Publication Date:
2016-07-17
Description:
There have recently been several reports of apparently periodic variations in the light curves of quasars, e.g. PG 1302–102 by Graham et al. Any quasar showing periodic oscillations in brightness would be a strong candidate to be a close binary supermassive black hole and, in turn, a candidate for gravitational wave studies. However, normal quasars – powered by accretion on to a single, supermassive black hole – usually show stochastic variability over a wide range of time-scales. It is therefore important to carefully assess the methods for identifying periodic candidates from among a population dominated by stochastic variability. Using a Bayesian analysis of the light curve of PG 1302–102, we find that a simple stochastic process is preferred over a sinusoidal variation. We then discuss some of the problems one encounters when searching for rare, strictly periodic signals among a large number of irregularly sampled, stochastic time series, and use simulations of quasar light curves to illustrate these points. From a few thousand simulations of steep spectrum (‘red noise’) stochastic processes, we find many simulations that display few-cycle periodicity like that seen in PG 1302–102. We emphasize the importance of calibrating the false positive rate when the number of targets in a search is very large.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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