Search Publications
Variability in the noise properties of Cygnus X-1.
Belloni, T.; Hasinger, G.
The aperiodic variability in the X-ray emission of Cyg X-1 is examined from 13 Exosat observations covering the period July 1983 - October 1985, by means of broad band 0.001-100 H3 power spectra. The total rms fractional variations in the range 0.016-14 Hz is between 28 and 38 percent. The power distribution is flat below a characteristic break fr…
An atlas of aperiodic variability in HMXB.
Belloni, T.; Hasinger, G.
From EXOSAT archive data of 14 X-ray binaries (12 high mass X-ray binaries and 2 low mass X-ray binaries) broad band power spectra and color- color diagrams are produced. The aperiodic (noise) variability of the sources is studied. All the X-ray pulsars present in the sample show strong noise components in the emission, with a fractional variabili…
X-ray emission from solar neighbourhood flare stars : a comprehensivesurvey of EXOSAT results.
Tagliaferri, G.; Stella, L.; Pallavicini, R.
Exosat Observatory observations of flare stars pertaining to 25 separate sources are examined. The Exosat instrumentation is described, and a data sample is presented. Quiescent and flaring emissions from UV Ceti-type flare stars as observed with a low-energy experiment on Exosat are discussed, and the results of the timing analysis of the low-ene…
A Study of BL Lacertae--Type Objects with EXOSAT. I. Flux Correlations, Luminosity, Variability, and Spectral Variability
Maccagni, D.; Giommi, P.; Garilli, B. +2 more
Results are presented from a uniform and systematic analysis of more than 200 X-ray observations of 36 BL Lacertae objects obtained from the EXOSAT archive. All but two objects were detected at least once. Strong luminosity variability, which reached a factor of 30 in one case, was a common property of these objects. Variability was usually more p…
The X-ray emission of 3C 273.
Koyama, K.; Nandra, K.; Tanaka, Y. +12 more
The X-ray emission of 3C273 in the 0.2-35 keV band has been studied, with EXOSAT and Ginga, over the period 1983 December to 1985 December. The overall flux is variable on a time-scale of weeks by a factor of 2. The spectral index in the 2-10 keV band (α ~0.5) is significantly different from the 'canonical' AGN index of α = 0.7. There are small bu…
X-Ray Eclipse Mapping of AR Lacertae
Parmar, A. N.; White, N. E.; Culhane, J. L. +2 more
Exosat data of the binary cycle of the eclipsing RS CVn system AR Lac are studied. The data reveal that at energies of less than 1 ke V a factor of 2 intensity modulation is detected and at energies greater than 1 ke V no orbital modulation is observed. The chi-squared and maximum entropy methods are used to model low-energy modulation. The relati…
The variable X-ray absorption and soft X-ray excess of the QSO MR 2251-178.
Pounds, K. A.; Stewart, G. C.; Pan, H. C.
We present the results from a programme of 15 EXOSAT observations in which we monitored the QSO MR2251 - 178. Over the period 1983 October-1984 December the hard X-ray flux varied by a factor of 2. While there is an overall correlation between the ME(2-10 keV) and LE(0.1-2 keV) fluxes the pattern of variability is such that it cannot be described …
The X-Ray Absorption Spectrum of VELA X-1
Haberl, F.; White, N. E.
Exosat observations of the massive X-ray binary Vela X-1 are spectrally analyzed. The absorbing column density shows erratic factor of 10 increases up to 5 x 10 to the 23rd H/sq cm that can occur at any orbital phase and last from minutes to many hours. These variations occur on top of a base level of absorption that increases from 3 x 10 to the 2…
Correlation of X-Ray Burst Properties with Source State in the ``Atoll'' Source 4U/MXB 1636-53
van Paradijs, J.; van der Klis, M.; Lewin, W. H. G. +3 more
A series of Exosat observations of the 'atoll' source 4U/MXB 1636 - 53 shows that duration and temperature of the X-ray bursts strongly correlate with the X-ray spectral and fast variability characteristics of the persistent emission of the source. This implies that spectral shape, fast variability, and burst duration and temperature all correlate…
The ephemeris of X 1822-371.
Hellier, Coel; Mason, Keith O.; Smale, Alan P. +1 more
From the analysis of eclipses observed with the HEAO-1, Einstein, Exosat and Ginga satellites, it is shown that the orbital period of X1822 - 371 is increasing on a time-scale of 2.9 million yr. The period change, one of only three determined amongst the LMXBs, is in the opposite sense to the expected evolutionary trend, but in the same sense as t…