Measuring the distance to the black hole candidate X-ray binary MAXI J1348-630 using H"‰I absorption

We present neutral hydrogen (H I) absorption spectra of the black hole candidate X-ray binary (XRB) MAXI J1348–630 using the Australian Square Kilometre Array Pathfinder (ASKAP) and MeerKAT. The ASKAP H I spectrum shows a maximum negative radial velocity (with respect to the local standard of rest) of −31 ± 4 km s⁻¹ for MAXI J1348–630, as compared to −50 ± 4 km s⁻¹ for a stacked spectrum of several nearby extragalactic sources. This implies a most probable distance of 2.2⁺₋⁰₀⁵₆ kpc for MAXI J1348–630, and a strong upper limit of the tangent point distance at 5.3 ± 0.1 kpc. Our preferred distance implies that MAXI J1348–630 reached 17 ± 10 per cent of the Eddington luminosity at the peak of its outburst, and that the source transited from the soft to the hard X-ray spectral state at 2.5 ± 1.5 per cent of the Eddington luminosity. The MeerKAT H I spectrum of MAXI J1348–630 (obtained from the older, low-resolution 4k mode) is consistent with the re-binned ASKAP spectrum, highlighting the potential of the eventual capabilities of MeerKAT for XRB spectral line studies.