562 research outputs found
Short Timescale AGN X-ray Variability with EXOSAT: Black hole mass and Normalised Variability Amplitude
The old EXOSAT medium energy measurements of high frequency (HF) AGN power
spectral normalisation are re-examined in the light of accurate black hole mass
determinations which were not available when these data were first published
(Green et al 1993). It is found that the normalised variability amplitude
(NVA), measured directly from the power spectrum, is proportional to M^{beta}
where beta ~ -0.54 +/- 0.08. As NVA is the square root of the power, these
observations show that the normalisation of the HF power spectrum for this
sample of AGN varies very close to inversely with black hole mass. Almost the
same value of is obtained whether the quasar 3C273 is included in the
sample or not, suggesting that the same process that drives X-ray variability
in Seyfert galaxies applies also to 3C273. These observations support the work
of Gierlinski et al (2008) who show that an almost exactly linear
anticorrelation is required if the normalisations of the HF power spectra of
AGN and X-ray binary systems are to scale similarly. These observations are
also consistent with a number of studies showing that the short timescale
variance of AGN X-ray lightcurves varies approximately inversely with mass.Comment: 6 pages, 2 figures, accepted for publication in Monthly Notices as a
lette
The swansong in context: long-timescale X-ray variability of NGC 4051
On 9-11 May 1998, the highly-variable, low luminosity Seyfert 1 galaxy
NGC4051 was observed in an unusual low flux state by BeppoSAX (Guainazzi et al.
1998) RXTE and EUVE. We present fits of the 4-15 keV RXTE spectrum and BeppoSAX
MECS spectrum obtained during this observation, which are consistent with the
interpretation that the source had switched off, leaving only the spectrum of
pure reflection from distant cold matter. We place this result in context by
showing the X-ray lightcurve of NGC4051 obtained by our RXTE monitoring
campaign over the past two and a half years, which shows that the low state
lasted for ~150 days before the May observations (implying that the reflecting
material is > 10^17 cm from the continuum source) and forms part of a
lightcurve showing distinct variations in long-term average flux over
timescales > months. We show that the long-timescale component to X-ray
variability is intrinsic to the primary continuum and is probably distinct from
the variability at shorter timescales, possibly associated with variations in
the accretion flow of matter onto the central black hole. As the source
approaches the low state, the variability process becomes non-linear. NGC4051
may represent a microcosm of all X-ray variability in radio quiet active
galactic nuclei (AGNs), displaying in a few years a variety of flux states and
variability properties which more luminous AGNs may pass through on timescales
of decades to thousands of years.Comment: 5 pages, accepted for publication in MNRA
Measuring the broadband power spectra of active galactic nuclei with RXTE
We have developed a Monte Carlo technique to test models for the true power
spectra of intermittently sampled lightcurves against the noisy, observed power
spectra, and produce a reliable estimate of the goodness of fit of the given
model. We apply this technique to constrain the broadband power spectra of a
sample of four Seyfert galaxies monitored by the Rossi X-ray Timing Explorer
(RXTE) over three years. We show that the power spectra of three of the AGN in
our sample (MCG-6-30-15, NGC5506 and NGC3516) flatten significantly towards low
frequencies, while the power spectrum of NGC5548 shows no evidence of
flattening. We fit two models for the flattening, a `knee' model, analogous to
the low-frequency break seen in the power spectra of BHXRBs in the low state
(where the power-spectral slope flattens to \alpha=0) and a `high-frequency
break' model (where the power-spectral slope flattens to \alpha=1), analogous
to the high-frequency break seen in the high and low-state power spectra of the
classic BHXRB Cyg X-1. Both models provide good fits to the power spectra of
all four AGN. For both models, the characteristic frequency for flattening is
significantly higher in MCG-6-30-15 than in NGC 3516 (by factor ~10) although
both sources have similar X-ray luminosities, suggesting that MCG-6-30-15 has a
lower black hole mass and is accreting at a higher rate than NGC 3516. Assuming
linear scaling of characteristic frequencies with black hole mass, the high
accretion rate implied for MCG-6-30-15 favours the high-frequency break model
for this source and further suggests that MCG-6-30-15 and possibly NGC 5506,
may be analogues of Cyg X-1 in the high state [ABRIDGED].Comment: 23 pages, accepted for publication in MNRA
The unusual host galaxy of the BL Lac object PKS 1413+135
The BL Lacertae object PKS 1413+135 is associated with a disk dominated
galaxy which heavily absorbs the BL Lac nucleus at optical and X-ray
wavelengths. It has been argued whether this galaxy is actually the host galaxy
of PKS 1413+135 or whether the BL Lac is a background QSO, gravitationally
lensed by the apparent host galaxy. We have obtained deep high resolution
H-band images of this unusual BL Lac object using the UKIRT IRCAM3. Our
observations show that the BL Lac nucleus is centered within < 0.05 arcsec of
the galaxy. Based on this result we assess the probability for the lensing
scenario and come to the conclusion that the disk galaxy is indeed the host of
PKS 1413+135. The galaxy shows peanut-shaped isophotes, suggesting the presence
of a central bar which is a common feature of AGNComment: accepted for publication in MNRAS, 5 pages with 2 Postscript figure
General relativistic modelling of the negative reverberation X-ray time delays in AGN
We present the first systematic physical modelling of the time-lag spectra
between the soft (0.3-1 keV) and the hard (1.5-4 keV) X-ray energy bands, as a
function of Fourier frequency, in a sample of 12 active galactic nuclei which
have been observed by XMM-Newton. We concentrate particularly on the negative
X-ray time-lags (typically seen above Hz) i.e. soft band variations
lag the hard band variations, and we assume that they are produced by
reprocessing and reflection by the accretion disc within a lamp-post X-ray
source geometry. We also assume that the response of the accretion disc, in the
soft X-ray bands, is adequately described by the response in the neutral iron
line (Fe k) at 6.4 keV for which we use fully general relativistic
ray-tracing simulations to determine its time evolution. These response
functions, and thus the corresponding time-lag spectra, yield much more
realistic results than the commonly-used, but erroneous, top-hat models.
Additionally we parametrize the positive part of the time-lag spectra
(typically seen below Hz) by a power-law. We find that the
best-fitting BH masses, M, agree quite well with those derived by other
methods, thus providing us with a new tool for BH mass determination. We find
no evidence for any correlation between M and the BH spin parameter, ,
the viewing angle, , or the height of the X-ray source above the disc,
. Also on average, the X-ray source lies only around 3.7 gravitational radii
above the accretion disc and the viewing angles are distributed uniformly
between 20 and 60 degrees. Finally, there is a tentative indication that the
distribution of spin parameters may be bimodal above and below 0.62.Comment: Accepted for publication in MNRAS. The paper is 22 pages long and
contains 19 figures and 2 table
Consistency between the radio and MIR faint source counts using the radio-MIR correlation
We show from the recent extrapolation of the radio-FIR correlation to the MIR
that the 20 cm and 15 um differential source counts are likely to come from the
same parent population.Comment: 2 pages, 2 figures, to appear in proceedings of 'Multi-wavelength AGN
surveys', Cozumel, 200
Extensive X-ray variability studies of NGC 7314 using long XMM-Newton observations
We present a detailed X-ray variability study of the low mass Active Galactic
Nuclei (AGN) NGC 7314 using the two newly obtained XMM-Newton observations
( and ks), together with two archival data sets of shorter duration
( and ks). The relationship between the X-ray variability
characteristics and other physical source properties (such as the black hole
mass) are still relatively poorly defined, especially for low-mass AGN. We
perform a new, fully analytical, power spectral density (PSD) model analysis
method, which will be described in detail in a forthcoming paper, that takes
into consideration the spectral distortions, caused by red-noise leak. We find
that the PSD in the keV energy range, can be represented by a bending
power-law with a bend around Hz, having a slope of
and below and above the bend, respectively. Adding our bend time-scale
estimate, to an already published ensemble of estimates from several AGN,
supports the idea that the bend time-scale depends linearly only on the black
hole mass and not on the bolometric luminosity. Moreover, we find that as the
energy range increases, the PSD normalization increases and there is a hint
that simultaneously the high frequency slope becomes steeper. Finally, the
X-ray time-lag spectrum of NGC 7314 shows some very weak signatures of
relativistic reflection, and the energy resolved time-lag spectrum, for
frequencies around Hz, shows no signatures of X-ray
reverberation. We show that the previous claim about ks time-delays in this
source, is simply an artefact induced by the minuscule number of points
entering during the time-lag estimation in the low frequency part of the
time-lag spectrum (i.e. below Hz).Comment: Accepted for publication in MNRAS. The paper is 21 pages long and
contains 15 figures and 3 table
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