NANOGrav 15-year gravitational-wave signals from binary supermassive black-holes seeded by primordial black holes

TL;DR

This paper proposes that primordial black holes with specific masses and abundances can explain the NANOGrav 15-year gravitational-wave background through binary supermassive black hole mergers, while remaining consistent with 21cm line emission constraints.

Contribution

It introduces a novel scenario linking primordial black holes to gravitational wave signals and cosmological observations, fitting data without conflicting with existing bounds.

Findings

Primordial black holes can account for the NANOGrav gravitational-wave background.

The model fits the data within specific mass and abundance ranges.

Future observations can test this primordial black hole scenario.

Abstract

In this paper, we explain the recently reported a nHz-band gravitational-wave background from NANOGrav 15-year through the merger of binary super-massive black holes with masses of $10^9 M_{\odot}$ formed by the growth of primordial black holes. When a primordial black hole accretes at a high accretion rate, it emits a large number of high-energy photons. These heat the plasma, causing high-redshift cosmological 21cm line emission. Since this has not been detected, there is a strict upper bound on the accretion rate. We have found that with the primordial black hole abundance $10^{-14} \lesssim f_{\rm PBH} \lesssim 10^{-12}$ and the mass $1 M_{\odot} \lesssim m_{\rm PBH} \lesssim 10^3 M_{\odot}$, we successfully fit the nHz band gravitational wave background from NANOGrav 15-year while avoiding the 21 cm line emission. We propose that future observations of the gravitational wave background and the cosmological 21cm line can test this scenario.