834 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
A Slower Superluminal Velocity for the Quasar 1156+295
As part of an ongoing effort to observe high energy gamma-ray blazars with
VLBI, we have produced 8 and 2 GHz VLBI images, at ten epochs spanning the
years 1988 to 1996, of the quasar 1156+295. The VLBI data have been taken from
the Washington VLBI correlator's geodetic database. We have detected detected
four components and have measured their apparent speeds to be 8.8 +/- 2.3, 5.3
+/- 1.1, 5.5 +/- 0.9, and 3.5 +/- 1.2 h^{-1}c from the outermost component
inwards. (H_{0}=100h km/(s Mpc), q_{0}=0.5 throughout paper). These velocities
contradict a previously published very high superluminal velocity of 26 h^{-1}c
for this source.Comment: 8 pages, 2 figures, accepted to ApJ Letter
Generating artificial light curves: Revisited and updated
The production of artificial light curves with known statistical and
variability properties is of great importance in astrophysics. Consolidating
the confidence levels during cross-correlation studies, understanding the
artefacts induced by sampling irregularities, establishing detection limits for
future observatories are just some of the applications of simulated data sets.
Currently, the widely used methodology of amplitude and phase randomisation is
able to produce artificial light curves which have a given underlying power
spectral density (PSD) but which are strictly Gaussian distributed. This
restriction is a significant limitation, since the majority of the light curves
e.g. active galactic nuclei, X-ray binaries, gamma-ray bursts show strong
deviations from Gaussianity exhibiting `burst-like' events in their light
curves yielding long-tailed probability distribution functions (PDFs). In this
study we propose a simple method which is able to precisely reproduce light
curves which match both the PSD and the PDF of either an observed light curve
or a theoretical model. The PDF can be representative of either the parent
distribution or the actual distribution of the observed data, depending on the
study to be conducted for a given source. The final artificial light curves
contain all of the statistical and variability properties of the observed
source or theoretical model i.e. same PDF and PSD, respectively. Within the
framework of Reproducible Research, the code, together with the illustrative
example used in this manuscript, are both made publicly available in the form
of an interactive Mathematica notebook.Comment: Accepted for publication in MNRAS. The paper is 23 pages long and
contains 21 figures and 2 tables. The Mathematica notebook can be found in
the web as part of this paper (Online Material) or at
http://www.astro.soton.ac.uk/~de1e08/ArtificialLightCurves
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
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