Introduction to the Murchison Widefield Array Project Alan R. Whitney MIT Haystack Observatory

Outline The genesis The process of defining the project A glimpse of the science objectives Challenges to be overcome A feel for what all is involved Some results from early deployment studies

Mid-late 1990s - Haystack was looking to get into arrays Low frequency arrays presented the most exciting opportunity – Unexplored territory – Exciting science – Digital telescopes - rapidly becoming technologically feasible – Affordable hardware The Genesis

So where do we start?

The key science objectives Epoch of Reionization –Power spectrum –Strömgren spheres Solar/Heliospheric –Faraday rotation, B-field –Interplanetary Scintillation (IPS) –Solar burst imaging Transients –Deep blind survey Other –Pulsars –ISM survey –Recombination lines –Etc. Frequency range Collecting area Array configuration Bandwidth Frequency resolution Time resolution Location Calibration requirements Data analysis and processing approach Logisitics Technological feasibility Computational feasibility Available knowhow

The Epoch of Re-Ionization ~300,000 years after Big Bang, hydrogen formed (opaque) After ~1 billion years hydrogen is ionized by stars (transparent) In between are the dark ages The MWA can see through the hydrogen

Cosmic Re-ionization Nick Gnedin

The Sun-Earth Connection Bow Shock Magnetopause CME Solar Wind Direction Travel Time = Days

Geomagnetic Storms Disrupt Technological Systems Radiation Hazards Damage to Satellites Communications Failures GPS Navigation Problems The need for predicting Space Weather

Direct Imaging of CMEs CMEs also visible directly –Synchrotron emission –Polarimetry yields transverse B-field information –Complementary to IPS data Faraday rotation measurements –Measure longitudinal B- field Complete characterization of particles and field …

Principle of Faraday Rotation For a rotation measure of 1 rad m -2  = 1° at 2.3 GHz ( =0.13 m)  = 90° at 240 MHz ( =1.25 m)  = 530° at 100 MHz ( =3.00 m) Observe a known linearly polarised background source through the magneto-ionic medium and use the observed changes in plane of polarisation to model the medium  = 2 c  n e B.ds = 2 RM

The origin of IPS Plane wavefront incident from a distant compact source The density fluctuations in the Solar Wind act like a medium with fluctuating refractive index, leading to corrugations in the emerging wavefront These phase fluctuations develop into intensity fluctuations by the time they reach the observer The resulting interference pattern sweeps past the telescope, leading to IPS

What transient sources might we find? Many rare giant pulsar pulses (2 now known) Radio bursts from cosmic ray neutrinos hitting the Moon Bright stellar radio flares Gamma ray burst afterglows Microlensing events involving AGNs Coherent burst emission from magnetar glitches Black hole/neutron star in-spiral events Coherent burst emission from planets and extra-solar planets Many unsuspected phenomena?

The MWA – a state of the art instrument Major new instrument to explore the low end of the radio-frequency spectrum MHz Being developed by Haystack scientists and engineers with collaborators Fully digital; no moving parts Situated in Western Australia – because of low RFI environment Depends on massive computing power – largely a “software telescope”

Murchison RFI Levels

U.S. RFI Levels 20

Murchison Widefield Array Specs Frequency range MHz Number of receptors 8192 dual polarization dipoles Number of tiles 512 Collecting area ~8000 m2 (at 200 MHz) Field of View ~15°-50° (1000 deg2 at 200 MHz) Configuration Core array ~1.5 km diameter (95%, 3.4’) + extended array ~3 km diameter (5%, 1.7’) Bandwidth 220 MHz (Sampled); 31 MHz (Processed) # Spectral channels 1024 (3072) Temporal resolution 8 sec (0.5 sec) [wide-field sky image rate] Polarization Full Stokes Point source sensitivity 20mJy in 1 sec (32 MHz, 200 MHz) 0.34mJy in 1 hr Multi-beam capability 32, single polarization Number of baselines (VLA: 351, GMRT: 435)

MWA Antennas

Interesting visitors!

Image Creation Real-time imaging supercomputer must absorb and process up to 160 Gbps continuously from the correlator to create a new calibrated high-resolution sky image every 8 seconds! Thousands of foreground stars and galaxies must be accurately substracted from these images to reveal the target background radiation These foreground-subtracteded images form the primary dataset for a large fraction of MWA science goals

Implementation Phases 32-tile system (32T) –Conceived as engineering test bed –Evolved into validation platform for MWA technologies –Successful ly completed and tested in tile system (512T) –Scoped for calibratability and key science capability –Additional development required (software + firmware) –Buildout to 128T currently in progress; budget limitations are delaying buildout to 512T

The MWA site – The Shire of Murchison

Aerial view of 32T

Challenges Calibration of ionospheric effects Foreground subtraction for EoR analysis High dynamic range wide field of view imaging

Calibration Issues Wide field of view –Tens of thousands of sources visible –Sky model requires large number of parameters High dynamic ranges required –Lots of very bright sources –Sensitive instrument –Peak/rms of at least 10 5 needed Complicated ionosphere –Rapidly changing –Each station sees different irregular screen across field –Many parameters required for each station/time

Calibration Regimes

Calibration Regimes (cont’d)

Solar Burst Activity Single baseline, amplitude vs freq and time 20 ~1 min x 4 MHz strips over 1 hour period

The Big Challenges: Multiple simultaneous technical innovations –This is new ground, in many different ways Short timescale –Driven by political and financial necessity Distributed project team –Physical distance and timezones –Cultural differences, some subtle

Summary MWA is a major technical innovation to enable exploration of a new frequency regime with major new capabilities; first real-time imaging array, made possible by massive computing power Risks are high, but potential payoffs also high 32T system has proven major parts of MWA concept, but 512T system needed to verify some aspects of design, particularly calibration 128T buildout currently in progress; full 512T system will require significant new funding that NSF is presently unable to provide As a pathfinder for SKA, a successful MWA is a critical steppingstone to HERA2 and the SKA array

Questions?