A Prospective 690 GHz Suborbital-Ground VLBI Concept: Detecting the M87 Black Hole Photon Ring

Theoretical studies and recent epoch making VLBI observations of the black hole at the center of the M87 galaxy by the Event Horizon Telescope have predicted the existence, and quantified the expected parameters, of a finely structured set of rings arising from light itself going into orbit around the black hole: the photon ring. The EHT observation detected the gravitationally lensed component (n = 0), but not the orbit component (n ≥ 1, where n is the number of half orbits executed). Observing and studying the n ≥ 1 general relativistic effects require spatial resolution capabilities currently not available. Nominally, such resolutions can only be reached with a space-VLBI instrument operating at 230/345 GHz. However, if observations can be extended to 690 GHz, it will be possible to achieve the required resolutions with a combination of ground and suborbital antennas. As very few ground sites can support 690 GHz observations - just one with large collecting area (ALMA) - we are considering a baseline between ALMA and a modest sized suborbital antenna. While suborbital platforms have focused on bands where the atmosphere is opaque, there are key benefits for VLBI in atmospheric transmission windows: (1) as the highest resolution VLBI data are obtained at very high air masses, even for modest sized suborbital apertures, the equivalent ground apertures are significantly large (2) long, variable and otherwise unachievable baselines are possible, uniquely enhancing u-v plane coverage, even for 230/345 GHz VLBI. We present a preliminary exploration of this suborbital-ground hybrid opportunity and outline the prospects and advantages. Prospective balloon flight paths based on completed NASA engineering flights allow the required baseline coverage. We are also engaged in a preliminary exploration of the possibility of hosting initial VLBI experiments on the ASTHROS (Astrophysics Stratospheric Telescope for High Spectral Resolution Observations at Submillimeter-wavelengths; 2.5 m) balloon borne telescope. Such observations have the potential to detect the characteristic signature of the photon ring, and therefore, test general relativity under strong gravity. An initial detection of the signatures consistent with theoretical predictions can provide their first verification, paving the way for future more extensive experiments. Suborbital VLBI is the only way to reach the spatial resolutions required to unambiguously observe the n = 1 photon ring component due to photons going into orbit around a black hole, without going to space.