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Published byRichard Hutchinson Modified over 9 years ago
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Observing Cosmic Dawn with the LWA-1 PIs: Judd Bowman (ASU), Greg Taylor (UNM) Jake Hartman (JPL) Jayce Dowell, Joe Craig (UNM) Steve Ellingson (Virginia Tech) Jackie Monkiewicz Arizona State University
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The “Dark Ages” and Cosmic Dawn 2/20 Dark Ages: z = 1,100 to z~ 40 matter-dominated H & He are neutral 1st structures collapsing Cosmic Dawn: z = 40-20 1st stars & galaxies 1st QSOs? Early heating, reionization of small bubbles
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Cosmic Dawn project purpose: 3/20 Detect/constrain signal of 1st generation of stars in 21-cm absorption of hydrogen at z ~ 30 REQUIRES: 1.Low frequency experiment, 10-100 MHz LWA-1 2.Long Integration time 3.Very accurate bandpass calibration 4. Novel beamforming techniques
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Cosmic Dawn in 21 cm: (Furlanetto 2006, Pritchard & Loeb 2010) 4/20 T b = T s – T cmb Seen against CMB: Thermal History of IGM
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Cosmic Dawn in 21 cm: 1 st stars create absorption trough Additional heating sources mitigate trough (Furlanetto 2006, Pritchard & Loeb 2010) 5 T b = T s - T cmb
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Observing strategy: REQUIREMENTS: Very good bandpass calibration! Looking for broad, shallow absorption trough need > 10 4 S/N in any spectral channel But only need ~10% accuracy in absolute power… 6/20 A E D B C 20406080100120140160180 ν (MHz) Δ T 21 (mK) +50 0 −50 −100 (Pritchard & Loeb 2010)
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Observing strategy: STRATEGY: Simultaneously observe bright calibrators & dark (low T sys ) science field 2 x 19.6 MHz beams on bright calibrator 2 x 19.6 MHz beams on science field 520 hours on-sky 6/20
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Observing Strategy -- COMPLICATION: Frequency variation of beam shape couples of foreground structure to sidelobes mistake sources drifting through sidelobes for 21-cm spectral features? 8/20 1.0 0.8 0.6 0.4 0.2 0.0 −15−10−5051015 Offset (degrees) Relative gain 74 MHz 38 MHz
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Novel Beamforming Strategies: Mitigate potential foreground-frequency coupling of sidelobes: 9/20 1. Defocusing (e.g. gaussian smoothing) 2. Sidelobe steering 3. Nulling 4. Sidelobe shimmering 5. “Optimized” beam-forming (account for mutual coupling of antennas)
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Work to date: 10/20 Learning the LWA Software Library! (and PYTHON in general) Phase-and-Sum Beamforming with TBN Raster Mapping of TBN Beam (pseudo-beams)
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Phase-and-Sum Beamforming: 11/20 TBN data: narrow bandwidth ( < 100 kHz) Commissioning scripts for TBN (J.Dowell): (follows “Fun with TBN” memo, S. Ellingson) 1. Fringe all antenna outlier stand #258 (simpleFringe.py ) 2.Fringe stop on bright source --- Cyg A or flaring Sun back out delay coefficients (solveCoeffs.py) 3.Use array geometry to point beam (formBeam.py)
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Phase-and-Sum Beamforming: 12/20 Find bursting Sun produces much better coefficients than Cyg A --- not surprising?
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Raster Mapping: 13/20 Use bright source in TBN data to map structure of sidelobes --- “Pseudo-beam”
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Raster Mapping – Variation with elevation 14/20 Cyg A: Transit EL = 83 deg -1 hour EL = 76 deg -2 hours EL = 65 deg
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15/20 Cas ANCP @ Cyg A transit EL = 46 deg -2 hours before Cyg A transit EL = 34 deg @ Cyg A transit EL = 34 deg … What is going on in the North/Northeast during the Cyg A transit on Sept 21, 2011??
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16 PASI started recording Sept 23, 2011: http://www.phys.unm.edu/~lwa/lwatv/55827.mov
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Pseudo-beam Maps over full frequency range: 17/20 Acquired TBN observations of 4 frequency groups: 87 MHz 80 MHz 73 MHz 71 MHz 64 MHz 57 MHz …corresponding to 4 DRX tunings for main Cosmic Dawn observations 55 MHz 48 MHz 41 MHz 39 MHz 32 MHz 25 MHz Beam 1 Tuning 1 Beam 1 Tuning 2 Beam 2 Tuning 1 Beam 2 Tuning 2
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Test our Beamforming Strategies: 18/17 1. Defocusing (e.g. gaussian smoothing) 2. Sidelobe steering 3. Nulling 4. Sidelobe shimmering 5. “Optimized” beam-forming (account for mutual coupling of antennas) Which is the “quickest and dirtiest”?
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Test our Beamforming Strategies: 19/17 1. Defocusing (e.g. gaussian smoothing) 2. Sidelobe steering 3. Nulling 4. Sidelobe shimmering 5. “Optimized” beam-forming (account for mutual coupling of antennas) Which is the “quickest and dirtiest”?
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Pseudo-beam Maps over full frequency range: 20/20 Apply some of our novel beam-forming strategies Defocusing is simplest, fastest apply Gaussian to antenna gains \ Acquire raster maps of customized DRX beams, compare with TBN predictions, confirm shape
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LWA-OCD Project Outputs: 21/20 Beamforming: Detailed measurements of beam Strategies for custom beamforming Deep integrations: Very high S/N spectra of bright calibrators Very high S/N spectra of diffuse Galaxy (including high-level H recombination lines) Serendipitous radio transients Lots of opportunity for RFI mitigation! Detection/contraints on First Light absorption trough. END
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RFI environment at LWA: 22
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The “Dark Ages” and Cosmic Dawn 23/17 z = redshift (decreases with increasing time) z = ∞ z = 0
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The “Dark Ages” and Cosmic Dawn 24/17
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Wouthuysen-Field Effect: 25
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Phase-and-sum Beamforming: 26/17 Use bright source to back out coefficients Only works for narrow (< 10 KHz bandwidth) insensitive to 2 offsets Delay-and-sum Beamforming: Do phase-and-sum over full LWA frequency range Solve for true delays for each antenna
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System noise for LWA: 27 From Pihlstrom, 2012, internal memo
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Acronyms: 28
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