Time Domain Search Techniques for Fast Radio Transients (With LOFAR) Jason Hessels (ASTRON/UvA) LOFAR and the Transient Radio Sky Amsterdam - Dec 15th, 2008 and the LOFAR Pulsar Working Group 

Transient Radio Sky The transient radio sky is a mostly unexplored domain, especially at high time resolution...  Difficult to get required sensitivity and large field of view.  Difficult to get large field of view and good spatial resolution.  Much higher data rates than with photon detectors (especially with short samples).  Propagation effects very important at short timescales and at low frequencies.  F.O.M. A*(Ω/ΔΩ)*(T/∆T) should be large. 

“Beam-formed”, i.e. timeseries data 

(Fender et al. 2008) “Fast” Radio Transients  Timescales of ns - seconds.  Internal source variability and singular bursts.  Probed only by non- imaging (timeseries) techniques.  Propagation effects in ISM (e.g. scattering and dispersion) very important.  RFI contamination. Imaging surveys Pulsar-like surveys Large FoV for rare events Large instantaneous sensitivity for weak source classes 

Fast Transients at Low Frequency Advantages Disadvantages  Steep spectral indices?  Only visible at low frequency?  Low-DM sources distinguishable from RFI.  Large field of view / dwell times (high F.O.M.).  Respond to high-frequency triggers (DM delay).  Scattering ∝ ν-4.4  Many dispersion trials (∝ ν-2).  Lower effective time resolution.  Ionosphere. νsky ~ 30 - 300 MHz Not pure evil... shows signal is astronomical. 

Galactic Distance Limit At ~120 MHz, one is likely limited to d < 2kpc for most ms bursts in galactic plane. BUT some sources may be visible at significantly larger distances because scattering is not too bad or the events are very bright. 

Known and expected source classes  Ultra-high-energy particles  The Sun (Type II and III bursts)  Flare Stars  AGN  Brown dwarfs (scaling from NSs?)  Planets (Jupiter, Saturn, Exoplanets)  ETI  Annihilating black holes, coalescing NSs  Supernovae  Neutron Stars (Pulsars), e.g. Rotating Radio Transients (RRATs: nullers,burpers,etc.), “sometimes a pulsars”, radio magnetars Cover over three orders of magnitude in time-scale in a large field survey: ~1-1000ms. Complementary to imaging transient searches. 

Single pulse searches can be more sensitive than periodicity searches for pulsars...  All pulsars show pulse-to-pulse intensity variations, but some pulsars show “giant” pulses.  McLaughlin & Cordes 2003: for certain pulse- amplitude distributions and time-series lengths, single pulse searches are more sensitive.  Good for finding very fast spinning pulsars and pulsars in short binary orbits?  Single-pulse search becomes exponentially better once the pulse width exceeds the pulse period (think scattering). 

Single Pulse Searches When are these better than periodicity searches? 700 Pulses J0054+66 P ~ 1.4s Hessels et al. 2007 Will detect ~1/3 of “normal” pulsars as well Peak ~100x rms (c.f. 0656) 

10,000+ DM trials required at 150MHz!!! 

Single Pulse Search Techniques Flow chart Raw Data (e.g. 195-kHz complex subbands) DM Trials (~10,000) Dedisperse Filterbank Data Channelize Excise RFI Match Filtering Bright Pulses Sift Astronomical Signals Large range of timescales Different pulse shapes Zero-DM’ing, DM-law Basic statistics (e.g. DM excess) Diagnostic plots Machine learning Trained monkeys Coherent Dedispersion? 

Single Pulse Search Techniques Diagnostic Plot (visual inspection): LOFAR data rates will require: Automation Machine Learning? Other criteria: scattering tail pulse shape DM-law polarization compare beams 

Matched filtering (pretty cheap c.w. DM trials) When scattering is important, it is optimal to use an asymmetric matched filter. More work, but useful for RFI rejection? with scattering... Match RFI and grade candidates (beyond S/N)? RFI Rejection 

RFI Rejection “Zero DM’ing” (Lyne, Keane et al.) Remove non-dedisperse RFI DM law mapping (Chennamangalam, Deshpande et al.) Identify swept frequency RFI 

PSR J0242+6256 • Identified in single pulse search: • DM ~ 4 pc cm-3 • D ~ 400pc! • L400 ~ 0.2 mJy kpc2 • Discovery depends on low observing frequency. • Type of source LOFAR will find. Hessels et al. 2007 

New very low DM source: DM = 1.8 pc cm-3, D ~ 200 pc? (LOFAR-type source) Seen only once in ~3 hours of observing Only ~ 1.5 ms wide 10 ms Follows expected DM sweep, but could be swept frequency RFI Needs confirmation! 

PSR B0329+54 With LOFAR Period = 714.5ms Bright single pulses detected with LOFAR HBA (160-240MHz) One of the brightest pulses 

Summary  The “transient radio sky” is a largely unexplored realm, with the potential to give unique insight into the physics of compact objects and their dynamical processes.  The LOFAR telescope is a major pathfinder to the “SKA” and will provide our best opportunity yet for all-sky monitoring.  LOFAR will be an excellent telescope for discovering fast radio transients and pulsars, of which hundreds could be discovered in an all-sky survey. 

Hiring!  Science Postdoc (ASTRON): Pulsars and transients with LOFAR/WSRT. Interested? Info: [email protected]