Constraining the Mass of a Hypothetical Secondary Black Hole in M87 with the NANOGrav 15-Year Data Set

TL;DR

This study constrains the possible mass and orbital parameters of a hypothetical secondary black hole in M87 by analyzing VLBI jet observations and gravitational wave data from NANOGrav, providing insights into SMBH binary systems.

Contribution

It introduces a novel method combining jet periodicity and gravitational wave constraints to limit the properties of a secondary black hole in M87.

Findings

Allowed mass ratio range for 11-year period: 6.9e-3 to 4.2e-2.

Allowed mass ratio range for 0.9-year period: 3.7e-2 to 1.

Identifies the need for VLBI monitoring to further constrain parameters.

Abstract

Galaxy mergers, each hosting a supermassive black hole (SMBH), are thought to form SMBH binaries. Motivated by recent observations from the East Asian VLBI Network (EAVN) showing periodic behavior in the M87 jet, a precession of about 11 years and a transverse oscillation of about 0.9 years, we constrain the mass of a hypothetical secondary black hole orbiting the primary SMBH in M87. To constrain the mass ratio between the primary SMBH ($M_{1}$) and the secondary black hole ($M_{2}$) defined as $q \equiv M_{2}/M_{1} \leq 1$, and the length of the semimajor axis of the binary system ($a$), we impose the following three constraints: (i) the lower limit of $a$, below which the SMBH binary is expected to merge. (ii) the strain amplitude of the gravitational wave background (GWB) at nanohertz frequencies shown in the NANOGrav 15-year dataset. (iii) a finite length of the semimajor axis of $M_{1}$, that can induce periodic behavior in the jet. By combining these constraints, we obtain the allowed parameter space for $q$ and $a$. If either of the EAVN-detected periods ($T$) corresponds to the binary's orbital period, the allowed range of $q$ is $6.9 \times 10^{-3} \le q \le 4.2 \times 10^{-2}$ for $T \approx 11$ years, and $3.7\times 10^{-2} \le q \le 1$ for $T \approx 0.9$ years. VLBI astrometric monitoring of the jet base of M87 is essential to explore the allowed parameter space for $q$ and $a$.