MeerKAT: a journey from commissioning to science
- Authors: Hugo, Benjamin Vorster
- Date: 2024-10-11
- Subjects: MeerKAT , Interferometry , Ionosphere , Pulsars
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/466871 , vital:76794 , DOI https://doi.org/10.21504/10962/466871
- Description: This dissertation presents a collection of work completed for the South African Radio Astronomy Observatory in characterizing calibrator fields PKS B1934-638, PKS B0407-65 and 3C286, the development of a facet-based multi-direction peeling scheme for the CUBICAL calibration framework and incorporation into an end-to-end containerized data reduction framework, a study of a transitional millisecond pulsar candidate, and characterization of baseline dependent archiving tooling for MeerKAT. Our long term studies of PKS B1934-638, PKS B0407-65 indicate that these bandpass and flux calibrators are stable over multiple years. We also find that, especially at low frequencies in the UHF band, the population of sources surrounding these stellar Gigahertz Peaked Sources (GPS) can contribute to errors two to three orders of magnitude above desired bandpass calibration solution stability, if left unmodeled. We derive new new full sky models of these fields, currently in use by the MeerKAT Science Data Processor. We characterize the MeerKAT feed alignment using the refraction-driven linearly polarized thermal light from the Moon in order to derive a new model for the linear polarization of the stable quasar 3C286 down to 544 MHz. Part of this work includes characterization of ionospheric corrections using the International Global Navigation Satelite System Service and direct measurement of total electron content above the MeerKAT site using interchange data from the South African TrigNET service. We find that current commonly-employed techniques achieve corrections to ionospheric Faraday rotation no better than 1 rad m2. This is the main limitation on the accuracy of polarimetric observation using the MeerKAT array. We find that 3C286 intrinsically depolarizes at frequencies below 1 GHz, with an associated non-linear increase in the intrinsic source rotation measure. We present an improvement to workflows using the CUBICAL calibration framework, developed at Rhodes University. Modern radio interferometers presents a significant challenge to calibrate, often necessitating memory and computeintensive direction-dependent calibration towards many directions in order to improve the fidelity of radio images in order to meet scientific goals. We developed a framework to simplify the model prediction aspect of these direction-dependent calibration workflows using targeted faceting. Using our scheme users use models derived from the DDFACET imaging package and only need to provide lattices to mark regions of sky to which direction-dependent calibration solutions need to be solved for. This simplifies a laborious multi-step process in traditional calibration packages that need to be executed per direction. The approach is compared to an image-space corrective regime and incorporated into the VERMEERKAT end-to-end calibration framework for MeerKAT data. The improved direction-dependent calibration techniques were then applied in an analysis of the transitional millisecond pulsar candidate CXOU J110926.4-650224. The link between accreting binary systems (where emission is dominated by the synchrotron emission of relativistic jets from thermo-nuclear reaction onto the Neutron Star surface by the infalling matter) and binary radio pulsars is currently elusive. This is due to the lack of a large population of such transitional systems — only three confirmed transitional systems are known at the time of writing. It is thought that infalling matter effectively quenches the radio pulsar mechanism. Our candidate was found to be variable in the optical and the X-ray, with transitions between low, high and flaring states lasting anywhere from a tens of seconds to tens of minutes, seen in archival observations spanning nearly three decades. For the first time we detect low level synchrotron emission (_ 50 mJy beam1) coincident with this system using MeerKAT, including a flare within minutes of a flare detected in X-ray using the XMM-Newton observatory. Our analysis indicate that there is no clear anti-correlated behaviour between radio and X-ray state transitions in this system, unlike other candidate systems—indicating that such transitional systems may not exhibit homogenous behaviour. This suggests that the processes driving the X-ray mode-switching in this system are not directly linked to the processes responsible for emitting radio synchrotron radiation. Finally, we consider the problem of MeerKAT data archiving. We present a qualification analysis, using MeerKAT data, of the Rhodes University baseline-dependent archiving package XOVA, which can be used to compress and archive MeerKAT data in interchange standard-compliant format. The data rates from interferometric array radio telescopes, such as MeerKAT, grow as the square of the number of antennas in such an array. For the sake of reproducibility and future reanalysis it is important to archive calibrated visibility products. The degree to which calibrated visibility products can be compressed, by averaging, depends on the amount of smearing that can be tolerated at a fixed distance from the center of the images synthesized from these visibility products. This is, traditionally, set by the longest spacing in the interferometric array, with all other spacings averaged to the same integration and channelization as the longest spacing. We find that, using baseline-dependent averaging techniques – where averaging intervals are set per interferometric spacing – we can achieve space savings an order of magnitude better than traditional averaging approaches, with no appreciable loss of image fidelity when compared to traditional averaging approaches. , Thesis (PhD) -- Faculty of Science, Faculty of Science, Physics and Electronics, 2024
- Full Text:
- Date Issued: 2024-10-11
- Authors: Hugo, Benjamin Vorster
- Date: 2024-10-11
- Subjects: MeerKAT , Interferometry , Ionosphere , Pulsars
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/466871 , vital:76794 , DOI https://doi.org/10.21504/10962/466871
- Description: This dissertation presents a collection of work completed for the South African Radio Astronomy Observatory in characterizing calibrator fields PKS B1934-638, PKS B0407-65 and 3C286, the development of a facet-based multi-direction peeling scheme for the CUBICAL calibration framework and incorporation into an end-to-end containerized data reduction framework, a study of a transitional millisecond pulsar candidate, and characterization of baseline dependent archiving tooling for MeerKAT. Our long term studies of PKS B1934-638, PKS B0407-65 indicate that these bandpass and flux calibrators are stable over multiple years. We also find that, especially at low frequencies in the UHF band, the population of sources surrounding these stellar Gigahertz Peaked Sources (GPS) can contribute to errors two to three orders of magnitude above desired bandpass calibration solution stability, if left unmodeled. We derive new new full sky models of these fields, currently in use by the MeerKAT Science Data Processor. We characterize the MeerKAT feed alignment using the refraction-driven linearly polarized thermal light from the Moon in order to derive a new model for the linear polarization of the stable quasar 3C286 down to 544 MHz. Part of this work includes characterization of ionospheric corrections using the International Global Navigation Satelite System Service and direct measurement of total electron content above the MeerKAT site using interchange data from the South African TrigNET service. We find that current commonly-employed techniques achieve corrections to ionospheric Faraday rotation no better than 1 rad m2. This is the main limitation on the accuracy of polarimetric observation using the MeerKAT array. We find that 3C286 intrinsically depolarizes at frequencies below 1 GHz, with an associated non-linear increase in the intrinsic source rotation measure. We present an improvement to workflows using the CUBICAL calibration framework, developed at Rhodes University. Modern radio interferometers presents a significant challenge to calibrate, often necessitating memory and computeintensive direction-dependent calibration towards many directions in order to improve the fidelity of radio images in order to meet scientific goals. We developed a framework to simplify the model prediction aspect of these direction-dependent calibration workflows using targeted faceting. Using our scheme users use models derived from the DDFACET imaging package and only need to provide lattices to mark regions of sky to which direction-dependent calibration solutions need to be solved for. This simplifies a laborious multi-step process in traditional calibration packages that need to be executed per direction. The approach is compared to an image-space corrective regime and incorporated into the VERMEERKAT end-to-end calibration framework for MeerKAT data. The improved direction-dependent calibration techniques were then applied in an analysis of the transitional millisecond pulsar candidate CXOU J110926.4-650224. The link between accreting binary systems (where emission is dominated by the synchrotron emission of relativistic jets from thermo-nuclear reaction onto the Neutron Star surface by the infalling matter) and binary radio pulsars is currently elusive. This is due to the lack of a large population of such transitional systems — only three confirmed transitional systems are known at the time of writing. It is thought that infalling matter effectively quenches the radio pulsar mechanism. Our candidate was found to be variable in the optical and the X-ray, with transitions between low, high and flaring states lasting anywhere from a tens of seconds to tens of minutes, seen in archival observations spanning nearly three decades. For the first time we detect low level synchrotron emission (_ 50 mJy beam1) coincident with this system using MeerKAT, including a flare within minutes of a flare detected in X-ray using the XMM-Newton observatory. Our analysis indicate that there is no clear anti-correlated behaviour between radio and X-ray state transitions in this system, unlike other candidate systems—indicating that such transitional systems may not exhibit homogenous behaviour. This suggests that the processes driving the X-ray mode-switching in this system are not directly linked to the processes responsible for emitting radio synchrotron radiation. Finally, we consider the problem of MeerKAT data archiving. We present a qualification analysis, using MeerKAT data, of the Rhodes University baseline-dependent archiving package XOVA, which can be used to compress and archive MeerKAT data in interchange standard-compliant format. The data rates from interferometric array radio telescopes, such as MeerKAT, grow as the square of the number of antennas in such an array. For the sake of reproducibility and future reanalysis it is important to archive calibrated visibility products. The degree to which calibrated visibility products can be compressed, by averaging, depends on the amount of smearing that can be tolerated at a fixed distance from the center of the images synthesized from these visibility products. This is, traditionally, set by the longest spacing in the interferometric array, with all other spacings averaged to the same integration and channelization as the longest spacing. We find that, using baseline-dependent averaging techniques – where averaging intervals are set per interferometric spacing – we can achieve space savings an order of magnitude better than traditional averaging approaches, with no appreciable loss of image fidelity when compared to traditional averaging approaches. , Thesis (PhD) -- Faculty of Science, Faculty of Science, Physics and Electronics, 2024
- Full Text:
- Date Issued: 2024-10-11
Systematic effects and mitigation strategies in observations of cosmic re-ionisation with the Hydrogen Epoch of Reionization Array
- Authors: Charles, Ntsikelelo
- Date: 2024
- Subjects: Cosmology , Astrophysics , Radio astronomy , Hydrogen Epoch of Reionization Array , Epoch of reionization
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/432605 , vital:72886 , DOI 10.21504/10962/432605
- Description: The 21 cm transition from neutral Hydrogen promises to be the best observational probe of the Epoch of Reionisation (EoR). It has driven the construction of the new generation of lowfrequency radio interferometric arrays, including the Hydrogen Epoch of Reionization Array (HERA). The main difficulty in measuring the 21 cm signal is the presence of bright foregrounds that require very accurate interferometric calibration. However, the non-smooth instrumental response of the antenna as a result of mutual coupling complicates the calibration process by introducing non-smooth calibration errors. Additionally, incomplete sky models are typically used in calibration due to the limited depth and resolution of current source catalogues. Combined with the instrumental response, the use of incomplete sky models during calibration can result in non-smooth calibration errors. These, overall, impart spectral structure on smooth foregrounds, leading to foreground power leakage into the EoR window. In this thesis we explored the use of fringe rate filters (Parsons et al., 2016) as a mean to mitigate calibration errors resulting from the effects of mutual coupling and the use of an incomplete sky model during calibration. We found that the use of a simple notch filter mitigates calibration errors reducing the foreground power leakage into the EoR window by a factor of ∼ 102. Thyagarajan et al. (2018) proposed the use of closure phase quantities as a means to detect the 21 cm signal, which has the advantage of being independent (to first order) from calibration errors and, therefore, bypasses the need for accurate calibration. In this thesis, we explore the impact of primary beam patterns affected by mutual coupling on the closure phase. We found that primary beams affected by mutual coupling lead to a leakage of foreground power into the EoR window, which can be up to ∼ 104 times and is mainly caused by the unsmooth spectral structure primary of primary beam sidelobes affected by mutual coupling. This power leakage was confined to k < 0.3 pseudo h Mpc−1. Lastly, we also proposed and demonstrated an analysis technique that can be used to derive a flux scale correction in post-calibrated HERA data. We found that after applying flux scale correction to calibrated HERA data, the bandpass error reduces significantly, with an improvement of 6%. The derived flux scale correction was antenna-independent, and it can be applied to fix the overall visibility spectrum scale of H4C data post-calibration in a fashion similar to Jacobs et al. (2013). , Thesis (PhD) -- Faculty of Science, Physics and Electronics, 2024
- Full Text:
- Date Issued: 2024
- Authors: Charles, Ntsikelelo
- Date: 2024
- Subjects: Cosmology , Astrophysics , Radio astronomy , Hydrogen Epoch of Reionization Array , Epoch of reionization
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/432605 , vital:72886 , DOI 10.21504/10962/432605
- Description: The 21 cm transition from neutral Hydrogen promises to be the best observational probe of the Epoch of Reionisation (EoR). It has driven the construction of the new generation of lowfrequency radio interferometric arrays, including the Hydrogen Epoch of Reionization Array (HERA). The main difficulty in measuring the 21 cm signal is the presence of bright foregrounds that require very accurate interferometric calibration. However, the non-smooth instrumental response of the antenna as a result of mutual coupling complicates the calibration process by introducing non-smooth calibration errors. Additionally, incomplete sky models are typically used in calibration due to the limited depth and resolution of current source catalogues. Combined with the instrumental response, the use of incomplete sky models during calibration can result in non-smooth calibration errors. These, overall, impart spectral structure on smooth foregrounds, leading to foreground power leakage into the EoR window. In this thesis we explored the use of fringe rate filters (Parsons et al., 2016) as a mean to mitigate calibration errors resulting from the effects of mutual coupling and the use of an incomplete sky model during calibration. We found that the use of a simple notch filter mitigates calibration errors reducing the foreground power leakage into the EoR window by a factor of ∼ 102. Thyagarajan et al. (2018) proposed the use of closure phase quantities as a means to detect the 21 cm signal, which has the advantage of being independent (to first order) from calibration errors and, therefore, bypasses the need for accurate calibration. In this thesis, we explore the impact of primary beam patterns affected by mutual coupling on the closure phase. We found that primary beams affected by mutual coupling lead to a leakage of foreground power into the EoR window, which can be up to ∼ 104 times and is mainly caused by the unsmooth spectral structure primary of primary beam sidelobes affected by mutual coupling. This power leakage was confined to k < 0.3 pseudo h Mpc−1. Lastly, we also proposed and demonstrated an analysis technique that can be used to derive a flux scale correction in post-calibrated HERA data. We found that after applying flux scale correction to calibrated HERA data, the bandpass error reduces significantly, with an improvement of 6%. The derived flux scale correction was antenna-independent, and it can be applied to fix the overall visibility spectrum scale of H4C data post-calibration in a fashion similar to Jacobs et al. (2013). , Thesis (PhD) -- Faculty of Science, Physics and Electronics, 2024
- Full Text:
- Date Issued: 2024
MeerKAT observations of three high-redshift galaxy clusters
- Authors: Manaka, Sinah Mokatako
- Date: 2023-03-29
- Subjects: MeerKAT , Galaxies Clusters , Calibration , Radio interferometers , Radio halo
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/422367 , vital:71936
- Description: Galaxy clusters are the largest, gravitationally-bound structures in the Universe, formed through the hierarchical merger of smaller structures. The most accepted view is that the merging process injects energy into the intracluster medium (ICM) and re-accelerates pre-existing particles and compresses magnetic fields, generating, eventually, synchrotron emission. Such radio emission appears as radio halos, i.e. central Mpc-size diffuse structures, mostly visible in merging or unrelaxed clusters and with a spatial correspondence with the thermal gas component of the ICM. Observations have probed radio halo properties mostly for clusters withM500 > 6×1014 M⊙ at intermediate redshifts (0.3 < z < 0.4), providing support to their connection between mergers, which provide the necessary energy to re-accelerate particles via turbulence. Probing the redshift evolution of radio halos is an important test of the turbulent re-acceleration scenario, as fewer halos are expected at high redshift, given the same mass interval. In this thesis, we present MeerKAT observations at 1.28 GHz of three high-redshift (PSZ2G254.08- 58.45, PSZ2G255.60-46.18 and PSZ2G277.76-51.74, in the 0.42 ≲ z ≲ 0.46 range) clusters, with masses M500 ≳ 6.2 × 1014 M⊙, selected for their disturbed dynamical state – inferred from existing X-ray observations. Our observations reached rms noise values between 20 and 23.5 μJy beam−1, with ∼ 4′′ angular resolution. No evidence of diffuse emission is found at ii full resolution. Low-resolution (∼ 30′′) images achieved rms noise levels of 30-50 μJy beam−1, amongst the deepest observations of high-redshift targets. One radio halo was detected in the least massive cluster PSZ2G254.08-58.45 extending over ∼ 500 kpc, with a 1.20 } 0.08 mJy integrated flux density. We placed a ∼1 mJy upper limit at 95% confidence level on the radio halo flux density for the other two targets. The radio-halo detection is consistent with the recent P1.4 GHz − M500 correlation from Cuciti et al. (2021b), while the upper limit on PSZ2G255.60-46.18 is consistent with being on the correlation. On the other hand, the upper limit on PSZ2G277.76-51.74 places the radio halo well below the correlation. Recently a 1.5 GHz survey (Giovannini et al., 2020) detected a slightly higher fraction of radio halos in clusters in the same redshift range, with power and size typically higher than what we found in our observations. Both observations are, however, not inconsistent with each other. Our results, although with limited statistics, do not disfavour the current scenario of radiohalo formation based on the turbulent re-acceleration model. , Thesis (MSc) -- Faculty of Science, Physics and Electronics, 2023
- Full Text:
- Date Issued: 2023-03-29
- Authors: Manaka, Sinah Mokatako
- Date: 2023-03-29
- Subjects: MeerKAT , Galaxies Clusters , Calibration , Radio interferometers , Radio halo
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/422367 , vital:71936
- Description: Galaxy clusters are the largest, gravitationally-bound structures in the Universe, formed through the hierarchical merger of smaller structures. The most accepted view is that the merging process injects energy into the intracluster medium (ICM) and re-accelerates pre-existing particles and compresses magnetic fields, generating, eventually, synchrotron emission. Such radio emission appears as radio halos, i.e. central Mpc-size diffuse structures, mostly visible in merging or unrelaxed clusters and with a spatial correspondence with the thermal gas component of the ICM. Observations have probed radio halo properties mostly for clusters withM500 > 6×1014 M⊙ at intermediate redshifts (0.3 < z < 0.4), providing support to their connection between mergers, which provide the necessary energy to re-accelerate particles via turbulence. Probing the redshift evolution of radio halos is an important test of the turbulent re-acceleration scenario, as fewer halos are expected at high redshift, given the same mass interval. In this thesis, we present MeerKAT observations at 1.28 GHz of three high-redshift (PSZ2G254.08- 58.45, PSZ2G255.60-46.18 and PSZ2G277.76-51.74, in the 0.42 ≲ z ≲ 0.46 range) clusters, with masses M500 ≳ 6.2 × 1014 M⊙, selected for their disturbed dynamical state – inferred from existing X-ray observations. Our observations reached rms noise values between 20 and 23.5 μJy beam−1, with ∼ 4′′ angular resolution. No evidence of diffuse emission is found at ii full resolution. Low-resolution (∼ 30′′) images achieved rms noise levels of 30-50 μJy beam−1, amongst the deepest observations of high-redshift targets. One radio halo was detected in the least massive cluster PSZ2G254.08-58.45 extending over ∼ 500 kpc, with a 1.20 } 0.08 mJy integrated flux density. We placed a ∼1 mJy upper limit at 95% confidence level on the radio halo flux density for the other two targets. The radio-halo detection is consistent with the recent P1.4 GHz − M500 correlation from Cuciti et al. (2021b), while the upper limit on PSZ2G255.60-46.18 is consistent with being on the correlation. On the other hand, the upper limit on PSZ2G277.76-51.74 places the radio halo well below the correlation. Recently a 1.5 GHz survey (Giovannini et al., 2020) detected a slightly higher fraction of radio halos in clusters in the same redshift range, with power and size typically higher than what we found in our observations. Both observations are, however, not inconsistent with each other. Our results, although with limited statistics, do not disfavour the current scenario of radiohalo formation based on the turbulent re-acceleration model. , Thesis (MSc) -- Faculty of Science, Physics and Electronics, 2023
- Full Text:
- Date Issued: 2023-03-29
Observations of cosmic re-ionisation with the Hydrogen Epoch of Reionization Array: simulations of closure phase spectra
- Authors: Charles, Ntsikelelo
- Date: 2021-04
- Subjects: Epoch of reionization , Space interferometry , Astronomy -- Observations , Closure phase spectra
- Language: English
- Type: thesis , text , Masters , MSc
- Identifier: http://hdl.handle.net/10962/174470 , vital:42480
- Description: The 21 cm transition from neutral Hydrogen promises to be the best observational probe of the Epoch of Reionisation. It has driven the construction of the new generation of low frequency radio interferometric arrays, including the Hydrogen Epoch of Reionization Array (HERA). The main difficulty in measuring the 21 cm signal is the presence of bright foregrounds that require very accurate interferometric calibration. Thyagarajan et al. (2018) proposed the use of closure phase quantities as a means to detect the 21 cm signal, which has the advantage of being independent (to first order) from calibration errors and therefore, bypasses the need for accurate calibration. Closure phases are, however, affected by so-called direction dependent effects, e.g. the fact that the dishes - or antennas - of an interferometric array are not identical to each other and , therefore, yield different antenna primary beam responses. In this thesis, we investigate the impact of direction dependent effects on closure quantities and simulate the impact that primary antenna beams affected by mutual coupling have on the foreground closure phase and its power spectrum i.e. the power spectrum of the bispectrum phase (Thyagarajan et al., 2020). Our simulations show that primary beams affected by mutual coupling lead to an overall leakage of foreground power in the so-called EoR window, i.e. power from smooth-spectrum foregrounds is confined to low k modes. We quantified this effect and found that the leakage is up to ~ 8 orders magnitude higher than the case of an ideal beam at kǁ > 0:5 h Mpc-1. We also found that the foreground leakage is worse when edge antennas are included, as they have a more different primary beam compared to antennas at the centre of the array. The leakage magnitude is worse when bright foregrounds appear in the antenna sidelobes, as expected. Our simulations provide a useful framework to interpret observations and assess which power spectrum region is expected to be most contaminated by foreground power leakage.
- Full Text:
- Date Issued: 2021-04
- Authors: Charles, Ntsikelelo
- Date: 2021-04
- Subjects: Epoch of reionization , Space interferometry , Astronomy -- Observations , Closure phase spectra
- Language: English
- Type: thesis , text , Masters , MSc
- Identifier: http://hdl.handle.net/10962/174470 , vital:42480
- Description: The 21 cm transition from neutral Hydrogen promises to be the best observational probe of the Epoch of Reionisation. It has driven the construction of the new generation of low frequency radio interferometric arrays, including the Hydrogen Epoch of Reionization Array (HERA). The main difficulty in measuring the 21 cm signal is the presence of bright foregrounds that require very accurate interferometric calibration. Thyagarajan et al. (2018) proposed the use of closure phase quantities as a means to detect the 21 cm signal, which has the advantage of being independent (to first order) from calibration errors and therefore, bypasses the need for accurate calibration. Closure phases are, however, affected by so-called direction dependent effects, e.g. the fact that the dishes - or antennas - of an interferometric array are not identical to each other and , therefore, yield different antenna primary beam responses. In this thesis, we investigate the impact of direction dependent effects on closure quantities and simulate the impact that primary antenna beams affected by mutual coupling have on the foreground closure phase and its power spectrum i.e. the power spectrum of the bispectrum phase (Thyagarajan et al., 2020). Our simulations show that primary beams affected by mutual coupling lead to an overall leakage of foreground power in the so-called EoR window, i.e. power from smooth-spectrum foregrounds is confined to low k modes. We quantified this effect and found that the leakage is up to ~ 8 orders magnitude higher than the case of an ideal beam at kǁ > 0:5 h Mpc-1. We also found that the foreground leakage is worse when edge antennas are included, as they have a more different primary beam compared to antennas at the centre of the array. The leakage magnitude is worse when bright foregrounds appear in the antenna sidelobes, as expected. Our simulations provide a useful framework to interpret observations and assess which power spectrum region is expected to be most contaminated by foreground power leakage.
- Full Text:
- Date Issued: 2021-04
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