MWA imaging and calibration – early science results Nadia Kudryavtseva, Stiven Tingay, Randall Wayth, David Kaplan, Aziz Jiwani Curtin University of Technology Australia

Outline MWA calibration challenge – list of problems which should be corrected Current status of calibration What will change with 128T compare to 512T? Future prospects

MWA calibration challenge Big field of view Large bandwidth Direction dependency Ionosphere

MWA calibration challenge Interstellar medium Ionosphere Correlated beam All changing with time and position in the sky Tile beam

Phases Ionosphere Large field Of view calibration Pointing error Correct using known positions of calibrators GPS station? Ionosphere Large field Of view calibration Fasceting imaging, multiple calibrators In the field of view Pointing error Calculating known offset and correcting For it in the beginning of calibration

Amplitudes Ionosphere? Pointing error Tile beam Correlated Correlated Very difficult task. Requires ionospheric modelling Ionosphere? Large field Of view calibration Fasceting imaging, multiple calibrators In the field of view Pointing error Requires knowledge of correlated beam Pattern and tile beam pattern Tile beam Direction dependent, known sources, Theoretical beam, two calibrators in the field Correlated beam Correlated beam Direction dependent, theoretical model? Bandpass Correlated beam Calibration using peeled calibrator Corrected ripples in bandpass using known Sources Sources In sidelobes Correlated beam Peeling off sources

PicA field

Ionosphere Compared positions with SUMMS catalogue Y [deg] X [deg] Compared positions with SUMMS catalogue 0.27 +-0.04 degrees offset

Tile beam

Tile beam Direction dependent and frequency-dependent Tile sensitivity

Tile beam

Tile beam Tile beam from PicA observations Flux difference [Jy] X [deg] Tile beam from PicA observations

Sources in sidelobes Noise from sources in sidelobes → Changing on a timescale of 5 mins

Correlated beam Dirty beam from miriad

Bandpass

Bandpass

Tile beam

Variability

Variability

Tile beam

Self-calibration

Quasar variability 0528-45

128T vs 512T 8 times less transient detections with 128T R ~S-3/2 (Macquart 2011)

Conclusions 1. RFI is less than 2 percents 2. Bandpass correction – much better when peeling off the calibrator source 3. Bandpass is stable over 1 hour 4. Self-calibration does not help 5. We need to understand tile and correlated beam 6. With current understanding of the beams it 6-10% error in calibrators' flux-densities

Future work 1. Understanding tile beam: - Map two calibrators in the field - Drift scans - Measure dipole beam and reconstruct the tile beam - Scanning with emitting antenna? 2. Understanding correlated beam: - find correlated noise - use model based on tile beam 3. Subtracting the data: - useful for transient science 4. Imaging: Change FFT to Fast Wavelet Transform

Tile beam