726 research outputs found
The Role of Millimeter VLBI Observations in AGN Research
VLBI at millimeter wavelengths (mm-VLBI) allows the detailed imaging of
compact galactic and extragalactic radio sources with micro-arcsecond scale
resolution, unaccessible by other observing techniques. Here we discuss the
scientific potential of mm-VLBI for present and future research on `Active
Galactic Nuclei' (AGN) and their powerful relativistic jets. With the new
generation of large radio telescopes and interferometer arrays operating in the
millimeter radio bands (e.g. ALMA), the ultimate vicinity of super massive
Black Holes, and eventually even their event horizon, could be imaged. With its
large collecting area, and in combination with these future telescopes, the
Sardinia Radio Telescope could form the World's `sharpest' astronomical imaging
machine.Comment: 10 pages, 1 table, 7 figures. An inveited talk held at the Sardinia
Radiotelescope Conference in Cagliari, Sardinia, on November 7-10, 200
Measuring the Black Hole Spin in Sgr A*
The polarized mm/sub-mm radiation from Sgr A* is apparently produced by a
Keplerian structure whose peak emission occurs within several Schwarzschild
radii (r_S=2GM/c^2) of the black hole. The Chandra X-ray counterpart, if
confirmed, is presumably the self-Comptonized component from this region. In
this paper, we suggest that sub-mm timing observations could yield a signal
corresponding to the period P_0 of the marginally stable orbit, and therefore
point directly to the black hole's spin a. Sgr A*'s mass is now known to be
(2.6\pm 0.2)\times 10^6 M_\odot (an unusually accurate value for supermassive
black hole candidates), for which 2.7 min<P_0<36 min, depending on the value of
a and whether the Keplerian flow is prograde or retrograde. A Schwarzschild
black hole (a=0) should have P_0 ~ 20 min. The identification of the orbital
frequency with the innermost stable circular orbit is made feasible by the
transition from optically thick to thin emission at sub-mm wavelengths. With
stratification in the emitter, the peak of the sub-mm bump in Sgr A*'s spectrum
is thus produced at the smallest radius. We caution, however, that theoretical
uncertainties in the structure of the emission region may still produce some
ambiguity in the timing signal. Given that Sgr A*'s flux at mm is
several Jy, these periods should lie within the temporal-resolving capability
of sub-mm telescopes using bolometric detectors. A determination of P_0 should
provide not only a value of a, but it should also define the angular momentum
vector of the orbiting gas in relation to the black hole's spin axis. In
addition, since the X-ray flux detected by Chandra appears to be the
self-Comptonized mm to sub-mm component, these temporal fluctuations may also
be evident in the X-ray signal.Comment: 15 pages, 1 figures. Accepted for publication in ApJ Letter
High-frequency VLBI observations of SgrA* during a multi-frequency campaign in May 2007
In May 2007 the compact radio source Sgr A* was observed in a global
multi-frequency monitoring campaign, from radio to X-ray bands. Here we present
and discuss first and preliminary results from polarization sensitive VLBA
observations, which took place during May 14-25, 2007. Here, Sgr A* was
observed in dual polarization on 10 consecutive days at 22, 43, and 86 GHz. We
describe the VLBI experiments, our data analysis, monitoring program and show
preliminary images obtained at the various frequencies. We discuss the data
with special regard also to the short term variability.Comment: 6 pages, 5 figures;necessary style files included; contribution for
the conference "The Universe under the Microscope" (AHAR 2008), held in Bad
Honnef (Germany) in April 2008, to be published in Journal of Physics:
Conference Series by Institute of Physics Publishing, R. Schoedel, A. Eckart,
S. Pfalzner, and E. Ros (eds.
Radio observations of active galactic nuclei with mm-VLBI
Over the past few decades, our knowledge of jets produced by active galactic
nuclei (AGN) has greatly progressed thanks to the development of
very-long-baseline interferometry (VLBI). Nevertheless, the crucial mechanisms
involved in the formation of the plasma flow, as well as those driving its
exceptional radiative output up to TeV energies, remain to be clarified. Most
likely, these physical processes take place at short separations from the
supermassive black hole, on scales which are inaccessible to VLBI observations
at centimeter wavelengths. Due to their high synchrotron opacity, the dense and
highly magnetized regions in the vicinity of the central engine can only be
penetrated when observing at shorter wavelengths, in the millimeter and
sub-millimeter regimes. While this was recognized already in the early days of
VLBI, it was not until the very recent years that sensitive VLBI imaging at
high frequencies has become possible. Ongoing technical development and wide
band observing now provide adequate imaging fidelity to carry out more detailed
analyses.
In this article we overview some open questions concerning the physics of AGN
jets, and we discuss the impact of mm-VLBI studies. Among the rich set of
results produced so far in this frequency regime, we particularly focus on
studies performed at 43 GHz (7 mm) and at 86 GHz (3 mm). Some of the first
findings at 230 GHz (1 mm) obtained with the Event Horizon Telescope are also
presented.Comment: Published in The Astronomy & Astrophysics Review. Open access:
https://link.springer.com/article/10.1007/s00159-017-0105-
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