Searching for Transients with LOFAR

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

Transcript

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.