Constraining the jet base emission of M87* with past and future Event Horizon Telescope observations

Abstract

We investigate the detectability of the jet base of M87* at Event Horizon Telescope (EHT) observing frequencies. Although M87 is known to host a prominent relativistic jet, detecting jet emission close to the black hole at horizon scales remains challenging. Our goal is to determine the minimum jet intensity that can be reliably detected with the recent EHT array configurations. We use synthetic EHT data generated for three array configurations corresponding to the 2021 and 2022 observing campaigns and to a near future EHT campaign. As input models, we employ semi analytic accretion jet models in which the jet emission can be tuned independently of the accretion flow. The synthetic data are reconstructed with regularized maximum likelihood and Bayesian imaging. Jet detectability is assessed through flux density recovery, image fidelity, and uncertainty maps. We find that jet detectability strongly depends on the jet intensity, the array configuration, and imaging methodology. Using our analysis, we determine a lower limit on the jet intensity that can be reliably recovered. The 2022 EHT array configuration represents a significant improvement over earlier arrays, enabling a more robust reconstruction of faint jet features. Our results indicate that the current EHT array is already sensitive to weak jet emission at horizon scales in M87*. The improved short-baseline coverage introduced in 2022 makes faint inner jet features more easily detectable. If the inner jet contributes a significant fraction of the unresolved compact flux, it should become visible in post-2021 observations. On the other hand, if no clear jet signature is found, this would suggest that the horizon-scale jet contributes only a small part of the compact emission. The continued expansion of the EHT will further improve the detection of such jet emission in M87*.