Spectroscopic Studies of an Ultraluminous Supersoft X-Ray Source in M81

TL;DR

This study presents the first optical spectroscopic observations of an ultraluminous supersoft X-ray source in M81, revealing accretion disk features and suggesting a halo system origin.

Contribution

First optical spectroscopy of a ULS in M81, showing accretion disk signatures and potential halo system association.

Findings

Balmer emission lines indicate accretion disk dynamics.

Spectral index aligns with multi-color disk model.

Hα velocity suggests halo system in M81.

Abstract

Ultraluminous supersoft X-ray sources (ULSs) exhibit supersoft X-ray spectra with blackbody temperatures below 0.1 keV and bolometric luminosities above 10$^{39}$ ergs s$^{-1}$. In this Letter, we report the first optical spectroscopic observations of a ULS in M81 using the LRIS spectrograph on the Keck I telescope. The detected Balmer emission lines show a mean intrinsic velocity dispersion of 400$\pm$80 km s$^{-1}$, which is consistent with that from an accretion disk. The spectral index of the continuum on the blue side is also consistent with the multi-color disk model. The H$_{\alpha}$ emission line exhibits a velocity of $\sim$180 km s$^{-1}$ relative to the local stellar environment, suggesting that this ULS may be a halo system in M81 belonging to an old population. No significant shift is found for the H$_{\alpha}$ emission line between two observations separated by four nights.