arXiv subjects

A. Zoghbi

Publications and source records attributed to A. Zoghbi.

At least 37 records · Page 2Linked to original sources

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

Supermassive Black Hole Spin and Reverberation

X-ray reverberation mapping has emerged as a powerful probe of microparsec scales around AGN, and with high sensitivity detectors, its full potential in echo-mapping the otherwise inaccessible disk-corona at the black hole horizon scale will be revealed.

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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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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

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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arXiv2017

X-ray Lags in PDS 456 Revealed by Suzaku Observations

X-ray reverberation lags from the vicinity of supermassive black holes have been detected in almost 30 AGN. The soft lag, which is the time delay between the hard and soft X-ray light curves, is usually interpreted as the time difference between the direct and reflected emission, but is alternatively suggested to arise from the direct and scattering emission from distant clouds. By analysing the archival Suzaku observations totalling an exposure time of ~ 770 ks, we discover a soft lag of $10\pm3.4$ ks at $9.58\times10^{-6}$ Hz in the luminous quasar PDS 456, which is the longest soft lag and lowest Fourier frequency reported to date. In this study, we use the maximum likelihood method to deal with non-continuous nature of the Suzaku light curves. The result follows the mass-scaling relation for soft lags, which further supports that soft lags originate from the innermost areas of AGN and hence are best interpreted by the reflection scenario. Spectral analysis has been performed in this work and we find no evidence of clumpy partial-covering absorbers. The spectrum can be explained by a self-consistent relativistic reflection model with warm absorbers, and spectral variations over epochs can be accounted for by the change of the continuum, and of column density and ionization states of the warm absorbers.

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arXiv2016

A global look at X-ray time lags in Seyfert Galaxies

X-ray reverberation, where light-travel time delays map out the compact geometry around the inner accretion flow in supermassive black holes, has been discovered in several of the brightest, most variable and well-known Seyfert galaxies. In this work, we expand the study of X-ray reverberation to all Seyfert galaxies in the XMM-Newton archive above a nominal rms variability and exposure level (a total of 43 sources). 50 per cent of source exhibit iron K reverberation, in that the broad iron K emission line responds to rapid variability in the continuum. We also find that on long timescales, the hard band emission lags behind the soft band emission in 85 per cent of sources. This `low-frequency hard lag' is likely associated with the coronal emission, and so this result suggests that most sources with X-ray variability show intrinsic variability from the nuclear region. We update the known iron K lag amplitude vs. black hole mass relation, and find evidence that the height or extent of the coronal source (as inferred by the reverberation time delay) increases with mass accretion rate.

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arXiv2016

The Accretion Disk Wind in the Black Hole GRS 1915+105

We report on a 120 ks Chandra/HETG spectrum of the black hole GRS 1915+105. The observation was made during an extended and bright soft state in June, 2015. An extremely rich disk wind absorption spectrum is detected, similar to that observed at lower sensitivity in 2007. The very high resolution of the third-order spectrum reveals four components to the disk wind in the Fe K band alone; the fastest has a blue-shift of v = 0.03c. Broadened re-emission from the wind is also detected in the first-order spectrum, giving rise to clear accretion disk P Cygni profiles. Dynamical modeling of the re-emission spectrum gives wind launching radii of r ~ 10^(2-4) GM/c^2. Wind density values of n ~ 10^(13-16) cm^-3 are then required by the ionization parameter formalism. The small launching radii, high density values, and inferred high mass outflow rates signal a role for magnetic driving. With simple, reasonable assumptions, the wind properties constrain the magnitude of the emergent magnetic field to B ~ 10^(3-4) Gauss if the wind is driven via magnetohydrodynamic (MHD) pressure from within the disk, and B ~ 10^(4-5) Gauss if the wind is driven by magnetocentrifugal acceleration. The MHD estimates are below upper limits predicted by the canonical alpha-disk model (Shakura & Sunyaev 1973). We discuss these results in terms of fundamental disk physics and black hole accretion modes.

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arXiv2015

NuSTAR and Suzaku X-ray Spectroscopy of NGC 4151: Evidence for Reflection from the Inner Accretion Disk

We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption, and reflection properties of the active galactic nucleus (AGN) by applying inner accretion disk reflection and absorption-dominated models. With a time-averaged spectral analysis, we find strong evidence for relativistic reflection from the inner accretion disk. We find that relativistic emission arises from a highly ionized inner accretion disk with a steep emissivity profile, which suggests an intense, compact illuminating source. We find a preliminary, near-maximal black hole spin a>0.9 accounting for statistical and systematic modeling errors. We find a relatively moderate reflection fraction with respect to predictions for the lamp post geometry, in which the illuminating corona is modeled as a point source. Through a time-resolved spectral analysis, we find that modest coronal and inner disk reflection flux variation drives the spectral variability during the observations. We discuss various physical scenarios for the inner disk reflection model, and we find that a compact corona is consistent with the observed features.

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arXiv2015

NuSTAR Reveals Relativistic Reflection But No Ultra-Fast Outflow In The Quasar PG 1211+143

We report on four epochs of observations of the quasar PG 1211+143 using NuSTAR. The net exposure time is 300 ks. Prior work on this source found suggestive evidence of an 'ultra-fast outflow' (or, UFO) in the Fe K band, with a velocity of approximately 0.1c. The putative flow would carry away a high mass flux and kinetic power, with broad implications for feedback and black hole-galaxy co-evolution. NuSTAR detects PG 1211+143 out to 30 keV, meaning that the continuum is well-defined both through and above the Fe K band. A characteristic relativistic disk reflection spectrum is clearly revealed, via a broad Fe K emission line and Compton back-scattering curvature. The data offer only weak constraints on the spin of the black hole. A careful search for UFO's show no significant absorption feature above 90% confidence. The limits are particularly tight when relativistic reflection is included. We discuss the statistics and the implications of these results in terms of connections between accretion onto quasars, Seyferts, and stellar-mass black holes, and feedback into their host environments.

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arXiv2014

Coronal Properties of the Seyfert 1.9 Galaxy MCG -05-23-016 Determined from Hard X-ray Spectroscopy with NuSTAR

Measurements of the high-energy cut-off in the coronal continuum of active galactic nuclei have long been elusive for all but a small number of the brightest examples. We present a direct measurement of the cut-off energy in the nuclear continuum of the nearby Seyfert 1.9 galaxy MCG -05-23-016 with unprecedented precision. The high sensitivity of NuSTAR up to 79 keV allows us to clearly disentangle the spectral curvature of the primary continuum from that of its reflection component. Using a simple phenomenological model for the hard X-ray spectrum, we constrain the cut-off energy to $116_{-5}^{+6}$ keV with 90% confidence. Testing for more complex models and nuisance parameters that could potentially influence the measurement, we find that the cut-off is detected robustly. We further use simple Comptonized plasma models to provide independent constraints for both the kinetic temperature of the electrons in the corona and its optical depth. At the 90% confidence level, we find $kT_e=29\pm2$ keV and $τ_e=1.23\pm0.08$ assuming a slab (disk-like) geometry, and $kT_e=25\pm2$ keV and $τ_e=3.5\pm0.2$ assuming a spherical geometry. Both geometries are found to fit the data equally well and their two principal physical parameters are correlated in both cases. With the optical depth in the optically thick regime, the data are pushing the currently available theoretical models of the Comptonized plasma to the limits of their validity. Since the spectral features and variability arising from the inner accretion disk have been observed previously in MCG -05-23-016, the inferred high optical depth implies that a spherical or disk-like corona cannot be homogeneous.

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arXiv2014

ASTRO-H White Paper - AGN Reflection

X-ray observations provide a powerful tool to probe the central engines of active galactic nuclei (AGN). A hard X-ray continuum is produced from deep within the accretion flow onto the supermassive black hole, and all optically thick structures in the AGN (the dusty torus of AGN unification schemes, broad emission line clouds, and the black hole accretion disk) "light up" in response to irradiation by this continuum. This White Paper describes the prospects for probing AGN physics using observations of these X-ray reflection signatures. High-resolution SXS spectroscopy of the resulting fluorescent iron line in type-2 AGN will give us an unprecedented view of the obscuring torus, allowing us to assess its dynamics (through line broadening) and geometry (through the line profile as well as observations of the "Compton shoulder"). The broad-band view obtained by combining all of the ASTRO-H instruments will fully characterize the shape of the underlying continuum (which may be heavily absorbed) and reflection/scattering, providing crucial constraints on models for the Cosmic X-ray Background with a subsequent impact on understanding of supermassive black hole evolution. ASTRO-H will also permit the relativistically broadened reflection spectrum from the inner accretion disk to be robustly studied, even in complex systems with, for example, warm absorption and composite soft excesses. Finally, the HXI will allow the detection and study of reverberation delays between the continuum and the Compton reflection hump from the inner disk.

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arXiv2014

Iron K and Compton hump reverberation in SWIFT J2127.4+5654 and NGC 1365 revealed by NuSTAR and XMM-Newton

In the past five years, a flurry of X-ray reverberation lag measurements of accreting supermassive black holes have been made using the XMM-Newton telescope in the 0.3-10 keV energy range. In this work, we use the NuSTAR telescope to extend the lag analysis up to higher energies for two Seyfert galaxies, SWIFT J2127.4+5654 and NGC 1365. X-ray reverberation lags are due to the light travel time delays between the direct continuum emission and the reprocessed emission from the inner radii of an ionised accretion disc. XMM-Newton has been particularly adept at measuring the lag associated with the broad Fe K emission line, where the gravitationally redshifted wing of the line is observed to respond before the line centroid at 6.4 keV, produced at larger radii. Now we use NuSTAR to probe the lag at higher energies, where the spectrum shows clear evidence for Compton reflection, known as the Compton 'hump'. The XMM-Newton data show Fe K lags in both SWIFT J2127.4+5654 and NGC 1365. The NuSTAR data provide independent confirmation of these Fe K lags, and also show evidence for the corresponding Compton hump lags, especially in SWIFT J2127.4+5654. These broadband lag measurements confirm that the Compton hump and Fe K lag are produced at small radii. At low-frequencies in NGC 1365, where the spectrum shows evidence for eclipsing clouds in the line of sight, we find a clear negative (not positive) lag from 2--10 keV, which can be understood as the decrease in column density from a neutral eclipsing cloud moving out of our line of sight during the observation.

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arXiv2014

Suzaku observations of Mrk 335: Confronting partial covering and relativistic reflection

We report on the deepest X-ray observation of the narrow-line Seyfert 1 galaxy Mrk 335 in the low-flux state obtained with Suzaku. The data are compared to a 2006 high-flux Suzaku observation when the source was ~10-times brighter. Describing the two flux levels self-consistently with partial covering models would require extreme circumstances, as the source would be subject to negligible absorption during the bright state and ninety-five per cent covering with near Compton-thick material when dim. Blurred reflection from an accretion disc around a nearly maximum spinning black hole (a>0.91, with preference for a spin parameter as high as ~ 0.995) appears more likely and is consistent with the long-term and rapid variability. Measurements of the emissivity profile and spectral modelling indicate the high-flux Suzaku observation of Mrk 335 is consistent with continuum-dominated, jet-like emission (i.e. beamed away from the disc). It can be argued that the ejecta must be confined to within ~25 rg if it does not escape the system. During the low-flux state the corona becomes compact and only extends to about 5 rg from the black hole, and the spectrum becomes reflection-dominated. The low-frequency lags measured at both epochs are comparable indicating that the accretion mechanism is not changing between the two flux levels. Various techniques to study the spectral variability (e.g. principal component analysis, fractional variability, difference spectra, and hardness ratio analysis) indicate that the low-state variability is dominated by changes in the power law flux and photon index, but that changes in the ionisation state of the reflector are also required. Most notably, the ionisation parameter becomes inversely correlated with the reflected flux after a long-duration flare-like event during the observation.

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arXiv2014

Observations of MCG-5-23-16 with Suzaku, XMM-Newton and NuSTAR: Disk tomography and Compton hump reverberation

MCG-5-23-16 is one of the first AGN where relativistic reverberation in the iron K line originating in the vicinity of the supermassive black hole was found, based on a short XMM-Newton observation. In this work, we present the results from long X-ray observations using Suzaku, XMM-Newton and NuSTAR designed to map the emission region using X-ray reverberation. A relativistic iron line is detected in the lag spectra on three different time-scales, allowing the emission from different regions around the black hole to be separated. Using NuSTAR coverage of energies above 10 keV reveals a lag between these energies and the primary continuum, which is detected for the first time in an AGN. This lag is a result of the Compton reflection hump responding to changes in the primary source in a manner similar to the response of the relativistic iron K line.

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arXiv2013

Modelling the broad Fe Kα reverberation in the AGN NGC 4151

The recent detection of X-ray reverberation lags, especially in the Fe Kα line region, around Active Galactic Nuclei (AGN) has opened up the possibility of studying the time-resolved response (reflection) of hard X-rays from the accretion disk around supermassive black holes. Here, we use general relativistic transfer functions for reflection of X-rays from a point source located at some height above the black hole to study the time lags expected as a function of frequency and energy in the Fe Kα line region. We explore the models and the dependence of the lags on key parameters such as the height of the X-ray source, accretion disk inclination, black hole spin and black hole mass. We then compare these models with the observed frequency and energy dependence of the Fe Kα line lag in NGC 4151. Assuming the optical reverberation mapping mass of $4.6\times10^7~M_\odot$ we get a best fit to the lag profile across the Fe Kα line in the frequency range $(1-2)\times10^{-5}$ Hz for an X-ray source located at a height $h = 7^{+2.9}_{-2.6}~R_G$ with a maximally spinning black hole and an inclination $i < 30^\circ$.

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arXiv2013

Hard X-ray Lags in Active Galactic Nuclei: Testing the Distant Reverberation Hypothesis with NGC 6814

We present an X-ray spectral and temporal analysis of the variable active galaxy NGC 6814, observed with Suzaku during November 2011. Remarkably, the X-ray spectrum shows no evidence for the soft excess commonly observed amongst other active galaxies, despite its relatively low level of obscuration, and is dominated across the whole Suzaku bandpass by the intrinsic powerlaw-like continuum. Despite this, we clearly detect the presence of a low frequency hard lag of ~1600s between the 0.5-2.0 and 2.0-5.0 keV energy bands at greater than 6-sigma significance, similar to those reported in the literature for a variety of other AGN. At these energies, any additional emission from e.g. a very weak, undetected soft excess, or from distant reflection must contribute less than 3% of the observed countrates (at 90% confidence). Given the lack of any significant continuum emission component other than the powerlaw, we can rule out models that invoke distant reprocessing for the observed lag behavior, which must instead be associated with this continuum emission. These results are fully consistent with a propagating fluctuation origin for the low frequency hard lags, and with the interpretation of the high frequency soft lags - a common feature seen in the highest quality AGN data with strong soft excesses - as reverberation from the inner accretion disk.

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arXiv2013

Discovery of high-frequency iron K lags in Ark 564 and Mrk 335

We use archival XMM-Newton observations of Ark 564 and Mrk 335 to calculate the frequency dependent time-lags for these two well-studied sources. We discover high-frequency Fe K lags in both sources, indicating that the red wing of the line precedes the rest frame energy by roughly 100 s and 150 s for Ark 564 and Mrk 335, respectively. Including these two new sources, Fe K reverberation lags have been observed in seven Seyfert galaxies. We examine the low-frequency lag-energy spectrum, which is smooth, and shows no feature of reverberation, as would be expected if the low-frequency lags were produced by distant reflection off circumnuclear material. The clear differences in the low and high frequency lag-energy spectra indicate that the lags are produced by two distinct physical processes. Finally, we find that the amplitude of the Fe K lag scales with black hole mass for these seven sources, consistent with a relativistic reflection model where the lag is the light travel delay associated with reflection of continuum photons off the inner disc.

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