806 research outputs found
Discovery of multiple Lorentzian components in the X-ray timing properties of the Narrow Line Seyfert 1 Ark 564
We present a power spectral analysis of a 100 ksec XMM-Newton observation of
the narrow line Seyfert 1 galaxy Ark~564. When combined with earlier RXTE and
ASCA observations, these data produce a power spectrum covering seven decades
of frequency which is well described by a power law with two very clear breaks.
This shape is unlike the power spectra of almost all other AGN observed so far,
which have only one detected break, and resemble Galactic binary systems in a
soft state. The power spectrum can also be well described by the sum of two
Lorentzian-shaped components, the one at higher frequencies having a hard
spectrum, similar to those seen in Galactic binary systems. Previously we have
demonstrated that the lag of the hard band variations relative to the soft band
in Ark 564 is dependent on variability time-scale, as seen in Galactic binary
sources. Here we show that the time-scale dependence of the lags can be
described well using the same two-Lorentzian model which describes the power
spectrum, assuming that each Lorentzian component has a distinct time lag. Thus
all X-ray timing evidence points strongly to two discrete, localised, regions
as the origin of most of the variability. Similar behaviour is seen in Galactic
X-ray binary systems in most states other than the soft state, i.e. in the
low-hard and intermediate/very high states. Given the very high accretion rate
of Ark 564 the closest analogy is with the very high (intermediate) state
rather than the low-hard state. We therefore strengthen the comparison between
AGN and Galactic binary sources beyond previous studies by extending it to the
previously poorly studied very high accretion rate regime.Comment: 11 pages, 11 figures, accepted for publication in MNRA
Simultaneous EUV and X-ray variability of NGC 4051
We present a flux variability study of simultaneous RXTE and EUVE
observations of the highly variable Seyfert galaxy NGC4051. We find a strong
correlation between variability in the EUV and medium energy X-ray
bands,indicating that both are sampling the same power-law continuum. The lag
between the two bands is less than 20 ks and, depending on model assumptions,
may be <1 ks. We examine the consequences of such a small lag in the context of
simple Comptonisation models for the production of the power-law continuum. A
lag of <1 ks implies that the size of the Comptonising region is less than 20
Schwarzschild radii for a black hole of mass >1E6 solar masses.Comment: 8 pages, accepted for publication in MNRA
Higher order glass-transition singularities in colloidal systems with attractive interactions
The transition from a liquid to a glass in colloidal suspensions of particles
interacting through a hard core plus an attractive square-well potential is
studied within the mode-coupling-theory framework. When the width of the
attractive potential is much shorter than the hard-core diameter, a reentrant
behavior of the liquid-glass line, and a glass-glass-transition line are found
in the temperature-density plane of the model. For small well-width values, the
glass-glass-transition line terminates in a third order bifurcation point, i.e.
in a A_3 (cusp) singularity. On increasing the square-well width, the
glass-glass line disappears, giving rise to a fourth order A_4 (swallow-tail)
singularity at a critical well width. Close to the A_3 and A_4 singularities
the decay of the density correlators shows stretching of huge dynamical
windows, in particular logarithmic time dependence.Comment: 19 pages, 12 figures, Phys. Rev. E, in prin
X-ray spectral variability of the Seyfert galaxy NGC 4051
We report on the X-ray spectral variability of the Seyfert 1 galaxy NGC 4051
observed with the Rossi X-ray Timing Explorer (RXTE) during a 1000 day period
between May 1996 and March 1999. The spectra were obtained as part of
monitoring observations and from two long observations using the RXTE
Proportional Counter Array (PCA). During the monitoring period the 2-10 keV
flux of NGC 4051 varied between 10E-12 and 7x 10E11 (cgs). We re-analysed RXTE
PCA observations from a distinct low state in May 1998 using the latest
background and detector response models. The RXTE and BeppoSAX observations of
NGC 4051 during the low state show a very hard spectrum with a strong
unresolved fluorescence line. This emission, probably due to reflection from a
molecular torus, is likely to be constant over long time-scales and is
therefore assumed as an underlying component at all flux states. By subtracting
the torus component we are able to determine the spectral variability of the
primary continuum. In the variable component we observe a strong
anti-correlation of X-ray flux and spectral hardness in the PCA energy band. We
show that the changes in hardness are caused by slope variability of the
primary power law spectrum rather than by changing reflection or variable
photoelectric absorption. The primary spectral index varies between Gamma=1.6
for the faintest states and Gamma=2.3 during the brightest states, at which
level the spectral index approaches an asympotic value. We find that the
response of the flux of the 6.4 keV iron fluorescence line to changes in the
continuum flux depends on the timescale of the observation. The profile of the
line is very broad and indicates an origin in the innermost regions of the
accretion disk.Comment: accepted for publication in MNRA
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