Abstract
ABSTRACT Infrared interferometry is a new frontier for precision ground-based observing, with new instrumentation achieving milliarcsecond (mas) spatial resolutions for faint sources, along with astrometry on the order of 10 microarcseconds (μas). This technique has already led to breakthroughs in the observations of the supermassive black hole at the Galactic centre and its orbiting stars, active galactic nucleus, and exo-planets, and can be employed for studying X-ray binaries (XRBs), microquasars in particular. Beyond constraining the orbital parameters of the system using the centroid wobble and spatially resolving jet discrete ejections on mas scales, we also propose a novel method to discern between the various components contributing to the infrared bands: accretion disc, jets, and companion star. We demonstrate that the GRAVITY instrument on the Very Large Telescope Interferometer should be able to detect a centroid shift in a number of sources, opening a new avenue of exploration for the myriad of transients expected to be discovered in the coming decade of radio all-sky surveys. We also present the first proof-of-concept GRAVITY observation of a low-mass XRB transient, MAXI J1820+070, to search for extended jets on mas scales. We place the tightest constraints yet via direct imaging on the size of the infrared emitting region of the compact jet in a hard state XRB.
Highlights
Radio wave interferometry has been in development for decades, culminating in the exquisite precision of Very Long Baseline Interferometry (VLBI)
Based on the radio-VLBI observations, we expect these bright ejecta to be spatially resolved with GRAVITY, with motions of ∼10s–100 mas d−1
For both HMXB and low-mass XRBs (LMXBs), astrometry with the current specifications of GRAVITY on the VLTI should be achievable if the target has K 16 and another K 11 mag star lies within 2 arcsec from the target, or vice versa
Summary
Radio wave interferometry has been in development for decades, culminating in the exquisite precision of Very Long Baseline Interferometry (VLBI). The fringes of the target can be actively stabilized with respect to the reference source, allowing for integration times long enough that the target can be much fainter than the reference object. An instrument with these capabilities is in operation on the VLTI, GRAVITY (Gravity Collaboration et al 2017a). GRAVITY is a second generation interferometric instrument, commissioned on the VLTI in 2016 It allows observing two objects (one bright fringe-tracking, phase reference object and one fainter science object).
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