X-ray Variability (x-ray + variability)

Distribution by Scientific Domains

Selected Abstracts

3D models of radiatively driven colliding winds in massive O + O star binaries , III.

Thermal X-ray emission
ABSTRACT The X-ray emission from the wind,wind collision in short-period massive O + O star binaries is investigated. The emission is calculated from 3D hydrodynamical models which incorporate gravity, the driving of the winds, orbital motion of the stars and radiative cooling of the shocked plasma. Changes in the amount of stellar occultation and circumstellar attenuation introduce phase-dependent X-ray variability in systems with circular orbits, while strong variations in the intrinsic emission also occur in systems with eccentric orbits. The X-ray emission in eccentric systems can display strong hysteresis, with the emission softer after periastron than at corresponding orbital phases prior to periastron, reflecting the physical state of the shocked plasma at these times. Our simulated X-ray light curves bear many similarities to observed light curves. In systems with circular orbits the light curves show two minima per orbit, which are identical (although not symmetric) if the winds are identical. The maxima in the light curves are produced near quadrature, with a phase delay introduced due to the aberration and curvature of the wind collision region. Circular systems with unequal winds produce minima of different depths and duration. In systems with eccentric orbits the maxima in the light curves may show a very sharp peak (depending on the orientation of the observer), followed by a precipitous drop due to absorption and/or cooling. We show that the rise to maximum does not necessarily follow a 1/dsep law. Our models further demonstrate that the effective circumstellar column can be highly energy dependent. Therefore, spectral fits which assume energy-independent column(s) are overly simplified and may compromise the interpretation of observed data. To better understand observational analyses of such systems we apply Chandra and Suzaku response files, plus Poisson noise, to the spectra calculated from our simulations and fit these using standard xspec models. We find that the recovered temperatures from two- or three-temperature mekal fits are comparable to those from fits to the emission from real systems with similar stellar and orbital parameters/nature. We also find that when the global abundance is thawed in the spectral fits, subsolar values are exclusively returned, despite the calculations using solar values as input. This highlights the problem of fitting oversimplified models to data, and of course is of wider significance than just the work presented here. Further insight into the nature of the stellar winds and the wind,wind collision region in particular systems will require dedicated hydrodynamical modelling, the results of which will follow in due course. [source]

Jets from black hole X-ray binaries: testing, refining and extending empirical models for the coupling to X-rays

R. P. Fender
ABSTRACT In this paper we study the relation of radio emission to X-ray spectral and variability properties for a large sample of black hole X-ray binary systems. This is done to test, refine and extend , notably into the timing properties , the previously published ,unified model' for the coupling of accretion and ejection in such sources. In 14 outbursts from 11 different sources we find that in every case the peak radio flux, on occasion directly resolved into discrete relativistic ejections, is associated with the bright hard to soft state transition near the peak of the outburst. We also note the association of the radio flaring with periods of X-ray flaring during this transition in most, but not all, of the systems. In the soft state, radio emission is in nearly all cases either undetectable or optically thin, consistent with the suppression of the core jet in these states and ,relic' radio emission from interactions of previously ejected material and the ambient medium. However, these data cannot rule out an intermittent, optically thin, jet in the soft state. In attempting to associate X-ray timing properties with the ejection events we find a close, but not exact, correspondence between phases of very low integrated X-ray variability and such ejections. In fact the data suggest that there is not a perfect one-to-one correspondence between the radio, X-ray spectral or X-ray timing properties, suggesting that they may be linked simply as symptoms of the underlying state change and not causally to one another. We further study the sparse data on the reactivation of the jet during the transition back to the hard state in decay phase of outbursts, and find marginal evidence for this in one case only. In summary we find no strong evidence against the originally proposed model, confirming and extending some aspects of it with a much larger sample, but note that several aspects remain poorly tested. [source]

Cross-spectral analysis of the X-ray variability of Markarian 421

Y. H. Zhang
ABSTRACT Using the cross-spectral method, we confirm the existence of the X-ray hard lags discovered with cross-correlation function technique during a large flare of Mrk 421 observed with BeppoSAX. For the 0.1,2 versus 2,10 keV light curves, both methods suggest sub-hour hard lags. In the time domain, the degree of hard lag, i.e. the amplitude of the 3.2,10 keV photons lagging the lower energy ones, tends to increase with the decreasing energy. In the Fourier frequency domain, by investigating the cross-spectra of the 0.1,2/2,10 keV and the 2,3.2/3.2,10 keV pairs of light curves, the flare also shows hard lags at the lowest frequencies. However, with the present data, it is impossible to constrain the dependence of the lags on frequencies even though the detailed simulations demonstrate that the hard lags at the lowest frequencies probed by the flare are not an artefact of sparse sampling, Poisson and red noise. As a possible interpretation, the implication of the hard lags is discussed in the context of the interplay between the (diffusive) acceleration and synchrotron cooling of relativistic electrons responsible for the observed X-ray emission. The energy-dependent hard lags are in agreement with the expectation of an energy-dependent acceleration time-scale. The inferred magnetic field (B, 0.11 G) is consistent with the value inferred from the spectral energy distributions of the source. Future investigations with higher quality data that show whether or not the time-lags are energy-/frequency-dependent will provide a new constraint on the current models of the TeV blazars. [source]

The X-ray variability of the Seyfert 1 galaxy MCG,6-30-15 from long ASCA and RXTE observations

Julia C. Lee
We present an analysis of the long Rossi X-ray Timing Explorer (RXTE) observation of the Seyfert 1 galaxy MCG,6-30-15, taken in 1997 July. We have previously used the data to place constraints for the first time on the iron abundance,reflection fraction relationship, and now expand the analysis to investigate in detail the spectral X-ray variability of the object. Our results show that the behaviour is complicated. We find clear evidence from colour ratios and direct spectral fitting that changes to the intrinsic photon index are taking place. In general, spectral hardening is evident during periods of diminished intensity, and in particular, a general trend for harder spectra is seen in the period following the hardest RXTE flare. Flux-correlated studies further show that the 3,10 keV photon index ,3,10 steepens, while that in the 10,20 keV band, ,10,20, flattens with flux. The largest changes come from the spectral index below 10 keV; however, changes in the intrinsic power-law slope (shown by changes in ,3,10), and reflection (shown by changes in ,10,20) both contribute in varying degrees to the overall spectral variability. We find that the iron-line flux FK, is consistent with being constant over large time intervals on the order of days (although tentative evidence exists which show that FK, changes on shorter time intervals of order ,10 ks during time periods surrounding flare events), and has an equivalent width which anticorrelates with the continuum flux and reflection fraction. A possible interpretation for the iron-line flux constancy and the relative Compton reflection increase with flux from the flux-correlated data is an increasing ionization of the emitting disc surface, while spectral analysis of short time intervals surrounding flare events hints tentatively at observed spectral responses to the flare. We present a simple model for partial ionization where the bulk of the variability comes from within 6rg. Temporal analysis further provides evidence for possible time (,1000 s) and phase (,,0.6 rad) lags. Finally, we report an apparent break in the power density spectrum (,4,510,6 Hz) and a possible 33-hour period. Estimates for the mass of the black hole in MCG,6-30-15 are discussed in the context of spectral and temporal findings. [source]

Supersoft AGNs and their relations to Galactic binaries

Th. Boller
Abstract I review some basic results on AGN with supersoft X-ray spectra and their relations to Galactic binaries in their soft high states. This paper is based on a talk given at the Supersoft Sources Workshop at ESTEC in May 2009. Given the length of the talk and the number of pages the review cannot be complete and is biased towards my personal view. I demonstrate that at high accretion rates supersoft AGNs and Galactic binaries share steep soft X-ray spectra, that the X-ray variability of supersoft AGNs is more pronounced compared to Galactic binaries in their high states, that the X-ray variability of supersoft novae and supersoft AGNs is similar, and that in Galactic binaries mostly positive time lags are seen, while negative time lags are observed in some supersoft AGN ( 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]