the lofar snapshot calibrator survey
TRANSCRIPT
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The LOFAR Snapshot Calibrator Survey Identifying Calibrators for Long Baseline LOFAR Observations Adam Deller, Javier Moldon & the Long Baseline Working Group
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The Long Baseline Situation
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The Long Baseline Problem, (1)
LOFAR enemy #1: Ionosphere
On a short baseline, the absolute ionospheric path delay difference is small, and the differential spatial gradients are small
On a long baseline, the absolute ionospheric path delay difference is large (phase changes rapidly with frequency) and the differential spatial gradients are large (phase changes rapidly with direction)
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The Long Baseline Problem, (II)
3C123: amplitude scale is (roughly) kJy
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The Long Baseline Problem
To summarise: � Your calibrators are going to be (much)
fainter and you can’t even average in frequency to (partially) compensate
� Oh, and they need to be close on the sky! � The standard LOFAR data reduction
approach will not work (except for maybe the brightest few sources in the sky)
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The Long Baseline Solutions
� Create a “super station” by phasing up all of the core antennas (ΣCS* è TS001)
� Use “VLBI” tools to coherently combine more data (delay/rate search, i.e. linear phase gradient in frequency and time) ◦ Imperfect solution, since ionospheric delay is
dispersive; ok for small bandwidths
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Calibration requirements
� LOFAR theoretical sensitivity suggests a calibrator for our current approach needs ≥100 mJy flux density in a compact component (@150 MHz)
� Experience agrees, and adds that this calibrator should be within ~1 degree
� The €64,000 question: are there enough bright compact sources?
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LOFAR Snapshot Calibrator Survey
� Designed to answer this question � Observe sources with S150MHz > 100 mJy � 16 subbands = 3 MHz / beam � 30 beams / scan � 4 minutes / scan � 360 sources inspected per hour � Advantage: No uv shifting means simple/
fast processing and smaller data volumes
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LOFAR Snapshot Calibrator Survey
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LOFAR Snapshot Calibrator Survey
� Two 1-hour observations: ◦ 02 May 2013 (targeting sources with 327 MHz
flux density > 200 mJy) ◦ 07 November 2013 (targeting sources with
327 MHz flux density 80 – 250 mJy)
� Calibrate and phase-up core stations � Then solve for delay for every target
source individually (minimum S/N 8) � Delay solutions are primary observable
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LOFAR Snapshot Calibrator Survey
Taking only the best solutions, fit and subtract average delay
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LOFAR Snapshot Calibrator Survey
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LOFAR Snapshot Calibrator Survey
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LOFAR Snapshot Calibrator Survey
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LOFAR Snapshot Calibrator Survey
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LOFAR Snapshot Calibrator Survey
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LOFAR Snapshot Calibrator Survey
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Results
� Preliminary! Analysis shown here was finished yesterday.
� Much more information than I can present here in the time available
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Results
Predicted LOFAR flux density (mJy)
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Results
0
50
100
150
200
Nu
mb
er
of
sou
rces
Spectral index distribution WENSS - NVSS
q = 1
q = 2
q = 3
−1.6 −1.4 −1.2 −1.0 −0.8 −0.6 −0.4 −0.2 0.0
Spectral index
0.0
0.2
0.4
0.6
0.8
1.0
%
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Results
0
20
40
60
80
100
120
140
Nu
mb
er
of
sou
rces
Spectral index distribution VLSS-WENSS
q = 1
q = 2
q = 3
−1.6 −1.4 −1.2 −1.0 −0.8 −0.6 −0.4 −0.2 0.0
Spectral index
0.0
0.2
0.4
0.6
0.8
1.0
%
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Results
� We classified 86 out of 620 sources as “good”, from total area of ~400 sq. deg. ◦ But search was not exhaustive, not every
source >100 mJy in those 400 sq. deg. was inspected (in fact only about ¼ were) ◦ Controlling for selection effects, we find the
effective search area was 89 sq. deg. ◦ So: density of good calibrators ~1 per sq. deg.
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Conclusions � Enough calibrators: density ~1/sq deg,
enough to calibrate virtually anywhere at 150 MHz. Bright sources with a spectral turnover most likely to be compact.
� We can find them: observing technique + pipeline can identify suitable long baseline LOFAR calibrator in ~15 minutes
� Looking ahead: Our pipeline to be developed into observatory pipeline for general long baseline reduction