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Mitchell, Daniel

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Mitchell

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Daniel

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Mitchell, Daniel
Author's Publications: search.filters.isAuthorOfPublication.e32f6d1c-4ff0-430c-bb4b-cca43a0eec6f

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    Low frequency observations of linearly polarized structures in the interstellar medium near the south Galactic pole
    (American Astronomical Society, 2016) Lenc, E.; Gaensler, Bryan; Sun, X; Sadler, Evan; Willis, A. G.; Barry, Nicholas; Beardsley, A. P.; Bell, Marjorie; Bernardi, G.; Bowman, Jason; Briggs, Florence; Callingham, J. R.; Cappallo, R. J.; Carroll, P.; Corey, B. E.; Oliveira-Costa, A. de; Deshpande, A. A.; Dillon, J. S.; Dwarkanath, K. S.; Emrich, D.; Ewall-Wice, A.; Feng, L.; For, B.-Q.; Goeke, R.; Greenhill, Lincoln; Hancock, P.; Hazelton, B. J.; Hewitt, Justina; Hindson, L.; Hurley-Walker, N.; Johnston-Hollitt, M.; Jacobs, Daniel; Kapińska, A. D.; Kaplan, Daniel; Kasper, Justin; Kim, Andrew Hyung-Do; Kratzenberg, E.; Line, J.; Loeb, Abraham; Lonsdale, C. J.; Lynch, M. J.; McKinley, B.; McWhirter, Sarah; Mitchell, Daniel; Morales, M. F.; Morgan, E.; Morgan, James; Murphy, Teresa; Neben, A. R.; Oberoi, D.; Offringa, A. R.; Ord, Stephen; Paul, S.; Pindor, B.; Pober, J. C.; Prabu, T.; Procopio, P.; Riding, J.; Rogers, Adrianne; Roshi, A.
    We present deep polarimetric observations at 154 MHz with the Murchison Widefield Array (MWA), covering 625 deg^2 centered on RA=0 h, Dec=-27 deg. The sensitivity available in our deep observations allows an in-band, frequency-dependent analysis of polarized structure for the first time at long wavelengths. Our analysis suggests that the polarized structures are dominated by intrinsic emission but may also have a foreground Faraday screen component. At these wavelengths, the compactness of the MWA baseline distribution provides excellent snapshot sensitivity to large-scale structure. The observations are sensitive to diffuse polarized emission at ~54' resolution with a sensitivity of 5.9 mJy beam^-1 and compact polarized sources at ~2.4' resolution with a sensitivity of 2.3 mJy beam^-1 for a subset (400 deg^2) of this field. The sensitivity allows the effect of ionospheric Faraday rotation to be spatially and temporally measured directly from the diffuse polarized background. Our observations reveal large-scale structures (~1 deg - 8 deg in extent) in linear polarization clearly detectable in ~2 minute snapshots, which would remain undetectable by interferometers with minimum baseline lengths >110 m at 154 MHz. The brightness temperature of these structures is on average 4 K in polarized intensity, peaking at 11 K. Rotation measure synthesis reveals that the structures have Faraday depths ranging from -2 rad m^-2 to 10 rad m^-2 with a large fraction peaking at ~+1 rad m^-2. We estimate a distance of 51+/-20 pc to the polarized emission based on measurements of the in-field pulsar J2330-2005. We detect four extragalactic linearly polarized point sources within the field in our compact source survey. Based on the known polarized source population at 1.4 GHz and non-detections at 154 MHz, we estimate an upper limit on the depolarization ratio of 0.08 from 1.4 GHz to 154 MHz.
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    CHIPS: The Cosmological HI Power Spectrum Estimator
    (American Astronomical Society, 2016) Trott, Cathryn; Pindor, Bart; Procopio, Pietro; Wayth, Randall; Mitchell, Daniel; McKinley, Benjamin; Tingay, Steven; Barry, N.; Beardsley, A.; Bernardi, G.; Bowman, Judd; Briggs, F.; Cappallo, R.; Carroll, P.; de Oliveira-Costa, A.; Dillon, Joshua; Ewall-Wice, A.; Feng, L.; Greenhill, Lincoln; Hazelton, B.; Hewitt, J.; Hurley-Walker, N.; Johnston-Hollitt, M.; Jacobs, Daniel; Kaplan, D.; Kim, HS; Lenc, E.; Line, J.; Loeb, Abraham; Lonsdale, C.; Morales, M.; Morgan, E.; Neben, A.; Thyagarajan, Nithyanandan; Oberoi, D.; Offringa, A.; Ord, Stephen; Paul, S.; Pober, J.; Prabu, T.; Riding, J.; Shankar, N.; Sethi, Shiv; Srivani, K.; Subrahmanyan, R.; Sullivan, I.; Tegmark, M.; Webster, R.; Williams, A.; Williams, C.; Wu, C.; Wyithe, J.
    Detection of the cosmological neutral hydrogen signal from the Epoch of Reionization, and estimation of its basic physical parameters, is the principal scientific aim of many current low-frequency radio telescopes. Here we describe the Cosmological HI Power Spectrum Estimator (CHIPS), an algorithm developed and implemented with data from the Murchison Widefield Array (MWA), to compute the two-dimensional and spherically-averaged power spectrum of brightness temperature fluctuations. The principal motivations for CHIPS are the application of realistic instrumental and foreground models to form the optimal estimator, thereby maximising the likelihood of unbiased signal estimation, and allowing a full covariant understanding of the outputs. CHIPS employs an inverse-covariance weighting of the data through the maximum likelihood estimator, thereby allowing use of the full parameter space for signal estimation ("foreground suppression"). We describe the motivation for the algorithm, implementation, application to real and simulated data, and early outputs. Upon application to a set of 3 hours of data, we set a 2σ upper limit on the EoR dimensionless power at k=0.05~h.Mpc−1 of Δ2k<7.6×104~mK2 in the redshift range z=[6.2−6.6], consistent with previous estimates.
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    Enabling a High Throughput Real Time Data Pipeline for a Large Radio Telescope Array with GPUs
    (Elsevier, 2010) Pfister, Hanspeter; Edgar, Richard G; Mitchell, Daniel; Ord, Stephen; Greenhill, Lincoln; Clark, Michael A.; Dale, Kevin; Wayth, Randall B.
    The Murchison Widefield Array (MWA) is a next-generation radio telescope currently under construction in the remote Western Australia Outback. Raw data will be generated continuously at 5 GiB s\(^{−1}\), grouped into 8 s cadences. This high throughput motivates the development of on-site, real time processing and reduction in preference to archiving, transport and off-line processing. Each batch of 8 s data must be completely reduced before the next batch arrives. Maintaining real time operation will require a sustained performance of around 2.5 TFLOP s\(^{−1}\) (including convolutions, FFTs, interpolations and matrix multiplications). We describe a scalable heterogeneous computing pipeline implementation, exploiting both the high computing density and FLOP-per-Watt ratio of modern GPUs. The architecture is highly parallel within and across nodes, with all major processing elements performed by GPUs. Necessary scatter-gather operations along the pipeline are loosely synchronized between the nodes hosting the GPUs. The MWA will be a frontier scientific instrument and a pathfinder for planned peta- and exascale facilities.