Continuum reverberation mapping of accretion discs surrounding supermassive black hole binaries: observational signatures

ABSTRACT It has remained challenging to reliably identify sub-parsec supermassive black hole binaries (SMBHBs), despite them being expected to be ubiquitous. We propose a new method using multiband continuum reverberation mapping to identify low-mass-ratio SMBHBs in active galactic nuclei. The basic principle is that, due to the presence of a low-density cavity between the mini-discs and the circumbinary disc, the continuum emissions show a deficit at certain wavelengths, leading to a distinguishing feature in the relation between the inter-band time lag and wavelengths $\tau (\lambda)$. Specifically, the relation appears flat at short wavelengths because of the truncated sizes of the mini-discs and transits to a power law $\lambda ^{4/3}$ at long wavelength stemming from the circumbinary disc. This transition feature is distinct from the uniform relation $\lambda ^{4/3}$ of the standard accretion disc around a single black hole. Using the lamp-post scenario and assuming that only the secondary black hole is active in a low-mass-ratio SMBHB, we design a simple continuum reverberation model to calculate the transfer function of the accretion discs and the resulting $\tau (\lambda)$ relations for various SMBHB orbital parameters. The transition wavelength typically can lie at UV/optical bands, mainly depending on the total mass and orbital separation of the SMBHB. We apply our SMBHB model to the intensive multiwavelength monitoring data of the SMBHB candidate PG1302–102 and find that the SMBHB model can reproduce the inter-band time lags. Remarkably, the inferred total mass and orbital period from the SMBHB fitting are consistent with values derived from other independent methods.