Recent Developments on FRATS

Sander ter Veen

Transcript

1. Recent developments on FRATS Sander ter Veen Heino Falcke, Emilio Enriquez, Anya Bilous, Jörg Rachen, Pim Schellart In collaboration with Cosmic Ray KSP and Pulsar Working Group 1
2. FRATS Real-time detection of Fast Radio Transients 2 •  Science: •  Millisecond pulses •  Searching for Unknown Objects •  Searching for Unknown classes/ processes •  Statistics on known sources •  Fishing expedition •  Fast: beamforming mode
3. Objects emitting millisecond pulses 3 Pulsars/ RRATs (exo) planets Humans Flare stars
4. Catch them all! Fish in the big pool 4 Incoherent summation of the station beam Keeps large FoV Parallel observing to other observations FoV (sq. degr) LBA INNER LBA OUTER 30 MHz 419 65.8 60 MHz 105 16.5 HBA CORE HBA 150 MHz 18.4 10.3
5. Disperion “refractive index of space” 5 •  Dispersion effect: Signal at lower frequencies arrive later. •  Data is split in small channels (~ kHz ) •  Channels are summed according to dispersion law •  Many (>100) DM trials Lorimer et al. 2007 Trigger moment Stop delay
6. What is it? 6
7. What is it? Is it celestial? 7 Lorimer Burst Lorimer et al. 2007 Is it a Peryton? Burke-Spoloar et al. 2010 Need an array for confirmation! Two more found!
8. Trigger on source Real-time detection. 8 Dedispersion in multiple frequency bands Coincidence requirement between the bands
9. Transient Buffer Board (TBBs) RAM Ring buffer for each LBA/HBA element Data stored, stopped, dumped to disk. Reproduce any LOFAR signal; Also your data! Limited memory => limited time Beamsize: LBA: All-sky (30000 sq. degr) HBA: 500 sq. degr. 9
10. Use TBB data to identify the source Real-time detection. 10 Dedispersion in multiple frequency bands Coincidence requirement between the bands Image from TBB data Use to identify source +Multiwavelength Follow-up 4 stations See Presentation by Emilio at 17:05
11. FRATS + MSSS LBA •  MSSS LBA •  All-sky observations •  3 beams of 8x2 MHz •  Test observations have run •  100 hours remaining •  1 locus node per beam •  150 DM trials 11
12. Search parameters: Maximum DM •  After trigger, same bandwidth should be available in TBB data for easy checking •  Dispersion delay •  ΔtDM =1.3 s (buffer length) •  Max DM depends on frequency 12
13. DM step and time window 13 •  Error in DM trial vs DM pulse •  Decreases efficiency •  Compared samples summed for for one DM trial compared to the next •  150-200 DM trials in real-time •  ΔDM=0.1 reasonable •  Lower frequencies problematic, skip? •  Time window 200 ms
14. Coincidence between bands single triggers 14
15. Coincidence between bands dual channel triggers 15
16. Coincidence between bands triple channel triggers 16
17. RFI mitigation: Raw data 17
18. RFI mitigation: Divide by spectrum 18
19. RFI mitigation: Flagging method •  Gaussian data: •  Flag channels that are outliers •  Narrow time flagging: -  Dedisperse DM=0 -  Coincidence of 2 bands 19 n ⌅ i=1 a2 i ⇥ n ⌅ i=1 ai ⇤2 = C
20. RFI mitigation: Flag RFI 20
21. Different averages per block 21 Top to bottom: 73-75 MHz 65-67 MHz 60-62 MHz 54-56 MHz 48-50 MHz 42-44 MHz 36-38 MHz 30-32 MHz Arbitrary offset between channels
22. Artifacts in dedispersed data 22 Top to bottom: 73-75 MHz 65-67 MHz 60-62 MHz 54-56 MHz 48-50 MHz 42-44 MHz 36-38 MHz 30-32 MHz Arbitrary offset between channels
23. RFI mitigation: Subtract average timeseries (zeroDM-ing) 23
24. Sometimes there is just too much RFI 24
25. Manual trigger verification 25 BAD
26. Manual trigger verification Real pulse from B1919+21 26 Good
27. Trigger settings MSSS LBA 8x 2 MHz bands at: 30-32 MHz 36-38 MHz 42-44 MHz 48-50 MHz 54-56 MHz 60-62 MHz 65-67 MHz 73-75 MHz DM range 0-15 pc cm-3 DM step: 0.01 pc cm-3 Coincidence time window: 200 ms Coincidences required: 3 bands 27
28. Log N / Log S 1 hour of data 4x11 minute 28
29. Status and outlook •  Trigger algorithm now runs 150 DM trials real-time •  Trigger parameters defined •  First test observations have been run (3x 4 hours) -  Handled triggers were all RFI events, apart from a trigger on B1919+21 •  To do: -  Run during remaining MSSS LBA observation (100 h) -  Run during MSSS HBA observations (full sky) -  Run during other observations? -  Make FRATS an easy add-on in LOFAR online system -  Further tuning of trigger algorithm -  Only use core-stations for incoherent beam •  TBB localisation (see talk by Emilio Enriquez) •  Trigger from ARTEMIS and other sources 29
30. Summary FRATS searches for dispersed millisecond pulses from various objects with a wide FoV parallel to other observations. Transient Buffer Boards see the whole sky for 1.3 seconds and thereby are a powerful tool to identify short duration signals First commissioning observations have been run 30
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32. Scattering Low DM, low scattering 32
33. Trigger parameters LBA bright small-band pulses observed or Scintillation effects? 33 Subband group 38 MHz 54 MHz Sample number Detected pulses in each band