Kavli Affiliate: Roger D. Blandford
| First 5 Authors: Adam D. Hincks, Adam D. Hincks, , ,
| Summary:
Large sinusoidal variations in the radio light curves of the blazars PKS
J0805$-$0111 and PKS 2131$-$021 have recently been discovered with an 18-year
monitoring programme at the Owens Valley Radio Observatory, making these
systems strong supermassive black hole binary (SMBHB) candidates. The
sinusoidal variations in PKS 2131$-$021 dominate its light curves from 2.7 GHz
to optical frequencies. We report sinusoidal variations observed in both
objects with the Atacama Cosmology Telescope (ACT) at 95, 147 and 225 GHz
consistent with the radio light curves. The ACT 95 GHz light curve of PKS
2131$-$021 agrees well with the contemporaneous 91.5 GHz ALMA light curve and
is comparable in quality, while the ACT light curves of PKS J0805$-$0111, for
which there are no ALMA or other millimetre light curves, show that PKS
2131$-$021 is not an isolated case, and that this class of AGN exhibits the
following properties: (a) the sinusoidal pattern dominates over a broad range
of frequencies; (b) the amplitude of the sine wave compared to its mean value
is monochromatic (i.e., nearly constant across frequencies); (c) the phase of
the sinusoid phase changes monotonically as a function of frequency; (d) the
sinusoidal variations are intermittent. We describe a physical model for SMBHB
systems, the modified Kinetic Orbital model, that explains all four of these
phenomena. Monitoring of $sim8000$ blazars by the Simons Observatory over
the next decade should provide a large number of SMBHB candidates that will
shed light on the nature of the nanohertz gravitational-wave background.
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