LBA observations for project V607 semester 2021OCTS

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High-mass X-ray binaries hosting a neutron star accretor have only recently been detected as radio sources. While for transient systems, their radio emission is likely caused by jets launched from their accretion flow, the origin of this emission remains unconfirmed for the persistently accreting sources. Lacking large variations in accretion properties that can be tied to changes in radio, three options remain for these persistent sources: (i) thermal/free-free stellar wind emission, or non-thermal/synchrotron emission from (ii) jets or (iii) accretion wake shocks. The stellar wind emission is expected to have a low brightness temperature (1e4-1e5 K) and large extent (100-200 binary orbits), especially compared to higher brightness temperatures (up to 1e12 K) possible for the synchrotron processes. Therefore, we propose a 12-hour single-run LBA observation of the radio-brightest neutron star high-mass X-ray binary, 4U 1700-37. If the source is unresolved at a 2 mas resolution, we can rule out the thermal emission scenario from both brightness temperature and size arguments. If the emission is instead resolved, its morphology will distinguish between a wind or jet scenario. Then, we will independently measure the wind mass loss rates and velocity, or the jet emission height, depending on the emission origin, while we place limits on the other mechanism.