20 research outputs found
Shock Excited 1720 MHz Masers
1720 MHz OH masers have been detected towards a number of supernova remnants (SNRs) at the shock interface where the SNR slams into the interstellar medium. Models indicate that these masers are shock excited and can only be produced under tight constraints of the physical conditions. In particular, the masers can only form behind a C-type shock. Jets from newlyformed
stars plow into the surrounding gas, creating nebulous regions known as Herbig Haro (HH) objects. Signatures of C-type shocks have been found in many HH objects. If conditions behind the shock fronts of HH objects are able to support 1720 MHz OH masers they would be a usefull diagnostic tool for star formation. A survey toward HH objects detected a number of 1720 MHz OH lines in emission, but future observations with arrays are required
to confirm the presence of masers.PhysicsM.Sc. (Astronomy
Radio astronomy techniques : the use of radio instruments from single dish radio telescopes to radio interferometers
New radio telescopes under development, will significantly enhance the capabilities
of radio astronomy in the Southern Hemisphere. South Africa, in
particular, is actively involved in the development of a new array (MeerKAT)
as well as in the expansion of existing very long baseline interferometer arrays
in the south. Participation in these new developments demands a thorough
understanding of radio astronomy techniques, and data analysis, and this
thesis focusses on two projects with the aim of gaining such experience.
The Southern Hemisphere very long baselines array is not well served
with calibrator sources and there are significant gaps in the present calibrator
distribution on the sky. An adequately dense, well distributed, set of strong,
compact calibrator or reference sources is needed. With this in mind, observations
using the Southern Hemisphere long baseline array were conducted to
investigate a sample of candidate calibrator sources. The compactness of the
sources was investigated and new potential calibrators have been identified.
Single antenna radio spectroscopy of OH masers has identified sources
of 1720 MHz emission associated with supernova remnants at the shock interface
between the expanding supernova remnant and a molecular cloud.
Models indicate that these masers are shock excited and can only be produced
under tight physical constraints. Out
ows from newly-formed stars
create nebulous regions known as Herbig-Haro objects when they interact
with the surrounding medium, and these regions are potentially similar to
those seen in supernova remnants. If conditions behind the shock fronts of
Herbig-Haro objects are able to support 1720-MHz OH masers they could
be a useful diagnostic tool for star formation. A survey toward Herbig-Haro
objects using a single-dish radio telescope did detect 1720-MHz OH lines in
emission, but neither their spectral signature nor follow-up observations with
the Very Large Array showed evidence of maser emission.Mathematical SciencesPh.D. (Astronomy
Hartebeesthoek Radio Astronomy Observatory (HartRAO) antenna axis offset determined by geodetic VLBI analysis and ground survey
In the Very Long Baseline Interferometry (VLBI) space geodetic technique, various stationspecific
error sources corrupt the observable VLBI delay. An antenna axis offset (AO) model is
applied in the VLBI data analysis for antennas with non-intersecting rotational axes, such as the
26-m and 15-m antennas for the Hartebeesthoek Radio Astronomy Observatory (HartRAO). The
a priori AO values recommended by the International VLBI Service for Geodesy and Astrometry
(IVS) for use in geodetic VLBI data analysis are taken, where possible, from values measured in
ground surveys. The a priori AO values used for the HartRAO antennas in geodetic VLBI analysis
have been identified as possible sources of error. The a priori AO value of 6695.3 mm for the 26-m
antenna originates from a 2003 co-locational ground survey, conducted before a major bearing
repair in 2008, which could have changed the AO. The a priori AO value of 1495.0 mm for the 15-
m antenna was determined in 2007 in only a preliminary GPS survey. In this study, the respective
AO values of the HartRAO 26-m and 15-m antennas were estimated from a VLBI analysis using the
Vienna VLBI and Satellite Software (VieVS) and compared with measurements from co-locational
ground surveys. It was found that the VLBI estimated values do not agree within the formal margins
of error with the ground survey values, in that they differ by up to eight millimetres (8 mm) for the
26-m antenna and up to five millimetres (5 mm) for the 15-m antenna. As the ground survey values are considered to be more accurate than the VLBI estimated values, a further investigation of the
site-specific error sources that may be contaminating the accuracy of VLBI results is required.ACKNOWLEDGEMENTS : This research was financially supported by the National Research Foundation (NRF) and the Department of Science and Technology.https://www.sajg.org.za/index.php/sajgam2024Geography, Geoinformatics and MeteorologyNon
Observations of radio sources near the Sun
Geodetic Very Long Baseline Interferometry (VLBI) data are capable of
measuring the light deflection caused by the gravitational field of the Sun and
large planets with high accuracy. The parameter of the parametrized
Post-Newtonian (PPN) formalism estimated using observations of reference radio
sources near the Sun should be equal to unity in the general relativity. We
have run several VLBI experiments tracking reference radio sources from 1 to 3
degrees from the Sun. The best formal accuracy of the parameter
achieved in the single-session mode is less than 0.01 percent, or better than
the formal accuracy obtained with a global solution included all available
observations at arbitrary elongation from the Sun. We are planning more
experiments starting from 2020 using better observing conditions near the
minimum of the Solar activity cycle.Comment: Proceeding of the EVGA 2019 Meeting. arXiv admin note: substantial
text overlap with arXiv:1806.1129
Position and Proper Motion of Sagittarius A* in the ICRF3 Frame from VLBI Absolute Astrometry
Sagittarius A* (Sgr A*) is a strong, compact radio source believed to be powered by a supermassive black hole at the galactic center. Extinction by dust and gas in the galactic plane prevents observing it optically, but its position and proper motion have previously been estimated using radio interferometry. We present new VLBI absolute astrometry measurements of its precise position and proper motion in the frame of the third realization of the International Celestial Reference Frame (ICRF3). The observations used were made at 52 epochs on the VLBA at K band (24 GHz) between 2006 June and 2022 August. We find the proper motion of Sgr A* to be −3.128 ± 0.042 mas yr ^−1 in R.A. and −5.584 ± 0.075 mas yr ^−1 in decl., or 6.400 ± 0.073 mas yr ^−1 at a position angle of 209.°26 ± 0.°51. We also find its J2000 ICRF3 coordinates at the 2015.0 proper motion epoch to be 17 ^h 45 ^m 40.ˢ034047 ± 0.ˢ000018, −29°00′28.″21601 ± 0.″00044. In galactic coordinates, Sgr A* shows proper motion of −6.396 ± 0.071 mas yr ^−1 in galactic longitude and −0.239 ± 0.045 mas yr ^−1 in galactic latitude, indicating solar motion of 248.0 ± 2.8 km s ^−1 in the galactic plane and 9.3 ± 1.9 km s ^−1 toward the north galactic pole
Overview and Status of the International Celestial Reference Frame as Realized by VLBI
Accurate measurement of angular positions on the sky requires a well-defined system of reference that is realized with accessible objects. The purpose of this study is to review the international standard realization of such a system, the International Celestial Reference Frame (ICRF). The ICRF uses the Very Long Baseline Interferometry (VLBI) technique as it has the highest resolution of any current astrometric technique for reference frames in order to observe Active Galactic Nuclei (AGN) which are at such great distances (typical redshift ∼1) that there is currently no observed parallax or proper motion of these objects thus giving the frame excellent stability. We briefly review the history of the transition from the Fundamental Katalog 5 (FK5) optical frame to VLBI-based frames with attention to each of the three generations: ICRF-1, ICRF-2, and ICRF-3. We present some of the more prominent applications of the ICRF and outline the methods used to construct the ICRF. Next we discuss in more detail the current standard ICRF-3—which is the first frame to be realized at multiple wavelengths (S/X, K, X/Ka-bands)—including an estimate of its accuracy and limiting errors. We conclude with an overview of future plans for improving the ICRF
Overview and Status of the International Celestial Reference Frame as Realized by VLBI
Accurate measurement of angular positions on the sky requires a well-defined system of reference that is realized with accessible objects. The purpose of this study is to review the international standard realization of such a system, the International Celestial Reference Frame (ICRF). The ICRF uses the Very Long Baseline Interferometry (VLBI) technique as it has the highest resolution of any current astrometric technique for reference frames in order to observe Active Galactic Nuclei (AGN) which are at such great distances (typical redshift ∼1) that there is currently no observed parallax or proper motion of these objects thus giving the frame excellent stability. We briefly review the history of the transition from the Fundamental Katalog 5 (FK5) optical frame to VLBI-based frames with attention to each of the three generations: ICRF-1, ICRF-2, and ICRF-3. We present some of the more prominent applications of the ICRF and outline the methods used to construct the ICRF. Next we discuss in more detail the current standard ICRF-3—which is the first frame to be realized at multiple wavelengths (S/X, K, X/Ka-bands)—including an estimate of its accuracy and limiting errors. We conclude with an overview of future plans for improving the ICRF
High-resolution radio astronomy: An outlook for Africa
Very Long Baseline Interferometry (VLBI) offers unrivalled resolution in studies of celestial radio sources. The subjects of interest of the current IAU Symposium, the Active Galactic Nuclei (AGN) of all types, constitute the major observing sample of modern VLBI networks. At present, the largest in the world in terms of the number of telescopes and geographical coverage is the European VLBI Network (EVN), which operates under the "open sky"policy via peer-reviewed observing proposals. Recent EVN observations cover a broad range of science themes from high-sensitivity monitoring of structural changes in inner AGN areas to observations of tidal eruptions in AGN cores and investigation of redshift-dependent properties of parsec-scale radio structures of AGN. All the topics above should be considered as potentially rewarding scientific activities of the prospective African VLBI Network (AVN), a natural "scientific ally"of EVN. This contribution briefly describe the status and near-term strategy for the AVN development as a southern extension of the EVN-AVN alliance and as an eventual bridge to the Square Kilometre Array (SKA) with its mid-frequency core in South Africa.</p
Astrometric Apparent Motion of High-redshift Radio Sources
Radio-loud quasars at high redshift (z ≥ 4) are rare objects in the universe and rarely observed with Very Long Baseline Interferometry (VLBI). But some of them have flux density sufficiently high for monitoring of their apparent position. The instability of the astrometric positions could be linked to the astrophysical process in the jetted active galactic nuclei in the early universe. Regular observations of the high-redshift quasars are used for estimating their apparent proper motion over several years. We have undertaken regular VLBI observations of several high-redshift quasars at 2.3 GHz (S band) and 8.4 GHz (X band) with a network of five radio telescopes: 40 m Yebes (Spain), 25 m Sheshan (China), and three 32 m telescopes of the Quasar VLBI Network (Russia)—Svetloe, Zelenchukskaya, and Badary. Additional facilities joined this network occasionally. The sources have also been observed in three sessions with the European VLBI Network in 2018-2019 and one Long Baseline Array experiment in 2018. In addition, several experiments conducted with the Very Long Baseline Array in 2017-2018 were used to improve the time sampling and the statistics. Based on these 37 astrometric VLBI experiments between 2017 and 2021, we estimated the apparent proper motions of four quasars: 0901+697, 1428+422, 1508+572, and 2101+600.</p