Search Publications
The EXOSAT Data on GX 339-4: Further Evidence for an ``Intermediate'' State
Méndez, Mariano; van der Klis, Michiel
We have studied the fast timing and spectral behavior of the black hole candidate (BHC) GX 339-4 using all 1-20 keV EXOSAT ME data: the 1983 July, 1984 March, 1984 May, and 1985 April observations. In 1985 April, GX 339-4 was in a weak low state (0.03 × 10-9 ergs cm-2 s-1, 2-10 keV). The X-ray spectrum was a hard p…
GX17+2: X-ray spectral and timing behaviour of a bursting Z source
van Paradijs, J.; van der Klis, M.; Lewin, W. H. G. +2 more
We have investigated the properties of the Z pattern of the low-mass X-ray binary GX17+2 in the X-ray colour-colour diagrams (CDs) and hardness-intensity diagrams (HIDs), and of the power spectra as a function of position in the Z. We used all EXOSAT ME data on GX17+2, a total of six observations during 1983-86. We find that the Z pattern in the C…
On the Relationship between the Periodic and Aperiodic Variability of Accreting X-Ray Pulsars
Stella, Luigi; Lazzati, Davide
Besides the narrow peaks originating from the periodic signal, the power spectra of accreting X-ray pulsars display continuum components usually increasing toward low frequencies; these arise from the source aperiodic variability. Most studies up to the present have adopted the view that the periodic and aperiodic variations are independent. Howev…
Correlated X-ray spectral and fast-timing behaviour of 4U 1636-53.
van der Klis, M.; Prins, S.
We present a study of the persistent X-ray emission from the low-mass X-ray binary 4U 1636-53 based on the entire archival EXOSAT ME data set. We performed a homogeneous analysis of all five EXOSAT observations in terms of the correlated rapid X-ray variability and X-ray spectral properties by means of power spectra, and colour-colour and hardness…
Analyzing X-ray variability by linear state space models
Timmer, J.; Konig, M.
In recent years, autoregressive models have had a profound impact on the description of astronomical time series as the observation of a stochastic process. These methods have advantages compared with common Fourier techniques concerning their inherent stationarity and physical background. However, if autoregressive models are used, it has to be t…
Evidence for an Anomalous State in the Black Hole Candidate 4U 1630-47
Kuulkers, Erik; van der Klis, Michiel; Parmar, Arvind N.
We have reanalyzed two EXOSAT medium-energy observations during part of the 1984 X-ray outburst of the black hole candidate transient system 4U 1630-47. One observation (May 10) shows fast-timing properties (on timescales shorter than ~250 s) typical of those of other black hole candidates in their high states. The power spectrum shows a power law…
Testing the X-ray variability of active galactic nuclei with the non-linear prediction method
Czerny, Bozena; Lehto, Harry J.
The analysis of eight EXOSAT X-ray light curves of six active galactic nuclei by non-linear prediction methods indicates that the observed variability is truly stochastic and is not caused by deterministic chaos. This result favours X-ray emission models with multiple centres of production of hot, possibly relativistic, electrons.
Doppler imaging of stellar surface structure. III. The X-ray source HD 116544 = IN Virginis.
Strassmeier, K. G.
We present the first Doppler image of the EXOSAT X-ray source EXO 1321.8-0203, recently identified to be a chromospherically active star exhibiting periodic light variations and consequently named INVirginis. Our high-resolution spectra show INVirginis to be a single-lined spectroscopic binary with an orbital period of 8.2 days, very strong CaII e…
The ultraviolet to X-ray spectrum of the Seyfert 1 galaxy E 1615+061: accretion disk and reflection models.
Piro, L.; Perola, G. C.; Fiore, F. +4 more
The Seyfert 1 galaxy E 1615+061 is a candidate for the strongest variability in soft X-rays, changing from a very steep high state ({GAMMA}=~4), as implied by HEAO1 observations performed in 1977, to a two orders of magnitude dimmer state with a flatter spectrum ({GAMMA}=~2), as observed by EXOSAT in 1985. In this paper we present the results of a…
Correlation between variability time scale and X-ray spectral index in AGN.
Wilms, J.; Staubert, R.; Koenig, M.
We have analysed EXOSAT ME X-ray light curves of a sample of AGN assuming a stochastic process in these sources. The result is a significant anti-correlation between the relaxation timescale τ, derived from this stochastic model fit, and the photon index {GAMMA} of the X-ray spectrum. This establishes for the first time a relationship between vari…