Search Publications
Multi-frequency variations of the Wolf-rayet system HD 193793 - I. Infrared, X-ray and radio observations.
van der Woerd, H.; van der Hucht, K. A.; Pollock, A. M. T. +3 more
Observations made of the Wolf-Rayet system HD 193793 over the 1979-1989 period in the radio, X-ray, and IR regions reveal great variations in all three regimes. The most complete coverage obtained was in the IR; attention is given to an IR outburst in 1985 during which the IR flux increased by as much as an order of magnitude over the course of se…
The ultra-compact broad emision line region in NGC 4151.
Perola, G. C.; Boksenberg, A.; Penston, M. V. +6 more
New observations of the UV continuum and emission lines of a Seyfert I galaxy are presented in which the time resolution is adequate to describe the character of variability. Using the IUE satellite, the nucleus of NGC 4151 was observed with a mean interval of 3.4 d during 2 months. The flux of the continuum and of most emission lines underwent la…
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…
A model for the planetary nebula NGC 7027.
Middlemass, D.
Extensive photoionization modeling of NGC 7027 is presented using the most recent observations available. The effects of the differential extinction caused by the local dust cloud are considered and corrections are made to large-aperture line fluxes. Abundances are derived from a large number of collisionally excited and recombination emission lin…
Simultaneous multifrequency observations of the Seyfert 1 galaxy NGC 4051 : constant optical-infrared emission observed during large-amplitude X-ray variability.
Done, C.; Fabian, A. C.; Wamsteker, W. +5 more
We report simultaneous infrared, optical, ultraviolet and X-ray observations of the low-luminosity Seyfert 1 galaxy, NGC 4051. A new method of reduction was developed to correct the optical flux for atmospheric variations due to seeing and extinction. The X-ray flux varied by factors of up to 2 on time-scales of tens of minutes whilst the optical …
Fast spectroscopic variations on rapidly rotating, cool dwarfs III. Masses of circumstellar absorbing clouds on AB Doradus.
Penston, M. V.; Collier Cameron, A.; Ehrenfreund, P. +5 more
We present new time-resolved Hα, Ca II H and K and Mg II h and k spectra of the rapidly rotating KO dwarf star AB Doradus (= HD 36705). The transient absorption features seen in the Hα line are also present in the Ca II and Mg II resonance lines. New techniques are developed for measuring the average strength of the line absorption along lines-of-…
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 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…
Localization of one of the galactic centre X-ray burst sources.
Skinner, G. K.; Eyles, C. J.; Foster, A. J. +1 more
Observations of an X-ray burst which occurred during the Spacelab-2 observations of the Galactic Center region are reported, and it is shown that it originated in the more southerly component of a previously reported persistent X-ray object, now resolved into two sources separated by 3 arcmin. Present knowledge of the number and location of the Ga…
Ultraviolet variability of NGC 4151 : a study using principal component analysis.
Penston, M. V.; Snijders, M. A. J.; Mittaz, J. P. D.
In an attempt at a decomposition of the different sources of variation in the spectra of NGC 4151, we have carried out a principal component analysis of spectra, seeking the number and form of the significant linearly independent spectra that are needed to reconstruct the data within the errors. Methods are outlined with particular emphasis on the…