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Optical followup of LOFAR transients with the Liverpool Telescope

Optical followup of LOFAR transients with the Liverpool Telescope

David Bersier
LOFAR and the Transient Radio Sky, Amsterdam, December 2008



June 18, 2012


  1. Followup of LOFAR transients with the Liverpool Telescope David Bersier,

    C. Simpson, I. Steele, Z. Cano, C. Mottram, N. Clay (LJMU), R. Fender (Southampton), B. Stappers (Manchester), R. Wijers (Amsterdam)
  2. Basic Specification •  Fully opening enclosure (no delays for ToO’s)

    •  2.0 metre f/10 ALT/AZ •  2 degree / second slew speed •  Deployable, folding mirror, allowing support of up to 5 instruments •  Instrument change time < 30 seconds •  Common User Facility (typically 40-50 science programmes from around 30 different institutes, allocated by TAC’s) •  Fully Robotic (no night time supervision apart from start of night photometricity check, weekdays there is a daytime daily visit) •  LT is owned, operated and maintained by Liverpool JMU (30% of time for internal use). Instrument development done in house
  3. Current instruments RATCam - optical CCD camera: •  2048 x

    2048 pixels •  0.135 arcsec/pixel, ~ 4.6arcmin field of view •  Binning 1x1, 2x2, 3x3, 4x4 (default 2x2) •  No windowed modes •  Sloan u’g’r’i’z’, BV, Hα SupIRCam - JH near-IR camera •  256 x 256 HgCdTe, 1.7 arcmin field •  Pre-defined exp times and dither patterns •  An equal length dark frame is always taken before and after the dither.
  4. Current instruments (2) RINGO - optical polarimeter: •  objects are

    rings; broad “V+R” filter •  Same CCD as RATCam but only cooled to -10° (dark current ~ 1 electron/sec) •  Ability to measure optical polarization variations on short (seconds - minutes) timescales Meaburn - Low Res optical Spectrograph: •  Three fixed wavelength ranges, at 4 Angstrom resolution •  49 x 1.7 arcsec fibres RISE - fast Readout CCD: •  9.2x9.2 arcmin, 0.54 “/pix •  Frame-transfer CCD (no readout overhead)
  5. Future instruments FRODOSpec (Medium Res spectrograph 2009) •  IFU 11x11

    0.9” fibers •  2600/5500 resolving power •  Blue/red arms “Wide” field CCD (10x10 arcmin) late 2009 •  Standard filter suite (ugriz BV + 2) SupIRCam2 (6x6 arcmin) late 2010 Internal tip-tilt. JH filters Simultaneous optical+IR
  6. Three Basic Operating Modes •  Background mode (does standards!) – 

    Nothing to schedule –  Seeing > 3 arcseconds (or unknown) –  Something is broken (e.g. out of focus!) •  “Science Control Agent” - phase 2 database driven •  Target of Opportunity Mode –  Immediate abort of current observing –  Driven by scripts
  7. Phase 2 database •  Specifies the observational setup •  Current

    Methods of data entry: –  Phase 2 forms via email –  Menus for a specific science programme –  Robotic Telescope Markup Language (RTML) via unix socket or Web Services •  Future Methods of data entry: –  User Tool (Web based - Java Web Start) –  RTML via Web Services
  8. The LT “edge”: Target of Opportunity •  Rapid response: simultaneous

    observations radio+optical. NOT available to non-JMU •  Average reaction time ~ 3 minutes (slew) •  A client script (csh, Python) running at the telescope (e.g. GRB followup) •  An intelligent agent submitting RTML with the appropriate priority flag •  May include data reduction, decisions made on-the-fly (interrupt obs, instrument change, etc.)
  9. GRB observations (rapid response) Guidorzi et al., 2006, PASP Data

    reduction (CCD proc, WCS fit, phot) part of pipeline.
  10. Two-steps strategy Initially (until ~ spring 2009): low alert rate,

    delayed: manual response is enough. Optical only, simple observing sequences. Second step (summer 2009): Alerts sent via VOEvents. Automatic response. Software to parse alert, decide whether to trigger or not, choose appropriate observations (flare star of know opt mag: bright --> spec, faint --> imaging; unknown transient: fast+deep opt)
  11. Step 1: Manual response READY!

  12. IR imaging Spec Opt imaging Data reduction Data reduction: Is

    there a transient? Updated observations Continue observations Initial response 5x2min R LOFAR Alert Long-term goal ? ? ? Response in override mode: Data reduction allows to change observing strategy in real time. Filters
  13. What to observe Not all sources will need optical followup.

    LOFAR will do its own followup. Some sources do not need ToO mode, standard proposal will do. •  We can’t trigger on each and every transient. •  How will “optically interesting sources” be selected? “Feed us!” •  Known sources displaying >nσ brightness change •  “Uninteresting”: objects for which optical data not necessary or not urgent, e.g. monitoring. Already implemented! •  No counterpart at any wavelength: fast+deep