Quasar Long Term Variability & Time Dilation
One of the criticisms of Hawkins’ findings that Quasars do not exhibit time dilation with redshift was that quasars are chaotic and do not exhibit periodic or structured changes. That criticism is not correct.
One of the unsolved puzzles for the quasar engine is how to adjust the mechanism to explain why quasars exhibit long term cyclic changes in intensity which gradually monotonically increase in amplitude.
For example the quasar Q0957+561 (The quasar from which a detailed of over twenty years provided evidence to support the assertion that black holes have an intrinsic magnetic field) has a medium cycle period of 405 days. The super cycle period (the luminosity changes monotonically increase) is not known but is based on observation in excess of 40 years.
Hawkins is one of the leading researchers in the study of the long cyclic changes in quasars. Hawkins found there was no time dilation with redshift of this medium term cyclic quasar variation.
This paper summaries the puzzle of the medium and long term quasar cyclic changes and attempts to explain the cyclic quasar variability with accretion disc changes. The MECO paper discusses the same issue and provides evidence for quasar structures and changes that cannot be explained by accretion disc instabilities. The MECO paper provides an alternative mechanism for the quasar.
Structure function analysis of Long-term quasar Variability, by W. DE VRIES and R.BECKER
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We find the following: (1) The outbursts have an asymmetric light-curve profile, with a fast-rise, slow-decline shape; this argues against a scenario in which microlensing events along the line of sight to the quasars are dominating the long-term variations in quasars. (2) There is no turnover in the structure function of the quasars up to timescales of 40 yr, and the increase in variability with increasing time lags is monotonic and constant. (3) Consequently, there is not a single preferred characteristic outburst timescale for the quasars, but most likely a continuum of outburst timescales. (4) The magnitude of the quasar variability is a function of wavelength: variability increases toward the blue part of the spectrum. (5) High-luminosity quasars vary less than low-luminosity quasars, consistent with a scenario in which variations have limited absolute magnitude. On this basis, we conclude that quasar variability is intrinsic to the active galactic nucleus and is caused by chromatic outbursts or flares that have limited luminosity range, varying timescales, and an overall asymmetric light-curve shape. Currently, the model that has the most promise of fitting the observations is based on accretion disk instabilities.
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Comment:
Hawkins' sarcastically refers to this paper in his second paper, on quasar cyclic variability in which he provides further data and analysis that confirms the finding of his 2001 paper, that the medium term quasar cyclic variation does not exhibit time dilation with redshift. The variations he notes must be studied in the frequency domain as the complete cycle is not known, rather than the time domain, as the above authors did. He notes that if time dilation is forced on quasar spectrum that mathematical manipulation creates a brake in the spectrum which would be not be in agreement with quasar theory with the disc accretion mechanism.
It is only recently with multiwave length observations that it has been possible to test quasar theoretical models.
Variability in active galactic nuclei: confrontation of models with observations by M. Hawkins. M. Hawkins
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Forty years after the discovery of quasars there is still much uncertainty about their structure. There is, arguably, broad agreement about the basic nature and overall arrangement of the various components of a ‘unified model’ for active galactic nuclei (AGN) (Antonucci 1993), but the details have proved very hard to tie down. In particular, the nature of the central engine and the radiative transfer processes are the subject of much debate.
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