566 research outputs found
Assessing statistical significance of periodogram peaks
The least-squares (or Lomb-Scargle) periodogram is a powerful tool which is
used routinely in many branches of astronomy to search for periodicities in
observational data. The problem of assessing statistical significance of
candidate periodicities for different periodograms is considered. Based on
results in extreme value theory, improved analytic estimations of false alarm
probabilities are given. They include an upper limit to the false alarm
probability (or a lower limit to the significance). These estimations are
tested numerically in order to establish regions of their practical
applicability.Comment: 7 pages, 6 figures, 1 table; To be published in MNRA
Modeling the X-ray fractional variability spectrum of Active Galactic Nuclei using multiple flares
Using Monte-Carlo simulations of X-ray flare distributions across the
accretion disk of active galactic nuclei (AGN), we obtain modeling results for
the energy-dependent fractional variability amplitude. Referring to previous
results of this model, we illustrate the relation between the shape of the
point-to-point fractional variability spectrum, F_pp, and the time-integrated
spectral energy distribution, F_E. The results confirm that the spectral shape
and variability of the iron Kalpha line are dominated by the flares closest to
the disk center.Comment: 2 pages, 1 figure, conference proceedings of the AGN meeting held in
October 2006 in Xi'an, China. To appear in "The Central Engine of Active
Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP
QPO in RE J1034+396: model constraints from observed trends
We analyze the time variability of the X-ray emission of RE J1034+396, an
active galactic nucleus with the first firm detection of a quasi-periodic
oscillations (QPO). Based on the results of a wavelet analysis, we find a drift
in the QPO central frequency. The change inthe QPO frequency correlates with
the change in the X-ray flux with a short time delay. Linear structures such as
shocks, spiral waves, orvery distant flares seem to be a favored explanation
for this particular QPO event.Comment: to appear in the proceedings to "The Central Kiloparsec in Galactic
Nuclei (AHAR2011)", Journal of Physics: Conference Series (JPCS), IOP
Publishin
Conditions for the Thermal Instability in the Galactic Centre Mini-spiral region
We explore the conditions for the thermal instability to operate in the
mini-spiral region of the Galactic centre (Sgr A*), where both the hot and cold
media are known to coexist. The photoionisation Cloudy calculations are
performed for different physical states of plasma. We neglect the dynamics of
the material and concentrate on the study of the parameter ranges where the
thermal instability may operate, taking into account the past history of Sgr A*
bolometric luminosity. We show that the thermal instability does not operate at
the present very low level of the Sgr A* activity. However, Sgr A* was much
more luminous in the past. For the highest luminosity states the two-phase
medium can be created up to 1.4 pc from the centre. The presence of dust grains
tends to suppress the instability, but the dust is destroyed in the presence of
strong radiation field and hot plasma. The clumpiness is thus induced in the
high activity period, and the cooling/heating timescales are long enough to
preserve later the past multi-phase structure. The instability enhances the
clumpiness of the mini-spiral medium and creates a possibility of episodes of
enhanced accretion of cold clumps towards Sgr A*. The mechanism determines the
range of masses and sizes of clouds; under the conditions of Sgr A*, the likely
values come out - for the cloud typical mass.Comment: Accepted for publication in MNRAS, 10 pages, 7 figure
Constraining global parameters of accreting black holes by modeling magnetic flares
We present modeling results for the reprocessed radiation expected from
magnetic flares above AGN accretion disks. Relativistic corrections for the
orbital motion of the flare and for the curved space-time in the vicinity of
the black hole are taken into account. We investigate the local emission
spectra, as seen in a frame co-orbiting with the disk, and the observed spectra
at infinity. We investigate long-term flares at different orbital phases and
short-term flares for various global parameters of the accreting black hole.
Particular emphasis is put on the relation between the iron Kalpha line and the
Compton hump as these two features can be simultaneously observed by the Suzaku
satellite and later by Simbol-X.Comment: 4 pages, 1 figure, 1 table, proceedings for a poster at the
international conference "The Extreme Universe in the Suzaku Era" held in
Kyoto, Japan, December 4-8, 200
Variability of accretion flow in the core of the Seyfert galaxy NGC 4151
We analyze observations of the Seyfert galaxy NGC 4151 covering 90 years in
the optical band and 27 years in the 2-10 keV X-ray band. We compute the
Normalized Power Spectrum Density (NPSD), the Structure Function (SF) and the
Autocorrelation Function (ACF) for these data. The results show that the
optical and X-ray variability properties are significantly different. X-ray
variations are predominantly in the timescale range of 5 - 1000 days. The
optical variations have also a short timescale component which may be related
to X-ray variability but the dominant effect is the long timescale variability,
with timescales longer than 10 years. We compare our results with
observations of NGC 5548 and Cyg X-1. We conclude that the long timescale
variability may be caused by radiation pressure instability in the accretion
disk, although the observed timescale in NGC 4151 is by a factor of few longer
than expected. X-ray variability of this source is very similar to what is
observed in Cyg X-1 but scaled with the mass of the black hole, which suggests
that the radiation pressure instability does not affect considerably the X-ray
production.Comment: 21 pages, 19 figures, 4 tables, accepted for publication in MNRA
Iron lines from transient and persisting hot spots on AGN accretion disks
[abridged] We model the X-ray reprocessing from a strong co-rotating flare
above an accretion disk in active galactic nuclei. We explore the horizontal
structure and evolution of the underlying hot spot. To obtain the spectral
evolution seen by a distant observer, we apply a general relativity ray-tracing
technique. We concentrate on the energy band around the iron K-line, where the
relativistic effects are most pronounced. Persistent flares lasting for a
significant fraction of the orbital time scale and short, transient flares are
considered. In our time-resolved analysis, the spectra recorded by a distant
observer depend on the position of the flare/spot with respect to the central
black hole. If the flare duration significantly exceeds the light travel time
across the spot, then the spot horizontal stratification is unimportant. On the
other hand, if the flare duration is comparable to the light travel time across
the spot radius, the lightcurves exhibit a typical asymmetry in their time
profiles. The sequence of dynamical spectra proceeds from more strongly to less
strongly ionized re-emission. At all locations within the spot the spectral
intensity increases towards edge-on emission angles, revealing the limb
brightening effect. Future X-ray observatories with significantly larger
effective collecting areas will enable to spectroscopically map out the
azimuthal irradiation structure of the accretion disk and to localize
persistent flares. If the hot spot is not located too close to the marginally
stable orbit of the black hole, it will be possible to probe the reflecting
medium via the sub-structure of the iron K-line. Indications for transient
flares will only be obtained from analyzing the observed lightcurves on the
gravitational time scale of the accreting supermassive black hole.Comment: 15 pages, 8 figures, accepted by Astronomy & Astrophysic
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