4 results
Gridded and direct Epoch of Reionisation bispectrum estimates using the Murchison Widefield Array
- Cathryn M. Trott, Catherine A. Watkinson, Christopher H. Jordan, Shintaro Yoshiura, Suman Majumdar, N. Barry, R. Byrne, B. J. Hazelton, K. Hasegawa, R. Joseph, T. Kaneuji, K. Kubota, W. Li, J. Line, C. Lynch, B. McKinley, D. A. Mitchell, M. F. Morales, S. Murray, B. Pindor, J. C. Pober, M. Rahimi, J. Riding, K. Takahashi, S. J. Tingay, R. B. Wayth, R. L. Webster, M. Wilensky, J. S. B. Wyithe, Q. Zheng, David Emrich, A. P. Beardsley, T. Booler, B. Crosse, T. M. O. Franzen, L. Horsley, M. Johnston-Hollitt, D. L. Kaplan, D. Kenney, D. Pallot, G. Sleap, K. Steele, M. Walker, A. Williams, C. Wu
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- Journal:
- Publications of the Astronomical Society of Australia / Volume 36 / 2019
- Published online by Cambridge University Press:
- 18 July 2019, e023
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We apply two methods to estimate the 21-cm bispectrum from data taken within the Epoch of Reionisation (EoR) project of the Murchison Widefield Array (MWA). Using data acquired with the Phase II compact array allows a direct bispectrum estimate to be undertaken on the multiple redundantly spaced triangles of antenna tiles, as well as an estimate based on data gridded to the uv-plane. The direct and gridded bispectrum estimators are applied to 21 h of high-band (167–197 MHz; z = 6.2–7.5) data from the 2016 and 2017 observing seasons. Analytic predictions for the bispectrum bias and variance for point-source foregrounds are derived. We compare the output of these approaches, the foreground contribution to the signal, and future prospects for measuring the bispectra with redundant and non-redundant arrays. We find that some triangle configurations yield bispectrum estimates that are consistent with the expected noise level after 10 h, while equilateral configurations are strongly foreground-dominated. Careful choice of triangle configurations may be made to reduce foreground bias that hinders power spectrum estimators, and the 21-cm bispectrum may be accessible in less time than the 21-cm power spectrum for some wave modes, with detections in hundreds of hours.
A High-Resolution Foreground Model for the MWA EoR1 Field: Model and Implications for EoR Power Spectrum Analysis
- Part of
- P. Procopio, R. B. Wayth, J. Line, C. M. Trott, H. T. Intema, D. A. Mitchell, B. Pindor, J. Riding, S. J. Tingay, M. E. Bell, J. R. Callingham, K. S. Dwarakanath, Bi-Qing For, B. M. Gaensler, P. J. Hancock, L. Hindson, N. Hurley-Walker, M. Johnston-Hollitt, A. D. Kapińska, E. Lenc, B. McKinley, J. Morgan, A. Offringa, L. Staveley-Smith, Chen Wu, Q. Zheng
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- Journal:
- Publications of the Astronomical Society of Australia / Volume 34 / 2017
- Published online by Cambridge University Press:
- 10 August 2017, e033
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The current generation of experiments aiming to detect the neutral hydrogen signal from the Epoch of Reionisation (EoR) is likely to be limited by systematic effects associated with removing foreground sources from target fields. In this paper, we develop a model for the compact foreground sources in one of the target fields of the MWA’s EoR key science experiment: the ‘EoR1’ field. The model is based on both the MWA’s GLEAM survey and GMRT 150 MHz data from the TGSS survey, the latter providing higher angular resolution and better astrometric accuracy for compact sources than is available from the MWA alone. The model contains 5 049 sources, some of which have complicated morphology in MWA data, Fornax A being the most complex. The higher resolution data show that 13% of sources that appear point-like to the MWA have complicated morphology such as double and quad structure, with a typical separation of 33 arcsec. We derive an analytic expression for the error introduced into the EoR two-dimensional power spectrum due to peeling close double sources as single point sources and show that for the measured source properties, the error in the power spectrum is confined to high k⊥ modes that do not affect the overall result for the large-scale cosmological signal of interest. The brightest 10 mis-modelled sources in the field contribute 90% of the power bias in the data, suggesting that it is most critical to improve the models of the brightest sources. With this hybrid model, we reprocess data from the EoR1 field and show a maximum of 8% improved calibration accuracy and a factor of two reduction in residual power in k-space from peeling these sources. Implications for future EoR experiments including the SKA are discussed in relation to the improvements obtained.
The Low-Frequency Environment of the Murchison Widefield Array: Radio-Frequency Interference Analysis and Mitigation
- Part of
- A. R. Offringa, R. B. Wayth, N. Hurley-Walker, D. L. Kaplan, N. Barry, A. P. Beardsley, M. E. Bell, G. Bernardi, J. D. Bowman, F. Briggs, J. R. Callingham, R. J. Cappallo, P. Carroll, A. A. Deshpande, J. S. Dillon, K. S. Dwarakanath, A. Ewall-Wice, L. Feng, B.-Q. For, B. M. Gaensler, L. J. Greenhill, P. Hancock, B. J. Hazelton, J. N. Hewitt, L. Hindson, D. C. Jacobs, M. Johnston-Hollitt, A. D. Kapińska, H.-S. Kim, P. Kittiwisit, E. Lenc, J. Line, A. Loeb, C. J. Lonsdale, B. McKinley, S. R. McWhirter, D. A. Mitchell, M. F. Morales, E. Morgan, J. Morgan, A. R. Neben, D. Oberoi, S. M. Ord, S. Paul, B. Pindor, J. C. Pober, T. Prabu, P. Procopio, J. Riding, N. Udaya Shankar, S. Sethi, K. S. Srivani, L. Staveley-Smith, R. Subrahmanyan, I. S. Sullivan, M. Tegmark, N. Thyagarajan, S. J. Tingay, C. M. Trott, R. L. Webster, A. Williams, C. L. Williams, C. Wu, J. S. Wyithe, Q. Zheng
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- Journal:
- Publications of the Astronomical Society of Australia / Volume 32 / 2015
- Published online by Cambridge University Press:
- 03 March 2015, e008
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The Murchison Widefield Array is a new low-frequency interferometric radio telescope built in Western Australia at one of the locations of the future Square Kilometre Array. We describe the automated radio-frequency interference detection strategy implemented for the Murchison Widefield Array, which is based on the aoflagger platform, and present 72–231 MHz radio-frequency interference statistics from 10 observing nights. Radio-frequency interference detection removes 1.1% of the data. Radio-frequency interference from digital TV is observed 3% of the time due to occasional ionospheric or atmospheric propagation. After radio-frequency interference detection and excision, almost all data can be calibrated and imaged without further radio-frequency interference mitigation efforts, including observations within the FM and digital TV bands. The results are compared to a previously published Low-Frequency Array radio-frequency interference survey. The remote location of the Murchison Widefield Array results in a substantially cleaner radio-frequency interference environment compared to Low-Frequency Array’s radio environment, but adequate detection of radio-frequency interference is still required before data can be analysed. We include specific recommendations designed to make the Square Kilometre Array more robust to radio-frequency interference, including: the availability of sufficient computing power for radio-frequency interference detection; accounting for radio-frequency interference in the receiver design; a smooth band-pass response; and the capability of radio-frequency interference detection at high time and frequency resolution (second and kHz-scale respectively).
A Change Agenda for Reserving. Report of the General Insurance Reserving Issues Taskforce (GRIT)
- A. R. Jones, P. J. Copeman, E. R. Gibson, N. J. S. Line, J. A. Lowe, P. Martin, P. N. Matthews, D. S. Powell
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- Journal:
- British Actuarial Journal / Volume 12 / Issue 3 / 01 September 2006
- Published online by Cambridge University Press:
- 10 June 2011, pp. 435-599
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Reserving is important to our profession as it is a core activity for actuaries. The members of the General Insurance Reserving Issues Taskforce (GRIT) have been considering how actuaries can improve the way in which we do reserving in general insurance. We gathered our thoughts and recommendations together in a Consultation Paper which has been discussed widely in the profession. We are very grateful to everyone who shared their views and comments with us, particularly those who gave us written feedback. We have considered carefully all the feedback which we received and adapted our final report in response to this.
Given the scope and importance of our remit, it is perhaps not surprising that this is not a short paper. We hope that Section 1 provides a reasonable summary.
Generally, our view is that there are many things on which our profession should focus. However, it is also important to remind ourselves of the positive items of feedback which we heard from our stakeholders. In addition to many suggestions for things to do better, we consistently heard the message that actuaries play an extremely valuable role in general insurance.
This is a major testimony to the progress which the actuarial profession has made in recent years in its ability to contribute to the general insurance industry. Perhaps it is because of this progress that now is an appropriate time for us, as a profession, to take a hard look at what we do in reserving, and ask ourselves whether there are any things which we could do differently. We hope that GRIT's report will facilitate this debate.
GRIT's recommendations fall under the following key themes:
— Providing more transparency to our reserving methods and helping our stakeholders have more insight into the key reserving assumptions and decisions.
— Providing more information on uncertainty in our reserve estimates. In particular, we recommend that actuaries provide a quantitative indication of the range of outcomes for future claim payments, and that our profession defines a common vocabulary for communicating uncertainty.
— Understanding better the business we are reserving. We suggest a range of analyses and activities for doing this.
— Applying our standard actuarial reserving methods more consistently. We identify a list of specific areas where we believe that there is scope for improvement. Also, we believe that the actuarial training syllabus should be extended, and this leads to consideration of whether a more specialised general insurance actuarial qualification is needed.
— Understanding the implications of the underwriting cycle, which, we believe, influences the behaviour of claims development in a way that our reserving models do not currently capture. We suggest what we believe may be the foundations of a potentially more cycle proof methodology, but this is an area which we believe will require much more research.
— Helping actuaries understand how behaviour can affect the reserve estimation process, particularly in the face of uncertainty. We make various suggestions in this area, including helping actuaries manage pressure from third parties.
We are convinced that, for our profession to implement these suggestions, it will require a concerted change management strategy and set of actions to embed changes into the way in which actuaries work. We believe that this will include:
— increasing the level of debate and research in the profession on claims reserving;
— a broader communication programme with the general insurance industry, covering, amongst other things, uncertainty and data quality;
— a sub-group of the GI Board with a specific focus on reserving, responsible for implementing GRIT's recommendations and dealing with new issues as they arise; and
— our profession resolving the conflicting pressures which will arise out of the extra work required for reserving by the GRIT recommendations.
There is one specific item where we have not made a recommendation. It has been suggested to us that many of the standard reserving methods in common use, such as the chain ladder, are not sophisticated, and that more sophisticated mathematical and statistical methods should be a priority. We do not agree with this. Whereas, in the longer term, this might be an important issue for our profession, we believe that the current focus for actuaries should be in the areas set out in this paper, such as understanding the business better.
GRIT believes that the issues which we have identified are important for the future of our profession and the contribution which we can make to the general insurance industry.