Shallow & deep integrations with the MWA Gianni Bernardi

1. Shallow & deep integrations with the MWA Gianni Bernardi Harvard-Smithsonian Center for Astrophysics (cooperative effort with D. Mitchell, L. Greenhill, S. Ord, N. Shravan, R. Wayth & the whole MWA collaboration)

2. MWA is (will be) the largest N-array (128 elements, 512?) dipole layout areal filling antenna spacings 800m u,vplane filled inside 1.1km 6.3´ @ 150 MHz 0.004 < k< 0.4 Mpc-1 filling suppresses artifacts in continuum fg subtraction 5x5m tiles

3. 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)

4. 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);

5. Flow chart Select a subset of visible sources from image data Generate the FM (the synthesized beam) for each source using current best parameter estimate (position, flux) Simultaneous fit for all the source parameters through a non linear minimization No Add to sky model Convergence? Yes Subtract sources from sky model Yes Are there unmodeled sources? No Done

6. Deconvolution can actually be represented by matrix algebra: • for M sourcesand N image pixels,the following system of linearized equations is solved at each iteration: 3M vector of parameter estimates Jacobian matrix weight matrix N vector of data points • get a new parameter estimate xi:

7. A 512T application: initial image (101 sources + thermal noise) Peak: 87 Jy rms: 105 mJy/beam DNR (apparent) ~ 800

8. Non linear minimization for the 15 brightest sources (5 iterations)

9. Non linear minimization for the 50 brightest sources (5 iterations)

10. 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)

11. MWA 32 tiles (32T) 400m …5% prototype for 80-300 MHz

12. 32T Pipeline First Light 103 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. 134 MHz 180 MHz 20°x 20° Zenith

13. Essential features of calibration and imaging are in place… What does it still need? • sky models • beam measurements/characterization • imaging improvements

14. 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

15. 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)

16. 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);

17. Sky coverage

18. Selected areas (undeconvolved images)

20. Zoom in (1) CenA Galactic centre CenB PKS1610-60

21. Zoom in (1.1) GRS 006.60-00.20 & GRS 006.60-00.10 SNR: G348.5+00.0 SgrA star

22. Zoom in (2) FornaxA Vela + Puppis PicA HydA

23. Zoom in (3) PKS 2153-69

24. Moving up north… HydA

25. (cont’d) HerA 3C253 SNR W44

26. (cont’d) VirA HerA

27. 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%

28. 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);

29. Primary beam measurements J0444-2905

30. Fitting a simple primary beam model The beam is accurate at a 5% level

31. 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 array Snapshot images co-added Multi-frequency synthesis (but in the image plane)

34. Conclusions: • the MWA calibration and imaging pipeline has been tested. It is working well, it will be refined – significant choices will have to be made; • real science data will be ready to come out in the next months; • a 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; • MWA will expand to 128 tiles in the next year, expanding its science targets; Thank you!