Towards a dynamical mass of the ultraluminous X-ray source NGC 5408 X-1

TL;DR

This study used multi-epoch optical spectroscopy to constrain the black hole mass in ULX NGC 5408 X-1, finding it likely less than 510 solar masses, with possibilities extending to stellar-mass black holes.

Contribution

First multi-epoch optical spectroscopic analysis of NGC 5408 X-1 providing constraints on its binary parameters and black hole mass.

Findings

Black hole mass less than ~510 solar masses.

No significant periodic signal detected.

Optical spectra show broad HeII line consistent with accretion disc origin.

Abstract

We obtained multi-epoch Very Large Telescope (VLT) optical spectroscopic data in 2011 and 2012 on the ultraluminous X-ray source (ULX) NGC 5408 X-1. We confirm that the HeII\lambda4686 line has a broad component with an average FWHM of v=780\pm64 km/s with a variation of ~13% during observations spanning over 4 years, and is consistent with the origin in the accretion disc. The deepest optical spectrum does not reveal any absorption line from a donor star. Our aim was to measure the radial velocity curve and estimate the parameters of the binary system. We find an upper limit on the semi-amplitude of the radial velocity of K=132\pm42 km/s. A search for a periodic signal in the data resulted in no statistically significant period. The mass function and constraints on the binary system imply a black hole mass of less than ~510 M_sun. Whilst, a disc irradiation model may imply a black hole mass smaller than ~431-1985 M_sun, depending on inclination. Our data can also be consistent with an unexplored orbital period range from a couple of hours to a few days, thus with a stellar-mass black hole and a subgiant companion.