Accretion disk parameters in HLX-1

TL;DR

This paper estimates the size of the accretion disk in HLX-1 using optical and X-ray data, revealing a very small disk size and high binary eccentricity, along with H-alpha emission observations.

Contribution

It provides new estimates of the accretion disk size and eccentricity in HLX-1, and reports H-alpha emission during an outburst, offering insights into the system's geometry and dynamics.

Findings

Disk is an order of magnitude smaller than the orbital semimajor axis.

Binary system likely has eccentricity >~ 0.95.

H-alpha emission with a narrow, single-peaked profile was detected.

Abstract

We estimate the outer radius of the accretion disk in HLX-1 from its optical brightness and from the exponential timescale of the decline in the X-ray lightcurve after an outburst. We find that the disk is an order of magnitude smaller than the semimajor axis of the orbit. If the disk size is determined by the circularization radius near periastron, the eccentricity of the binary system must be >~ 0.95. We report on the discovery of H-alpha emission during the 2012 outburst, with a single-peaked, narrow profile (consistent with a nearly face-on view), and a central velocity displaced by ~490 km/s from that of the host galaxy.