A population of short-period variable quasars from PTF as supermassive black hole binary candidates
Supermassive black hole binaries (SMBHBs) at sub-parsec separations should be common in galactic nuclei, as a result of frequent galaxy mergers. Hydrodynamical simulations of circum-binary discs predict strong periodic modulation of the mass accretion rate on time-scales comparable to the orbital period of the binary. As a result, SMBHBs may be recognized by the periodic modulation of their brightness. We conducted a statistical search for periodic variability in a sample of 35 383 spectroscopically confirmed quasars in the photometric data base of the Palomar Transient Factory (PTF). We analysed Lomb–Scargle periodograms and assessed the significance of our findings by modelling each individual quasar's variability as a damped random walk (DRW). We identified 50 quasars with significant periodicity beyond the DRW model, typically with short periods of a few hundred days. We find 33 of these to remain significant after a re-analysis of their periodograms including additional optical data from the intermediate-PTF and the Catalina Real-Time Transient Survey. Assuming that the observed periods correspond to the redshifted orbital periods of SMBHBs, we conclude that our findings are consistent with a population of unequal-mass SMBHBs, with a typical mass ratio as low as q ≡ M_2/M_1 ≈ 0.01.
© 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2016 July 22. Received 2016 July 21. In original form 2016 April 5. First published online July 28, 2016. We thank Jules Halpern and Daniel D'Orazio for useful discussions, Ying Zu for assisting with the Javelin code, and Marcel Agueros for providing the light curves and the stellar rotation periods from the Praesepe cluster for an initial test to our Lomb–Scargle algorithm. For our analysis, we made use of the High Performace Computing cluster Yeti at Columbia University. Additional resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center. This work was supported in part by NASA grants NNX11AE05G and NNX15AB19G (to ZH). MJG acknowledges support from NSF grant AST-1518308. This paper is based on observations obtained with the Samuel Oschin Telescope as part of the PTF project, a scientific collaboration between the California Institute of Technology, Columbia University, Las Cumbres Observatory, the Lawrence Berkeley National Laboratory, the National Energy Research Scientific Computing Center, the University of Oxford, and the Weizmann Institute of Science. The CSS survey is funded by the National Aeronautics and Space Administration under Grant No. NNG05GF22G issued through the Science Mission Directorate Near-Earth Objects Observations Program. The CRTS survey is supported by the US National Science Foundation under grants AST-0909182 and AST-1313422. This paper was approved for publication by the LIGO Scientific Collaboration, with document number LIGO-P1600113.
Submitted - 1604.01020v2.pdf
Published - MNRAS-2016-Charisi-2145-71.pdf