 0.5−25 keV with AstroSat. The spectra can be modelled using a multicolour disc emission (DISKBB) convolved with a thermal Comptonization component (thcomp). The disc temperature decreases from 0.6 to 0.1 keV during the decay with a corresponding decrease in photon index (Γ) from 4.6 to 1.8. The fraction of Compton-scattered photons (fcov) remains &amp;lt;0.3 during the decay upto 2020 mid-January and gradually increases to 1 as the source reaches hard state. Power density spectra generated in the 0.01−100 Hz range display no quasi-periodic oscillations, although band-limited noise is seen towards the end of 2020 January. During AstroSat observations, Γ lies in the range 2.3−2.6 and rms increases from 11 to 20 per cent, suggesting that the source was in an intermediate state till 2019 November 21. Spectral fitting with the relativistic disc model (kerrbb), in conjunction with the soft-hard transition luminosity, favour a black hole with mass $3\!-\!19\, \mathrm{ M}_{\odot }$ with retrograde spin at a distance &amp;lt;15 kpc. Finally, we discuss the possible implications of our findings.

Mass, Spin, and Ultralight Boson Constraints from the Intermediate-mass Black Hole in the Tidal Disruption Event 3XMM J215022.4–055108

Abstract We simultaneously and successfully fit the multiepoch X-ray spectra of the tidal disruption event (TDE) 3XMM J215022.4−055108 using a modified version of our relativistic slim disk model that now accounts for angular momentum losses from radiation. We explore the effects of different disk properties and of uncertainties in the spectral hardening factor f c and redshift z on the estimation of the black hole mass M • and spin a •. Across all choices of theoretical priors, we constrain M • to less than 2.2 × 104 M ⊙ at 1σ confidence. Assuming that the TDE host is a star cluster associated with the adjacent, brighter, barred lenticular galaxy at z = 0.055, we constrain M • and a • to be 1.75 − 0.05 + 0.45 × 10 4 M ⊙ and 0.8 − 0.02 + 0.12 , respectively, at 1σ confidence. The high, but sub-extremal, spin suggests that, if this intermediate-mass black hole (IMBH) has grown significantly since formation, it has acquired its last e-fold in mass in a way incompatible with both the “standard” and “chaotic” limits of gas accretion. Ours is the first clear IMBH with a spin measurement. As such, this object represents a novel laboratory for astroparticle physics; its M • and a • place tight limits on the existence of ultralight bosons, ruling out those with masses from ∼10−15 to 10−16 eV.

Photospheric Radius Expansion and a Double-peaked Type-I X-Ray Burst from GRS 1741.9–2853

Abstract We present an analysis of two type-I X-ray bursts observed by NuSTAR originating from the very faint transient neutron star (NS) low-mass X-ray binary GRS 1741.9–2853 during a period of outburst in 2020 May. We show that the persistent emission can be modeled as an absorbed, Comptonized blackbody in addition to Fe Kα emission, which can be attributed to relativistic disk reflection. We measure a persistent bolometric, unabsorbed luminosity of L bol = 7.03 − 0.05 + 0.04 × 10 36 erg s − 1 , assuming a distance of 7 kpc, corresponding to an Eddington ratio of 4.5%. This persistent luminosity combined with light-curve analysis leads us to infer that the bursts were the result of pure He burning rather than mixed H/He burning. Time-resolved spectroscopy reveals that the bolometric flux of the first burst exhibits a double-peaked structure, placing the source within a small population of accreting NSs that exhibit multiple-peaked type-I X-ray bursts. We find that the second, brighter burst shows evidence for photospheric radius expansion (PRE) and that at its peak, this PRE event had an unabsorbed bolometric flux of F peak = 2.94 − 0.26 + 0.28 × 10 − 8 erg cm − 2 s − 1 . This yields a new distance estimate of d = 9.0 ± 0.5 kpc, assuming that this corresponds to the Eddington limit for pure He burning on the surface of a canonical NS. Additionally, we performed a detailed timing analysis that failed to find evidence for quasi-periodic oscillations or burst oscillations, and we place an upper limit of 16% on the rms variability around 589 Hz, the frequency at which oscillations have previously been reported.

Estimating the Black Hole Spin for the X-Ray Binary MAXI J1820+070

Abstract MAXI J1820+070 is a newly discovered black hole X-ray binary, whose dynamical parameters, namely the black hole mass, the inclination angle, and the source distance, have recently been estimated. Insight-HXMT have observed its entire outburst from 2018 March 14. In this work, we attempted to estimate the spin parameter a *, using the continuum-fitting method and applying a fully relativistic thin disk model to the soft-state spectra obtained by Insight-HXMT. It is well known that a * is strongly dependent on three dynamical parameters in this method, and we have examined two sets of parameters. Adopting our preferred parameters: M = 8.48 − 0.72 + 0.79 M ⊙ , i = 63° ± 3°, and D = 2.96 ± 0.33 kpc, we found a slowly spinning black hole of a * = 0.14 ± 0.09 (1σ), which gives a prograde spin parameter as the majority of other systems show. It is also possible for the black hole to have a retrograde spin (less than 0) if different dynamical parameters are taken.

An AMUSING look at the host of the periodic nuclear transient ASASSN-14ko reveals a second AGN

ABSTRACT We present Multi-Unit Spectroscopic Explorer (MUSE) integral-field spectroscopy of ESO 253−G003, which hosts a known active galactic nucleus (AGN) and the periodic nuclear transient ASASSN-14ko, observed as part of the All-weather MUse Supernova Integral-field of Nearby Galaxies survey. The MUSE observations reveal that the inner region hosts two AGN separated by $1.4\pm 0.1~\rm {kpc}$ (≈1${_{.}^{\prime\prime}}$7). The brighter nucleus has asymmetric broad permitted emission-line profiles and is associated with the archival AGN designation. The fainter nucleus does not have a broad emission-line component but exhibits other AGN characteristics, including $\hbox{$v_{\rm {FWHM}}$} \approx 700~\hbox{km~s$^{-1}$}$ forbidden line emission, $\rm{\log _{10}(\rm{[O\,\small {III}]}/\rm{H\beta})} \approx 1.1$, and high-excitation potential emission lines, such as [Fe vii] λ6086 and He ii λ4686. The host galaxy exhibits a disturbed morphology with large kpc-scale tidal features, potential outflows from both nuclei, and a likely superbubble. A circular relativistic disc model cannot reproduce the asymmetric broad emission-line profiles in the brighter nucleus, but two non-axisymmetric disc models provide good fits to the broad emission-line profiles: an elliptical disc model and a circular disc + spiral arm model. Implications for the periodic nuclear transient ASASSN-14ko are discussed.

Exploring higher order images with Fe Kα-lines from relativistic discs: black hole spin determination and bias

ABSTRACT We study the contributions to the relativistic Fe K α line profile from higher order images (HOIs) produced by strongly deflected rays from the disc which cross the plunging region, located between the innermost stable circular orbit (ISCO) radius and the event horizon of a Kerr black hole. We investigate the characteristics features imprinted by the HOIs in the line profile for different black hole spins, disc emissivity laws, and inclinations. We find that they extend from the red wing of the profile up to energies slightly lower than those of the blue peak, adding ∼0.4–1.3 per cent to the total line flux. The contribution to the specific flux is often in the ∼1 to 7 per cent range, with the highest values attained for low and negative spin (a ≲ 0.3) black holes surrounded by intermediate inclination angle (i ∼ 40°) discs. We simulate future observations of a black hole X-ray binary system with the Large Area Detector of the planned X-ray astronomy enhanced X-ray Timing and Polarimetry Mission (eXTP) and find that the Fe Kα line profiles of systems accreting at ≲1 per cent the Eddington rate are affected by the HOI features for a range of parameters. This would provide evidence of the extreme gravitational lensing of HOI rays. Our simulations show also that not accounting for HOI contributions to the Fe Kα line profile may systematically bias measurements of the black hole spin parameter towards values higher by up to ∼0.3 than the inputted ones.

The Inner Accretion Flow in the Resurgent Seyfert-1.2 AGN Mrk 817

Abstract Accretion disks and coronae around massive black holes have been studied extensively, and they are known to be coupled. Over a period of 30 yr, however, the X-ray (coronal) flux of Mrk 817 increased by a factor of 40 while its UV (disk) flux remained relatively steady. Recent high-cadence monitoring finds that the X-ray and UV continua in Mrk 817 are also decoupled on timescales of weeks and months. These findings could require mechanical beaming of the innermost accretion flow, and/or an absorber that shields the disk and/or broad line region (BLR) from the X-ray corona. Herein, we report on a 135 ks observation of Mrk 817 obtained with NuSTAR, complemented by simultaneous X-ray coverage via the Neil Gehrels Swift Observatory. The X-ray data strongly prefer a standard relativistic disk reflection model over plausible alternatives. Comparable fits with related models constrain the spin to lie in the range of 0.5 ≤ a ≤ 1, and the viewing angle to lie between 10° ≤ θ ≤ 22° (including 1σ statistical errors and small systematic errors related to differences between the models). The spectra also reveal strong evidence of moderately ionized absorption, similar to but likely less extreme than obscuring events in NGC 5548 and NGC 3783. Archival Swift data suggest that the absorption may be variable. Particularly if the column density of this absorber is higher along the plane of the disk, it may intermittently mask or prevent coupling between the central engine, disk, and BLR in Mrk 817.

A full relativistic thin disc – the physics of the plunging region and the value of the stress at the ISCO

ABSTRACT The widely used Novikov–Thorne relativistic thin disc equations are only valid down to the radius of the innermost stable circular orbit (ISCO). This leads to an undetermined boundary condition at the ISCO, known as the inner stress of the disc, which sets the luminosity of the disc at the ISCO and introduces considerable ambiguity in accurately determining the mass, spin, and accretion rate of black holes from observed spectra. We resolve this ambiguity by self-consistently extending the relativistic disc solution through the ISCO to the black hole horizon by calculating the inspiral of an average disc particle subject to turbulent disc forces, using a new particle-in-disc technique. Traditionally it has been assumed that the stress at the ISCO is zero, with material plunging approximately radially into the black hole at close to the speed of light. We demonstrate that in fact the inspiral is less severe, with several (∼4–17) orbits completed before the horizon. This leads to a small non-zero stress and luminosity at and inside the ISCO, with a local surface temperature at the ISCO between ∼0.15 and 0.3 times the maximum surface temperature of the disc, in the case where no dynamically important net magnetic field is present. For a range of disc parameters we calculate the value of the inner stress/surface temperature, which is required when fitting relativistic thin disc models to observations. We resolve a problem in relativistic slim disc models in which turbulent heating becomes inaccurate and falls to zero inside the plunging region.

A Geometric Origin for Quasi-periodic Oscillations in Black Hole X-Ray Binaries

Abstract We expand the relativistic precession model to include nonequatorial and eccentric trajectories and apply it to quasi-periodic oscillations (QPOs) in black hole X-ray binaries (BHXRBs) and associate their frequencies with the fundamental frequencies of the general case of nonequatorial (with Carter's constant, ) and eccentric ( ) particle trajectories, around a Kerr black hole. We study cases with either two or three simultaneous QPOs and extract the parameters {e, r p , a, Q}, where r p is the periastron distance of the orbit, and a is the spin of the black hole. We find that the orbits with should have e ≲ 0.5 and r p ∼ 2–20 for the observed range of QPO frequencies, where a ∈ [0, 1], and that the spherical trajectories {e = 0, } with Q = 2–4 should have r s ∼ 3–20. We find nonequatorial eccentric solutions for both M82 X-1 and GROJ 1655-40. We see that these trajectories, when taken together, span a torus region and give rise to a strong QPO signal. For two simultaneous QPO cases, we found equatorial eccentric orbit solutions for XTEJ 1550-564, 4U 1630-47, and GRS 1915+105, and spherical orbit solutions for BHXRBs M82 X-1 and XTEJ 1550-564. We also show that the eccentric orbit solution fits the Psaltis–Belloni–Klis correlation observed in BHXRB GROJ 1655-40. Our analysis of the fluid flow in the relativistic disk edge suggests that instabilities cause QPOs to originate in the torus region. We also present some useful formulae for trajectories and frequencies of spherical and equatorial eccentric orbits.

HFQPOs and discoseismic mode excitation in eccentric, relativistic discs. II. Magnetohydrodynamic simulations

ABSTRACT Trapped inertial oscillations (r modes) provide a promising explanation for high-frequency quasi-periodic oscillations (HFQPOs) observed in the emission from black hole X-ray binary systems. An eccentricity (or warp) can excite r modes to large amplitudes, but concurrently, the oscillations are likely damped by magnetohydrodynamic (MHD) turbulence driven by the magnetorotational instability (MRI). We force eccentricity in global, unstratified, zero-net-flux MHD simulations of relativistic accretion discs and find that a sufficiently strong disc distortion generates trapped inertial waves despite this damping. In our simulations, eccentricities above ∼0.03 in the inner disc excite trapped waves. In addition to the competition between r-mode damping and driving, we observe that larger amplitude eccentric structures modify and in some cases suppress MRI turbulence. Given the variety of distortions (warps as well as eccentricities) capable of amplifying r modes, the robustness of trapped inertial wave excitation in the face of MRI turbulence in our simulations provides support for a discoseismic explanation for HFQPOs.

HFQPOs and discoseismic mode excitation in eccentric, relativistic discs. I. Hydrodynamic simulations

ABSTRACT High-frequency quasi-periodic oscillations (HFQPOs) observed in the emission of black hole X-ray binary systems promise insight into strongly curved spacetime. ‘Discoseismic’ oscillations with frequencies set by the intrinsic properties of the central black hole, in particular ‘trapped inertial waves’ (r modes), offer an attractive explanation for HFQPOs. To produce an observable signature, however, such oscillations must be excited to sufficiently large amplitudes. Turbulence driven by the magnetorotational instability fails to provide the necessary amplification, but r modes may still be excited via interaction with accretion disc warps or eccentricities. We present 3D global hydrodynamic simulations of relativistic accretion discs, which demonstrate for the first time the excitation of trapped inertial waves by an imposed eccentricity in the flow. While the r modes’ saturated state depends on the vertical boundary conditions used in our unstratified, cylindrical framework, their excitation is unambiguous in all runs with eccentricity ≳ 0.005 near the innermost stable circular orbit. These simulations provide a proof of concept, demonstrating the robustness of trapped inertial wave excitation in a non-magnetized context. In a companion paper, we explore the competition between this excitation, and damping by magnetohydrodynamic turbulence.

ASASSN-15lh: a TDE about a maximally rotating 109 M⊙ black hole

ABSTRACT We model the light curves of the novel and extremely luminous transient ASASSN-15lh at nine different frequencies, from infrared to ultraviolet photon energies, as an evolving relativistic disc produced in the aftermath of a tidal disruption event (TDE). Good fits to all nine light curves are simultaneously obtained when Macc ≃ 0.07 M⊙ is accreted on to a black hole of mass M ≃ 109 M⊙ and near-maximal rotation a/rg = 0.99. The best-fitting black hole mass is consistent with a number of existing estimates from galactic scaling relationships. If confirmed, our results represent the detection of one of the most massive rapidly spinning black holes to date, and are strong evidence for a TDE origin for ASASSN-15lh. This would be the first TDE to be observed in the disc-dominated state at optical and infrared frequencies.

Studying the Reflection Spectra of the New Black Hole X-Ray Binary Candidate MAXI J1631−479 Observed by NuSTAR: A Variable Broad Iron Line Profile

Abstract We present results from the Nuclear Spectroscopic Telescope Array observations of the new black hole X-ray binary candidate MAXI J1631–479 at two epochs during its 2018–2019 outburst, which caught the source in a disk dominant state and a power-law dominant state. Strong relativistic disk reflection features are clearly detected, displaying significant variations in the shape and strength of the broad iron emission line between the two states. Spectral modeling of the reflection spectra reveals that the inner radius of the optically thick accretion disk evolves from &lt;1.9 to 12 ± 1 r g (statistical errors at 90% confidence level) from the disk dominant to the power-law dominant state. Assuming in the former case that the inner disk radius is consistent with being at the innermost stable circular orbit, we estimate a black hole spin of a* &gt; 0.94. Given that the bolometric luminosity is similar in the two states, our results indicate that the disk truncation observed in MAXI J1631–479 in the power-law dominant state is unlikely to be driven by a global variation in the accretion rate. We propose that it may instead arise from local instabilities in the inner edge of the accretion disk at high accretion rates. In addition, we find an absorption feature in the spectra centered at 7.33 ± 0.03 keV during the disk dominant state, which is evidence for the rare case that an extremely fast disk wind ( ) is observed in a low-inclination black hole binary, with the viewing angle of 29° ± 1° as determined by the reflection modeling.

Testing The Lamp-Post and Wind Reverberation Models with XMM-Newton Observations of NGC 5506.

The lamp-post geometry is often used to model X-ray data of accreting black holes. Despite its simple assumptions, it has proven to be powerful in inferring fundamental black hole properties such as the spin. Early results of X-ray reverberations showed support for such a simple picture, though wind-reverberation models have also been shown to explain the observed delays. Here, we analyze new and old XMM-Newton observations of the variable Seyfert-1 galaxy NGC 5506 to test these models. The source shows an emission line feature around 6.7 keV that is delayed relative to harder and softer energy bands. The spectral feature can be modeled with either a weakly relativistic disk line or by scattering in distant material. By modeling both the spectral and timing signatures, we find that the reflection fraction needed to explain the lags is larger than observed in the time-averaged spectrum, ruling out both a static lamp-post and simple wind reverberation models.