96 research outputs found
Jets in AGN at extremely high redshifts
A brief review of VLBI structures in extremely high-redshift AGN
(revised/edited).Comment: To appear in the Proceedings of the IAU Symposium No. 313:
Extragalactic jets from every angle, Galapagos, Ecuador, 15-19 September
2014, F. Massaro, C. C. Cheung, E. Lopez, and A. Siemiginowska (Eds.),
Cambridge University Pres
VLBI observations of a flared optical quasar CGRaBS J0809+5341
A bright optical flare was detected in the high-redshift () quasar
CGRaBS J0809+5341 on 2014 April 13. The absolute magnitude of the object
reached during the flare, making it the brightest one (in flaring
stage) among all known quasars so far. The 15 GHz flux density of CGRaBS
J0809+5341 monitored in the period from 2008 to 2016 also reached its peak at
the same time. To reveal any structural change possibly associated with the
flare in the innermost radio structure of the quasar, we conducted a pilot very
long baseline interferometry (VLBI) observation of CGRaBS J0809+5341 using the
European VLBI Network (EVN) at 5 GHz on 2014 November 18, about seven months
after the prominent optical flare. Three epochs of follow-up KaVA (Korean VLBI
Network and VLBI Exploration of Radio Astrometry Array) observations were
carried out at 22 and 43 GHz frequencies from 2015 February 25 to June 4, with
the intention of exploring a possibly emerging new radio jet component
associated with the optical flare. However, these high-resolution VLBI
observations revealed only the milliarcsecond-scale compact "core" that was
known in the quasar from earlier VLBI images, and showed no sign of any
extended jet structure. Neither the size, nor the flux density of the "core"
changed considerably after the flare according to our VLBI monitoring. The
results suggest that any putative radio ejecta associated with the major
optical and radio flare could not yet be separated from the "core" component,
or the newly-born jet was short-lived.Comment: 4 figures, 2 tables, accepted for publication in PAS
Antenna Design and Implementation for the Future Space Ultra-Long Wavelength Radio Telescope
In radio astronomy, the Ultra-Long Wavelengths (ULW) regime of longer than 10
m (frequencies below 30 MHz), remains the last virtually unexplored window of
the celestial electromagnetic spectrum. The strength of the science case for
extending radio astronomy into the ULW window is growing. However, the
opaqueness of the Earth's ionosphere makes ULW observations by ground-based
facilities practically impossible. Furthermore, the ULW spectrum is full of
anthropogenic radio frequency interference (RFI). The only radical solution for
both problems is in placing an ULW astronomy facility in space. We present a
concept of a key element of a space-borne ULW array facility, an antenna that
addresses radio astronomical specifications. A tripole-type antenna and
amplifier are analysed as a solution for ULW implementation. A receiver system
with a low power dissipation is discussed as well. The active antenna is
optimized to operate at the noise level defined by the celestial emission in
the frequency band 1 - 30 MHz. Field experiments with a prototype tripole
antenna enabled estimates of the system noise temperature. They indicated that
the proposed concept meets the requirements of a space-borne ULW array
facility.Comment: Submitted to Experimental Astronomy, 23 pages, 17 figure
The compact radio structure of the high-redshift blazar J1430+4204 before and after a major outburst
The high-redshift (z=4.72) blazar J1430+4204 produced an exceptional radio
outburst in 2006. We analyzed 15-GHz radio interferometric images obtained with
the Very Long Baseline Array (VLBA) before and after the outburst, to search
for possible structural changes on milli-arcsecond angular scales and to
determine physical parameters of the source.Comment: Proceedings of the 5th Workshop of Young Researchers in Astronomy and
Astrophysics, Budapest, 2009; to be published in J. Phys.: Conf. Series
(JPCS); 4 pages, 3 figure
Spacecraft VLBI and Doppler tracking: algorithms and implementation
We present the results of several multi-station Very Long Baseline
Interferometry (VLBI) experiments conducted with the ESA spacecraft Venus
Express as a target. To determine the true capabilities of VLBI tracking for
future planetary missions in the solar system, it is necessary to demonstrate
the accuracy of the method for existing operational spacecraft. We describe the
software pipeline for the processing of phase referencing near-field VLBI
observations and present results of the ESA Venus Express spacecraft observing
campaign conducted in 2010-2011. We show that a highly accurate determination
of spacecraft state-vectors is achievable with our method. The consistency of
the positions indicates that an internal rms accuracy of 0.1 mas has been
achieved. However, systematic effects produce offsets up to 1 mas, but can be
reduced by better modelling of the troposphere and ionosphere and closer
target-calibrator configurations.Comment: 10 pages, 10 figures. Astronomy and Astrophysics, accepte
Orbital configurations of spaceborne interferometers for studying photon rings of supermassive black holes
Recent advances in technology coupled with the progress of observational
radio astronomy methods resulted in achieving a major milestone of astrophysics
- a direct image of the shadow of a supermassive black hole, taken by the
Earth-based Event Horizon Telescope (EHT). The EHT was able to achieve a
resolution of 20 as, enabling it to resolve the shadows of the black
holes in the centres of two celestial objects: the supergiant elliptical galaxy
M87 and the Milky Way Galaxy. The EHT results mark the start of a new round of
development of next generation Very Long Baseline Interferometers (VLBI) which
will be able to operate at millimetre and sub-millimetre wavelengths. The
inclusion of baselines exceeding the diameter of the Earth and observation at
as short a wavelength as possible is imperative for further development of high
resolution astronomical observations. This can be achieved by a spaceborne VLBI
system. We consider the preliminary mission design of such a system,
specifically focused on the detection and analysis of photon rings, an
intrinsic feature of supermassive black holes. Optimised Earth, Sun-Earth L2
and Earth-Moon L2 orbit configurations for the space interferometer system are
presented, all of which provide an order of magnitude improvement in resolution
compared to the EHT. Such a space-borne interferometer would be able to conduct
a comprehensive survey of supermassive black holes in active galactic nuclei
and enable uniquely robust and accurate tests of strong gravity, through
detection of the photon ring features.Comment: Accepted for publication in Acta Astronautica. 40 pages, 13 figure
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