The black hole in NGC 1313 X-2

TL;DR

This study models NGC 1313 X-2's optical and X-ray data, suggesting its black hole is likely a massive 50-100 solar mass black hole, supporting the idea of ULXs containing very massive black holes formed in low metallicity environments.

Contribution

The paper provides the first detailed binary evolution modeling of NGC 1313 X-2, constraining the black hole mass and donor star type, and discusses implications for ULX formation scenarios.

Findings

The black hole in NGC 1313 X-2 is likely 50-100 solar masses.

A 20 solar mass black hole with a 12-15 solar mass giant donor is unlikely.

Results support formation of massive black holes in low metallicity environments.

Abstract

The amount of data available for NGC 1313 X-2 make it a cornerstone for the study of ultraluminous X-ray sources (ULXs). We modelled the optical and X-ray data of this ULX with a binary evolution code that takes into account X-ray irradiation. We restricted the candidate binary system to be either a 50-100 Msun black hole (BH) accreting from a 12-15 Msun main sequence star or a 20 Msun BH with a 12-15 Msun giant donor. If the orbital period of the system is ~6 days, a 20 Msun BH model becomes unlikely and we are left with the only possibility that the compact accretor in NGC1313 X-2 is a massive BH of 50-100 Msun. We briefly discuss these results within the framework of an alternative scenario for the formation of ULXs, in which a portion of them may contain BHs of >~30-40 Msun formed from very massive stars in a low metallicity environment.