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Register a new userTowards the Event Horizon - High Resolution VLBI Imaging of Nuclei of Active Galaxies
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Added by
Thomas P. Krichbaum
Publication date
2006
fields
Physics
and research's language is
English
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We report on new developments in VLBI, with emphasis on experiments performed at the highest frequencies possible to date (so called mm-VLBI). We have observed the nucleus of M 87 (Virgo A) with global VLBI at 3 mm. We show a new image of the inner-most jet region with an angular resolution of approx. 300 x 60 micro-arcseconds. In terms of Schwarzschild radii, this leads to an upper limit of the jet base of approx. 100 x 20 Schwarzschild radii. We also report on two VLBI pilot-experiments, which demonstrate the technical feasibility of global VLBI at 150 and 230 GHz (2 mm and 1.3 mm). The experiments lead to upper limits to the size of the unresolved AGN-cores in the 25 - 30 micro-arcsecond range. The participation of new and near-future mm-telescopes (like APEX, CARMA, SMA, LMT, ALMA, etc.) in global mm-VLBI will provide the necessary sensitivity for the imaging of black holes and their immediate environment.
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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.
A long standing goal in astrophysics is to directly observe the immediate environment of a black hole with angular resolution comparable to the event horizon. Realizing this goal would open a new window on the study of General Relativity in the strong field regime, accretion and outflow processes at the edge of a black hole, the existence of an event horizon, and fundamental black hole physics (e.g., spin). Steady long-term progress on improving the capability of Very Long Baseline Interferometry (VLBI) at short wavelengths has now made it extremely likely that this goal will be achieved within the next decade. The most compelling evidence for this is the recent observation by 1.3mm VLBI of Schwarzschild radius scale structure in SgrA*, the compact source of radio, submm, NIR and xrays at the center of the Milky Way. SgrA* is thought to mark the position of a ~4 million solar mass black hole, and because of its proximity and estimated mass presents the largest apparent event horizon size of any black hole candidate in the Universe. Over the next decade, existing and planned mm/submm facilities will be combined into a high sensitivity, high angular resolution Event Horizon Telescope that will bring us as close to the edge of black hole as we will come for decades. This white paper describes the science case for mm/submm VLBI observations of both SgrA* and M87 (a radio loud AGN of a much more luminous class that SgrA*). We emphasize that while there is development and procurement involved, the technical path forward is clear, and the recent successful observations have removed much of the risk that would normally be associated with such an ambitious project.
We summarize the present status of VLBI experiments at 3 mm (86 GHz), 2 mm (129-150 GHz) and 1.3 mm (215-230 GHz). We present and discuss a new 3 mm VLBI map of M87 (Virgo A), which has a spatial resolution of only approx. 20 Schwarzschild radii. We discuss recent VLBI results for SgrA* and argue in favor of new observations within an extended European mm-VLBI network, in order to search for variability. We discuss the possibilities to image the `event horizon of a super-massive black hole at wavelengths < 2mm, and conclude that the addition of large and sensitive millimetre telescopes such as CARMA, the SMA, the LMT and ALMA will be crucial for this.
A review is given on the current status and selected results from large VLBI surveys of compact extragalactic radio sources made between 13 cm and 3 mm wavelengths and covering the entire sky. More than 4200 objects are observed and imaged with dynamic ranges from a hundred to several thousand at (sub)parsec scales. Implications to the VSOP-2 project are discussed.
The Event Horizon Telescope (EHT) is a very long baseline interferometer built to image supermassive black holes on event-horizon scales. In this paper, we investigate candidate sites for an expanded EHT array with improved imaging capabilities. We use historical meteorology and radiative transfer analysis to evaluate site performance. Most of the existing sites in the EHT array have median zenith opacity less than 0.2 at 230 GHz during the March/April observing season. Seven of the existing EHT sites have 345 GHz opacity less than 0.5 during observing months. Out of more than forty candidate new locations analyzed, approximately half have 230 GHz opacity comparable to the existing EHT sites, and at least seventeen of the candidate sites would be comparably good for 345 GHz observing. A group of new sites with favorable transmittance and geographic placement leads to greatly enhanced imaging and science on horizon scales.
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