# An HI absorption distance to the black hole candidate X-ray binary MAXI J1535-571

## TL;DR

This study used ASKAP radio observations to measure HI absorption spectra, constraining the distance to the black hole candidate X-ray binary MAXI J1535-571, and analyzed its luminosity and spectral state transition at this distance.

## Contribution

First application of ASKAP HI absorption spectroscopy to determine the distance to a Galactic X-ray binary, demonstrating the method's effectiveness for transient sources.

## Key findings

- Distance to MAXI J1535-571 is approximately 4.1 kpc.
- The source's luminosity was >78% of the Eddington luminosity at the measured distance.
- The spectral state transition occurred at a very low Eddington ratio.

## Abstract

With the Australian Square Kilometre Array Pathfinder (ASKAP) we monitored the black hole candidate X-ray binary MAXI J1535--571 over seven epochs from 21 September to 2 October 2017. Using ASKAP observations, we studied the HI absorption spectrum from gas clouds along the line-of-sight and thereby constrained the distance to the source. The maximum negative radial velocities measured from the HI absorption spectra for MAXI J1535--571 and an extragalactic source in the same field of view are $-69\pm4$ km s$^{-1}$ and $-89\pm4$ km s$^{-1}$, respectively. This rules out the far kinematic distance ($9.3^{+0.5}_{-0.6}$ kpc), giving a most likely distance of $4.1^{+0.6}_{-0.5}$ kpc, with a strong upper limit of the tangent point at $6.7^{+0.1}_{-0.2}$ kpc. At our preferred distance, the peak unabsorbed luminosity of MAXI J1535--571 was $>78$ per cent of the Eddington luminosity, and shows that the soft-to-hard spectral state transition occurred at the very low luminosity of 1.2 -- 3.4 $\times$ 10$^{-5}$ times the Eddington luminosity. Finally, this study highlights the capabilities of new wide-field radio telescopes to probe Galactic transient outbursts, by allowing us to observe both a target source and a background comparison source in a single telescope pointing.