The ubiquity and behaviour of the rms-flux relation in Black Holes
Abstract
The X-ray timing properties of accreting compact objects provide an opportunity to study changes in the accretion flow and its properties very close to the accreting source. Among the more recent discoveries in this area is that of a simple linear scaling of rms variability amplitude and flux -the power spectrum is shifted to higher amplitudes during periods of high flux such that the rms over a frequency band scales with flux. This effect has been seen in several AGN (Seyfert 1 galaxies), one Neutron star (NS) X-ray binary (SAX J1808.4-3658) and a few Galactic black hole X-ray binaries (BH XRBs), notably Cygnus X-1. The prevalence of this relation would seem to indicate it is common and basic property of accretion onto compact objects (or, at least, luminous black holes and NSs). This simple rms-flux relation explains and predicts several observational properties such as millisecond flares in XRBs, the lognormal distribution of fluxes, and prolonged periods of low flux in AGN. It also has important consequences for models of the variability process: the linearity of the rms-flux relation over a wide range of time scales rules out simple shot noise models (which rely on each shot being independent), but are naturally explained by models that involve fluctuations propagating through the accretion flow and modulating the flow (and hence fluctuations) produced at different radii. In order to test the ubiquity of the linear rms-flux relation, and its dependence on timescale, energy, source state, and so on we have analysed most of the data available in the RXTE archive for 9 BH XRBs (2077 observations), the majority of which showed a clear rms-flux relation. This is an indicator that a linear rms-flux relation is indeed a common property of BH XRBs. Most of the observations have been taken when the sources are in outburst, the relation has been observed in a range of different states and variability levels. We have compared the gradient (k) and the x-intercept (C) over the course of these outbursts and observe not only clear differences in these parameters dependent on spectral hardness, but also striking similarities between different sources. By taking the average of the rms and flux in each observation it is also possible to study the long term rms-flux relation, this long term relationship shows changes and distinct patterns relating to hardness over the course of the outburst. We will also discuss the energy and frequency dependence of this relation in individual observations. Finally, we will discuss an analysis of the rms-flux properties of an Ultra-Luminous X-ray source (ULX) and the ubiquity of the rms-flux relation in accreting black holes of all scales.
- Publication:
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38th COSPAR Scientific Assembly
- Pub Date:
- 2010
- Bibcode:
- 2010cosp...38.2384H