arXiv subjects

E. Kara

Publications and source records attributed to E. Kara.

At least 73 records · Page 4Linked to original sources

arXiv2017

The high-Eddington NLS1 Ark 564 has the coolest corona

Ark 564 is an archetypal Narrow line Seyfert 1 that has been well observed in soft X-rays from 0.3-10 keV, revealing a steep spectrum, strong soft excess, iron K emission line and dramatic variability on the order of hours. Because of its very steep spectrum, observations of the source above 10 keV have been sparse. We report here on the first NuSTAR observation of Ark 564. The source was observed for 200 ks with NuSTAR, 50 ks of which were concurrent with Suzaku observations. NuSTAR and Suzaku observed a dramatic flare, in which the hard emission is clearly delayed with respect to the soft emission, consistent with previous detections of a low-frequency hard lag found in XMM-Newton data. The NuSTAR spectrum is well described by a low-temperature Comptonisation continuum (with an electron temperature of 15 +/- 2 keV), which irradiates a highly ionised disc. No further relativistic broadening or ionized absorption is required. These spectral results show that Ark 564 has one of the lowest temperature coronae observed by NuSTAR to date. We discuss possible reasons for low-temperature coronae in high-Eddington sources.

astro-ph.HE↗

arXiv2017

From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link

In recent work with high-resolution grating spectrometers (RGS) aboard XMM-Newton Pinto et al. (2016) have discovered that two bright and archetypal ultraluminous X-ray sources (ULXs) have strong relativistic winds in agreement with theoretical predictions of high accretion rates. It has been proposed that such winds can become optically thick enough to block and reprocess the disk X-ray photons almost entirely, making the source appear as a soft thermal emitter or ultraluminous supersoft X-ray source (ULS). To test this hypothesis we have studied a ULX where the wind is strong enough to cause significant absorption of the hard X-ray continuum: NGC 55 ULX. The RGS spectrum of NGC 55 ULX shows a wealth of emission and absorption lines blueshifted by significant fractions of the light speed (0.01 - 0.20)c indicating the presence of a powerful wind. The wind has a complex dynamical structure with the ionization state increasing with the outflow velocity, which may indicate launching from different regions of the accretion disk. The comparison with other ULXs such as NGC 1313 X-1 and NGC 5408 X-1 suggests that NGC 55 ULX is being observed at higher inclination. The wind partly absorbs the source flux above 1 keV, generating a spectral drop similar to that observed in ULSs. The softening of the spectrum at lower (~ Eddington) luminosities and the detection of a soft lag agree with the scenario of wind clumps crossing the line of sight, partly absorbing and reprocessing the hard X-rays from the innermost region.

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arXiv2017

KYNREFREV - implementation of an X-ray reverberation model in XSPEC

We present briefly the first results obtained by the application of the KYNREFREV-reverberation model, which is ready for its use in XSPEC. This model computes the time dependent reflection spectra of the disc as a response to a flash of primary power-law radiation from a point source corona located on the axis of the black-hole accretion disc. The assumptions of the model are: central Kerr black hole, surrounded by a Keplerian, geometrically thin, optically thick, ionised disc with the possibility of defining the radial density profile and a stationary hot point-like patch of plasma located on the system rotation axis and emitting isotropic power-law radiation (lamp-post geometry). Full relativistic ray-tracing code in vacuum is used for photon paths from the corona to the disc and to the observer and from the disc to the observer. The ionisation of the disc is set for each radius according to the amount of the incident primary flux and the density of the accretion disc. In this paper we comment on some preliminary results obtained through the analysis of X-ray reverberation time-lags from 1H 0707-495 and IRAS 13224-3809.

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arXiv2016

The Future of X-ray Reverberation from AGN

XMM-Newton is capable of making a transformational advance in our understanding of how luminous accreting black holes work, by dedicating about 10 per cent of future observing time to long observations, of order Megaseconds, to X-ray variable Active Galactic Nuclei (AGN) research. This would enable reverberation studies, already a commonplace feature of AGN, to proceed to the next level and follow the behaviour of the powerful dynamic corona. Such a dedicated legacy programme can only be carried out with XMM-Newton.

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arXiv2016

Highly ionized disc and transient outflows in the Seyfert galaxy IRAS 18325-5926

We report on strong X-ray variability and the Fe K band spectrum of the Seyfert galaxy IRAS 18325-5926 obtained from the 2001 XMM-Newton EPIC pn observation of a 120 ks duration. While the X-ray source is highly variable, the 8-10 keV band shows larger variability than that of the lower energies. Amplified 8-10 keV flux variations are associated with two prominent flares of the X-ray source during the observation. The Fe K emission is peaked at 6.6 keV with moderate broadening. It is likely to originate from a highly ionized disc with the ionization parameter of log xi ~3. The Fe K line flux responds to the major flare, supporting its disc origin. There is a short burst of the Fe line flux with no relation to the continuum brightness for which we have no clear explanation. We also find transient, blueshifted Fe K absorption features, which can be identified with high-velocity (~0.2 c) outflows of highly ionized gas, as found in other active galaxies. The deepest absorption feature appears only briefly (~1 hr) at the onset of the major flare and disappears when the flare is declining. The rapid evolution of the absorption spectrum makes this source peculiar among the active galaxies with high velocity outflows. Another detection of the absorption feature also precedes the other flare. The variability of the absorption feature partly accounts for the excess variability in the 8-10 keV band where the absorption feature appears. Although no reverberation measurement is available, the black hole mass of 2e6 Msun is inferred from the X-ray variability. When this mass is assumed, the black hole is accreting at around the Eddington limit, which may fit the highly ionized disc and strong outflows observed in this galaxy.

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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

Relativistic X-ray reverberation modelling of the combined time-averaged and lag-energy spectra in AGN

General relativistic ray tracing simulations of the time-averaged spectrum and energy-dependent time delays in AGN are presented. We model the lamp-post geometry in which the accreting gas is illuminated by an X-ray source located on the rotation axis of the black hole. The spectroscopic features imprinted in the reflection component are modelled using REFLIONX. The associated time delays after the direct continuum, known as reverberation lags, are computed including the full effects of dilution and ionization gradients on the disc. We perform, for the first time, simultaneous fitting of the time-averaged and lag-energy spectra in three AGN: Mrk 335, IRAS 13224-3809 and Ark 564 observed with XMM-Newton. The best fitting source height and central mass of each AGN partly agree with those previously reported. We find that including the ionization gradient in the model naturally explains lag-energy observations in which the 3 keV and 7-10 keV bands precede other bands. To obtain the clear 3 keV and 7-10 keV dips in the lag-energy profile, the model requires either a source height > 5$r_g$, or a disc that is highly ionized at small radii and is colder further out. We also show that fitting the lag or the mean spectra alone can lead to different results and interpretations. This is therefore important to combine the spectral and timing data in order to find the plausible but self-consistent fits which is achievable with our model.

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arXiv2016

Probing the effects of a thermonuclear X-ray burst on the neutron star accretion flow with NuSTAR

Observational evidence has been accumulating that thermonuclear X-ray bursts ignited on the surface of neutron stars influence the surrounding accretion flow. Here, we exploit the excellent sensitivity of NuSTAR up to 79 keV to analyze the impact of an X-ray burst on the accretion emission of the neutron star LMXB 4U 1608-52. The ~200 s long X-ray burst occurred during a hard X-ray spectral state, and had a peak intensity of ~30-50 per cent of the Eddington limit with no signs of photospheric radius expansion. Spectral analysis suggests that the accretion emission was enhanced up to a factor of ~5 during the X-ray burst. We also applied a linear unsupervised decomposition method, namely non-negative matrix factorization (NMF), to study this X-ray burst. We find that the NMF performs well in characterizing the evolution of the burst emission and is a promising technique to study changes in the underlying accretion emission in more detail than is possible through conventional spectral fitting. For the burst of 4U 1608-52, the NMF suggests a possible softening of the accretion spectrum during the X-ray burst, which could potentially be ascribed to cooling of a corona. Finally, we report a small (~3 per cent) but significant rise in the accretion emission ~0.5 h before the X-ray burst, although it is unclear whether this was related to the X-ray burst ignition.

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arXiv2015

The complex accretion geometry of GX 339-4 as seen by NuSTAR and Swift

We present spectral analysis of five NuSTAR and Swift observations of GX 339-4 taken during a failed outburst in summer 2013. These observations cover Eddington luminosity fractions in the range ~0.9-6%. Throughout this outburst, GX 339-4 stayed in the hard state, and all five observations show similar X-ray spectra with a hard power-law with a photon index near 1.6 and significant contribution from reflection. Using simple reflection models we find unrealistically high iron abundances. Allowing for different photon indices for the continuum incident on the reflector relative to the underlying observed continuum results in a statistically better fit and reduced iron abundances. With a photon index around 1.3, the input power-law on the reflector is significantly harder than that which is directly observed. We study the influence of different emissivity profiles and geometries and consistently find an improvement when using separate photon indices. The inferred inner accretion disk radius is strongly model dependent, but we do not find evidence for a truncation radius larger than 100 r_g in any model. The data do not allow independent spin constraints but the results are consistent with the literature (i.e., a>0). Our best-fit models indicate an inclination angle in the range 40-60 degrees, consistent with limits on the orbital inclination but higher than reported in the literature using standard reflection models. The iron line around 6.4 keV is clearly broadened, and we detect a superimposed narrow core as well. This core originates from a fluorescence region outside the influence of the strong gravity of the black hole and we discuss possible geometries.

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arXiv2015

NuSTAR and Suzaku observations of the hard state in Cygnus X-1: locating the inner accretion disk

We present simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR ) and Suzaku observations of the X-ray binary Cygnus X-1 in the hard state. This is the first time this state has been observed in Cyg X-1 with NuSTAR, which enables us to study the reflection and broad-band spectra in unprecedented detail. We confirm that the iron line cannot be fit with a combination of narrow lines and absorption features, and instead requires a relativistically blurred profile in combination with a narrow line and absorption from the companion wind. We use the reflection models of Garcia et al. (2014) to simultaneously measure the black hole spin, disk inner radius, and coronal height in a self-consistent manner. Detailed fits to the iron line profile indicate a high level of relativistic blurring, indicative of reflection from the inner accretion disk. We find a high spin, a small inner disk radius, and a low source height, and rule out truncation to greater than three gravitational radii at the 3σ confidence level. In addition, we find that the line profile has not changed greatly in the switch from soft to hard states, and that the differences are consistent with changes in the underlying reflection spectrum rather than the relativistic blurring. We find that the blurring parameters are consistent when fitting either just the iron line or the entire broad-band spectrum, which is well modelled with a Comptonized continuum plus reflection model.

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arXiv2015

Properties of AGN coronae in the NuSTAR era

The focussing optics of NuSTAR have enabled high signal-to-noise spectra to be obtained from many X-ray bright Active Galactic Nuclei (AGN) and Galactic Black Hole Binaries (BHB). Spectral modelling then allows robust characterization of the spectral index and upper energy cutoff of the coronal power-law continuum, after accounting for reflection and absorption effects. Spectral-timing studies, such as reverberation and broad iron line fitting, of these sources yield coronal sizes, often showing them to be small and in the range of 3 to 10 gravitational radii in size. Our results indicate that coronae are hot and radiatively compact, lying close to the boundary of the region in the compactness - temperature diagram which is forbidden due to runaway pair production. The coincidence suggests that pair production and annihilation are essential ingredients in the coronae of AGN and BHB and that they control the shape of the observed spectra.

astro-ph.HE↗

arXiv2015

A NuSTAR observation of disk reflection from close to the neutron star in 4U 1608-52

Studying the reflection of X-rays off the inner edge of the accretion disk in a neutron star low-mass X-ray binary, allows us to investigate the accretion geometry and to constrain the radius of the neutron star. We report on a NuSTAR observation of 4U 1608-52 obtained during a faint outburst in 2014 when the neutron star, which has a known spin frequency of 620 Hz, was accreting at ~1-2% of the Eddington limit. The 3-79 keV continuum emission was dominated by a Gamma~2 power law, with a ~1-2% contribution from a kTbb~0.3-0.6 keV black body component. The high-quality NuSTAR spectrum reveals the hallmarks of disk reflection; a broad iron line peaking near 7~keV and a Compton back-scattering hump around ~20-30 keV. Modeling the disk reflection spectrum points to a binary inclination of i~30-40 degrees and a small `coronal' height of h<8.5 GM/c2. Furthermore, our spectral analysis suggests that the inner disk radius extended to Rin~7-10 GM/c2, close to the innermost stable circular obit. This constrains the neutron star radius to R<21 km and the redshift from the stellar surface to z>0.12, for a mass of M=1.5 Msun and a spin parameter of a=0.29.

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arXiv2015

Discovery of a ~2 hr high frequency X-ray QPO and iron K alpha reverberation in the active galaxy MS 2254.9-3712

We report the discovery of a $\sim 1.5 \times 10^{-4}$ Hz ($\sim 2$ hr) X-ray quasi-periodic oscillation (QPO) in the active galaxy MS 2254.9-3712, using a $\sim 70$ ks XMM-Newton observation. The QPO is significantly detected ($\sim 3.3 σ$) in the $1.2 - 5.0$ keV band only, connecting its origin with the primary X-ray power-law continuum. We detect a highly coherent soft lag between the $0.3 - 0.7$ keV and $1.2 - 5.0$ keV energy bands at the QPO frequency and at a frequency band in a 3:2 ratio, strongly suggesting the presence of a QPO harmonic. An iron K$α$ reverberation lag is found at the harmonic frequency, indicating the reflecting material subtends some angle to the primary continuum, which is modulated by the QPO mechanism. Frequency resolved spectroscopy reveals the QPO and harmonic to have a hard energy dependence. These properties of the QPO variability, together with the current black hole mass estimate, $M_{\rm bh} \sim 4 \times 10^{6} M_{\rm sun}$, are consistent with the QPO originating from the same process as the high frequency QPO phenomenon observed in black hole X-ray binaries. Principle component analysis reveals the spectral variability in MS 2254.9-3712 is similar to that of the active galaxy RE J1034+396, a source which also displays an X-ray QPO. This suggests a distinct spectral variability pattern for accreting black holes when in a state where QPOs are present.

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arXiv2015

The Compton hump and variable blue wing in the extreme low-flux NuSTAR observations of 1H0707-495

The Narrow-line Seyfert I galaxy, 1H0707-495, has been well observed in the 0.3-10 keV band, revealing a dramatic drop in flux in the iron K alpha band, a strong soft excess, and short timescale reverberation lags associated with these spectral features. In this paper, we present the first results of a deep 250 ks NuSTAR observation of 1H0707-495, which includes the first sensitive observations above 10 keV. Even though the NuSTAR observations caught the source in an extreme low-flux state, the Compton hump is still significantly detected. NuSTAR, with its high effective area above 7 keV, clearly detects the drop in flux in the iron K alpha band, and by comparing these observations with archival XMM-Newton observations, we find that the energy of this drop increases with increasing flux. We discuss possible explanations for this, the most likely of which is that the drop in flux is the blue wing of the relativistically broadened iron K alpha emission line. When the flux is low, the coronal source height is low, thus enhancing the most gravitationally redshifted emission.

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arXiv2014

Revealing the X-ray Variability of AGN with Principal Component Analysis

We analyse a sample of 26 active galactic nuclei with deep XMM-Newton observations, using principal component analysis (PCA) to find model independent spectra of the different variable components. In total, we identify at least 12 qualitatively different patterns of spectral variability, involving several different mechanisms, including five sources which show evidence of variable relativistic reflection (MCG-6-30-15, NGC 4051, 1H 0707-495, NGC 3516 and Mrk 766) and three which show evidence of varying partial covering neutral absorption (NGC 4395, NGC 1365, and NGC 4151). In over half of the sources studied, the variability is dominated by changes in a power law continuum, both in terms of changes in flux and power law index, which could be produced by propagating fluctuations within the corona. Simulations are used to find unique predictions for different physical models, and we then attempt to qualitatively match the results from the simulations to the behaviour observed in the real data. We are able to explain a large proportion of the variability in these sources using simple models of spectral variability, but more complex models may be needed for the remainder. We have begun the process of building up a library of different principal components, so that spectral variability in AGN can quickly be matched to physical processes. We show that PCA can be an extremely powerful tool for distinguishing different patterns of variability in AGN, and that it can be used effectively on the large amounts of high-quality archival data available from the current generation of X-ray telescopes.

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arXiv2014

New Constraints on the Black Hole Low/Hard State Inner Accretion Flow with NuSTAR

We report on an observation of the Galactic black hole candidate GRS 1739-278 during its 2014 outburst, obtained with NuSTAR. The source was captured at the peak of a rising "low/hard" state, at a flux of ~0.3 Crab. A broad, skewed iron line and disk reflection spectrum are revealed. Fits to the sensitive NuSTAR spectra with a number of relativistically blurred disk reflection models yield strong geometrical constraints on the disk and hard X-ray "corona". Two models that explicitly assume a "lamppost" corona find its base to have a vertical height above the black hole of h = 5 (+7, -2) GM/c^2 and h = 18 +/-4 GM/c^2 (90% confidence errors); models that do not assume a "lamppost" return emissivity profiles that are broadly consistent with coronae of this size. Given that X-ray microlensing studies of quasars and reverberation lags in Seyferts find similarly compact coronae, observations may now signal that compact coronae are fundamental across the black hole mass scale. All of the models fit to GRS 1739-278 find that the accretion disk extends very close to the black hole - the least stringent constraint is r = 5 (+3,-4) GM/c^2. Only two of the models deliver meaningful spin constraints, but a = 0.8 +/-0.2 is consistent with all of the fits. Overall, the data provide especially compelling evidence of an association between compact hard X-ray coronae and the base of relativistic radio jets in black holes.

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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

The changing X-ray time lag in MCG-6-30-15

MCG-6-30-15 is one of the most observed Narrow Line Seyfert 1 galaxies in the X-ray band. In this paper we examine the X-ray time lags in this source using a total of 600 ks in observations (440 ks exposure) taken with the XMM-Newton telescope (300 ks in 2001 and 300 ks in 2013). Both the old and new observations show the usual hard lag that increases with energy, however, the hard lag turns over to a soft lag at frequencies below ~1e-4 Hz. The highest frequencies (~1e-3 Hz) in this source show a clear soft lag, as previously presented for the first 300 ks observation, but no clear iron K lag is detected in either the old or new observation. The soft lag is more significant in the old observation than the new. The observations are consistent with a reverberation interpretation, where the soft, reflected emission is delayed with respect to the hard powerlaw component. These spectral timing results suggest that two distinct variability mechanisms are important in this source: intrinsic coronal variations (which lead to correlated variability in the reprocessed emission), and geometrical changes in the corona. Variability due to geometrical changes does not result in correlated variability in the reflection, and therefore inhibits the clear detection of an iron K lag.

astro-ph.HE↗