Search Publications
On the nature of rapid X-ray variability in active galactic nuclei.
McHardy, I. M.; Lehto, H. J.; Green, A. R.
We systematically analyse all (110) EXOSAT AGN medium-energy light curves of length greater than 20 ks to search for and quantify their flux variability. We also perform extensive simulations to quantify the selection effects (such as the source brightness or the length of observation) that might prevent the detection of variability. We quantify v…
ROSAT studies of the composition and structure of DA white dwarf atmospheres.
Barstow, M. A.; Koester, D.; Sansom, A. E. +7 more
We present here a detailed study of a sample of hot I)A white dwarfs detected in the EUV and soft X-ray bands by ROSAT during its all-sky survey. The survey data are combined with additional ROSAT pointed-phase observations, earlier EXOSAT results and the available optically determined effective temperatures and gravities. We show that the spectra…
Further probing of the X-ray source in NGC 4151 : new constraints on the nuclear geometry.
Sokoloski, J. L.; Yamauchi, M.; Warwick, R. S. +4 more
We report on the results of new Ginga observations of the archetypal Seyfert galaxy, NGC 4151, performed over an 18-month period between early 1990 and mid-1991. The present programme complements earlier extensive monitoring of this source in the medium-energy X-ray band by both EXOSAT and Ginga to give a combined data set of some 36 separate obse…
X-ray orbital modulations in intermediate polars.
Hellier, Coel; Mason, Keith O.; Garlick, Mark A.
A systematic search of the EXOSAT observations of intermediate polars has revealed that a majority of systems show orbital-cycle X-ray modulations. The modulations are irregular, have the characteristic energy dependence of photoelectric absorption and, where it can be determined, have minima preferentially at orbital phase about 0.8. We remark on…
A detailed X-ray and radio study of the supernova remnant W 44.
Angelini, Lorella; Jones, L. R.; Smith, A.
We analyze EXOSAT and Einstein Observatory X-ray spectra, an Einstein X-ray image, and a detailed VLA radio map of the SNR W44. The diffuse soft X-ray emission, which is only observed from the interior of the SNR, is shown to be largely of thermal origin. The absence of soft X-ray emission from the shell can be explained by the measured line-of-si…
The soft X-ray excesses of high-luminosity AGN.
Saxton, R. D.; Turner, M. J. L.; Ohashi, T. +3 more
LE and ME spectra of 12 quasars from the EXOSAT data base have been analysed in a search for soft excesses. We have been able to quantify the soft excesses more accurately by using spectral indices for the ME taken from the much more sensitive Ginga observations of the same quasars. Eight quasars show a significant soft excess and the remaining fo…
Cosmic-abundance absorption dips in X1755-33.
Church, M. J.; Balucinska-Church, M.
Results of spectral analysis for the source X1755-33 using the 1984 EXOSAT/ME data are presented, with spectral fitting made to the complete 18-h observation by dividing it into 314 consecutive 400-s timebins. It is found that spectral changes take place on time scales of this order and that analysis of longer sections of data can lead to errors. …
The broad-band X-ray spectral variability of MRK 841.
Nandra, K.; Turner, T. J.; Done, C. +3 more
The results of a detailed spectral analysis of four X-ray observations of the luminous Seyfert 1.5 galaxy Mrk 841, performed using the EXOSAT and Ginga satellites over the period 1984 June to 1990 July, are reported. Preliminary results from a short ROSAT PSPC observation of Mrk 841 in 1990 July are also presented. Variability is apparent in both …
AGN X-ray light curves - shot noise or low-dimensional attractor ?
McHardy, Ian M.; Czerny, Bozena; Lehto, Harry J.
We use correlation integrals to determine whether the X-ray variability of AGN, as seen in EXOSAT observations, arises from chaotic systems. We distinguish three characteristic types of behaviour. We also analyse a number of simulated light curves, to investigate the poorly understood limitations to the correlation integral method caused by noise …
Further constraints on the warm absorber in MR 2251-17.8 from GINGA and EXOSAT observations.
Mineo, T.; Stewart, G. C.
Analysis of a Ginga observation of MR 2251 - 17.8 is presented. The source is in a relatively low emission state, with a high absorbing column density. This result is consistent with the previous EXOSAT observations, where a strong correlation between the flux and the column density was detected. The Ginga spectrum and the entire EXOSAT spectral d…