V-FASTR: Searching for fast transients with the VLBA

Adam Deller
LOFAR TKP Meeting, Amsterdam, June 2011

Transcript

1. Searching for fast radio transients with the VLBA Adam Deller, 28 June 2011
2. The V-FASTR experiment  Pathfinder for the CRAFT high time resolution survey on ASKAP  Commensal observation on the Very Long Baseline Array, all projects  Membership is a CRAFT subset  Curtin: Steven Tingay (PI), Randall Wayth  NRAO: Walter Brisken  ASTRON: Adam Deller  NASA JPL: David Thompson, Kiri Wagstaff, Walid Majid
3. The Very Long Baseline Array • 10 x 25m dishes • 8,000 km baselines • Frequency range covering 0.3 - 86 GHz • Synthesized beam ~mas; coherent search not possible!
4. The V-FASTR experiment Existing (V-FASTR adds few % overhead) 1 3U server; ~$6k (2x6 core, big disks)
5. DiFX/VLBA-specific challenges  Incoherent filterbank spectra (< 1 MHz freq. resolution) comes out as UDP packets from multiple processing nodes @ ms time resolution:  Some packets get lost  Data must be sorted into time order  During re-ordering, other unwanted signals such as strong RFI and noise calibration must also be excised
6. Data processing frequency time raw filterbank data bandpass, tcal corrected data Also: boxcar averaging to match longer pulse widths
7. V-FASTR properties  RFI robustness (different sites); detection thresholds can be lowered (just like ARTEMIS on LOFAR)  Ability to accurately localize transients:  Detection implies sufficient S/N to fringe fit  Even S/N ~ 8 gives delay precision <10 ns  With 8,000 km baselines, that gives sky localization to sub-arcsecond precision  Imaging ability (scattering, short term motion / near field)
8. V-FASTR properties  Baseband data still readily available (like hours of “transient buffer board”); can be automatically captured and reprocessed at higher time resolution  Competitive sensitivity to other transient searches:  Within primary beam 9 x VLBA antennas = 43 m equivalent for the incoherent search, but 75 m equivalent when coherently combined in follow-up analysis  Out of primary beam considerably more sensitive than e.g. Parkes multibeam
9. Initial V-FASTR results  Single pulse detections of B0329+54 and the Crab
10. Initial V-FASTR results  Single pulse detections of B0329+54 and the Crab The single detected Crab giant pulse
11. Initial V-FASTR results  Automated dump of baseband data and reprocessing at higher time resolution + generating cross-correlations for imaging Raw visibilities (no dedispersion)
12. Initial V-FASTR results  Automated dump of baseband data and reprocessing at higher time resolution + generating cross-correlations for imaging Post-correlation dedispersed (ready to image)
13. Initial V-FASTR results  Imaging of B0329+54 from a single pulse, no external calibration (position correct to several arcsecond accuracy) Cleaned image from ~5ms data
14. Processing challenges / progress  VLBA is not designed for high quality single dish work - gain variations, local RFI rarely interfere with VLBI but are obvious to V-FASTR  Flagging known RFI a priori is useful  (Somewhat) surprising results so far:  “Robust” estimator, where several of the highest and/or lowest value antennas are discarded, strongly outperforms simple sum  High pass filter (tau ~ 0.1 second) eliminates slow gain variations at cost of reduced sensitivity to longer transients
15. Processing challenges / progress
16. Processing challenges / progress
17. Processing challenges / progress
18. Current status  Two papers in press:  Experiment description, sensitivity estimates  Algorithm evaluation  Multi-purpose proposal approved for repeated 330 MHz observations of M81/M82 field:  Deep P-band continuum + slow transients (image) + potential fast transients  Long time series of stable setup  Now running full-time on VLBA
19. Future work  Improved follow-up analysis  Automatic higher time resolution recorrelation of the dumped baseband data  Trial coherent dedispersion (pre-correlation) for candidates which are unresolved in time  User-friendly visualisation of the results (waterfall plots, frequency averaged dedispersed data, DM sensitivity plots)  Automated dissemination of candidate results (daily email digest, statistics) + automated update to V-FASTR webpage  Detection/limits paper (~3 months obs)