The Unanticipated Phenomenology of the Blazar PKS 2131-021: A Unique Supermassive Black Hole Binary Candidate
Abstract
Most large galaxies host supermassive black holes in their nuclei and are subject to mergers, which can produce a supermassive black hole binary (SMBHB), and hence periodic signatures due to orbital motion. We report unique periodic radio flux density variations in the blazar PKS 2131-021, which strongly suggest an SMBHB with an orbital separation of ~0.001-0.01 pc. Our 45.1 yr radio light curve shows two epochs of strong sinusoidal variation with the same period and phase to within ≲2% and ~10%, respectively, straddling a 20 yr period when this variation was absent. Our simulated light curves accurately reproduce the "red noise" of this object, and Lomb-Scargle, weighted wavelet Z-transform and least-squares sine-wave analyses demonstrate conclusively, at the 4.6σ significance level, that the periodicity in this object is not due to random fluctuations in flux density. The observed period translates to 2.082 ± 0.003 yr in the rest frame at the z = 1.285 redshift of PKS 2131-021. The periodic variation in PKS 2131-021 is remarkably sinusoidal. We present a model in which orbital motion, combined with the strong Doppler boosting of the approaching relativistic jet, produces a sine-wave modulation in the flux density that easily fits the observations. Given the rapidly developing field of gravitational-wave experiments with pulsar timing arrays, closer counterparts to PKS 2131-021 and searches using the techniques we have developed are strongly motivated. These results constitute a compelling demonstration that the phenomenology, not the theory, must provide the lead in this field.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2022
- DOI:
- 10.3847/2041-8213/ac504b
- arXiv:
- arXiv:2111.02436
- Bibcode:
- 2022ApJ...926L..35O
- Keywords:
-
- 1390;
- 164;
- 1663;
- 16;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 24 pages, 13 figure, 3 Tables, accepted for publication in APJL