Prospects for submillimeter VLBI of black hole photon rings with a stratospheric balloon-borne telescope
Abstract
Submillimeter very long baseline interferometry (VLBI) achieves higher angular resolution and probes more optically thin emission near supermassive black holes, with less interstellar scattering toward Sgr A*. Above 345 GHz, however, the atmosphere confines ground-based VLBI to a few high, dry sites. Using 2012-2022 weather, we forecast what a 2.5 m telescope floating at 35-40 km, a successor to the 22 GHz Balloon-borne VLBI Experiment (BVEX) pathfinder, would add to ground-VLBI arrays at 345-690 GHz. The balloon adds little baseline length but reaches higher frequencies, where a fixed baseline is longer in wavelengths. On an ALMA-balloon baseline, Sgr A* at 460 GHz is at the detection threshold, with ten or more 20 s detections on about half of the simulated April days. These detections lie beyond the polarimetric photon ring transition, where the n=1 ring dominates the polarized visibility amplitudes. M87* at 460 GHz would need frequency phase transfer from a 230 GHz reference to be detected. With phase transfer, most Sgr A* detections beyond the transition at 460 GHz are on balloon baselines, whose thermal noise meets the linear-polarization threshold within one 300 s scan. At 690 GHz such detections require phase transfer that stays coherent at three times the reference frequency. In Fisher forecasts, a 460 GHz ring-geometry constraint requires balloons but is weaker than at 345 GHz, where balloons add little improvement. High-resolution 460 GHz VLBI science would require a 32 GHz bandwidth backend, submillimeter receivers, and simultaneous 230 GHz reference recording at every station.
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BibTeX
@misc{bagchi2026prospects,
title = {{Prospects for submillimeter VLBI of black hole photon rings with a stratospheric balloon-borne telescope}},
author = {Mayukh Bagchi and Dominic W. Pesce and Felix M. Thiel and Laura M. Fissel and Lindy Blackburn},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.24396},
eprint = {2610.24396},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.24396},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Prospects for submillimeter VLBI of black hole photon rings with a stratospheric balloon-borne telescope
AU - Mayukh Bagchi
AU - Dominic W. Pesce
AU - Felix M. Thiel
AU - Laura M. Fissel
AU - Lindy Blackburn
PY - 2026
DA - 2026/10/17
PB - arXiv
DO - 10.48550/arxiv.2610.24396
UR - https://arxiv.org/abs/2610.24396
AB - Submillimeter very long baseline interferometry (VLBI) achieves higher angular resolution and probes more optically thin emission near supermassive black holes, with less interstellar scattering toward Sgr A^*. Above 345 GHz, however, the atmosphere confines ground-based VLBI to a few high, dry sites. Using 2012-2022 weather, we forecast what a 2.5 m telescope floating at 35-40 km, a successor to the 22 GHz Balloon-borne VLBI Experiment (BVEX) pathfinder, would add to ground-VLBI arrays at 345-690 GHz. The balloon adds little baseline length but reaches higher frequencies, where a fixed baseline is longer in wavelengths. On an ALMA-balloon baseline, Sgr A^* at 460 GHz is at the detection threshold, with ten or more 20 s detections on about half of the simulated April days. These detections lie beyond the polarimetric photon ring transition, where the n=1 ring dominates the polarized visibility amplitudes. M87^* at 460 GHz would need frequency phase transfer from a 230 GHz reference to be detected. With phase transfer, most Sgr A^* detections beyond the transition at 460 GHz are on balloon baselines, whose thermal noise meets the linear-polarization threshold within one 300 s scan. At 690 GHz such detections require phase transfer that stays coherent at three times the reference frequency. In Fisher forecasts, a 460 GHz ring-geometry constraint requires balloons but is weaker than at 345 GHz, where balloons add little improvement. High-resolution 460 GHz VLBI science would require a 32 GHz bandwidth backend, submillimeter receivers, and simultaneous 230 GHz reference recording at every station.
ER -