Abstract
Due to its proximity, M87 is a prime target for next-generation high-resolution VLBI at short millimeter wavelengths, by which the jet launching region and the black hole shadow are expected to be resolved and imaged sometime soon. Along with this situation, high-quality VLBI imaging and monitoring at lower frequencies play an important role in complementing the high-frequency data. Here, we present our recent and ongoing observational studies of the M87 jet on pc-to-subpc scales based on ultra-deep VLBI imaging programs at 86 GHz and 15 GHz. The high-dynamic-range images have allowed us to obtain some remarkably improved views on this jet. We also introduce the KVN and VERA Array (KaVA), a new regularly-operating VLBI network in East Asia, which is quite suitable for studying the structure and propagation of relativistic jets. Some early results from our pilot study for M87—including the detection of superluminal motions near the jet base—implying an efficient magnetic-to-kinetic conversion at these scales, are reported.
Highlights
It has been nearly 100 years since the first extragalactic jet was discovered in the nearby elliptical galaxy M87
The jet of M87 is privileged because one can resolve the jet formation scales near the central engine thanks to its proximity (∼16 Mpc) and the large mass of the central black hole
The Event Horizon Telescope (EHT) is quite promising to image the shadow of the central black hole and the jet launching within the inner part of the accretion disk
Summary
It has been nearly 100 years since the first extragalactic jet was discovered in the nearby elliptical galaxy M87. The effect of gravitational lensing can enhance the brightness of the counter-jet near the black hole, which might further enable us to constrain the mass loading radius of the jet [8,9] Along with this situation, the traditional lower-frequency VLBI gains in importance. The M87 jet has intensively been monitored by many VLBI programs over the past years, pinpointing the sites of active gamma-ray flares (e.g., [13,14,15,16,17]) as well as measuring the jet motions (e.g., [18,19,20,21]) Such excellent capabilities of imaging and monitoring of the low-frequency VLBI can be further improved by the recent rapid improvement of data recording rate.
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