ramesh bhat centre for astrophysics & supercomputing swinburne university of technology
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Ramesh BhatRamesh BhatCentre for Astrophysics & Supercomputing Centre for Astrophysics & Supercomputing
Swinburne University of TechnologySwinburne University of Technology
Time Domain Astronomy Meeting, Marsfield, 24 October 2011
Searching for Fast Transients with
Interferometric Arrays
An Australia-India collaborative project
Developing new scientific capabilities for the GMRT Transient detection pipeline High time resolution pulsar science VLBI between GMRT and Australian LBA
Collaborating institutions: Swinburne, Curtin/ICRAR, CASS (Australia) National Centre for Radio Astrophysics (India)
Project team:Matthew Bailes (Swinburne) Ben Barsdell (Swinburne)
Ramesh Bhat (Swinburne) Sarah Burke-Spolaor (JPL)
Jayaram Chengalur (NCRA) Peter Cox (Swinburne)Yashwant Gupta (NCRA) Chris Phillips (CASS) Jayanti Prasad (IUCAA) Jayanta Roy (NCRA) Steven Tingay (Curtin) Tasso Tzioumis (CASS)
W van Straten (Swinburne) Randall Wayth (Curtin)
In This Talk:
Searching for fast transients - important considerations
GMRT as a test bed instrument Transient detection pipeline Event analysis methodology
Searching for fast radio transients: Important considerations
Detection sensitivity, survey speed, and search volume -- Figure of Merit (FoM)
Propagation effects: e.g. dispersion, scattering, and scintillation due to the intervening media
Parameter space to search for: DM, time scale; computational requirements
Radio frequency interference (RFI) -- a major impediment in the detection of fast transients!
Detection algorithms; candidate identification and verification strategies
De-dispersion
DM = Dispersion Measure (in units of pc cm-3)
Dispersion smearing can be quite severe at low obs frequencies
Processing will involve searching over a large range of dispersion measure (DM)
Low frequencies will require very fine steps in DM (e.g. ~1000 trial DMs @325 MHz)
Incoherent dedispersion: channelise data, shift and align the channels, then sum
Searching for “events” in the time - DM parameter space
Detections of single pulses from J0628+0909
Standard search strategy: Dedispersion + matched filtering
Each “event” is characterised by its amplitude, width, time of arrival and dispersion measure
(DM)
Matched filtering
Time domain clustering
Matched filtering
Observational Parameter Space
S (x, t, , )x : Location of the station
: Direction on sky
t : Time domain
: Radio frequency
RFI is site-specific & direction dependent: function of x and Effective use of “coincidence” or “anti-coincidence” filters
Celestial transients vs. RFI:
• May have similar -t signature (e.g. swept-frequency radar and pulsars)
• Will have very different occupancy of x- space:
Detecting fast transients: search algorithms and strategies
PSR J1129-53 - an RRAT discovered by Burke-Spolaor & Bailes (2010)
Transient Exploration with GMRT
30 x 45m dishes, collecting area ~ 3% SKA Modest number of elements, long baselinesAdvent of GMRT software backend (GSB) Demonstration of multibeaming across FoV Superb event localisation capabilities (~5”)Computational requirements are significant, however
affordable
GMRT makes an excellent test-bed for developing the techniques and strategies applicable for next-
generation (array type) instruments
Considerations for sub-arraying: False alarm
probabilities
N independent elements Multiple sub-arrays, p = N/nIncoherent combination
14 km
1 km x 1 km
RFI environment is known to vary significantly across the array; e.g. between the arms; between the central square and the arms (east, west, south)
Considerations for sub-arraying: RFI environment
Local RFI sources: • TV boosters• Cell phone towers• Power lines
Antenna locations are marked in red
Locations of RFI sources are marked in blue
courtesy: Ue-Li Pen
+
GMRT software backend (GSB)GMRT + configurability
Transient Detection Pipeline for GMRT
Real-time processing and Trigger generation + Local recording of Raw Data
GMRT array GSB cluster Transient Detector Trigger Generator@ 2 GB/sec
512 MB/sec
(Ndm/Nchan) x 64 MB/sec
Salient features of GMRT transient project
The GMRT + GSB combination offers some unique features for efficient transient surveys at low radio frequencies Long baselines: powerful discrimination between signals of
RFI origin vs celestial origin (via effective coincidence filtering)High resolution imaging: event localization (~ 5”-10”) possible
through imaging the field of view and/or full beam synthesisSoftware phasing (offline): sensitive phased array beams
toward candidate directions (~5 x sensitivity); base-band data benefits (e.g. coherent de-dispersion)
Search strategy: commensal mode with other observing programs; real-time processing and local recording
Pilot transient surveys with the GMRT
Primary goals: Technical development Efficacies at low frequencies
Survey region: -10o < l < 50o , | b | < 1o @ 610 -10o < l < 50o, 1o < | b | 3o @ 325
Data recording Software backend’s “raw dump”
DR = 2 x 30 x (32 MHz)-1 x 4 bpsData from the surveys are used to
develop the transient processing and the event analysis pipelines
Transient Detection Pipeline
RFI + quality checks
Form N Sub-arrays
De-dispersion
Transient detection
Event identification
Coincidence filter
Trigger generation
Data extraction
Event analysis
Examples from the pipeline: a real astrophysical signal
Examples from the pipeline: spurious signals (local RFI)
Spectral Kurtosis Filter for RFI excision: Implementation on CASPSR
Andrew Jameson (Swinburne)
Need for high resolutions in time, frequency and DM space
Signals can be as short as tens of micro seconds at GMRT frequencies Maximum achievable time resolution ~ 30 us with the current pipeline
An example from the GMRT transient detection pipeline (mode: 7 sub-arrays)
A Giant Pulse from Crab Pulsar at GMRT 610 MHz, Time duration ~ 50 us
Processing Requirements
Benchmark with current software: data at full resolution (30 us, 512 channel FB) 15 x real-time on a dual quad-core Dell PE1950 equivalent to 133 Gflops (theoretical)
Net processing requirement: 15 x 133 Gflops = 2 Tflops (per beam!)
Possible (practical) solutions: Data down sampling (degrading resolution in f-t) by a factor 4 4 machines per beam OR 16 machines for 4 subarray beams Alternatively, 4 x GPUs, each of 0.5 Tflops
De-dispersion (searching in DM parameter space) is the most computationally intensive part of the pipeline
30 us, 512-channels
16 bit data samples
DM range: 0 - 500
tolerance level: T1.25
GPU dedispersion code by Ben Barsdell (Swinburne)
Considerations for the real-time system: false positives and RFI signals
Considerations for the real-time system: (false positives + RFI) + real
signal
Event Analysis (offline) Pipeline
Localisation of the event on sky + phasing up + further checks
FLAGCAL: A flagging and
calibration package
Description of the FLAGCAL pipeline in Prasad & Chengalur (2011)
Snapshot imaging for event localisation
Currently FLAGCAL + AIPS; will soon be integrated into the main event analysis pipeline
“Dirty” imageSingle pulse from J1752-2806
“dirty” image
After cleaning and self-cal
Signal peak ~ 0.27 Jy
rms ~ 6 mJy; beam ~ 59” x 10”
Example from Event Analysis Pipeline
On
phas
ing
upD
etec
tion
Pha
se u
p th
e ar
ray
Summary and Concluding Remarks
Searching for fast transients with multi-element instruments involve several considerations and challenges; propagation effects, RFI, signal processing, etc.
The GMRT makes a powerful test bed for developing and demonstrating novel transient detection techniques and methodologies applicable for next-generation (LNSD type) instruments such as ASKAP
Transient detection pipeline for GMRT - development nearly complete; the commensal surveys to start by early 2012; the system will be extended to larger bandwidths
The VLBA and GMRT based efforts will help demonstrate the advantages of multiple stations and long baselines for transient exploration; valuable lessons for the SKA-era
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