# Sub-milliarcsecond Optical Interferometry of the HMXB BP Cru with VLTI/GRAVITY

## TL;DR

This study uses VLTI/GRAVITY interferometry to resolve the structure of the high-mass X-ray binary BP Cru at sub-milliarcsecond scales, revealing asymmetric emission and providing insights into the accretion environment.

## Contribution

First high-resolution optical interferometric observations of BP Cru, demonstrating the ability to resolve HMXBs at accretion scales and modeling the extended wind and gas stream structures.

## Key findings

- Detected differential visibility and phase signatures across spectral lines.
- Observed asymmetric, extended emission with wavelength dependence.
- Achieved microarcsecond astrometric precision.

## Abstract

We observe the HMXB BP Cru using interferometry in the near-infrared K band with VLTI/GRAVITY. Continuum visibilities are at most partially resolved, consistent with the predicted size of the hypergiant. Differential visibility amplitude ($\Delta |V| \sim 5\%$) and phase ($\Delta \phi \sim 2 \deg$) signatures are observed across the HeI $2.059 \mu$m and Br$\gamma$ lines, the latter seen strongly in emission, unusual for the donor star's spectral type. For a baseline $B \sim 100$m, the differential phase RMS $\sim 0.2 \deg$ corresponds to an astrometric precision of $\sim 2 \mu$as. A model-independent analysis in the marginally resolved limit of interferometry reveals asymmetric and extended emission with a strong wavelength dependence. We propose geometric models based on an extended and distorted wind and/or a high density gas stream, which has long been predicted to be present in this system. The observations show that optical interferometry is now able to resolve HMXBs at the spatial scale at which accretion takes place, and therefore probe the effects of the gravitational and radiation fields of the compact object on its environment.