Unveiling sub-parsec supermassive black hole binary candidates in active galactic nuclei

TL;DR

This paper introduces a new method to identify supermassive black hole binary candidates in active galactic nuclei using periodic signals in X-ray light curves, leading to the detection of promising candidates and validation with known sources.

Contribution

It presents a novel technique based on the Fisher's exact g-test for detecting periodicity in AGN X-ray light curves, accounting for colored noise, and applies it to identify SMBHB candidates.

Findings

Identified Seyfert 1.5 Mrk 915 as a potential SMBHB with a 35-month period.

Confirmed a 26.3-month period in MCG+11-11-032, consistent with previous Fe Kα line analysis.

Developed a new statistical method for SMBHB candidate selection in X-ray data.

Abstract

Elusive supermassive black hole binaries (SMBHBs) are thought to be the penultimate stage of galaxy mergers, preceding a final coalescence phase. SMBHBs are sources of continuous gravitational waves, possibly detectable by pulsar timing arrays; the identification of candidates could help in performing targeted gravitational wave searches. Due to their origin in the innermost parts of active galactic nuclei (AGN), X-rays are a promising tool to unveil the presence of SMBHBs, by means of either double Fe K$\alpha$ emission lines or periodicity in their light curve. Here we report on a new method to select SMBHBs by means of the presence of a periodic signal in their Swift-BAT 105-months light curves. Our technique is based on the Fisher's exact g-test and takes into account the possible presence of colored noise. Among the 553 AGN selected for our investigation, only the Seyfert 1.5 Mrk 915 emerged as possible candidate for a SMBHB; from the subsequent analysis of its light curve we find a period $P_0=35\pm2$ months, and the null hypothesis is rejected at the $3.7\sigma$ confidence level. We also present a detailed analysis of the BAT light curve of the only previously X-ray-selected binary candidate source in the literature, the Seyfert 2 galaxy MCG+11-11-032. We find $P_0=26.3\pm0.6$ months, consistent with the one inferred from previously reported double Fe K$\alpha$ emission lines.