Zooming towards the Event Horizon - mm-VLBI today and tomorrow

TL;DR

This paper reviews the current state and future prospects of millimeter and sub-millimeter Very Long Baseline Interferometry (mm-VLBI) for imaging active galactic nuclei and black hole event horizons, highlighting recent results and technological advancements.

Contribution

It presents new multi-frequency mm-VLBI imaging results of OJ287 and reports a record-breaking 1.3mm VLBI observation with APEX, paving the way for future high-resolution black hole imaging.

Findings

Multi-frequency mm-VLBI reveals structural changes in AGN during outbursts.

Record angular resolution achieved with 1.3mm VLBI at APEX.

Future prospects include imaging black hole event horizons with ALMA.

Abstract

Global VLBI imaging at millimeter and sub-millimeter wavelength overcomes the opacity barrier of synchrotron self-absorption in AGN and opens the direct view into sub-pc scale regions not accessible before. Since AGN variability is more pronounced at short millimeter wavelength, mm-VLBI can reveal structural changes in very early stages after outbursts. When combined with observations at longer wavelength, global 3mm and 1mm VLBI adds very detailed information. This helps to determine fundamental physical properties at the jet base, and in the vicinity of super-massive black holes at the center of AGN. Here we present new results from multi-frequency mm-VLBI imaging of OJ287 during a major outburst. We also report on a successful 1.3mm VLBI experiment with the APEX telescope in Chile. This observation sets a new record in angular resolution. It also opens the path towards future mm-VLBI with ALMA, which aims at the mapping of the black hole event horizon in nearby galaxies, and the study of the roots of jets in AGN.