1,664 research outputs found
Investigating a Fluctuating-accretion Model for the Spectral-timing Properties of Accreting Black Hole Systems
The fluctuating accretion model of Lyubarskii (1997) and its extension by
Kotov et al. (2001), seeks to explain the spectral-timing properties of the
X-ray variability of accreting black holes in terms of inward-propagating mass
accretion fluctuations produced at a broad range of radii. The fluctuations
modulate the X-ray emitting region as they move inwards and can produce
temporal-frequency-dependent lags between energy bands, and energy-dependent
power spectral densities (PSDs) as a result of the different emissivity
profiles, which may be expected at different X-ray energies. Here we use a
simple numerical implementation to investigate in detail the X-ray
spectral-timing properties of the model and their relation to several
physically interesting parameters, namely the emissivity profile in different
energy bands, the geometrical thickness and viscosity parameter of the
accretion flow, the strength of damping on the fluctuations and the temporal
coherence (measured by the `quality-factor', Q) of the fluctuations introduced
at each radius. We find that a geometrically thick flow with large viscosity
parameter is favoured, and confirm that the predicted lags are quite robust to
changes in the emissivity profile, and physical parameters of the accretion
flow, which may help to explain the similarity of the lag spectra in the
low/hard and high/soft states of Cyg X-1. We also demonstrate the model regime
where the light curves in different energy bands are highly spectrally
coherent. We compare model predictions directly to X-ray data from the Narrow
Line Seyfert~1 galaxy NGC 4051 and the BHXRB Cyg X-1 in its high/soft state and
show that this general scheme can reproduce simultaneously the time lags and
energy-dependence of the PSD.Comment: 15 pages, accepted for publication in MNRA
On sign-changeable interaction in FLRW cosmology
We investigate an interacting two-fluid model in a spatially flat
Friedmann-Lema\^itre-Robertson-Walker (FLRW) Universe, when the energy transfer
between these two dark components is produced by a factorisable nonlinear
sign-changeable interaction depending linearly on the energy density and
quadratically on the deceleration parameter. We solve the source equation and
obtain the effective energy densities of the dark sector and their components.
We show that the effective equation of state of the dark sector includes some
of the several kind of Chaplygin gas equations of state as well as a
generalization of the polytropic equation of state. We use bayesian statistics
methods to constrain free parameters in the models during its most recent
evolution considering supernovae type Ia and measurements of the Hubble
expansion rate. The resulting constraints provide new information on
sign-changeable interactions, its equivalences and compatibility with previous
models and novel late time universe dynamics.Comment: 8 figure
The Energy-dependent X-ray Timing Characteristics of the Narrow Line Seyfert 1 Mkn 766
We present the energy-dependent power spectral density (PSD) and
cross-spectral properties of Mkn 766, obtained from combining data obtained
during an XMM-Newton observation spanning six revolutions in 2005 with data
obtained from an XMM-Newton long-look in 2001. The PSD shapes and rms-flux
relations are found to be consistent between the 2001 and 2005 observations,
suggesting the 2005 observation is simply a low-flux extension of the 2001
observation and permitting us to combine the two data sets. The resulting PSD
has the highest temporal frequency resolution for any AGN PSD measured to date.
Applying a broken power-law model yields break frequencies which increase in
temporal frequency with photon energy. Obtaining a good fit when assuming
energy-independent break frequencies requires the presence of a Lorentzian at
4.6+/-0.4 * 10^-4 Hz whose strength increases with photon energy, a behavior
seen in black hole X-ray binaries. The cross-spectral properties are measured;
temporal frequency-dependent soft-to-hard time lags are detected in this object
for the first time. Cross-spectral results are consistent with those for other
accreting black hole systems. The results are discussed in the context of
several variability models, including those based on inwardly-propagating
viscosity variations in the accretion disk.Comment: Accepted for publication in The Astrophysical Journal. 18 pages, 9
figures. Uses emulateapj5.st
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