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Shallow & deep integrations with the MWA
Gianni BernardiHarvard-Smithsonian Center for Astrophysics
(cooperative effort with D. Mitchell, L. Greenhill, S. Ord, N. Shravan, R. Wayth & the whole MWA collaboration)
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MWA is (will be) the largest N-array (128 elements, 512?)
800m
u,v plane filled inside 1.1km
6.3´ @ 150 MHz0.004 < k < 0.4 Mpc-1
filling suppresses artifacts in
continuum fg subtraction
dipole layout areal filling antenna spacings
5x5mtiles
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The Real-Time calibration and imaging System (RTS)
(Mitchell et al., 2008, IEEE, 2, 707)(Ord et al., 2010, PASP, )
(GB et al., 2011, MNRAS, 413, 411)
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Deconvolution/source subtraction via forward modeling
(or further development on image based deconvolution)
(Bowman, Morales & Hewitt, ApJ, 695, 183)(Geil, Gaensler & Whyithe, 2010, 2010arXiv1011.2321G)
(Pindor et al., 2011, PASA, 28, 46)(GB et al., 2011, MNRAS, 413, 411)
•in the MWA case, visibility data are not stored – causing a possible limitation in the deconvolution accuracy;
•once the uv plane includes time and position dependent primary beam and ionospheric correction the synthesized beam is position dependent and there is no standard
deconvolution method applicable (no Clean, no Cotton-Schwab method);•it is a valuable method to evaluate the statistics of the residual visibilities (in the light
of EoR detection);
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Flow chartSelect a subset of visible sources from image data
Generate the FM (the synthesized beam) for each sourceusing current best parameter estimate (position, flux)
Simultaneous fit for all the source parameters through anon linear minimization
Convergence?
Subtract sources from sky model
Are there unmodeled sources?
No
Yes
Add to sky model
Yes
No
Done
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Deconvolution can actually be represented by matrix algebra:
•for M sources and N image pixels, the following system of linearized equations is solved at each iteration:
1( )T Tx J WJ J W m
1i ix x x • get a new parameter estimate xi:
3M vector of parameter estimates
Jacobian matrix weight matrix N vector of data points
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A 512T application: initial image(101 sources + thermal noise)
Peak: 87 Jy
rms: 105 mJy/beam
DNR (apparent) ~ 800
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Non linear minimization for the 15 brightest sources (5 iterations)
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Non linear minimization for the 50 brightest sources (5 iterations)
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Non linear minimization for all the 101 sources (5 iterations)
rms ~ 25 mJy/beam
final DNR ~ 3400
(Source subtraction for the EoR: ~1200 sources
down 1 mJy in a 20° FoV - MWA will be
confusion limited before that)
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400m
…5% prototype for 80-300 MHz
MWA 32 tiles (32T)
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32T Pipeline
First Light
20°x 20°Zenith
103 MHz
134 MHz
180 MHz
• Observations from January 2010
• 3 x 7hr tracks with real-time calibration, peeling, imaging, resampling and
averaging.
• HEALPIX projection of Pic A field.
• Pic A peeled to reveal many secondary sources.
• Several simplifying assumptions relative to the 512T version, but all of the
pieces are in place.
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Essential features of calibration and imaging are in place… What does it still need?
•sky models•beam measurements/characterization
•imaging improvements
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Sky survey: observing specs & strategy
• Coverage of the whole southern sky @100 MHz & @180 MHz
• Meridian survey: 136 fields + 34 calibrators
• Each field is observed close to meridian (within a few minutes of transit) for ~5 min
observations of similar HA -> similar primary beam for each scan -> similar synthesized beam and sidelobe structure
• 30.72 MHz bandwidth & ~ 30’ angular resolution
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Calibration and imaging
• calibration and imaging performed through the Real Time System running on CPU/GPU in an off-line mode;
• choice of a few calibrators per pointing direction to determine passband and gain solutions (both direction independent). Gains are computed over ~ 8 MHz bandwidth (after bandpass fitting). The converged solutions are transferred to fields where there is not enough signal-to noise-ratio for selfcalibration. A few calibrators are sufficient to cover the whole sky for δ > -70°:
CenA (4500 Jy @ 80 MHz) PicA (345 Jy @ 109.44 MHz) HydA (351 Jy @ 109.44 MHz) HerA (650 Jy @ 109.44 MHz) VirA (1260 Jy @ 109.44 MHz) TauA (150 Jy @ 109.44 MHz)
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Calibration and imaging (cont’d)
• each 8 sec-0.6 MHz snapshot is re-sampled into the Healpix frame and weighted by the (model) tile primary beam. The short baselines are down weighted using a Gaussian taper with σ = 15 u;
• snapshot images are co-added up to 5 min;
• mosaicking is performed in the traditional way, by combining the various pointings weighted by their primary beam (the tile beams are considered to be all the same);
• dirty images need (offline) deconvolution (both point sources and diffuse emission). We have been doing work in this direction (deconvolution through forward modeling);
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Sky coverage
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Selected areas (undeconvolved images)
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Zoom in (1)
Galactic centre
CenA
CenB
PKS1610-60
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Zoom in (1.1)
SNR: G348.5+00.0
GRS 006.60-00.20 & GRS 006.60-00.10
SgrA star
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Zoom in (2)
Vela + Puppis
FornaxA
PicA
HydA
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Zoom in (3)
PKS 2153-69
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Moving up north…
HydA
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(cont’d)
HerA
3C253
SNR W44
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(cont’d)
HerA
VirA
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Deconvolving real data: an example
Source J0523-36 is modeled in the same
way that the pointing is processed via the RTS
(beams, cadence, frequency)
Convergence after 2 iterations. Positional
error ~ 15’, flux error ~ 10%
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Primary beam measurements
• the sky drifts overhead while the tiles point at zenith;
• ~30 bandwidth centered @ 185 MHz;
• snapshot images (one every 5 min) are used to measure the beam response towards the J0444-2905 (which is ~ 37 Jy @ 185 MHz);
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Primary beam measurements
J0444-2905
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Fitting a simple primary beam model
The beam is accurate at a 5% level
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Extending the beam work: zooming in to HydA field
HydA
Observations span slightly more than 5 hours (total) over 110-200 MHz:21 tiles available
HydA provides the direction independent calibration of the arraySnapshot images co-added
Multi-frequency synthesis (but in the image plane)
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Conclusions:
othe MWA calibration and imaging pipeline has been tested. It is working well, it will be refined – significant choices will have to be made;
oreal science data will be ready to come out in the next months;
oa first all sky survey (catalogue of point sources brighter than 10 Jy, images of the A-team source) will be available in the next months;
oMWA will expand to 128 tiles in the next year, expanding its science targets;
Thank you!
