Transient Cosmic Phenomena and their Influence on the Design of the SKA Radio Telescope

Transcription

1 Transient Cosmic Phenomena and their Influence on the Design of the SKA Radio Telescope Research Review Curtin Institute of Radio Astronomy 5 May 2009

2 ToC Parameter space Discovering phenomena ASKAP & the SKA The digital age

3 PhD objectives Compendium of time-domain phenomena Enabling technological factors to fully explore the time-domain parameter space Transient capability of the SKA and other arrays Integrate results into the SKA performance and cost modelling effort

4 Transient cosmic phenomena - the high time resolution universe Dynamic radio sky is poorly understood First steps are rare but bright events with pulse-like emission Lorimer-type extra-galactic bursts, giant pulses etc. No guarantee of periodicity Not explicitly an SKA Key Science Project, but there is: A growing interest in the transient universe A desire to explore the unknown Serendipity! We have a generally accepted idea of how to build a good synthesis imaging telescope... Can the same be said of a high time resolution aperture synthesis telescope?

5 Parameter (phase) space Cosmic Discovery (Harwit, M., 1981) Phase space of E-M observations Spectral frequency Spectral resolution Temporal resolution Angular resolution Polarisation (Intensity) Assumes that positional information is unimportant! Figure: Properties of a photon: frequency, time, angular direction, spatial position and spin (adapted from Harwit, 81).

6 Filtering the observational parameter space We want to explore temporal resolution; and possibly spectral resolution Wavelength limited by instruments Pulse-like phenomena - consider event rates Point sources, so high angular resolution not advantageous for detection Event rate search space: FoV + observing time Figure: Phase space filter. Small shaded cube is what the instrument dectects (source: Harwit, 81).

7 Harwit: Analysis vs Discovery For a single pulse, these need to occur simultaneously! Analysis Discovery Detailed information Explore parameter space Figure: Applying filters to the observational parameter space (source: Robinson, 87)

8 Monitoring the sky Event rates Field of View + observing time increases likelihood of dectection Figure: Telescopes overlaid on minimum detectable flux density vs. instantaneous solid angle (SKA Memo 97, Cordes 08) ν 0 = 1 GHz, BW = 300 MHz t int = 1 sec, η a = 0.6, T sys = 25 K

9 The which with what and how?

10 ASKAP observational strategy Searching at ms time resolution with a wide FoV, but also good verification and localisation Incoherent search with triggered coherent buffer Incoherently sum 36 ASKAP antennas 6 x sensitivity, 30 deg 2 FoV Data streams scale with number of PAF primary beams Inherently flexible - throttle data rate: t samp, N channels, N bits, N beams, Still many real-time challenges! Rolling buffer PAF primary beam voltages Allows for analysis of event at full sensitivity Excellent for commensal observing Within similar regions of the sky, any pointing is as good as any other? Large observed volume of sky

11 SKA observational strategy Expecting a dense core of dishes Searching the primary FoV Pixelise the FoV with tied-array beams ( Scale as FoVexpansion b ) 2 D Note the inefficiencies for ASKAP! Snapshot imaging High correlator dump-rate Sky pixelisation still required Search on each pixel Comensal observing possible? Figure: Pixelising the primary FoV with tied-array beams (SKA Memo 97, Cordes 07). Also see Smits et al, 09.

12 Why ASKAP? SKA Pathfinder Event verification - only one chance! Radio frequency interference excision Novel experiments? Weighting on the PAF beamformer? Number dishes Relative sensitivity Relative FoV Incoherent ASKAP Correlated ASKAP Correlated ASKAP Table: Relative sensitivity and FoV compared to Parkes MB (assuming same Tsys, aperture efficiency etc.)

13 Realities Bottlenecks Data transport How soon can we get it off-site? Real-time processing Searching for the unknown can be very intensive Storage capability We have the space, but can we get it onto disk? Power Over-arching concern for any activity on-site Questions to be answered Where can we take shortcts or make tradeoffs? Limiting our parameter space search in different directions Is an incoherent telescope best for the near future? Where does Moore s law fail us? How do we not design out certain features?

14 SKA performance and costing tool Relevance of the tool Dish based models Preliminary SKA Specification Process (SKA Memo 100) Aperture array models SKA Design Studies program New version of tool under initial use by SKADS and some engineers in the SKA Program Development Office Using the tool Tool has use in the design stage of the SKA Operational decisions yet to be made (esp. power) Identify bottlenecks and limitations of the system

15 The path ahead Use ASKAP and other pathfinder telescopes as case studies ASKAP Survey Science Projects is a useful process Sept 09 - extended ( 9 month) Manchester visit Interaction with engineers at the SPDO Further development of cost tool European thinking on transients Exercise costing tool for insights on SKA design and operational considerations for high time resolution science Show path for ASKAP and SKA as a credible transient telescopes Write it up!