Dynamical mass constraints on the ultraluminous X-ray Source NGC 1313 X-2

TL;DR

This study uses dynamical measurements and light curve modeling to constrain the mass of the black hole in ULX NGC 1313 X-2, suggesting it could be either a massive stellar black hole or an intermediate mass black hole, depending on further observations.

Contribution

First dynamical mass constraints on NGC 1313 X-2 using HST light curves and radial velocity data, exploring models for stellar and intermediate mass black holes.

Findings

Black hole mass could be a few tens of solar masses or over 1000 solar masses.

Secondary star likely 40-50 Myrs old in specific evolutionary phase.

Further observations needed to distinguish between stellar black hole and IMBH.

Abstract

Dynamical mass measurements hold the key to answering whether ultraluminous X-ray sources (ULXs) are intermediate mass black holes (IMBHs) or stellar mass black holes with special radiation mechanisms. NGC 1313 X-2 is so far the only ULX with HST light curves, the orbital period, and the black hole's radial velocity amplitude based on the He II $\lambda4686$\AA\ disk emission line shift of $\sim200$ km/s. We constrain its black hole mass and other parameters by fitting observations to a binary light curve code with accommodations for X-ray heating of the accretion disk and the secondary. Given the dynamical constraints from the observed light curves and the black hole radial motion and the observed stellar environment age, the only acceptable models are those with 40-50 Myrs old intermediate mass secondaries in their helium core and hydrogen shell burning phase filling 40%-80% of their Roche lobes. The black hole can be a massive black hole of a few tens of $M_\odot$ that can be produced from stellar evolution of low metalicity stars, or an IMBH of a few hundred to above 1000$M_\odot$ if its true radial velocity $2K^\prime<40$ km/s. Further observations are required to better measure the black hole radial motion and the light curves in order to determine whether NGC 1313 X-2 is a stellar black hole or an IMBH.