153 research outputs found
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
XMM-Newton study of the complex and variable spectrum of NGC 4051
We study the X-ray spectral variability of the Narrow Line Seyfert 1 galaxy
NGC 4051 as observed during two XMM-Newton observations. The data show evidence
for a neutral and constant reflection component and for constant emission from
photoionized gas, which are included in all spectral models. The nuclear
emission can be modelled both in terms of a ``standard model'' (pivoting power
law plus a black body component for the soft excess) and of a two--component
one (power law plus ionized reflection from the accretion disc). The standard
model results indicate that the soft excess does not follow the standard black
body law. Moreover, although the spectral slope is correlated with flux, which
is consistent with spectral pivoting, the hardest photon indexes are so flat as
to require rather unusual scenarios. These problems can be solved in terms of
the two-component model in which the soft excess is not thermal, but due to the
ionized reflection component. The variability of the reflection component from
the inner disc closely follows the predictions of the light bending model,
suggesting that most of the primary nuclear emission is produced in the very
innermost regions, only a few gravitational radii from the central black hole.
(abridged)Comment: accepted for publication in MNRA
Have we detected the most luminous ULX so far?
We report the XMM-Newton detection of a moderately bright X-ray source
superimposed on the outer arms of the inactive spiral galaxy MCG-03-34-63
(z=0.0213). It is clearly offset from the nucleus (by about 19'') but well
within the D25 ellipse of the galaxy, just along its bar axis. The field has
also been observed with the HST enabling us to compute a lower limit of > 94 on
the X-ray to optical flux ratio which, together with the X-ray spectrum of the
source, argues against a background AGN. On the other hand, the detection of
excess X-ray absorption and the lack of a bright optical counterpart argue
against foreground contamination. Short-timescale variability is observed,
ruling out the hypothesis of a particularly powerful supernova. If it is
associated with the apparent host galaxy, the source is the most powerful ULX
detected so far with a peak luminosity of 1.35x10^41 erg/s in the 0.5-7 keV
band. If confirmed by future multi-wavelength observations, the inferred
bolometric luminosity (about 3x10^41 erg/s) requires a rather extreme beaming
factor (larger than 115) to accommodate accretion onto a stellar-mass black
hole of 20 solar masses and the source could represent instead one of the best
intermediate-mass black hole candidate so far. If beaming is excluded, the
Eddington limit implies a mass of >2300 solar masses for the accreting compact
object.Comment: MNRAS Letters in press; minor correction at the end of Section
Accretion Disk Illumination in Schwarzschild and Kerr Geometries: Fitting Formulae
We describe the methodology and compute the illumination of geometrically
thin accretion disks around black holes of arbitrary spin parameter exposed
to the radiation of a point-like, isotropic source at arbitrary height above
the disk on its symmetry axis. We then provide analytic fitting formulae for
the illumination as a function of the source height and the black hole
angular momentum . We find that for a source on the disk symmetry axis and
, the main effect of the parameter is allowing the disk to extend
to smaller radii (approaching as ) and thus allow the
illumination of regions of much higher rotational velocity and redshift. We
also compute the illumination profiles for anisotropic emission associated with
the motion of the source relative to the accretion disk and present the
fractions of photons absorbed by the black hole, intercepted by the disk or
escaping to infinity for both isotropic and anisotropic emission for
and . As the anisotropy (of a source approaching the disk) increases
the illumination profile reduces (approximately) to a single power-law, whose
index, , because of absorption of the beamed photons by the black hole,
saturates to a value no higher than . Finally, we compute the
fluorescence Fe line profiles associated with the specific illumination and
compare them among various cases.Comment: 26 pages, 21 b/w figures, accepted for publication in the
Astrophysical Journal as of 4/16/200
X-ray Spectral and Variability Properties of Low-Mass AGN
We study the X-ray properties of a sample of 14 optically-selected low-mass
AGN whose masses lie within the range 1E5 -2E6 M(solar) with XMM-Newton. Only
six of these low-mass AGN have previously been studied with sufficient quality
X-ray data, thus, we more than double the number of low-mass AGN observed by
XMM-Newton with the addition of our sample. We analyze their X-ray spectral
properties and variability and compare the results to their more massive
counterparts. The presence of a soft X-ray excess is detectable in all five
objects which were not background dominated at 2-3 keV. Combined with previous
studies, this gives a total of 8 low-mass AGN with a soft excess. The low-mass
AGN exhibit rapid, short-term variability (hundreds to thousands of seconds) as
well as long-term variability (months to years). There is a well-known
anti-correlation between black hole mass and variability amplitude (normalized
excess variance). Comparing our sample of low-mass AGN with this relation we
find that all of our sample lie below an extrapolation of the linear relation.
Such a flattening of the relation at low masses (below about 1E6 M(solar)) is
expected if the variability in all AGN follows the same shape power spectrum
with a break frequency that is dependent on mass. Finally, we also found two
objects that show significant absorption in their X-ray spectrum, indicative of
type 2 objects, although they are classified as type 1 AGN based on optical
spectra.Comment: 12 pages, 5 figures, 7 tables, accepted for publication in MNRA
Suzaku observations of Markarian 335: evidence for a distributed reflector
We report on a 151 ks net exposure Suzaku observation of the Narrow Line
Seyfert 1 galaxy Mrk 335. The 0.5-40 keV spectrum contains a broad Fe line, a
strong soft excess below about 2 keV and a Compton hump around 20-30 keV. We
find that a model consisting of a power law and two reflectors provides the
best fit to the time-averaged spectrum. In this model, an ionized, heavily
blurred, inner reflector produces most of the soft excess, while an almost
neutral outer reflector (outside ~40 r_g) produces most of the Fe line
emission. The spectral variability of the observation is characterised by
spectral hardening at very low count rates. In terms of our power-law +
two-reflector model it seems like this hardening is mainly caused by pivoting
of the power law. The rms spectrum of the entire observation has the curved
shape commonly observed in AGN, although the shape is significantly flatter
when an interval which does not contain any deep dip in the lightcurve is
considered. We also examine a previous 133 ks XMM-Newton observation of Mrk
335. We find that the XMM-Newton spectrum can be fitted with a similar
two-reflector model as the Suzaku data and we confirm that the rms spectrum of
the observation is flat. The flat rms spectra, as well as the high-energy data
from the Suzaku PIN detector, disfavour an absorption origin for the soft
excess in Mrk 335.Comment: 13 pages, 13 figures. Accepted for publication in MNRA
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
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.Comment: Resubmitted to MNRAS after minor corrections. 11 pages, 10 figure
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