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
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2. FRATS
   Real-time detection of Fast Radio Transients
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   •  Science:
   •  Millisecond pulses
   •  Searching for Unknown Objects
   •  Searching for Unknown classes/ processes
   •  Statistics on known sources
   •  Fishing expedition
   •  Fast: beamforming mode
   

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3. Objects emitting millisecond pulses
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   Pulsars/ RRATs
   (exo) planets
   Humans
   Flare stars
   

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4. Catch them all!
   Fish in the big pool
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   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
   

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5. Disperion
   “refractive index of space”
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   •  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
   

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6. What is it?
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7. What is it?
   Is it celestial?
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   Lorimer Burst
   Lorimer et al. 2007
   Is it a Peryton?
   Burke-Spoloar et al. 2010
   Need an array for confirmation!
   Two more found!
   

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8. Trigger on source
   Real-time detection.
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   Dedispersion in multiple frequency bands
   Coincidence requirement between the bands
   

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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.
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10. Use TBB data to identify the source
    Real-time detection.
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    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
    

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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
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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
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13. DM step and time window
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    •  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
    

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14. Coincidence between bands
    single triggers
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15. Coincidence between bands
    dual channel triggers
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16. Coincidence between bands
    triple channel triggers
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17. RFI mitigation: Raw data
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18. RFI mitigation: Divide by spectrum
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19. RFI mitigation: Flagging method
    •  Gaussian data:
    •  Flag channels that are outliers
    •  Narrow time flagging:
    -  Dedisperse DM=0
    -  Coincidence of 2 bands
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    n
    ⌅
    i=1
    a2
    i
    ⇥
    n
    ⌅
    i=1
    ai
    ⇤2
    = C
    

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20. RFI mitigation: Flag RFI
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21. Different averages per block
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    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
    
    

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22. Artifacts in dedispersed data
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    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
    
    

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23. RFI mitigation: Subtract average timeseries (zeroDM-ing)
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24. Sometimes there is just too much RFI
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25. Manual trigger verification
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    BAD
    

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26. Manual trigger verification
    Real pulse from B1919+21
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    Good
    

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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
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28. Log N / Log S
    1 hour of data 4x11 minute
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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
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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
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32. Scattering
    Low DM, low scattering
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33. Trigger parameters
    LBA bright small-band pulses observed or Scintillation effects?
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    Subband group
    38 MHz
    54 MHz
    Sample number
    Detected pulses in each band
    

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