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For refcode 1989MNRAS.238.1029M:
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Copyright by Royal Astronomical Society. 1989MNRAS.238.1029M The flux and spectral variability of NGC 6814 as observed with EXOSAT J. P. D. Mittaz and G. Branduardi-Raymont Mullard Space Science Laboratory, University Collage London, Holmbury St Mary, Dorking, Surrey RH5 6NT Accepted 1989 January 6. Received 1989 January 5; in original form 1988 June 21 Summary. We present five EXOSAT observations of the Seyfert galaxy NGC 6814 which were carried out over the period 1983 September to 1985 October at intervals of a few months to a year. Over these time-scales the soft X-ray (0.02-2.5 keV) flux of NGC 6814 varies by large factors, decreasing by over and order of magnitude in the first 9 months; this is found to be partially due to variable low-energy absorption in the spectrum. The medium energy (2-6 keV) flux also varies on these long time-scales, but with a lower amplitude than the soft X-ray flux. Within the individual observations the 2-6 keV flux is always rapidly variable down to time-scale at least as short as 1000 s. We also see a dramatic factor of 2 drop in the count-rate in ~300 s. What makes these observations so interesting, though, is the presence of periodic flaring on a time-scale of approximately 12 000 s during the longest (~1 day) observation of NGC 6814. Analysis of one of the shorter observations, when the source was at its lowest flux level, also suggests the presence of quasi-periodic oscillations on a time-scale of 3000 s. Because of the periodic behaviour of the ME flux we conclude that the temporal characteristics of NGC 6814 in the EXOSAT observations are unusual with respect to those of other Seyfert galaxies. Since NGC 6814 has a low inclination on the sky, we could be looking down a jet into regions close to the nuclear black hole, where periodic flares may originate from instabilities in a pair-dominated plasma, or where disc instabilities cause flaring and oscillations. We therefore argue that it is possible to observe characteristic time-scales in the X-ray variability of AGN, but that this may only occur under particular conditions.
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