A fast test for the identification and confirmation of massive black hole binaries

TL;DR

The paper introduces a rapid observational test to identify massive black hole binaries by analyzing the cross-correlation of broad emission line segments, distinguishing binaries from single black holes with shorter monitoring times.

Contribution

A novel, fast spectroscopic test for detecting massive black hole binaries using cross-correlation analysis of broad emission lines, applicable over shorter timescales than existing methods.

Findings

The test can distinguish binaries from single black holes based on cross-correlation variability.

Dividing broad lines into two equal-flux parts reduces noise impact.

The method requires shorter monitoring campaigns compared to previous techniques.

Abstract

We present a new observational test to identify massive black hole binaries in large multi-epoch spectroscopical catalogues and to probe the real nature of already proposed binary candidates. The test is tailored for binaries with separations large enough to allow each black hole to retain its own broad line region. In this limit the fast AGN variability typically observed over months cannot be associated to the much longer binary period, and it is assumed (as for the case of single black holes) to be the consequence of the evolution of the innermost regions of the two accretion discs. A simple analysis of the cross-correlation between different parts of individual broad emission lines can therefore identify the presence of two massive black holes whose continua vary independently of each other. Our analysis indicates that to be less affected by the noise in the spectra the broad lines should be divided in two close-to-equal-flux parts. This ensures that in the single massive black hole scenario the cross-correlation will always be high. With monitoring campaigns similar to those performed for reverberation mapping studies, on the other way, a binary can show any value of the cross-correlation and can therefore be distinguished from a standard AGN. The new test can be performed over timescales orders of magnitude shorter than the alternative tests already discussed in literature, and can be a powerful complement to the massive black hole binary search strategies already in place.