Searching for Transients with LOFAR Casey Law (Amsterdam) 

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 

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 

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

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! 

High-cadence imaging with LOFAR: new science - Extragalactic transients - Electron distribution of the Galaxy - Unexplored time scales 

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 

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 

What will this produce? •  Image ~300 deg2/s •  Studies sky on range of timescales •  Lightcurves of all sources •  Public access to database 

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! 

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. 

Example lightcurves:   3C 390.3:   1 hour integrations   30 min integrations 

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. 

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 

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.