685 research outputs found
The curious time lags of PG 1244+026: Discovery of the iron K reverberation lag
High-frequency iron K reverberation lags, where the red wing of the line
responds before the line centroid, are a robust signature of relativistic
reflection off the inner accretion disc. In this letter, we report the
discovery of the Fe K lag in PG 1244+026 from ~120 ks of data (1 orbit of the
XMM-Newton telescope). The amplitude of the lag with respect to the continuum
is 1000 s at a frequency of ~1e-4 Hz. We also find a possible
frequency-dependence of the line: as we probe higher frequencies (i.e. shorter
timescales from a smaller emitting region) the Fe K lag peaks at the red wing
of the line, while at lower frequencies (from a larger emitting region) we see
the dominant reflection lag from the rest frame line centroid. The mean energy
spectrum shows a strong soft excess, though interestingly, there is no
indication of a soft lag. Given that this source has radio emission and it has
little reported correlated variability between the soft excess and the hard
band, we explore one possible explanation in which the soft excess in this
source is dominated by the steep power-law like emission from a jet, and that a
corona (or base of the jet) irradiates the inner accretion disc, creating the
blurred reflection features evident in the spectrum and the lag. General
Relativistic ray-tracing models fit the Fe K lag well, with the best-fit giving
a compact X-ray source at a height of 5 gravitational radii and a black hole
mass of 1.3e7 Msun.Comment: 6 pages, 6 figures, resubmitted to MNRAS after moderate revisions.
This paper focuses on the discovery of the Fe K reverberation lag in PG
1244+026. We point the interested reader to Alston, Done & Vaughan (See
today: arXiv:submit/0851673), which focuses on the soft lags in this sourc
Caught in the act: Measuring the changes in the corona that cause the extreme variability of 1H 0707-495
The X-ray spectra of the narrow line Seyfert 1 galaxy, 1H 0707-495, obtained
with XMM-Newton, from time periods of varying X-ray luminosity are analysed in
the context of understanding the changes to the X-ray emitting corona that lead
to the extreme variability seen in the X-ray emission from active galactic
nuclei (AGN). The emissivity profile of the accretion disc, illuminated by the
X-ray emitting corona, along with previous measurements of reverberation time
lags are used to infer the spatial extent of the X-ray source. By fitting a
twice-broken power law emissivity profile to the relativistically-broadened
iron K fluorescence line, it is inferred that the X-ray emitting corona expands
radially, over the plane of the accretion disc, by 25 to 30 per cent as the
luminosity increases, contracting again as the luminosity decreases, while
increases in the measured reverberation lag as the luminosity increases would
require also variation in the vertical extent of the source above the disc. The
spectrum of the X-ray continuum is found to soften as the total X-ray
luminosity increases and we explore the variation in reflected flux as a
function of directly-observed continuum flux. These three observations combined
with simple, first-principles models constructed from ray tracing simulations
of extended coron self-consistently portray an expanding corona whose average
energy density decreases, but with a greater number of scattering particles as
the luminosity of this extreme object increases.Comment: 12 pages, 4 figures. Accepted for publication in MNRA
Revealing the X-ray source in IRAS 13224-3809 through flux-dependent reverberation lags
IRAS 13224-3809 was observed in 2011 for 500 ks with the XMM-Newton
observatory. We detect highly significant X-ray lags between soft (0.3 - 1 keV)
and hard (1.2 - 5 keV) energies. The hard band lags the soft at low frequencies
(i.e. hard lag), while the opposite (i.e. soft lag) is observed at high
frequencies. In this paper, we study the lag during flaring and quiescent
periods. We find that the frequency and absolute amplitude of the soft lag is
different during high-flux and low-flux periods. During the low flux intervals,
the soft lag is detected at higher frequencies and with smaller amplitude.
Assuming that the soft lag is associated with the light travel time between
primary and reprocessed emission, this behaviour suggests that the X-ray source
is more compact during low-flux intervals, and irradiates smaller radii of the
accretion disc (likely because of light bending effects). We continue with an
investigation of the lag dependence on energy, and find that isolating the
low-flux periods reveals a strong lag signature at the Fe K line energy,
similar to results found using 1.3 Ms of data on another well known Narrow-Line
Seyfert I galaxy, 1H0707-495.Comment: 6 pages, 8 figures, accepted for publication in MNRA
X-ray reverberation around accreting black holes
Luminous accreting stellar mass and supermassive black holes produce
power-law continuum X-ray emission from a compact central corona. Reverberation
time lags occur due to light travel time-delays between changes in the direct
coronal emission and corresponding variations in its reflection from the
accretion flow. Reverberation is detectable using light curves made in
different X-ray energy bands, since the direct and reflected components have
different spectral shapes. Larger, lower frequency, lags are also seen and are
identified with propagation of fluctuations through the accretion flow and
associated corona. We review the evidence for X-ray reverberation in active
galactic nuclei and black hole X-ray binaries, showing how it can be best
measured and how it may be modelled. The timescales and energy-dependence of
the high frequency reverberation lags show that much of the signal is
originating from very close to the black hole in some objects, within a few
gravitational radii of the event horizon. We consider how these signals can be
studied in the future to carry out X-ray reverberation mapping of the regions
closest to black holes.Comment: 72 pages, 32 figures. Accepted for publication in The Astronomy and
Astrophysics Review. Corrected for mostly minor typos, but in particular
errors are corrected in the denominators of the covariance and rms spectrum
error equations (Eqn. 14 and 15
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.Comment: 10 pages, 12 figures, accepted for publication in MNRA
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.Comment: 11 pages, 8 figures, accepted for publication in MNRA
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.Comment: 5 pages, 4 figures, 1 table, MNRAS Letters in pres
The Closest Look at 1H0707-495: X-ray Reverberation Lags with 1.3 Ms of Data
Reverberation lags in AGN were first discovered in the NLS1 galaxy,
1H0707-495. We present a follow-up analysis using 1.3 Ms of data, which allows
for the closest ever look at the reverberation signature of this remarkable
source. We confirm previous findings of a hard lag of ~100 seconds at
frequencies v ~ [0.5 - 4] e-4 Hz, and a soft lag of ~30 seconds at higher
frequencies, v ~ [0.6 - 3] e-3 Hz. These two frequency domains clearly show
different energy dependences in their lag spectra. We also find evidence for a
signature from the broad Fe K line in the high frequency lag spectrum. We use
Monte Carlo simulations to show how the lag and coherence measurements respond
to the addition of Poisson noise and to dilution by other components. With our
better understanding of these effects on the lag, we show that the lag-energy
spectra can be modelled with a scenario in which low frequency hard lags are
produced by a compact corona responding to accretion rate fluctuations
propagating through an optically thick accretion disc, and the high frequency
soft lags are produced by short light-travel delay associated with reflection
of coronal power-law photons off the disc.Comment: 11 pages, 10 figures. Accepted for publication in MNRA
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