False periodicities in quasar time-domain surveys

S. Vaughan, P. Uttley, A. G. Markowitz, D. Huppenkothen, M. J. Middleton, W. N. Alston, J. D. Scargle, W. M. Farr

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Abstract

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.
Original languageEnglish
Pages (from-to)3145-3152
JournalRoyal Astronomical Society. Monthly Notices
Volume461
Issue number3
Early online date21 Jun 2016
DOIs
Publication statusPublished - 21 Sept 2016

Keywords

  • methods: data analysis
  • methods: statistical
  • quasars: general

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