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J. M. Miller

Publications and source records attributed to J. M. Miller.

At least 55 records · Page 3Linked to original sources
arXiv2020

Blueshifted absorption lines from X-ray reflection in IRAS 13224-3809

We explore a disc origin for the highly-blueshifted, variable absorption lines seen in the X-ray spectrum of the Narrow Line Seyfert 1 galaxy IRAS13224-3809. The blueshift corresponds to a velocity of about 0.25c. Such features in other Active Galactic Nuclei are often interpreted as UltraFast Outflows (UFOs). The velocity is of course present in the orbital motions of the inner disk. The absorption lines in IRAS13224-3809 are best seen when the flux is low and the reflection component of the disk is strong relative to the power-law continuum. The spectra are consistent with a model in which the reflection component passes through a thin, highly-ionized absorbing layer at the surface of the inner disc, the blue-shifted side of which dominates the flux due to relativistic aberration (the disc inclination is about 70 deg). No fast outflow need occur beyond the disc.

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arXiv2020

A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608-52

It is commonly assumed that the properties and geometry of the accretion flow in transient low-mass X-ray binaries (LMXBs) significantly change when the X-ray luminosity decays below $\sim 10^{-2}$ of the Eddington limit ($L_{\rm Edd}$). However, there are few observational cases where the evolution of the accretion flow is tracked in a single X-ray binary over a wide dynamic range. In this work, we use NuSTAR and NICER observations obtained during the 2018 accretion outburst of the neutron star LMXB 4U 1608-52, to study changes in the reflection spectrum. We find that the broad Fe-K$α$ line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is $\sim 10^{37}$ erg/s ($\sim 0.05$ $L_{\rm Edd}$), disappear during the decay of the outburst when the source luminosity drops to $\sim 4.5 \times 10^{35}$ erg/s ($\sim 0.002$ $L_{\rm Edd}$). We show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise at lower flux, but is instead caused by physical changes in the accretion flow. Simulating synthetic NuSTAR observations on a grid of inner disk radius, disk ionisation, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disk ionisation ($\log ξ\geq 4.1$) or a much decreased reflection fraction. A changing disk truncation alone, however, cannot account for the lack of reprocessed Fe-K$α$ emission. The required increase in ionisation parameter could occur if the inner accretion flow evaporates from a thin disk into a geometrically thicker flow, such as the commonly assumed formation of an radiatively inefficient accretion flow at lower mass accretion rates.

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arXiv2020

Observations of a GX 301-2 Apastron Flare with the X-Calibur Hard X-Ray Polarimeter Supported by NICER, the Swift XRT and BAT, and Fermi GBM

The accretion-powered X-ray pulsar GX 301-2 was observed with the balloon-borne X-Calibur hard X-ray polarimeter during late December 2018, with contiguous observations by the NICER X-ray telescope, the Swift X-ray Telescope and Burst Alert Telescope, and the Fermi Gamma-ray Burst Monitor spanning several months. The observations detected the pulsar in a rare apastron flaring state coinciding with a significant spin-up of the pulsar discovered with the Fermi GBM. The X-Calibur, NICER, and Swift observations reveal a pulse profile strongly dominated by one main peak, and the NICER and Swift data show strong variation of the profile from pulse to pulse. The X-Calibur observations constrain for the first time the linear polarization of the 15-35 keV emission from a highly magnetized accreting neutron star, indicating a polarization degree of (27+38-27)% (90% confidence limit) averaged over all pulse phases. We discuss the spin-up and the X-ray spectral and polarimetric results in the context of theoretical predictions. We conclude with a discussion of the scientific potential of future observations of highly magnetized neutron stars with the more sensitive follow-up mission XL-Calibur.

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arXiv2019

A Comprehensive Chandra Study of the Disk Wind in the Black Hole Candidate 4U 1630-472

The mechanisms that drive disk winds are a window into the physical processes that underlie the disk. Stellar-mass black holes are an ideal setting in which to explore these mechanisms, in part because their outbursts span a broad range in mass accretion rate. We performed a spectral analysis of the disk wind found in six Chandra/HETG observations of the black hole candidate 4U~1630$-$472, covering a range of luminosities over two distinct spectral states. We modeled both wind absorption and extended wind re-emission components using PION, a self-consistent photoionized absorption model. In all but one case, two photoionization zones were required in order to obtain acceptable fits. Two independent constraints on launching radii, obtained via the ionization parameter formalism and the dynamical broadening of the re-emission, helped characterize the geometry of the wind. The innermost wind components ($r \simeq {10}^{2-3}$ $GM/{c}^{2}$) tend towards small volume filling factors, high ionization, densities up to $n \simeq {10}^{15-16} {\text{cm}}^{-3}$, and outflow velocities of $\sim 0.003c$. These small launching radii and large densities require magnetic driving, as they are inconsistent with numerical and analytical treatments of thermally driven winds. Outer wind components ($r \simeq {10}^{5}$ $GM/{c}^{2}$) are significantly less ionized and have filling factors near unity. Their larger launching radii, lower densities ($n \simeq {10}^{12} {\text{cm}}^{-3}$), and outflow velocities ($\sim 0.0007c$) are nominally consistent with thermally driven winds. The overall wind structure suggests that these components may also be part of a broader MHD outflow and perhaps better described as magneto-thermal hybrid winds.

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arXiv2019

MAXI J1820+070 with NuSTAR I. An increase in variability frequency but a stable reflection spectrum: coronal properties and implications for the inner disc in black hole binaries

MAXI J1820+070 (optical counterpart ASASSN-18ey) is a black hole candidate discovered through its recent very bright outburst. The low extinction column and long duration at high flux allow detailed measurements of the accretion process to be made. In this work, we compare the evolution of X-ray spectral and timing properties through the initial hard state of the outburst. We show that the inner accretion disc, as measured by relativistic reflection, remains steady throughout this period of the outburst. Nevertheless, subtle spectral variability is observed, which is well explained by a change in coronal geometry. However, characteristic features of the temporal variability - low-frequency roll-over and QPO frequency - increase drastically in frequency, as the outburst proceeds. This suggests that the variability timescales are governed by coronal conditions rather than solely by the inner disc radius. We also find a strong correlation between X-ray luminosity and coronal temperature. This can be explained by electron pair production with a changing effective radius and a non-thermal electron fraction of ~20%.

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arXiv2019

A NICER Look at Strong X-ray Obscuration in the Seyfert-2 Galaxy NGC 4388

We present an analysis of the time-averaged spectrum of the Seyfert-2 active galaxy NGC 4388, obtained by NICER. The intrinsic strength of the reflection spectrum in NGC 4388, the large collecting area and favorable pass band of NICER, and a net exposure of 105.6 ks yielded an exceptionally sensitive spectrum. Using two independent families of models, the intrinsic spectrum from the central engine is found to be highly obscured but not Compton-thick. Enforcing physical self-consistency within each model, the independent treatments give formally consistent results: N_H = 2.67 (-0.03,+0.02) E+23 cm^-2 or N_H = 2.64 (-0.03, +0.03) E+23 cm^-2. Past measurements made with Suzaku and XMM-Newton are in broad agreement with these column density values. A more recent measurement with NuSTAR (in late 2013) recorded a column density about twice as large; the robustness of this variability is reinforced by the use of consistent models and procedures. The neutral Fe K-alpha line in the NICER spectrum is nominally resolved and consistent with an origin in the optical broad line region (BLR). The data also require ionized absorption in the Fe K band, similar to the "warm absorbers" detected in Seyfert-1 active galactic nuclei (AGN). The low-energy spectrum is consistent with a set of ionized plasma components. We discuss these findings and note that the geometric inferences that derive from this analysis can be tested with XRISM and Athena.

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arXiv2019

Observations of the Ultra-compact X-ray Binary 4U 1543-624 in Outburst with NICER, INTEGRAL, Swift, and ATCA

We report on X-ray and radio observations of the ultra-compact X-ray binary 4U 1543-624 taken in August 2017 during an enhanced accretion episode. We obtained NICER monitoring of the source over a $\sim10$ day period during which target-of-opportunity observations were also conducted with Swift, INTEGRAL, and ATCA. Emission lines were measured in the NICER X-ray spectrum at $\sim0.64$ keV and $\sim6.4$ keV that correspond to O and Fe, respectively. By modeling these line components, we are able to track changes in the accretion disk throughout this period. The innermost accretion flow appears to move inwards from hundreds of gravitational radii ($R_{g}=GM/c^{2}$) at the beginning of the outburst to $<8.7$ $R_{g}$ at peak intensity. We do not detect the source in radio, but are able to place a $3σ$ upper limit on the flux density at $27$ $μ$Jy beam$^{-1}$. Comparing the radio and X-ray luminosities, we find that the source lies significantly away from the range typical of black holes in the ${L}_{r}$-${L}_{x}$ plane, suggesting a neutron star (NS) primary. This adds to the evidence that NSs do not follow a single track in the ${L}_{r}$-${L}_{x}$ plane, limiting its use in distinguishing between different classes of NSs based on radio and X-ray observations alone.

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arXiv2019

Crust cooling of the neutron star in Aql X-1: Different depth and magnitude of shallow heating during similar accretion outbursts

The structure and composition of the crust of neutron stars plays an important role in their thermal and magnetic evolution, hence in setting their observational properties. One way to study the crust properties is to measure how it cools after it has been heated during an accretion outburst in a low-mass X-ray binary (LMXB). Such studies have shown that there is a tantalizing source of heat, of currently unknown origin, that is located in the outer layers of the crust and has a strength that varies between different sources and different outbursts. With the aim of understanding the mechanism behind this "shallow heating", we present Chandra and Swift observations of the neutron star LMXB Aql X-1, obtained after its bright 2016 outburst. We find that the neutron star temperature was initially much lower, and started to decrease at much later time, than observed after the 2013 outburst of the source, despite the fact that the properties of the two outbursts were very similar. Comparing our data to thermal evolution simulations, we infer that the depth and magnitude of shallow heating must have been much larger during the 2016 outburst than during the 2013 one. This implies that basic neutron star parameters that do not change between outbursts, do not play a strong role in shallow heating. Furthermore, it suggests that outbursts with a similar accretion morphology can give rise to very different shallow heating. We also discuss alternative explanations for the difference in quiescent evolution after the 2016 outburst.

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arXiv2019

Multiwavelength characterisation of the accreting millisecond X-ray pulsar and ultra-compact binary IGR J17062-6143

IGR J17062-6143 is an ultra-compact X-ray binary (UCXB) with an orbital period of 37.96 min. It harbours a millisecond X-ray pulsar that is spinning at 163 Hz and and has continuously been accreting from its companion star since 2006. Determining the composition of the accreted matter in UCXBs is of high interest for studies of binary evolution and thermonuclear burning on the surface of neutron stars. Here, we present a multi-wavelength study of IGR J17062-6143 aimed to determine the detailed properties of its accretion disc and companion star. The multi-epoch photometric UV to near-infrared spectral energy distribution (SED) is consistent with an accretion disc $F_ν\proptoν^{1/3}$. The SED modelling of the accretion disc allowed us to estimate an outer disc radius of $R_{out}=2.2^{+0.9}_{-0.4} \times 10^{10}$ cm and a mass-transfer rate of $\dot{m}=1.8^{+1.8}_{-0.5}\times10^{-10}$ M$_{\odot}$ yr$^{-1}$. Comparing this with the estimated mass-accretion rate inferred from its X-ray emission suggests that $\gtrsim$90% of the transferred mass is lost from the system. Moreover, our SED modelling shows that the thermal emission component seen in the X-ray spectrum is highly unlikely from the accretion disc and must therefore represent emission from the surface of the neutron star. Our low-resolution optical spectrum revealed a blue continuum and no emission lines, i.e. lacking H and He features. Based on the current data we cannot conclusively identify the nature of the companion star, but we make recommendations for future study that can distinguish between the different possible evolution histories of this X-ray binary. Finally, we demonstrate how multiwavelength observations can be effectively used to find more UCXBs among the LMXBs.

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arXiv2019

NICER Discovers Spectral Lines During Photospheric Radius Expansion Bursts from 4U 1820-30: Evidence for Burst-driven Winds

We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of narrow emission and absorption lines during photospheric radius expansion (PRE) X-ray bursts from the ultracompact binary 4U 1820-30. NICER observed the source in 2017 August accumulating about 60 ks of exposure. Five thermonuclear X-ray bursts were detected of which four showed clear signs of PRE. We extracted spectra during the PRE phases and fit each to a model that includes a comptonized component to describe the accretion-driven emission, and a black body for the burst thermal radiation. The temperature and spherical emitting radius of the fitted black body are used to assess the strength of PRE in each burst. The two strongest PRE bursts (burst pair 1) had black body temperatures of approximately 0.6 keV and emitting radii of 100 km (at a distance of 8.4 kpc). The other two bursts (burst pair 2) had higher temperatures (~0.67 keV) and smaller radii (75 km). All of the PRE bursts show evidence of narrow line emission near 1 keV. By co-adding the PRE phase spectra of burst pairs 1 and, separately, 2 we find, in both co-added spectra, significant, narrow, spectral features near 1.0 (emission), 1.7 and 3.0 keV (both in absorption). Remarkably, all the fitted line centroids in the co-added spectrum of burst pair 1 appear systematically blue-shifted by a factor of $1.046 \pm 0.006$ compared to the centroids of pair 2, strongly indicative of a gravitational red-shift, a wind-induced blue-shift, or more likely some combination of both effects. The observed shifts are consistent with this scenario in that the stronger PRE bursts in pair 1 reach larger photospheric radii, and thus have weaker gravitational red-shifts, and they generate faster outflows, yielding higher blue-shifts. We discuss possible elemental identifications for the observed features in the context of recent burst-driven wind models.

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arXiv2019

Continued cooling of the accretion-heated neutron star crust in the X-ray transient IGR J17480-2446 located in the globular cluster Terzan 5

We present a new Chandra observation (performed in July 2016) of the neutron star X-ray transient IGR J17480-2446, located in the globular cluster Terzan 5. We study the continued cooling of the neutron star crust in this system that was heated during the 2010 outburst of the source. This new observation was performed two years after the last observation of IGR J17480-2446, hence, significantly extending the cooling baseline. We reanalysed all available Chandra observations of the source (but excluding observations during which one of the known transients in Terzan 5 was in outburst) and fitted the obtained cooling curve with our cooling code NSCool, which allows for much improved modelling than what was previously performed for the source. The data and our fit models indicate that the crust was still cooling ~5.5 years after the outburst ended. The neutron star crust has likely not reached crust-core thermal equilibrium yet, and further cooling is predicted (which can be confirmed with additional Chandra observations in >5 years). Intriguingly, we find indications that the thermal conductivity might be relatively low in part of the crust compared to what has been inferred for other crust-cooling sources and tentatively suggest that this layer might be located around the neutron drip. The reason for this difference is unclear, but might be related to the fact that IGR J17480-2446 harbours a relatively slowly rotating neutron star (with a spin of 11 Hz) that has a relatively strong inferred surface magnetic field ($10^{9-10}$ Gauss) compared to what is known or typically assumed for other cooling sources.

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arXiv2019

A hard look at NGC 5347: revealing a nearby Compton-thick AGN

Current measurements show that the observed fraction of Compton-thick (CT) AGN is smaller than the expected values needed to explain the cosmic X-ray background. Prior fits to the X-ray spectrum of the nearby Seyfert-2 galaxy NGC 5347 ($z=0.00792,\, D =35.5 \rm ~Mpc $) have alternately suggested a CT and Compton-thin source. Combining archival data from $Suzaku$, $Chandra$, and - most importantly - new data from $NuSTAR$, and using three distinct families of models, we show that NGC 5347 is an obscured CTAGN ($N_{\rm H} > 2.23\times 10^{24}~\rm cm^{-2}$). Its 2-30~keV spectrum is dominated by reprocessed emission from distant material, characterized by a strong Fe K$α$ line and a Compton hump. We found a large equivalent width of the Fe K$α$ line ($\rm EW = 2.3 \pm 0.3$ keV) and a high intrinsic-to-observed flux ratio ($\sim 100$). All of these observations are typical for bona fide CTAGN. We estimate a bolometric luminosity of $L_{\rm bol} \simeq 0.014 \pm 0.005~L_{\rm Edd.}$. The $Chandra$ image of NGC 5347 reveals the presence of extended emission dominating the soft X-ray spectrum ($E < 2\,\rm keV$), which coincides with the [O III] emission detected in the $Hubble ~Space~ Telescope$ images. Comparison to other CTAGN suggests that NGC 5347 is broadly consistent with the average properties of this source class. We simulated $XRISM$ and $Athena$/X-IFU spectra of the source, showing the potential of these future missions in identifying CTAGN in the soft X-rays.

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arXiv2019

Accretion in Stellar-Mass Black Holes at High X-ray Spectral Resolution

Accretion disks around stellar-mass black holes offer unique opportunities to study the fundamental physics of standard thin disks, super-Eddington disks, and structure that may be connected to flux variability. These local analogues of active galactic nuclei (AGN) are particularly attractive for their proximity, high flux, and peak emissivity in the X-ray band. X-ray calorimeter spectrometers, with energy resolutions of 2-5 eV, are ideally suited to study accretion in stellar-mass black holes. The results will make strong tests of seminal disk theory that applies in a broad range of circumstances, help to drive new numerical simulations, and will inform our understanding of AGN fueling, evolution, and feedback.

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arXiv2019

Deep Learning for Multi-Messenger Astrophysics: A Gateway for Discovery in the Big Data Era

This report provides an overview of recent work that harnesses the Big Data Revolution and Large Scale Computing to address grand computational challenges in Multi-Messenger Astrophysics, with a particular emphasis on real-time discovery campaigns. Acknowledging the transdisciplinary nature of Multi-Messenger Astrophysics, this document has been prepared by members of the physics, astronomy, computer science, data science, software and cyberinfrastructure communities who attended the NSF-, DOE- and NVIDIA-funded "Deep Learning for Multi-Messenger Astrophysics: Real-time Discovery at Scale" workshop, hosted at the National Center for Supercomputing Applications, October 17-19, 2018. Highlights of this report include unanimous agreement that it is critical to accelerate the development and deployment of novel, signal-processing algorithms that use the synergy between artificial intelligence (AI) and high performance computing to maximize the potential for scientific discovery with Multi-Messenger Astrophysics. We discuss key aspects to realize this endeavor, namely (i) the design and exploitation of scalable and computationally efficient AI algorithms for Multi-Messenger Astrophysics; (ii) cyberinfrastructure requirements to numerically simulate astrophysical sources, and to process and interpret Multi-Messenger Astrophysics data; (iii) management of gravitational wave detections and triggers to enable electromagnetic and astro-particle follow-ups; (iv) a vision to harness future developments of machine and deep learning and cyberinfrastructure resources to cope with the scale of discovery in the Big Data Era; (v) and the need to build a community that brings domain experts together with data scientists on equal footing to maximize and accelerate discovery in the nascent field of Multi-Messenger Astrophysics.

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arXiv2019

NuSTAR Observations of the Accreting Atolls GX 3+1, 4U 1702-429, 4U 0614+091, and 4U 1746-371

Atoll sources are accreting neutron star (NS) low-mass X-ray binaries. We present a spectral analysis of four persistent atoll sources (GX 3+1, 4U 1702$-$429, 4U 0614+091, and 4U 1746$-$371) observed for $\sim20$ ks each with NuSTAR to determine the extent of the inner accretion disk. These sources range from an apparent luminosity of $0.006-0.11$ of the Eddington limit (assuming the empirical limit of $3.8\times10^{38}$ ergs s$^{-1}$). Broad Fe emission features shaped by Doppler and relativistic effects close to the NS were firmly detected in three of these sources. The position of the disk appears to be close to the innermost stable circular orbit (ISCO) in each case. For GX 3+1, we determine $R_{in}=1.8^{+0.2}_{-0.6}\ R_{\mathrm{ISCO}}$ (90% confidence level) and an inclination of $27^{\circ}-31^{\circ}$. For 4U 1702$-$429, we find a $R_{in}=1.5_{-0.4}^{+1.6}\ R_{\mathrm{ISCO}}$ and inclination of $53^{\circ}-64^{\circ}$. For 4U 0614+091, the disk has a position of $R_{in}=1.3_{-0.2}^{+5.4}\ R_{\mathrm{ISCO}}$ and inclination of $50^{\circ}-62^{\circ}$. If the disk does not extend to the innermost stable circular orbit, we can place conservative limits on the magnetic field strength in these systems in the event that the disk is truncated at the Alfvén radius. This provides the limit at the poles of $B\leq6.7\times10^{8}$ G, $3.3\times10^{8}$ G, and $14.5\times10^{8}$ G for GX 3+1, 4U 1702$-$429, and 4U 0614+091, respectively. For 4U 1746$-$371, we argue that the most plausible explanation for the lack of reflection features is a combination of source geometry and strong Comptonization. We place these sources among the larger sample of NSs that have been observed with NuSTAR.

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arXiv2018

X-ray and UV Monitoring of the Seyfert 1.5 Galaxy Markarian 817

We report the results of long-term simultaneous X-ray and UV monitoring of the nearby (z=0.03145) Seyfert 1.5 galaxy Mrk 817 using the Neil Gehrels Swift Observatory XRT and UVOT. Prior work has revealed that the X-ray flux from Mrk 817 has increased by a factor of 40 over the last 40 years, whereas the UV emission has changed by a factor of 2.3. The X-ray emission of Mrk 817 now compares to some of the brightest Seyferts, but it has been poorly studied in comparison. We find that the X-ray (0.3-10.0 keV) and the UVM2 (roughly 2000--2500 Angstrom) fluxes have fractional variability amplitudes of 0.35 and 0.18, respectively, over the entire monitoring period (2017 Jan. 2 to 2018 Apr. 20). A cross-correlation analysis is performed on the X-ray (0.3-10.0 keV) and UVM2 light curves over the entire monitoring period, a period of less frequent monitoring (2017 Jan. 2 to 2017 Dec. 11), and a period of more frequent monitoring (2018 Jan. 12 to 2018 Apr. 20). The analysis reveals no significant correlation between the two at any given lag for all monitoring periods. Especially given that reverberation studies have found significant lags between optical/UV continuum bands and broad optical lines in Mrk 817, the lack of a significant X-ray-UV correlation may point to additional complexities in the inner or intermediate disk. Mechanical (e.g.,a funnel in the inner disk) and/or relativistic beaming of the X-ray emission could potentially explain the lack of a correlation. Alternatively, scattering in an equatorial wind could also diminish the ability of more isotropic X-ray emission to heat the disk itself.

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arXiv2018

A NICER Discovery of a Low-Frequency Quasi-Periodic Oscillation in the Soft-Intermediate State of MAXI J1535-571

We present the discovery of a low-frequency $\approx 5.7$ Hz quasi-periodic oscillation (QPO) feature in observations of the black hole X-ray binary MAXI J1535-571 in its soft-intermediate state, obtained in September-October 2017 by the Neutron Star Interior Composition Explorer (NICER). The feature is relatively broad (compared to other low-frequency QPOs; quality factor $Q\approx 2$) and weak (1.9% rms in 3-10 keV), and is accompanied by a weak harmonic and low-amplitude broadband noise. These characteristics identify it as a weak Type A/B QPO, similar to ones previously identified in the soft-intermediate state of the transient black hole X-ray binary XTE J1550-564. The lag-energy spectrum of the QPO shows increasing soft lags towards lower energies, approaching 50 ms at 1 keV (with respect to a 3-10 keV continuum). This large phase shift has similar amplitude but opposite sign to that seen in Rossi X-ray Timing Explorer data for a Type B QPO from the transient black hole X-ray binary GX 339-4. Previous phase-resolved spectroscopy analysis of the Type B QPO in GX 339-4 pointed towards a precessing jet-like corona illuminating the accretion disk as the origin of the QPO signal. We suggest that this QPO in MAXI J1535-571 may have the same origin, with the different lag sign depending on the scale height of the emitting region and the observer inclination angle.

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arXiv2018

X-ray Structure between the Innermost Disk and Optical Broad Line Region in NGC 4151

We present an analysis of the narrow Fe K-alpha line in Chandra/HETGS observations of the Seyfert AGN, NGC 4151. The sensitivity and resolution afforded by the gratings reveal asymmetry in this line. Models including weak Doppler boosting, gravitational red-shifts, and scattering are generally preferred over Gaussians at the 5 sigma level of confidence, and generally measure radii consistent with R ~ 500-1000 GM/c^2. Separate fits to "high/unobscured" and "low/obscured" phases reveal that the line originates at smaller radii in high flux states; model-independent tests indicate that this effect is significant at the 4-5 sigma level. Some models and Delta t ~ 2 E+4 s variations in line flux suggest that the narrow Fe K-alpha line may originate at radii as small as R ~ 50-130 GM/c^2 in high flux states. These results indicate that the narrow Fe K-alpha line in NGC 4151 is primarily excited in the innermost part of the optical broad line region (BLR), or X-ray BLR. Alternatively, a warp could provide the solid angle needed to enhance Fe K-alpha line emission from intermediate radii, and might resolve an apparent discrepancy in the inclination of the innermost and outer disk in NGC 4151. Both warps and the BLR may originate through radiation pressure, so these explanations may be linked. We discuss our results in detail, and consider the potential for future observations with Chandra, XARM, and ATHENA to measure black hole masses and to study the intermediate disk in AGN using narrow Fe K-alpha emission lines.

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