211 research outputs found
Short term X-ray rms variability of Cyg X-1
A linear dependence of the amplitude of broadband noise variability on flux
for GBHC and AGN has been recently shown by Uttley & McHardy (2001). We present
the long term evolution of this rms-flux-relation for Cyg X-1 as monitored from
1998-2002 with RXTE. We confirm the linear relationship in the hard state and
analyze the evolution of the correlation for the period of 1996-2002. In the
intermediate and the soft state, we find considerable deviations from the
otherwise linear relationship. A possible explanation for the rms-flux-relation
is a superposition of local mass accretion rate variations.Comment: 3 pages, 3 figures, Proceedings of the 4th Microquasar Workshop, eds.
Ph Durouchoux, Y. Fuchs and J. Rodriguez, published by the Center for Space
Physics: Kolkat
INTEGRAL-RXTE observations of Cygnus X-1
We present first results from contemporaneous observations of Cygnus X-1 with
INTEGRAL and RXTE, made during INTEGRAL's performance verification phase in
2002 November and December. Consistent with earlier results, the 3-250 keV data
are well described by Comptonization spectra from a Compton corona with a
temperature of kT~50-90 keV and an optical depth of tau~1.0-1.3 plus reflection
from a cold or mildly ionized slab with a covering factor of Omega/2pi~0.2-0.3.
A soft excess below 10 keV, interpreted as emission from the accretion disk, is
seen to decrease during the 1.5 months spanned by our observations. Our results
indicate a remarkable consistency among the independently calibrated detectors,
with the remaining issues being mainly related to the flux calibration of
INTEGRAL.Comment: 6 pages, 3 figures. Figs. 2 and 3 are best viewed in color. Accepted
for publication in the INTEGRAL special edition of A&A
Spectro-timing analysis of Cygnus X-1 during a fast state transition
We present the analysis of two long, quasi-uninterrupted RXTE observations of
Cygnus X-1 that span several days within a 10 d interval. The spectral
characteristics during this observation cover the region where previous
observations have shown the source to be most dynamic. Despite that the source
behavior on time scales of hours and days is remarkably similar to that on year
time scales. This includes a variety of spectral/temporal correlations that
previously had only been observed over Cyg X-1's long-term evolution.
Furthermore, we observe a full transition from a hard to a soft spectral state
that occurs within less than 2.5 hours - shorter than previously reported for
any other similar Cyg X-1 transition. We describe the spectra with a
phenomenological model dominated by a broken power law, and we fit the X-ray
variability power spectra with a combination of a cutoff power law and
Lorentzian components. The spectral and timing properties are correlated: the
power spectrum Lorentzian components have an energy-dependent amplitude, and
their peak frequencies increase with photon spectral index. Averaged over
3.2-10 Hz, the time lag between the variability in the 4.5-5.7 keV and 9.5-15
keV bands increases with decreasing hardness when the variability is dominated
by the Lorentzian components during the hard state. The lag is small when there
is a large power law noise contribution, shortly after the transition to the
soft state. Interestingly, the soft state not only shows the shortest lags, but
also the longest lags when the spectrum is at its softest and faintest. We
discuss our results in terms of emission models for black hole binaries.Comment: 13 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
Short-Term Radio-X-ray Correlations of Cygnus X-1
We analyze simultaneous radio-X-ray data of Cygnus X-1 from the Ryle
telescope (RT) and RXTE over more than 4 a. We show that apparent correlations
on short time scales in the lightcurves of Cyg X-1 are probably the
coincidental outcome of white noise statistics.Comment: 1 page, 1 figure. Proceedings of the IAU Colloquium 194 "Compact
Binaries in the Galaxy and Beyond", to be published in: Revista Mexicana de
Astronomia y Astrofisica, Conference Serie
X-ray variation statistics and wind clumping in Vela X-1
We investigate the structure of the wind in the neutron star X-ray binary
system Vela X-1 by analyzing its flaring behavior. Vela X-1 shows constant
flaring, with some flares reaching fluxes of more than 3.0 Crab between 20-60
keV for several 100 seconds, while the average flux is around 250 mCrab. We
analyzed all archival INTEGRAL data, calculating the brightness distribution in
the 20-60 keV band, which, as we show, closely follows a log-normal
distribution. Orbital resolved analysis shows that the structure is strongly
variable, explainable by shocks and a fluctuating accretion wake. Analysis of
RXTE ASM data suggests a strong orbital change of N_H. Accreted clump masses
derived from the INTEGRAL data are on the order of 5 x 10^19 -10^21 g. We show
that the lightcurve can be described with a model of multiplicative random
numbers. In the course of the simulation we calculate the power spectral
density of the system in the 20-100 keV energy band and show that it follows a
red-noise power law. We suggest that a mixture of a clumpy wind, shocks, and
turbulence can explain the measured mass distribution. As the recently
discovered class of supergiant fast X-ray transients (SFXT) seems to show the
same parameters for the wind, the link between persistent HMXB like Vela X-1
and SFXT is further strengthened.Comment: 8 pages, 6 figures, accepted for publication in A&
Long term variability of Cygnus X-1, III. Radio-X-ray correlations
Long time scale radio-X-ray correlations in black holes during the hard state
have been found in many sources and there seems to emerge a universal
underlying relationship which quantitatively describes this behavior. Although
it would appear only natural to detect short term emission patterns in the
X-ray and - with a certain time lag - in the radio, there has been little
evidence for this up to now. The most prominent source for radio-X-ray
correlations on short time scales (minutes) so far remains GRS 1915+105 where a
single mass ejection could be detected successively in the X-ray, IR, and radio
wavebands. We analyze a database of more than 4 years of simultaneous
radio-X-ray data for Cygnus X-1 from the Ryle Telescope and RXTE PCA/HEXTE. We
confirm the existence of a radio-X-ray correlation on long time scales,
especially at hard energies. We show that apparent correlations on short time
scales in the lightcurves of Cygnus X-1 are most likely the coincidental
outcome of white noise statistics. Interpreting this result as a breakdown of
radio-X-ray correlations on shorter time scales, this sets a limit to the speed
of the jet.Comment: 8 pages, 7 figures, accepted for publication in A&
The rms-flux relations in different branches in Cyg X-2
In this paper, the rms-flux (root mean square-flux) relation along the
Z-track of the bright Z-Source Cyg X-2 is analyzed using the observational data
of Rossi X-ray Timing Explorer (RXTE). Three types of rms-flux relations, i.e.
positive, negative, and 'arch'-like correlations are found in different
branches. The rms is positively correlated with flux in normal branch (NB), but
anti-correlated in the vertical horizontal branch (VHB). The rms-flux relation
shows an 'arch'-like shape in the horizontal branch (HB). We also try to
explain this phenomenon using existing models.Comment: Accepted for publication in Astrophysics & Space Scienc
States and transitions in black-hole binaries
With the availability of the large database of black-hole transients from the
Rossi X-Ray Timing Explorer, the observed phenomenology has become very
complex. The original classification of the properties of these systems in a
series of static states sorted by mass accretion rate proved not to be able to
encompass the new picture. I outline here a summary of the current situation
and show that a coherent picture emerges when simple properties such as X-ray
spectral hardness and fractional variability are considered. In particular,
fast transition in the properties of the fast time variability appear to be
crucial to describe the evolution of black-hole transients. Based on this
picture, I present a state-classification which takes into account the observed
transitions. I show that, in addition to transients systems, other black-hole
binaries and Active Galactic Nuclei can be interpreted within this framework.
The association between these states and the physics of the accretion flow
around black holes will be possible only through modeling of the full time
evolution of galactic transient systems.Comment: 30 pages, 11 figures, To appear in Belloni, T. (ed.): The Jet
Paradigm - From Microquasars to Quasars, Lect. Notes Phys. 794 (2009
Long Term Variability of Cyg X-1, II. The rms-Flux Relation
We study the long term evolution of the relationship between the root mean
square (rms) variability and flux (the ``rms-flux relation'') for the black
hole Cygnus X-1 as monitored from 1996 to 2003 with the Rossi X-ray Timing
Explorer (RXTE). We confirm earlier results by Uttley & McHardy (2001) of a
linear relationship between rms and flux in the hard state on time scales > 5 s
reflecting in its slope the fractional rms variability. We demonstrate the
perpetuation of the linear rms-flux relation in the soft and the intermediate
state. The existence of a non-zero intercept in the linear rms-flux relation
argues for two lightcurve components, for example, one variable and one
non-variable component, or a possible constant rms component. The relationship
between these two hypothesized components can be described by a fundamental
dependence of slope and intercept at time scales ~< 10 ksec with long term
averages of the flux.Comment: 14 pages, 1 table, 15 figures, accepted for publication in A&A,
corrected layou
Patientsâ Perceptions of Memory Functioning Before and After Surgical Intervention to Treat Medically Refractory Epilepsy.
Purpose:One risk associated with epilepsy surgery is memory loss, but perhaps more important is how patients perceive changes in their memories. This longitudinal study evaluated changes in memory self-reports and investigated how self-reports relate to changes on objective memory measures in temporal or extratemporal epilepsy patients who underwent surgery.
Methods: Objective memory (Wechsler Memory ScaleâRevised) and subjective memory self-reports (Memory Assessment Clinics Self-Rating Scale) were individually assessed for 136 patients âŒ6 months before and 6 months after surgery. A measure of depressive affect (Beck Depression Inventoryâ2nd Edition) was used to control variance attributable to emotional distress.
Results: Despite a lack of significant correlational relationships between objective and subjective memory for the entire sample, significant correlations between objective memory scores and self-reports did emerge for a subset of patients who evidenced memory decline. Differences also were found in the subjective memory ratings of temporal lobe versus extratemporal patients. Temporal lobe patients rated their memories more negatively than did extratemporal patients and were more likely to report significant improvements in their memory after surgery.
Conclusions: In general, patients were not accurate when rating their memories compared to other adults. However, patients with significant declines in their memories were sensitive to actual changes in their memories over time relative to their own personal baselines
- âŠ