Formation of intermediate-mass black holes as primordial black holes in the inflationary cosmology with running spectral index

TL;DR

This paper proposes that primordial black holes formed during inflation with a large negative spectral index running can explain intermediate-mass black holes observed as ULXs, with testable predictions based on spectral parameters.

Contribution

It introduces a novel scenario linking inflationary spectral index running to the formation of intermediate-mass black holes as primordial black holes.

Findings

Primordial black holes can account for ultraluminous X-ray sources.

The mass of PBHs is determined by spectral index and its running.

The scenario is testable with future observations of spectral parameters.

Abstract

Formation of primordial black holes (PBHs) on astrophysical mass scales is a natural consequence of inflationary cosmology if the primordial perturbation spectrum has a large and negative running of the spectral index as observationally inferred today, because double inflation is required to explain it and fluctuations on some astrophysical scales are enhanced in the field oscillation regime in between. It is argued that PBHs thus produced can serve as intermediate-mass black holes (IMBHs) which act as the observed ultraluminous X-ray sources (ULXs) by choosing appropriate values of the model parameters in their natural ranges. Our scenario can be observationally tested in near future because the mass of PBHs is uniquely determined once we specify the values of the spectral index and its running on large scales.