Broad Components in Optical Emission Lines from the Ultra-Luminous X-ray Source NGC 5408 X-1

TL;DR

High-resolution optical spectra of NGC 5408 X-1 reveal a broad emission component likely from the accretion disk, with potential implications for estimating the mass of the compact object.

Contribution

First detection of a broad emission component in optical lines from NGC 5408 X-1, suggesting its origin in the accretion disk and providing a method to estimate the black hole mass.

Findings

Broad component width ~750 km/s in HeII and Hbeta lines.

Shift of 252 km/s in the HeII line suggests possible motion of the compact object.

Mass of the compact object estimated to be less than ~1800 solar masses.

Abstract

High-resolution optical spectra of the ultraluminous X-ray source NGC 5408 X-1 show a broad component with a width of ~750 km/s in the HeII and Hbeta lines in addition to the narrow component observed in these lines and [O III]. Reanalysis of moderate-resolution spectra shows a similar broad component in the HeII line. The broad component likely originates in the ULX system itself, probably in the accretion disk. The central wavelength of the broad HeII line is shifted by 252 \pm 47 km/s between the two observations. If this shift represents motion of the compact object, then its mass is less than ~1800 M_sun.