64 research outputs found
Towards modelling X-ray reverberation in AGN: Piecing together the extended corona
Models of X-ray reverberation from extended coronae are developed from
general relativistic ray tracing simulations. Reverberation lags between
correlated variability in the directly observed continuum emission and that
reflected from the accretion disc arise due to the additional light travel time
between the corona and reflecting disc. X-ray reverberation is detected from an
increasing sample of Seyfert galaxies and a number of common properties are
observed, including a transition from the characteristic reverberation
signature at high frequencies to a hard lag within the continuum component at
low frequencies, as well a pronounced dip in the reverberation lag at 3keV.
These features are not trivially explained by the reverberation of X-rays
originating from simple point sources. We therefore model reverberation from
coronae extended both over the surface of the disc and vertically. Causal
propagation through its extent for both the simple case of constant velocity
propagation and propagation linked to the viscous timescale in the underlying
accretion disc is included as well as stochastic variability arising due to
turbulence locally on the disc. We find that the observed features of X-ray
reverberation in Seyfert galaxies can be explained if the long timescale
variability is dominated by the viscous propagation of fluctuations through the
corona. The corona extends radially at low height over the surface of the disc
but with a bright central region in which fluctuations propagate up the black
hole rotation axis driven by more rapid variability arising from the innermost
regions of the accretion flow
A soft x-ray reverberation lag in the agn ESO 113-G010
Reverberation lags have recently been discovered in a handful of nearby,
variable AGN. Here, we analyze a ~100 ksec archival XMM-Newton observation of
the highly variable AGN, ESO 113-G010 in order to search for lags between hard,
1.5 - 4.5 keV, and soft, 0.3 - 0.9 keV, energy X-ray bands. At the lowest
frequencies available in the lightcurve (<1.5E-4 Hz), we find hard lags where
the power-law dominated hard band lags the soft band (where the reflection
fraction is high). However, at higher frequencies in the range (2-3)E-4 Hz we
find a soft lag of -325 +/- 89 s. The general evolution from hard to soft lags
as the frequency increases is similar to other AGN where soft lags have been
detected. We interpret this soft lag as due to reverberation from the accretion
disk, with the reflection component responding to variability from the X-ray
corona. For a black hole mass of 7E6 M(solar) this corresponds to a
light-crossing time of ~9 R_g/c, however, dilution effects mean that the
intrinsic lag is likely longer than this. Based on recent black hole
mass-scaling for lag properties, the lag amplitude and frequency are more
consistent with a black hole a few times more massive than the best estimates,
though flux-dependent effects could easily add scatter this large
Discovery of fe Kα X-Ray reverberation around the black holes in MCG-5-23-16 and NGC 7314
Several X-ray observations have recently revealed the presence of
reverberation time delays between spectral components in AGN. Most of the
observed lags are between the power-law Comptonization component, seen
directly, and the soft excess produced by reflection in the vicinity of the
black hole. NGC 4151 was the first object to show these lags in the iron K
band. Here, we report the discovery of reverberation lags in the Fe K band in
two other sources: MCG-5-23-16 and NGC 7314. In both objects, the 6-7 keV band,
where the Fe K line peaks, lags the bands at lower and higher energies with a
time delay of ~ 1 kilo-seconds. These lags are unlikely to be due to the narrow
Fe K line. They are fully consistent with reverberation of the
relativistically-broadened iron K line. The measured lags, their time-scale and
spectral modeling, indicate that most of the radiation is emitted at ~ 5 and 24
gravitational radii for MCG-5-23-16 and NGC 7314 respectively
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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 active galactic nuclei (AGNs). 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 ± 3.4 ks at 9.58 × 10−6 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 AGNs 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.EMC gratefully acknowledges support from the NSF through CAREER award number AST-1351222. CSR thanks NASA for support under grant NNX15AU54G. ACF acknowledges ERC Advanced Grant 340442
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
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 archive above a nominal rms variability and exposure level (a total of 43 sources). Approximately 50 per cent of sources 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 time-scales, 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 versus 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.EK thanks Ari Laor for interesting discussions on this work and acknowledges support from the International Space Science Institute. This work is based on observations obtained with XMM–Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. EK thanks the Gates Cambridge Scholarship and the Hubble Fellowship Program. Support for Program number HSTHF2-51360.001-A was provided by NASA through a Hubble Fellowship grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. EK, WNA, and ACF acknowledge support from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 312789, StrongGravity. CSR acknowledges support from NASA under grant NNX14AF86G. EMC gratefully acknowledges support from the National Science Foundation through CAREER award number 1351222
Intensive disc-reverberation mapping of Fairall 9: 1st year of Swift & LCO monitoring
We present results of time-series analysis of the first year of the Fairall 9 intensive disc-reverberation campaign. We used Swift and the Las Cumbres Observatory global telescope network to continuously monitor Fairall 9 from X-rays to near-infrared at a daily to sub-daily cadence. The cross-correlation function between bands provides evidence for a lag spectrum consistent with the scaling expected for an optically thick, geometrically thin blackbody accretion disc. Decomposing the flux into constant and variable components, the variable component's spectral energy distribution is slightly steeper than the standard accretion disc prediction. We find evidence at the Balmer edge in both the lag and flux spectra for an additional bound-free continuum contribution that may arise from reprocessing in the broad-line region. The inferred driving light curve suggests two distinct components, a rapidly variable ( days) component with an opposite lag to the reverberation signal
The remarkable X-ray variability of IRAS 13224-3809 - I. The variability process
We present a detailed X-ray timing analysis of the highly variable NLS1
galaxy, IRAS 13224-3809. The source was recently monitored for 1.5 Ms with
XMM-Newton which, combined with 500 ks archival data, makes this the best
studied NLS1 galaxy in X-rays to date. We apply standard time- and
Fourier-domain in order to understand the underlying variability process. The
source flux is not distributed lognormally, as would be expected for accreting
sources. The first non-linear rms-flux relation for any accreting source in any
waveband is found, with . The light
curves exhibit significant strong non-stationarity, in addition to that caused
by the rms-flux relation, and are fractionally more variable at lower source
flux. The power spectrum is estimated down to Hz and consists of
multiple peaked components: a low-frequency break at Hz, with
slope down to low frequencies; an additional component breaking at
Hz. Using the high-frequency break we estimate the black hole
mass , and mass accretion rate
in Eddington units, . The non-stationarity is
manifest in the PSD with the normalisation of the peaked components increasing
with decreasing source flux, as well as the low-frequency peak moving to higher
frequencies. We also detect a narrow coherent feature in the soft band PSD at
mHz, modelled with a Lorentzian the feature has and an
%. We discuss the implication of these results for
accretion of matter onto black holes
Accreting Millisecond X-Ray Pulsars
Accreting Millisecond X-Ray Pulsars (AMXPs) are astrophysical laboratories
without parallel in the study of extreme physics. In this chapter we review the
past fifteen years of discoveries in the field. We summarize the observations
of the fifteen known AMXPs, with a particular emphasis on the multi-wavelength
observations that have been carried out since the discovery of the first AMXP
in 1998. We review accretion torque theory, the pulse formation process, and
how AMXP observations have changed our view on the interaction of plasma and
magnetic fields in strong gravity. We also explain how the AMXPs have deepened
our understanding of the thermonuclear burst process, in particular the
phenomenon of burst oscillations. We conclude with a discussion of the open
problems that remain to be addressed in the future.Comment: Review to appear in "Timing neutron stars: pulsations, oscillations
and explosions", T. Belloni, M. Mendez, C.M. Zhang Eds., ASSL, Springer;
[revision with literature updated, several typos removed, 1 new AMXP added
Is there a UV/X-ray connection in IRAS 13224-3809?
We present results from the optical, ultraviolet and X-ray monitoring of the NLS1 galaxy IRAS 13224-3809 taken with Swift and XMM-Newton during 2016. IRAS 13224-3809 is the most variable bright AGN in the X-ray sky and shows strong X-ray reflection, implying that the X-rays strongly illuminate the inner disc. Therefore, it is a good candidate to study the relationship between coronal X-ray and disc UV emission. However, we find no correlation between the X-ray and UV flux over the available ~40 day monitoring, despite the presence of strong X-ray variability and the variable part of the UV spectrum being consistent with irradiation of a standard thin disc. This means either that the X-ray flux which irradiates the UV emitting outer disc does not correlate with the X-ray flux in our line of sight and/or that another process drives the majority of the UV variability. The former case may be due to changes in coronal geometry, absorption or scattering between the corona and the disc
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