How to build a TraP: An image-plane transient-discovery tool
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There are three main points summarized: 1. There are many interesting slow radio transients that could be detected through imaging surveys like accretion flares, orphan gamma-ray bursts, and flare stars. 2. Radio surveys are increasing in sensitivity and field of view by orders of magnitude with instruments like LOFAR, enabling the detection of more rare transient events. 3. TraP is a transient detection pipeline that works by extracting sources from radio images, matching to known sources, identifying new bright sources, analyzing light curves, and making the results accessible through a user-friendly web interface.
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How to build a TraP: An image-plane transient-discovery tool
- 1. How to build a TraP An image-plane transient-finding pipeline Tim Staley & TraP contributors. Lunchtime talk, Southampton, Jan 2015 WWW: 4pisky.org , timstaley.co.uk/talks
- 2. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Outline ’Slow’ radio transients How TraP works How do I use it? Future work Summary
- 3. ’Slow’ radio transients How TraP works How do I use it? Future work Summary What are we missing? Image surveys are best for finding ‘slow’ transients i.e. > 1 second timescale — excludes regular pulsars, etc. Such as. . .
- 4. ’Slow’ radio transients How TraP works How do I use it? Future work Summary What are we missing? Accretion flares Artist’s impression of the microquasar GRO J1655-40. Image credit: NASA/STScI
- 5. ’Slow’ radio transients How TraP works How do I use it? Future work Summary What are we missing? ‘Orphan’ gamma-ray burst afterglows Image credit: NASA’s Goddard Space Flight Center e.g. Ghirlanda 2014, http://adsabs.harvard.edu/abs/2014PASA...31...22G
- 6. ’Slow’ radio transients How TraP works How do I use it? Future work Summary What are we missing? Flare-star events Image credit: Casey Reed/NASA e.g. Osten 2010, http://ukads.nottingham.ac.uk/abs/2010ApJ...721..785O
- 7. ’Slow’ radio transients How TraP works How do I use it? Future work Summary What are we missing? Image surveys are best for finding ’slow’, i.e. > 1 second timescale transients (excludes regular pulsars, etc). AGN tidal disruption events Compact-object binary flares Orphan gamma-ray bursts Flare stars Nulling and eclipsing pulsars (e.g. J. Broderick et al, in press) (The unknown?)
- 8. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Proof of concept: ALARRM AMI-LA Rapid Response Mode Staley 2013, http://ukads.nottingham.ac.uk/abs/2013MNRAS.428.3114S
- 9. ’Slow’ radio transients How TraP works How do I use it? Future work Summary GRB140327A Anderson 2014, http://adsabs.harvard.edu/abs/2014MNRAS.440.2059A van der Horst 2014, http://adsabs.harvard.edu/abs/2014MNRAS.444.3151V
- 10. ’Slow’ radio transients How TraP works How do I use it? Future work Summary DG CVn M-dwarf superflare Fender 2014, http://adsabs.harvard.edu/abs/2014arXiv1410.1545F Osten et al (in prep)
- 11. ’Slow’ radio transients How TraP works How do I use it? Future work Summary So: Radio transients are out there. How do we find them directly?
- 12. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Bigger fields of view (Green = AMI-LA FoV)
- 13. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Bigger fields of view
- 14. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Bigger fields of view LOFAR-RSM footprint
- 15. ’Slow’ radio transients How TraP works How do I use it? Future work Summary The LOFAR ‘Radio Sky Monitor’ Eight 7-beam tiles in LBAs tiles out entire zenith strip (∼ 1800deg2 / ∼ 1 4 hemisphere) Sixteen 7-beam tiles in HBAs for a narrower strip (∼ 1000deg2 )
- 16. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Mini-summary There are interesting radio-transients waiting. Radio sensitivity / field of view is increasing by orders of magnitude.
- 17. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Mini-summary There are interesting radio-transients waiting. Radio sensitivity / field of view is increasing by orders of magnitude. =⇒ Many uninteresting pixels, and a few exciting rare events. =⇒ We need tools to search this data.
- 18. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Outline ’Slow’ radio transients How TraP works How do I use it? Future work Summary
- 19. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 1: PySE Python Source Extractor - Loosely based around S-Extractor algorithms, but tuned for radio data. Written solely in Python (extensive use of Numpy).
- 20. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Sourcefinding algorithms An illustration of the island deblending method pioneered by S-Extractor (Bertin et al 1996).
- 21. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 2: Load ‘extractedsources’ into SQL database NB: Store extractions without cross-matching initially.
- 22. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Brief aside on SQL
- 23. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Brief aside on SQL SQL is almost always the most efficient tool for searching large, well-parsed datasets. Are astronomers missing out?
- 24. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 3: Cross-match with known sources a.k.a. ‘association’ First calculate DeRuiter radius for candidate associations: i j αij ij α,i ,i rj = (Δαj)2 σ2 α, + σ2 α,j + (Δδj)2 σ2 δ, + σ2 δ,j
- 25. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 3: Cross-match with known sources a.k.a. ‘association’ First calculate DeRuiter radius for candidate associations: i j αij ij α,i ,i rj = (Δαj)2 σ2 α, + σ2 α,j + (Δδj)2 σ2 δ, + σ2 δ,j Handling meridian-wrap, celestial poles, left as exercise to reader.
- 26. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 3: Cross-match with known sources a.k.a. ‘association’ Mostly we just pick the closest match, and everything works out fine. But we also try to deal with some variable-PSF issues:
- 27. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 4: Identify bright new transients The problem: One of the neat features of TraP is that we can tell immediately when a bright new source appears. This may sound trivial initially, but...
- 28. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 4: Identify bright new transients The problem: One of the neat features of TraP is that we can tell immediately when a bright new source appears. This may sound trivial initially, but...
- 29. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 4: Identify new sources Tracking fields of view
- 30. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 4: Identify new sources Tracking detection limits 0 1 2 3 4 5 6 Epoch 3 4 5 6 7 8 Flux
- 31. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 4: Identify new sources Tracking detection limits 0 1 2 3 4 5 6 Epoch 3 4 5 6 7 8 Flux
- 32. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 4: Identify new sources Tracking detection limits 0 1 2 3 4 5 Epoch 3 4 5 6 7 8 Flux
- 33. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 4: Identify new sources Tracking detection limits 0 1 2 3 4 5 Epoch 3 4 5 6 7 8 Flux
- 34. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 4: Identify new sources Tracking detection limits 0 1 2 3 4 5 Epoch 3 4 5 6 7 8 Flux
- 35. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 4: Identify new sources Tracking detection limits 0 1 2 3 4 5 Epoch 3 4 5 6 7 8 Flux
- 36. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 5: Force measurement of any missing known sources 0 1 2 3 4 5 6 Epoch 3 4 5 6 7 8 Flux
- 37. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 5: Force measurement of any missing known sources 0 1 2 3 4 5 6 Epoch 3 4 5 6 7 8 Flux
- 38. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 6: Analyse lightcurves We keep a running aggregate (i.e. only need to include additional data, no recalculation of previous timesteps) for: Regular and weighted mean fluxes, μ & ξ ξN+1 = WN ξN + N+1 N+1 WN + N+1 (1) (2)
- 39. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Step 6: Analyse lightcurves We keep a running aggregate (i.e. only need to include additional data, no recalculation of previous timesteps) for: Regular and weighted mean fluxes, μ & ξ ξN+1 = WN ξN + N+1 N+1 WN + N+1 (1) (2) ‘Coefficient of variation’, V = σ/μ Calculate reduced χ-squared value (η), against fit to straight line at level of weighted-mean ξ.
- 40. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Outline ’Slow’ radio transients How TraP works How do I use it? Future work Summary
- 41. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Installation TraP can be run on a laptop. But . . . Makes heavy use of SQL database (most astronomers not familiar) Expected to be run on large datasets
- 42. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Installation TraP can be run on a laptop. But . . . Makes heavy use of SQL database (most astronomers not familiar) Expected to be run on large datasets Solution: Web-interface. Displays data in user-friendly fashion, works extremely well in server-client model.
- 43. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Demo =⇒ Demo.
- 44. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Development Facts ∼20,000 lines of code, ∼350 unit tests, 26K lines of docs 4 core developers, plus ∼4 testers, 3 continents Remote, collaborative, development model Issue tracking Open (going forward)
- 45. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Development Implications ( / Ruminations) Astronomy –> More software intensive Getting anything done requires better code re-use Core software efforts are larger, require ongoing effort from many contributors. (cf http://astropy.org!)
- 46. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Development Implications ( / Ruminations) Astronomy –> More software intensive Getting anything done requires better code re-use Core software efforts are larger, require ongoing effort from many contributors. (cf http://astropy.org!) You don’t have to be part of it, but, you should be aware of it Get to know the latest tools, maybe submit a bugfix here and there if you can Do better science, faster (hopefully!)
- 47. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Outline ’Slow’ radio transients How TraP works How do I use it? Future work Summary
- 48. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Different approaches Two main approaches to image-based transient surveys: Cataloguing: Extract source representations, store in database, analyze lightcurve catalogue. Difference image analysis a.k.a. image subtraction
- 49. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Lightcurve cataloguing Basic concept easily understood - ‘just’ glue together source extraction and lightcurve analysis. However, blind source extraction typically requires good signal to noise - will miss marginal sources. Crowded fields are also a problem.
- 50. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Difference image analysis Better at picking out faint sources in clean data, much better in crowded fields. Alard & Lupton 1997, http://adsabs.harvard.edu/abs/1998ApJ...503..325A Wyrzykowski et al, 2014, http://adsabs.harvard.edu/abs/2014AcA....64..197W
- 51. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Optical survey characteristics Which technique should we employ? Most transient surveys to date are optical: Fields often crowded or even confusion limited (best places to look for stellar flares, microlensing) PSF usually quite well behaved (smooth) Pixel noise usually uncorrelated / varies on a different scale to the PSF (dependent on sampling) =⇒ DIA is usually best approach.
- 52. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Radio survey characteristics: (Contrast with optical) Fields usually quite sparsely populated, at least with current generation of instruments. Noise is correlated on beam-width scale. Dirty beam / PSF may vary significantly from image to image. May be well modelled, but this depends on system characterisation. (Phased arrays e.g. LOFAR) May see artifacts due to side lobes from out-of-field bright sources. =⇒ DIA would cause many false positives, better to stick to high SNR cataloguing.
- 53. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Outline ’Slow’ radio transients How TraP works How do I use it? Future work Summary
- 54. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Summary TraP: Good for (wide-field) sparsely populated surveys. Can be used for real-time transient detection. Produces catalogue of all sources.
- 55. ’Slow’ radio transients How TraP works How do I use it? Future work Summary Summary TraP: Good for (wide-field) sparsely populated surveys. Can be used for real-time transient detection. Produces catalogue of all sources. Server-based / web-interface reduction model well suited to large, challenging datasets. Open-source Python / SQL, with comprehensive test-suite and documentation. http://ascl.net/1412.011