LOFAR Image Plane Transient Candidate #3

Adam Stewart

Transcript

1. Image Plane
   Transient Candidate #3
   Found in the NCP Field by the TraP
   Adam Stewart, Tom Hassall, Rob Fender,
   Jess Broderick, Gosia Pietka
   LOFAR TKP Meeting - Amsterdam - 8-10 January 2014
   

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2. Brief Reminder of NCP Data
   • Recorded simultaneously, using a
   single sub band, with the initial
   MSSS-LBA observing run in
   2011-2012
   • Each observation:
   • Is 11 minutes long
   • At 60 MHz
   • 200 kHz of bandwidth
   • When in sequence
   observations are 4 minutes
   apart.
   • 175 deg2 searchable area.
   • MSSS calibrator is used to process
   the data. msss.astron.nl
   

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3. NCP Transient Simulations
   

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4. Transient Simulations
   • Wanted to test how the NCP field would react to a
   transient.
   • As we know, LOFAR reduction is quite dependent on
   the sky model used.
   • Nevertheless I wanted to confirm that a bright
   transient, not in the model, would appear or at least
   leave hints.
   • Eg. removing 3C 61.1 from the sky model, while
   causing bad artifacts, still leaves the source visible.
   

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5. Transient Simulations
   • Wanted to test how the NCP field would react to a
   transient.
   • As we know, LOFAR reduction is quite dependent on
   the sky model used.
   • Nevertheless I wanted to confirm that a bright
   transient, not in the model, would appear or at least
   leave hints.
   • Eg. removing 3C 61.1 from the sky model, while
   causing bad artifacts, still leaves the source visible.
   

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6. Transient Simulations
   • Wanted to test how the NCP field would react to a
   transient.
   • As we know, LOFAR reduction is quite dependent on
   the sky model used.
   • Nevertheless I wanted to confirm that a bright
   transient, not in the model, would appear or at least
   leave hints.
   • Eg. removing 3C 61.1 from the sky model, while
   causing bad artifacts, still leaves the source visible.
   3C 61.1 removed from model
   

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7. Transient Simulations
   • Wanted to test how the NCP field would react to a
   transient.
   • As we know, LOFAR reduction is quite dependent on
   the sky model used.
   • Nevertheless I wanted to confirm that a bright
   transient, not in the model, would appear or at least
   leave hints.
   • Eg. removing 3C 61.1 from the sky model, while
   causing bad artifacts, still leaves the source visible.
   3C 61.1 removed from model
   

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8. Inserting a Transient
   • Aim was to test whether a transient would be seen with the
   reduction method.
   • Any brightness of transient.
   • Transient inserted into pre-processed data before being
   reduced through MSSS pipeline.
   • Using calibrator gain solutions to insert - ideally want field
   phase solutions but proved difficult to merge and use table.
   Calibrate
   calibrator
   Transfer
   calibrator gain
   solutions to
   NCP
   Phase-only
   calibration using
   VLSS based
   model
   Image using
   AWimager
   Assuming transient survives
   demixing, flagging etc
   

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9. Inserting a Transient
   • Aim was to test whether a transient would be seen with the
   reduction method.
   • Any brightness of transient.
   • Transient inserted into pre-processed data before being
   reduced through MSSS pipeline.
   • Using calibrator gain solutions to insert - ideally want field
   phase solutions but proved difficult to merge and use table.
   Calibrate
   calibrator
   Transfer
   calibrator gain
   solutions to
   NCP
   Phase-only
   calibration using
   VLSS based
   model
   Image using
   AWimager
   Pre-processed
   Data
   Solutions
   +
   Transient
   BBS
   Assuming transient survives
   demixing, flagging etc
   

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10. Inserting a Transient
    15 Jy 30 Jy
    It’s clear that at least something odd is in the dataset, and becomes
    clearer when brighter.
    

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11. Inserting a Transient
    15 Jy 30 Jy
    It’s clear that at least something odd is in the dataset, and becomes
    clearer when brighter.
    

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12. Works Very Nicely in RSM
    10 Jy transient source
    

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13. Works Very Nicely in RSM
    10 Jy transient source
    

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14. Inserting a Very Bright Transient
    150 Jy
    •Flux starts to
    be absorbed
    by 3C 61
    •Now ~170 Jy
    •Image is poor
    with other
    sources
    missing.
    

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15. Ghosts... • A mirrored
    source appears
    when the
    transient was
    bright 50 - 80 Jy
    • Opposite 3C 61.1
    - the brightest
    source in the
    field.
    • Ghost is brighter,
    roughly 60 - 20 Jy
    (in 80 Jy case)
    • Reminded me of
    something
    

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16. Ghosts... • A mirrored
    source appears
    when the
    transient was
    bright 50 - 80 Jy
    • Opposite 3C 61.1
    - the brightest
    source in the
    field.
    • Ghost is brighter,
    roughly 60 - 20 Jy
    (in 80 Jy case)
    • Reminded me of
    something
    

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17. View Slide

18. Placing Transient in the Model
    At Ghost Location At True Location
    Ghost disappears when the true source is in the sky model
    

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19. Placing Transient in the Model
    At Ghost Location At True Location
    Ghost disappears when the true source is in the sky model
    

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20. Ghosts Not Limited to One
    Location
    • At first we were
    worried that there
    was a special distance
    such as Bell #1 that
    would scale with
    frequency.
    • Sampling the whole
    field reveals ghosts can
    be created in various
    locations.
    • In the NCP cases
    tested it seems to be
    concentrated to the
    right-hand side of 3C
    61.1
    

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21. Ghosts Not Limited to One
    Location
    • At first we were
    worried that there
    was a special distance
    such as Bell #1 that
    would scale with
    frequency.
    • Sampling the whole
    field reveals ghosts can
    be created in various
    locations.
    • In the NCP cases
    tested it seems to be
    concentrated to the
    right-hand side of 3C
    61.1
    

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22. Ghosts Not Limited to One
    Location
    • At first we were
    worried that there
    was a special distance
    such as Bell #1 that
    would scale with
    frequency.
    • Sampling the whole
    field reveals ghosts can
    be created in various
    locations.
    • In the NCP cases
    tested it seems to be
    concentrated to the
    right-hand side of 3C
    61.1
    

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23. The Transient
    

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24. • An object only seen once and never again.
    • Snapshot taken on December 24th 2011 at
    04:33.
    TraP Discovered a Similar Event
    

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25. TraP Discovered a Similar Event
    

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26. TraP Discovered a Similar Event
    

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27. TraP Discovered a Similar Event
    

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28. TraP Discovered a Similar Event
    

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29. TraP Discovered a Similar Event
    TraP
    extracted
    this source
    

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30. TraP Discovered a Similar Event
    7 Jy
    Source?
    Ghost?
    13 Jy
    

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31. Can the Ghost be ?
    • With what was seen in the simulations the next step was to
    process the data again with suspected sources in the sky
    model.
    • Try a 20 Jy (13+7) point source at each location one at a time.
    7 Jy
    Source
    Ghost?
    13 Jy
    

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32. Transient in the Model
    20 Jy at ghost Location
    Both sources still visible
    20 Jy at Right/Source
    Location
    Ghost Source
    disappears!
    We came, we saw...
    

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33. Transient in Other Surrounding
    Snapshots Model
    • Process the surrounding 4 observations with the
    transient in the sky model.
    • The source appears strongly only in the original
    snapshot where it was discovered.
    

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34. Attempts to Kill the Transient
    • Tried numerous methods to remove or at least
    greatly effect the transient.
    Subtract 3C 61.1
    Different weighting scheme (here natural)
    • Also tried imaging using different
    baseline selections.
    • Different time compression
    before processing.
    • Checked other observations at
    the same LST - no hint of source.
    • It survived all these tests where
    somewhat similar candidates
    failed.
    • Though this is the only candidate
    which has a clear ghost source.
    

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35. There are similar objects found...
    • 8 more to be precise at similar fluxes of the initial source found 4 - 8 Jy
    1 Jan 2012 2 Jan 2012 21 Jan 2012
    11 Feb 2012
    11 Feb 2012 10 Mar 2012 31 Mar 2012
    

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36. There are similar objects found...
    • 8 more to be precise at similar fluxes of the initial source found 4 - 8 Jy
    1 Jan 2012 2 Jan 2012 21 Jan 2012
    11 Feb 2012
    11 Feb 2012 10 Mar 2012 31 Mar 2012
    Need Testing!
    NCP Deep Image can also help
    

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37. There are similar objects found...
    • 8 more to be precise at similar fluxes of the initial source found 4 - 8 Jy
    1 Jan 2012 2 Jan 2012 21 Jan 2012
    11 Feb 2012
    11 Feb 2012 10 Mar 2012 31 Mar 2012
    Need Testing!
    NCP Deep Image can also help
    

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38. If real - what do we know?
    • Duration of 11 mins.
    - When dataset split in half or
    thirds, the source is still
    present in each half/third.
    - Flux also relatively constant.
    - Processing effect? To test... Transient set split in half (in model)
    • Also not present in snapshots before or after. Combing next two also nothing.
    • Bright at ~ 25 +/- 5 Jy
    • Would suggest a rate of 1 / 2537 day-1 deg-2 with ∆t = 11 mins, 4 Jy limit (10σ
    TraP selection)
    • Not in EoR deep NCP map.
    • Localisation to ~120” (10 km baselines)
    - Will take a bit of care to incorporate longer baselines
    

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39. Optical Follow Up
    • r-band, slightly deeper than 21 magnitude.
    • With current localisation many sources are seen in the error box.
    • None show obvious variability on the timescales of minutes and one month.
    • Ongoing.
    Teo Muñoz-Darias
    

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40. What can we infer? (Reminder First)
    • Prospects of finding FRBs in image plane data detailed by
    Hassall, Keane & Fender 2013:
    How scattering effects the
    pulse
    Image plane can outperform beamformed in high scattering
    scenarios
    Low scattering the Beamformed > image plane
    FRBs/hour
    LBA
    

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41. • With 200 kHz of bandwidth - 11 minutes is due to
    scattering or intrinsic pulse width.
    - Because a DM of 6000 (very high! z~6) would mean a
    dispersive delay of 45s between 60 - 60.2 MHz.
    - Hence much more likely to be scattering or pulse width.
    What can we infer?
    T.Hassall
    

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42. • With 200 kHz of bandwidth - 11 minutes is due to
    scattering or intrinsic pulse width.
    - Because a DM of 6000 (very high! z~6) would mean a
    dispersive delay of 45s between 60 - 60.2 MHz.
    - Hence much more likely to be scattering or pulse width.
    • So lets take the upper bound flux of the first split 5 min
    image, 30 Jy, and the lower flux of the second 5 minute
    image, 20 Jy, and calculate the minimum DM.
    What can we infer?
    T.Hassall
    

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43. • With 200 kHz of bandwidth - 11 minutes is due to
    scattering or intrinsic pulse width.
    - Because a DM of 6000 (very high! z~6) would mean a
    dispersive delay of 45s between 60 - 60.2 MHz.
    - Hence much more likely to be scattering or pulse width.
    • So lets take the upper bound flux of the first split 5 min
    image, 30 Jy, and the lower flux of the second 5 minute
    image, 20 Jy, and calculate the minimum DM.
    • Using S = e(-t/tscatt) and the Bhat et al law:
    - t_scatt ~ 1600 hence a DM ~ 450
    - Reasonable value if considering DMs of FRBs, which
    could last minutes at low frequencies.
    What can we infer?
    T.Hassall
    

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44. • Can attempt to say something about spectral
    index:
    - Take FRBs at 1.4 GHz, the NCP will be brighter
    by (1.4 / 0.06)α
    - FRB 10 Jy x 5 ms = 0.01 Jy/s
    - NCP 25 Jy x 660s = 13200 Jy/s - a 106 ratio
    - But could have a spectral broadening term, β
    - (1.4 / 0.06)α+β ~ 10-6
    - Hence, α+β = -4 which is steep, but we should
    keep β in mind.
    What can we infer?
    T.Hassall
    

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45. Conclusions
    • Altering the model makes a
    definite impact to the data,
    removing the friend.
    • I cannot just create a source by
    inserting it into the sky model.
    • Doesn’t appear in other
    snapshots even when
    processing with it in model.
    • Follows transient simulation
    results.
    • Survives various tests thrown at
    it, when I have seen others not.
    For Against
    • Only seen in one snapshot.
    • If exponential decay then should be
    around ~10 Jy in next snapshot,
    cannot see anything.
    • No variation when splitting the
    dataset to image.
    • General uncertainty surrounding
    this LBA MSSS data - the data as a
    whole was never fully used for
    quality issues.
    • Do we understand artefacts in
    these short, limited UV coverage
    snapshots?
    A 25 Jy Transient at 60 MHz, ‘on’ for 11
    minutes or less - Is it real?
    

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46. Conclusions
    • Lack of other data (more bw) is a blessing and a curse.
    • Perhaps will never be 100% sure because of this, but we
    have to take what we see.
    • Bell #1 situation of not being sure
    - Sky model tests on this data do not effect the transient.
    • Other candidates to check, may help in ways to prove/
    disprove.
    • But this transient is tough to kill, it is yet to ever disappear.
    • RSM should be able to provide some answers...
    

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47. Finally thanks to...
    • The TraP team of John, Antonia, Gijs, Bart and Tim et al
    • Previously data had been combined to gather decent
    images for humans to work with.
    • The TraP in it’s current state made searching these
    images possible.
    

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48. DM Calculation
    • The flux of the pulse as a function of time is:
    • S = exp(-t/t_scatt)
    • so 30/20 = exp(-660/t_scatt)
    • t_scatt = -660/ln(20/30) ~ 1600
    • This corresponds to a DM~450 from the Bhat
    et al. law. Current DMs of FRBs are in the
    range 550-1100, so this seems very
    reasonable.
    

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49. PSF Example
    

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50. UV Coverage Example
    For transient observation on 24th Dec
    

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