Searching for Transients with LOFAR

Searching for Transients with LOFAR

Casey Law
LOFAR and the Transient Radio Sky, Amsterdam, December 2008

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transientskp

June 18, 2012
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  1. Searching for Transients with LOFAR Casey Law (Amsterdam)

  2. How to survey for radio transients? Traditional F.O.M.: Aeff *

    Ω/ΔΩ * T/Δt Imaging: GBT (Gregory & Taylor 1986) FIRST/NVSS (Levinson et al. 2002) VLA cal field (Bower et al. 2007) ✔ Good localization ✔ Large field of view ✖ Poor time resolution ✖ High data volumes Non-imaging: STARE (Katz et al. 2003) Nasu (Matsumura et al. 2007) Parkes MB (Lyne et al. 2000) ✔ High time resolution ✖ Poor localization ✖ RFI more problematic
  3. Imaging with LOFAR offers best of both worlds... ✔ Large

    field of view ✔ High time resolution F.O.M Large! ✔ Good localization ...But offers new challenges: ✖ Computational cost ~ nant 2 * npol * nchan ✖ Money cost ~ computational cost + development time + ... ✖ New analysis techniques
  4. High-cadence imaging with LOFAR: harder than is sounds Integration time

    ~ 1 second Dispersion delay (DM=60, 120-124 MHz) ~ 1 second Properties skewed!
  5. High-cadence imaging with LOFAR: harder than is sounds Integration time

    ~ 1 second Dispersion delay (DM=60, 120-124 MHz) ~ 1 second Properties skewed! Time-domain techniques!
  6. High-cadence imaging with LOFAR: new science - Extragalactic transients -

    Electron distribution of the Galaxy - Unexplored time scales
  7. And expand on the known... Class Timescale Rate GRB afterglows

    minutes - months ~100/yr Radio supernovae days - months ~5/yr Flare stars msec - hours 100-1000 Exoplanets min - hours 10-100
  8. How will this be done? The Radio Sky Monitor • 

    Imaging survey at 30, 120 MHz •  1 sec sensitivity: 500, 50 mJy •  Covers 1/4 of sky daily The RSM in action Sky coverage of the RSM
  9. What will this produce? •  Image ~300 deg2/s •  Studies

    sky on range of timescales •  Lightcurves of all sources •  Public access to database
  10. You know LOFAR... What is CS1? Possible layout of LOFAR

    stations Low-band antenna in CS1 CS1 intended for commissioning, but can also do science!
  11. The Search Data: •  LBA, near 45 MHz •  Bandwidth

    = 4 MHz •  24 hours Method: •  Image for times of   4, 8 min, ..., 4, 8 hrs •  Area of ~1200 deg2 •  Run source detection •  Search for good candidates Image of North Celestial Pole at 45 MHz. All 3C sources visible, some SNRs, M82. Search area shown in green.
  12. Example lightcurves:   3C 390.3:   1 hour integrations  

    30 min integrations
  13. Results For 10σ threshold, no transients. Nondetection limits transient rate,

    f. For Poisson statistics, P(Ndet=0) = e-fN For 95% confidence, P(Ndet=0) = 0.05. Constraint on transients in LOFAR CS1 survey.
  14. Comparison to Previous Results STARE (Katz et al. 2003) MOTER

    (Amy et al. 1989 Green Bank 300ft (Gregory & Taylor 1986) FIRST/NVSS (Levinson et al. 2002) STARE Green Bank MOTER FIRST/NVSS
  15. Summary 1. High-cadence imaging will be transformative. 2. LOFAR/CS1 is able to

    do science today! 3. A 24 hour study has placed weak constraints on low- frequency radio transients.